Introduction: Your First Lathe Project: the M40 Grenade From "Aliens"

This is an excellent starter project for the metal lathe; it requires several different operations, requires a minimal set of tooling, it can be done in lightweight 'free machining" aluminium, and aside from a couple of key places accuracy to within a thousandth of an inch is not required.

Materials needed;

1" Aluminium round bar (2011 T3 is a good choice for this)
3/8" Aluminium round bar
1/4" Brass round bar (I used C360 free machining brass)

You only need a foot or two of each of these (get enough for errors!) I bought mine at, which I have found cheap and fast to order from. Since you are turning all of this stock down anyways, you can make do with anything that is of at least the above dimensions.

5/16" x 1" compression spring.  (I used a Servalite #135U bought at Orchard Supply Hardware.)

Tools needed:

Right-hand turning tool, medium straight knurl, parting tool -- I'll discuss the tooling more within the Instructable.

Oh, yes; and a lathe.

Step 1: Don't Worry, I Have a Plan

The first step for any replica prop is to do the research, and draw up accurate plans.

The M41A Pulse Rifle, as created for the James Cameron classic action/SF film Aliens, is one of the top ten recognizable and emblematic weapons of the genre. Canonically, it fires 10x24 caseless armor-piercing bullets and 30mm rifle grenades in an over-and-under configuration. Only the M40 HEDP grenade is seen in the film, both as fired from the Pulse Rifle, and in a secondary role as a classic "push button, wait four seconds" hand grenade.

(Actually, it is hard to tell in the lighting of the scene, but at least one green-cap variant shows up in the Operations Room scene. "Don't touch that, honey; it's dangerous.")

According to the movie's armorer Simon Atherton, speaking in the DVD commentary, the original grenade props were manufactured from 12-gauge snap caps; soft aluminium or plastic dummy rounds used for training.

After some poking around at the Aliens Legacy forum and similar, I chose to make my own plans more-or-less from scratch. Which is a subject for another Instructable! The plans I made, I freely give back to the community.

Step 2: ..Some Kind of Rudimentary Lathe?

I've wanted a bench-top lathe for ever so long. But, actually, TechShop is a better deal. I get access to a full-sized gear-head engine lathe with DRO, plus a bunch of other tools, and I get a place to work that doesn't leave me with metal shavings in my soup (there are real downsides to building props in the same apartment you eat and sleep in).

TechShop will give you enough instruction to get started. But really learning how to use the thing is mostly up to you. I am self-taught, the grenade you see in the pictures is my first lathe project, and there's doubtless a lot of things I am not doing right. Despite that, I'm going to plow on anyway in hopes my own journey will be of help to others.

Yes, it is true. I made it at TechShop.

I'd recommend reading up a little. I read the manual (available in .pdf from Jet), and I read the perennial "Instructions on How to Use a Lathe," which can be found all over the web these days. And visited a number of helpful web pages and forums. And watched a bunch of YouTube videos, and of course asked questions of staff and other friendly people over at TechShop.

For me, the first big hurdle was just figuring out which knob and lever did what. Every lathe is different -- very different -- and the manuals are often not a help in figuring out how that specific lathe's controls interact with the basic and common principles of lead screw and half nut and so forth.

Also for me, the biggest "Aha!" moment was reading about the physics of what actually happens at the cutting edge. This is key to understanding why too slow a lathe speed is as problematic as too high a speed. If you've been using hacksaw and drill and grinder, you've probably gotten used to the idea that you get cleaner results by going slower. That is not necessarily so on a lathe.

This is because of what is actually happening at the cutting tip. The edge of the cutting tool is forced into the stock so quickly the metal heats up. Heats up a lot; heats up until it becomes soft enough to curl away from the blade in a nice chip. This is less of an issue with aluminium, which is naturally quite malleable, but even then the principles become clear in practice; at the right speed for the work, you form nice curls of metal for chips, and leave behind a smooth surface.

Okay, the first thing to remember is that the lathe is the most deceptively dangerous tool in the shop. It doesn't have exposed slicey blades, it doesn't come "whang!" down like a hydraulic press or whine like a table saw. It just sits there quietly spinning. Spinning at such speed, and with such torque, that it can pick you up and wrap you around the spindle on a second's notice. Enough speed that it can pick up a carelessly placed object like a chuck key left in the chuck and throw it at high velocity across the room.

Here's the basic things I try to do:

1) Stand clear of the chuck. Don't reach over the chuck while it is spinning.

2) Before leaning over the lathe to set up a finicky operation, turn off the power with the Emergency Stop switch.

3) Never take your hand off the chuck key. Insert, tighten, remove without letting go.

4) Before engaging power, swing the chuck around manually to check clearance.

5) Before starting a cut, double-check all your settings.

6) Before engaging power feed for the first time on a new setting, dial yourself well off the work and try it in empty space first.

7) Stay conscious of where the brake bar is so you can stomp it in case of trouble.

8) Don't hurry, and don't push on if something seems wrong. Cut the power, check to find what the trouble is. You can always continue a cut after you've confirmed everything is nominal.

Lathing is, like most things, both art and science. In practical terms, especially when hacking out a bit of free-machining aluminium to a standard of .01 inch or so, you can set all your speeds and feed rates by eye and instinct. But to develop that instinct, you need to spend time with the calculator getting the speeds book-right first.

I'll go into details on the choices I made with this project, again with the understanding that I am still learning and some of my choices may not have been optimal.

But that again is a wonderful thing about the lathe; there are almost always several ways to do an operation, and the choice of which is more-or-less up to the individual machinist in the situation they are operating in, the context of the piece being made, and their own experience.

Step 3: Op Orders

As in many projects, it matters what order you do things in. Some steps are difficult if not impossible if taken out of order.

Beyond that, you want to be efficient. Plan so you do as many operations as you can at the same time, without having to re-chuck the work or re-set the machine. A quick-change tool post makes this much simpler, as you can swap in and out tools and know you will be coming back to the same settings each time.

Here's the basic order of operations for the grenade:

1) Face off and install a live center

2) Turn down

3) Cut all grooves

4) Turn nose into groove.

5) Turn rear angle into the groove there.

6) Cut the nose radius

7) Knurl

8) Drill for the button

9) Part

At this point, you remove the grenade from the lathe and re-chuck it backwards.

10) Drill to button shoulder, then to plug shoulder

You remove the grenade body and set it aside. The next operations are performed on the 3/8" stock;

11) Face

12) Turn down, and turn down spring retainer nub

13) Chamfer

14) Part

Then another piece of the same 3/8" stock;

15) Face

16)  Turn to nearest thousandth of an inch, and turn down spring retainer nub.

17) Chamfer to make insertion easier

18) Part

The last piece of stock goes in the lathe now; this time the brass 1/4"

19) Face

20) Turn primer and chamfer slightly

21) Part

If all went well the grenade can be assembled now; button first into the hole, then spring, then the plug. The plug is cut just fractionally over-sized so once it is in, it will never come back out.

22) Arbor Press

Now the assembled grenade goes back in the lathe;

23) Face rear and face down to dimension

24) Turn rim

25) Cut shoulder

26) Chamfer rim

27) Drill primer hole

28) Chamfer edge of primer well

29) Insert primer

30) Grenade!!!

Step 4: Face Off

The first step is truing the material to the lathe.

Raw material straight from the mill is neither quite straight, quite round, nor does it have a smooth surface. Before you get into detailed measurements, you want to face it off and turn it down a little.

In the case of this grenade, it is just long enough in proportion to its diameter that we are better off supporting it at both ends.

So, in short, the first step is to chuck it, face it off, set a live center, turn it down, then zero the DRO.

Don't worry; I'm going to explain all of these terms as I go! That's the purpose of this Instructable, after all; a sort of walk through a typical lathe project for rank beginners like me.

First step is to chuck. For simplicity let's use the three-jaw chuck. Center the piece between the jaws, and turn one chuck key to close all three jaws. You want as little of the stock sticking out as will let you perform all the following operations without moving it. This is important, because a three-jaw chuck in perfect condition is barely going to get you three thous of runout. Which is to say, if you open the jaws, shift the piece a little because you don't have enough length exposed, and close the chuck again, you could be as much as six thousandths of an inch out of round.

At the same time, the more that is exposed, the more wobble. The rule of thumb is no more than three times the diameter of the stock should extend out of the jaws without some kind of secondary support.

We'll get there in a moment.

For the grenade, we will be parting at 2.4". The parting tool is 1/8" in width and should have some clearance with the jaws, so call it 3/8" extra for parting off. Because the end might not be particularly square (especially if you cut a hunk of stock with the bandsaw for this project) allow at least an eighth of an inch for cleaning up the face.

This leaves us with a bit under three inches that needs to be outside of the chuck jaws. Tighten well, but not gorilla-well. Turn the chuck by hand to make sure it clears, then stand back and spin it under power for a few moments to see how bad it wobbles.

If the stock is clean and the chuck is in good condition, you should see a slightly blurry surface as the stock revolves, but no visible shake or shimmy. If the lathe vibrates, chatters, or squeals, you've got a problem.

Step 5: Face Off, Part II

My first grenade, I re-sharpened some carbide insert bits that were lying around the shop. For the second one, I'd purchased a few general-purpose bits from McMaster-Carr. Bit shape, and bit grinding, is a bit much to get into at this moment so let's assume you have the general idea and a suitable bit.

Another mental stumble I had for the longest time came from my experience with woodworking. Here's the trick; in a metal lathe, the cut surface the operation leaves behind is not the surface the cutting edge of your bit is applied to.

Let me put that a different way. When you use a wood plane, the blade -- the cutting edge -- is against the wood and moves along the spot you are planing down. In a lathe, the bit moves across the fresh surface you are creating. The cuts are orthogonal to it.

Okay, let me try it again. When you face, the blade is cutting IN towards the center of the work. When you turn, the blade is cutting ALONG the length of the work.

Once you get this clear in your head (it took me a while!) it becomes a lot more clear how to set up the bit for a clean cut.

Understanding this is key to understanding relief. The edge of the tool angles away from the point where fresh metal enters the blade. The tool does not touch the work after the cut is made. Once you stop feeding, there is only the slightest contact between tool and work. Most of those fancy grinds on a tool bit are to achieve just this; make sure that only a small part of the blade contacts metal, and it only contacts that metal that has yet to be cut.

With a general-purpose bit, you can set up for facing with the tool roughly at right angles to the ways -- that is, to the spin axis of the lathe. You may need to angle the tool post a little in order to achieve a proper relief for the edge that otherwise would rest against the freshly-cut face.

The next step uses the most useful tool I own; a short piece of cut-off bar stock.

Using the carriage and cross-feed wheels, bring the tool into the side of the work with the lathe off and not spinning, and gently pin a short steel ruler or something like my Most Useful Tool between work and cutting edge.

What you are trying to achieve is a tool height that is exactly centered on the work. Using a piece of flat stock or a ruler like this, you can judge whether you are a little high or low by whether the ruler tilts away from you or towards you.

Turn the thumbwheel to raise or lower the tool holder on the quick-change tool post, lock down with the handle, then re-check. Always re-check after locking down -- in almost every setting you make.

Step 6: Face Off, Part III

Now that the bit is positioned correctly, set the speed for the lathe. The simple form of the calculation is SFM (Surface Feet per Minute) times 4, divided by the diameter of the work in inches, equals the RPM. You'll have to look up the SFM for your material. Aluminium is generally given as 250 FPM, so the calculation gives us an RPM of 1,000 for this particular piece.

As a rule of thumb, go up to 2x faster with carbide tools, and half the speed when parting.

The lathe I'm on offers 910 RPM as one of the gear settings, which is just fine for me. Aluminium is forgiving about being spun a little slow (feed rates are more critical with it).

The usual procedure is to "touch off" or "kiss" the piece. I don't usually do that. Instead I take my first cuts somewhere between roughing and finishing depth -- 20 to 40 thousandths of an inch -- and use that as my zero reference.

Once the lathe is turning, start off the end of the piece, bring the bit inside the diameter using the cross feed wheel, then carefully bring it up against the end of the piece using the carriage feed wheel. It helps to use both hands on the carriage wheel for this. When you've made a good "bite" -- enough to form well-defined chips -- lock off the carriage using the lever, then switch to the cross-feed wheel.

Turn smoothly at a steady rate until you've reached center and chips have stopped forming. If you are moving at the right speed you should get well-formed chips. Too fast, and you will get tiny broken chips.

Shut off the lathe, and eyeball the cut. If the face seems fairly smooth, this is now your established zero. If there is a slight nub left, you are too high or too low, and now is a good time to adjust that, too. If the cut is rough, check your RPM, your feed rate, check the bit for sharpness, lubricate. For a good finishing cut nothing beats using auto-feed -- but again, we'll get to that!

Step 7: Welcome to the DRO

Most modern lathes have a Digital Read-Out fitted to them. This is a lot easier than squinting at the tick-marks on a handwheel. 

The two metal lathes at TechShop San Francisco (Jet Model GW-1440 W3 if I recall) both have DROs.

So here's the idea; get a clean surface that is aligned to the lathe (by facing and turning). Now, without moving the tool from the last operation, zero the DRO. Now you know that any number you see on the DRO will be relative to the surface you just cut.

Following facing, you zero the Z axis (that's the spin axis) on the DRO. This means the cutting edge of my tool will display its position relative to the clean end of the piece.  Here's the trick; because of backlash on the carriage traverse, you have to do this in a specific way. Starting off the piece, turn the handwheel until you are cutting into the face. Now lock off the carriage using the lock-off lever. Use the cross-feed traverse to make your facing cut then, without touching anything, go to the DRO.

Hit "set zero" and then hit the button next to the Z axis.

You do the exact same thing to set the X axis on the DRO, except that you are working at right angles to the previous operation, and you don't need to lock the cross-feed.

There's another wrinkle to this, but before we do anything else to this piece we need to install the live center.

Step 8: Centerfire

The most accurate work-holding method on the lathe is turning between centers. That is the only method that allows you to restore a piece exactly after it has been removed. We're not going to do that here. Instead, we're putting in a live center to keep the end of the stock from flexing about quite so much.

That great old, oft-reprinted guide to the lathe has a little chart for picking the right center drill. Based on it, I'm going with a #2 Center Drill -- that will leave a hole about 9/64ths, which is well under the size of the holes I'll be drilling later.

There are rules of thumb for the RPM for drilling as well, but there are also online calculators, and a handy chart I found here. With that, I know that my previous RPM of 910 will work for this operation.

The trick to all drilling on the lathe is lock the tailstock, and advance the quill carefully using the handwheel, back off frequently, and lubricate well. Aluminium in particular clogs up the flutes, and those wads of aluminium chips will scrape against the sides of the hole and tear it up. This is less of a problem with center-drilling, since you aren't going very deep.

In fact, that's the other thing to remember for the center drill; don't go past the tapered portion. Stop about 2/3 of the way.

Once it is drilled, clean the hole, back off the tailstock, remove and put away the center drill. If you back off the quill all the way the drill chuck will drop off into your hand.  Do this, run the quill out an inch or so, wipe down the shank of the live center with a clean rag and press it in.

Now you can run the tailstock back up to your piece and lock it off. Turn the quill until you feel resistance, and the live center has seated firmly in the quill. Now lock the quill and manually spin the chuck to make sure everything looks good. Power on, and listen for noise and vibration.

As a practical matter, this grenade is so small there simply isn't room to get the tools into the piece on some of the operations. So you'll be backing off the live center as often as not. But it is nice to have it there when you can!

Step 9: Turnabout

So you've chucked the workpiece firmly and set a live center as additional support. You've selected a turning bit and clamped it into a tool holder, set that into the tool post and set the height. You've calculated the RPM and set the headstock gears.

Now you can turn down the piece.

But first, back to the DRO. Assume you've taken the first cut to center the remaining stock in the lathe. And perhaps a finishing-level cut just to have a nice surface to work from. Without moving the cross-feed, zero the X axis on the DRO.

The DRO will now tell us how far we are moving the tool. But it won't tell us the actual dimensions. For that, we need to apply calipers to the piece.

Again, the accuracy of even a good pair of dial calipers is generally considered at no better than a couple thousandths of an inch. Some may display up to a ten-thousandth (like our DRO), but they can't actually measure that with any reliability. So take several measurements, wipe off the blades frequently, and try to get a good average.

Now, you can do the rest of the project juggling numbers in your head; the actual diameter of the piece minus any tool bit movement recorded on the DRO. can go back to the zero button and enter the actual diameter of the piece into the display. (Or radius, if you prefer to work in radius.)

Some people like to do it the other way; they enter the difference between the current size, and the target size, so the number on the DRO will be zero by the time you complete the finishing pass. So far this is not for me -- but it does mean I have to juggle a different
set of numbers in my head; the difference between what my last cut was, and what my next cut should be.

With aluminium, you can hog out a hundred thou at a pass (remember, you are measuring in diameter but cutting in radius, so taking the diameter down by .02 inches is making a cut that is .01 inches in depth. This is a rare case in which it is good to "sneak up on it"; hog the first cuts, then take smaller cuts as you are converging on the final measurement, then one or two finishing passes for that last handful of thous. The finishing cut can be anywhere from 5 to 20 thous, depending on how fine a finish you want and how much confidence you have in your measurements.

Because things slip, tools wear, and calipers aren't perfect, you want to stop at least once (and very much you want to stop before the finishing pass) to apply the calipers again. I call this a sanity check, and I do it for all critical lathe operations.

For the grenade, most of my passes were in the range 40-120 thousandths. This may or may not be right, but it felt right, I had well-formed chips, and the surface finish was decent.

To get a clean surface, these things are key;

1) A clean and sharp tool. If necessary, re-grind, and stoning is recommended for the finest passes.

2) Higher speeds.  Use speeds near the higher end of your choices will usually improve the finish.

3) Sufficient lubrication.

4) Slow, controlled feeds. You will know when you have a good feed rate when the chips are well-formed. You will know you aren't moving at a steady rate when you see banding. The latter is toughest; use both hands on the wheel and practice a lot.

That last, though. What is really smooth and controlled? A gear.

Step 10: Full Auto

This is what makes an "Engine Lathe" out of a, well, lathe. It is that is has a feedscrew driven by the same motor that turns the headstock. One of the controls on the carriage will turn what is called the Half-Nut to lock it to this screw, driving the carriage automatically.

The feed speed is geared down to some fraction of the number of turns the headstock makes, and that ratio is selected by the complicated set of levers and dials on the lower part of the Jet Lathe's panel.

In the case of the Jet at TechShop San Francisco, there is both a feed rod (for making powered cutting passes) and a lead screw (for cutting screw threads). And as I mentioned before, every manufacturer is completely different in how they lay out these controls. It is as if a Ford has the modern layout, and a Chevy steered with a tiller, the gear shift was a set of buttons on the dash, and a lever extended from the roof of the car for a brake handle. And it had six wheels.

So all I can really describe is the controls specific to the lathe I am using at this time. Which you see in the picture.

I really don't know a good rule of thumb for a feed rate. Instead I know what the proper speed looks like. So I set the dials until the thing is feeding at about that speed. For this project, most of the cuts ended up being B range, setting F, and from 1 to 4.  Somewhat less speed for the cross-feed...and of course adjusted to match when I changed RPMs.

Once everything is set and the speed tested (with the tool safely off the work), power up the lathe, then push the half-nut lever on the carriage. Up is traverse, down is cross-feed -- and there is a little diagram by the lever if you forget. Keep one hand on the half-nut lever and lubricate with the other. When the cut gets close to the end (1/16" if you think you can handle it), throw off the half-nut and take up the motion immediately with the longitudinal traverse wheel until you've reached the end of the cut.

And that's a powered cut.

Step 11: Grooving

As passed over briefly above, you turn the grenade down 20-60 thousandths of an inch to clean it up for the next passes. Then you turn down the body to .8", with the last pass being a high-speed, slow-feed finishing pass, because this surface will be the bulk of the visible surface once the grenade is finished. This set of cuts is up to a shoulder at 2.22" from the front of the stock.

Now comes the delight of the quick-change tool post. Turn the lever to release the holder carrying the turning bit, and set that aside. Set a parting tool in the appropriate tool holder for that tool, slide that on to the tool post, and set the height. Getting the cutting edge of the tool exactly on center is even more important for parting tool (also called cutting bars).

The other thing that is really important is the tool must be exactly at right angles to the lathe axis. The simplest way to do this is to loosen the tool post (the big nut on top, which takes a wrench) and traverse the tool post into a flat portion of the chuck, between the jaws.

Tighten and re-check everything. Now, of course, your zero will be different, because you've changed tools (and possibly tool post angle). For cutting the grooves, I find it close enough to push the tool up against the end of the workpiece and use that as the Z-axis zero. For the X-axis zero, I will actually power up the lathe and bring in the tool until it just grazes the surface of the workpiece.

Given the rule of thumb of parting at half the RPM, I dialed down to 460 RPM. I advanced manually -- you don't want to use power feed for parting -- and lubricated copiously. If you hear squeaking, that usually means the tool isn't quite straight, and one side of the tool is rubbing against the side of the cut. If it chatters, you are either off in height, or you are being too tentative; push it in until you get nice curls out. If it doesn't cut at all, don't force it; again, you are probably off in height (or the tool needs to be sharpened).

The first grenade, I used a 1/8" blade for the rear groove, and the others were 3/32".  For the second grenade, I couldn't find the 1/8" parting bit, and I'd changed the drawing to reflect a slightly wider groove anyhow, so I made two parallel cuts. I got close enough in depth you can barely see the line between them.

Oh, yeah.

And although you see a 3/32" in my picture of the tools I used, I didn't have the appropriate tool holder for it. So instead I ground down the end of a broken-off stub of a 1/8" bar. Not to go into bit grinding, but once again the key is relief; parting tools are designed with either a T-shape or a similar hollow cut back from the cutting edge, so only the cutting edge touches the workpiece.

You can't grind high-speed steel on an ordinary stone, but fortunately TechShop has a wheel designed for grinding tool bits.

Step 12: Nose Art

Once the grooves are cut, the next shaping steps begin. 

Return the turning bit to the tool post and turn down the nose of the grenade.  The first passes are turning to the outside diameter of the little ring that functions as a retaining stub for the plastic safety cap. There's no need to worry about the shoulder as you are cutting towards the deep groove you already created.

The next passes are cut to the shoulder of the ring, and here it is more critical to have the tool angled correctly to make a good shoulder, and as well to keep a careful eye on the Z-axis measurement. These cuts are so short you will be doing them manually, not with power feed.

Step 13: For Special Services

The next two cuts are different. Although you can cut the nose radius by manipulating the cross feed and traverse controls, to get the most control of the depth and placement of this cut you will be using the compound slide. This also marks the last time you are going to need the current X and Y values on the DRO, and the current position of the tool rest. All the cuts that needed to be completed with those settings are now done.

The compound slide is the part of the lathe carriage that sits on top of the cross slide. Like the tool post, it can swivel; there are two set screws in the base, along with a marked indicator ring that will allow you to set it to a specific angle.

45 degrees is about right for the first operation. And conveniently enough, this is the normal position for the compound slide. You will have to unlock the slide in order to use it; there is a set screw in the side of it.

Turn the tool post until it is aligned with the compound slide, then adjust the traverse and cross feed to bring the tool up to where you want the cut to begin. Now lock off the carriage, back off the compound slide slightly, and power on the lathe. Use the compound wheel this time, to run the tool lightly into the work. Note that there is no DRO for this operation. In the case of the nose radius, just do it by eye.

There are radius tools available from McMaster-Carr. It is also possible to cut a radius using an ordinary bit, via either simultaneous manipulation of both wheels, or by rotating the tool post. I chose neither of those options, but instead used the corner of the grinding wheel to make my own radius tool.

You can get blanks of high-speed steel (aka tool steel) from the usual suspects, or you can re-grind a broken or well-used bit. Or the heel of a perfectly good tool, as I did. Having the radius on one end doesn't hurt the primary cutting end at all. Again, this kind of improvisation might not be suitable for production work on a steel workpiece, but for taking a sixteenth of an inch off a piece of soft aluminium it is quite sufficient.

The next cut is putting the angle in. For this, we set up as if turning, except that the bit is aligned to the compound slide instead, and the compound slide is set to the angle desired for the taper we are cutting. After that it is just like turning, except that after you use the cross slide to bring the tool in to the work the desired amount, you turn the compound slide wheel instead of the longitudinal traverse wheel (aka instead of traversing the carriage).

I don't have a good way to measure those cuts, except to mark the workpiece in ink and keep bringing the cuts in via the cross feed until the cutting edge finally touches the ink mark on the final pass.

Step 14: Duke of Knurl

Knurling is the formation of decorative patterns in the metal. Knurling is also the operation that is more dependent than any other on adequate support of the work.

Because knurling is not a cutting operation. The knurling wheel actually pushes the metal out of the way. This takes, as you can imagine, a fair amount of force. And the last thing you want is for your nearly-finished grenade body to pop out of the chuck and go flying across the room.

There is a school of thought that believes you need to multiply the pitch of the knurl wheel by the approximate diameter of your workpiece, then find the nearest whole number of knurls that will fit and turn the piece to that. I can understand the instinct but in practice it doesn't seem to work that way. The contact has a certain amount of "slop" in it, and given a chance the knurling tool will slip or climb enough in order to land in existing grooves.

That means the real trick to getting a good knurl is establishing those grooves.

Again, unlike many other operations in metal or wood working, being gentle will fail. You need to jam the tool in. Jam it in harder than you would think would be a good idea. Only then will you get the clean knurl. In the example below, I twitched a little; I didn't jam it and hold it tight, and it bounced out a little and made that double-cut you can see if you look closely. Once it was established, though, the wheel showed no tendency to slip out or otherwise double-track. I was able to clean up my poor initial pass slightly by adding a little more pressure and rotating a few more time.

You knurl at much lower speeds than you turn -- down near the lowest speeds available on the lathe.

And, well, that's about all there is too it. Hold the workpiece firmly, support it with the live center, line up the knurling tool as closely as you can, and use the compound to align the compound slide along the direction you will be pressing the knurl. Lock down everything, start the lathe, and twist the compound slide wheel hard to jam the knurl in. For a longer knurl, you'd immediately start the traverse as soon as you'd made one or two rotations. For this, you run it two or three times around and stop the lathe. You don't need to cut any deeper than that first deliberate pass.

Step 15: Drill, Baby, Drill

Now is the time to remove the live center for good, because we're putting a drill bit back in the tailstock.

The first hole is the diameter of the button itself, and only needs to go about an inch into the piece. The second is drilled part-way to create a shoulder; it retains the button so it doesn't fall out the front of the grenade.

Really, you shouldn't drill to diameter. Especially aluminium, which clots up the flutes of the drill bit and goes on to chew up the insides of the hole. You should drill undersize and use a boring bit to clean up the hole and bring it up to the finish dimension.

But boring tools in this size are expensive and fragile and really annoying to set up right, and these holes just have to be good enough to hold the button and spring in place, so...we drill.

Although it doesn't matter for the first hole, I'm going to go into how to get the depth correct. You will notice the DRO doesn't care what the tailstock is doing. Instead, there is a dial on the tailstock quill.

I don't have a better way of finding the zero for a twist drill other than drilling a short ways in, then with the power off advancing the drill bit until the outer corner of the flutes seems to be right at the surface of the workpiece. But once that is done; holding the quill firmly, turn the dial in front of the quill handle to zero it at this position.

Now, to drill to depth, turn the wheel ten times the depth in inches, and on the last turn/tenth of an inch use the dial to get to within the required thousandth of an inch.  What with backlash and the inaccuracy of the sighting method I described above, though, you will be lucky to be within half a hundredth.

In re the actual drilling; set the RPM according to one of the resources linked to previously. Advance the bit no more than a quarter-turn at a time; you should not be able to feel it bind. With each advance, back off half that and come in again. You want to move forward like the frog escaping from the well; three hundredths forwards, one hundredth back. About every three drill diameters, shut down the lathe and back the drill completely out to brush off the accumulated chips and re-soak it in lubricant or cutting oil.

You don't want to withdraw the bit under power, because if it does jam on clogged material, you can pull the drill chuck right out of the quill. And that would be bad.

Step 16: Parting Is Such Sweet...

..No, I'm not going to say it.

I am told many people fear parting. I can't say I've had any issues with it. For me, it is just like cutting a groove, except it takes longer.

For this grenade, something I learned on the second one; you might want to put a piece of wood under it before finishing the parting. Contrary to expectations, it doesn't go flying out of the lathe when you cut through, but it does fall, and my second grenade fell on some metal on its way down and got itself a couple of dings.

Oh, and the location of the part is completely arbitrary. The only thing that is nice is if you have it at a whole-number dimension from the front of the grenade it will make it easier to figure out the depths of the blind holes you'll be drilling from the other side. This is why the plans specify parting at 2.40 inches. But it doesn't matter if the cut is a little ragged (which it will be); we'll be trimming about .12 inches off the back of the grenade after the working parts are assembled.

Once the piece is parted, remove the remaining stock from the lathe, clip off any nub left from the parting operation, and wipe it down. You've made the first part of your grenade.

Step 17: Action

The next operations are performed on your billet of 3/8" stock.

Here's what we are after; a spring-loaded button you can actually depress. How is this achieved? I got the concept from Steven Pasek, whose very nifty construction drawings for an M40 grenade were linked to by the Aliens Legacy forum.

The button is turned to just slightly smaller than the front hole, and is long enough to travel smoothly in and out. Then the hole through the grenade body is widened (drilled or bored) to the next drill size up. The button has a t-shape; the base just fits down the larger hole, meaning the button will only protrude so far until it stops.

The spring goes in that same hole, and what retains it and keeps it under compression is an aluminium plug in the base of the hole. To make the process of pressing the plug in an the arbor press a little less scary I did my first ones with a shoulder on the plug as well. I've stopped doing that since, and redrawn the plans to reflect that choice.

The plug, of course, has to be press-fit, because the final operations are performed with the lathe on the back of the grenade. If the plug isn't a tight fit, it can be caught and pulled out during the lathing.

And this gives rise to a problem. To get this interference fit, the plug has to be within .5 to 1.5 thousandths of an inch larger than the actual hole as drilled. If the lathe is run out a bit, that hole can be oversized by that same amount. And if you drilled a larger hole for a shoulder -- as I did -- you end up very close to the starting diameter of the stock for the plug. 

Which means the the three thousandths or more runout of even a three-jaw chuck in good condition was unacceptable. By the time I turned the billet round and true to the lathe, it was too small for the plug.

And this leads us directly to learning how to use the four-jaw chuck.

Step 18: What's Up, Chuck

Before you remove the chuck on a lathe, find a piece of wood to lay across the ways. You don't want to ding those up because a fifty-pound chunk of metal slipped out of your grip.

With the chuck key, turn the three set screws closer to the headstock, the ones that hold the chuck on. Line them up to the indicator marks. The chuck may loosen up at this point, but more than likely you will need to give it a few friendly taps with a rubber mallet. Loosen it, pull it free, put it aside. Lift the four-jaw chuck into place and set all three screws. Then go around and tighten each of them, making sure they end up between the two caret-shaped indicator marks.

Rotate the chuck by hand as a sanity check.

With the four-jaw, each jaw moves independently. If you can get the jaws relatively close first, it will save a lot of time later. You can line each jaw up with one of the rings cut into the chuck face. Or you can also start with this, then turn the chuck handle for each jaw by the same amount (full turn, half turn, etc.)

Or, for some pieces, you can fake your way towards getting it close to centered by using the tailstock as a guide. For the grenade body, once the hole was drilled in it I could support it on the correct size drill bit and move the chuck jaws in to just barely touching (you don't want to tighten more than that, because you could break the drill bit doing that!) For small enough stock, you could chuck the work itself in a tailstock drill chuck.

In any case, you are only aiming for an approximate center. If you can get it to within a tenth, it will a lot easier to do the next steps.

Bring in all four jaws to just touching. Notice the jaws have a slight radius cut in them; you want to be inside this radius, not hanging up on the corners.

Now set up a dial indicator on the the cross feed slide. The complete dial indicator assembly has a magnetic base which you can clamp to the cross slide. Set up the arm so the gauge is face up, behind the work, with the indicator probe (plunger) pressed against the center of the work.

Hand-rotate the chuck (it helps to throw the lathe out of gear for this operation -- and don't forget to ensure it stays off by pushing in the emergency stop button), until two of the jaws are laying horizontal; parallel and in plane with the dial indicator. You will work on these two jaws first.

Turn the chuck 180 degrees from one jaw to the other in your starting pair. Make a note of which is the lowest reading on the dial indicator. When you've found that, put that jaw nearest the indicator, and then use the cross-feed slide wheel to move the entire dial indicator assembly. Turn the cross-feed wheel until the dial indicator reads zero.

Now rotate the chuck 180 to the other jaw in your first pair. Make a note of your new high reading. Now divide that number by two, and using the cross feed wheel again move the dial indicator assembly until you are showing half the high reading you were before. Check by rotating the chuck around. Your two selected jaws should now be equal amounts on either side of zero.

The next step is easier if you have two chuck keys. With one in each hand, and your first pair of jaws once again flat to the ways and parallel with the dial indicator, relax one and tighten the other until the indicator reads zero.

Rotate 180 until the other of the jaws of the first selected pair is towards the dial indicator. You should be very close to zero on this side, too. If there is a significant remainder, go through the same steps again; find the high side, split the difference on either side of zero, move the jaws into that zero.

Once you get within five thou or so, it is smart to switch to the other pair of jaws and ensure those are also converging on zero. If the piece is seated properly in all four jaws, all you will have to do is lightly tighten the second pair. If your piece is twisted a little, checking the second pair will reveal it. Bring this second pair to within your desired thou, then go back to the first pair.

Once you are within the degree of accuracy wanted (with a little practice you can get under a thou within a couple of minutes) freely rotate the chuck around a few times and see if the needle wobbles more than you want it to. Be careful during this process you haven't accidentally backed one jaw completely off the work; it helps to do a last pass tightening each pair of jaws whilst watching the dial indicator to make sure you aren't pushing the piece out of alignment.

Step 19: Button

The button and end plug are more of the same; chuck the stock, face it off, zero the DRO, then turn down to dimension.

The difference is that while you can get sloppy with the outside cuts on the grenade, the button has to be within a few thousandths of an inch in order to move smoothly. The plug needs to be to within even greater accuracy; half a thousandth or better if you can achieve it.

For the button, you turn down about 1- 1/4" inches of stock to the inside shoulder; 4-5 thou smaller than the main bore of the hole through the center of the grenade. If you drilled it with an 11/32" fractional drill, it will be about .344" in diameter. If you turn the shoulder of the button to .342, there will be .03 of metal resting against the constriction into the smaller hole in the front of the grenade (drilled at 5/16", which translates to .312")

(And, yes, you will be flipping back and forth between fractional and decimal frequently, with excursions into metric as well, when you do projects like this one. Get used to it.)

Once the shoulder is turned down to a loose fit, turn the button itself to just a hair under .312 (the final number depends on how smooth your hole is; if you had to clean up the hole with file or emery paper, as I did, it will be a few thous wider.) Start by turning down to a generous .316", and test-fit while the stock is still on the drill. Take finish passes of one or two thous until it slides smoothly.

The first time I did the button, I flipped it in the lath in order to cut the spring retainer nub. This is a stub of arbitrary length that is turned to comfortably smaller than the internal diameter of the spring. The purpose of it is to keep the spring from riding along one edge of the hole and perhaps getting pinched or otherwise hanging up. My spring measured out to .24 so I cut to .220".

For the grenade I documented for this Instructable, I simply used the parting tool to take a groove down to the required diameter, then moved the tool over one tool width to part the finished piece.

Don't forget to dress the front edge of the button before you finish with this piece. A light touch with a file is sufficient, but I'm not comfortable with filing on the lathe; instead I took a very light chamfer with my turning tool, just breaking the edge. 

(Err, that's what it is called when you dress the sharp edges left by a lathe operation; you "break" the edge.)

Step 20: Plug

Honestly? The tolerance required to make the plug right is really beyond that of this particular lathe and process. We're simply not set up to get within "tens" -- that is, ten-thousandth's of an inch accuracy. 

Fortunately aluminium is, again, forgiving. The piece can be up to .0015" large and it can still be crushed into more-or-less place by the arbor press.  The trick here is designing so the spring is already under a little bit of compression. Since the spring is 1.125" long, and the required button travel is only .240, we have a good .2" of play in how deep the plug seats. As the parts are designed, the spring is compressed to .9" already. This means if the plug jams by .1" the spring will still be under compression and the button will still sit out as designed.

In the other case, the final dimension of the grenade is .12" smaller than the current length -- extra that will be taken up when we dress the base. And the flat surface of the arbor press ensures the plug can't be driven more than a few hundredths of an inch deep, anyhow.

And this is how you do it. Cut the plug as close as you can to .0005" to .0015" larger in diameter than the hole. Give the inside edge a generous chamfer to help the plug ease in. Apply a light coating of oil, line it up carefully in line with the axis of the arbor press, and set it in. And if it sticks out just a little, it isn't a problem.

An alternate method is heat fitting.

Here's how that works. Metals -- like many materials -- expand when heated. The co-efficient of thermal expansion for pure aluminium is 23.1 x 10^-6 (that is, per Kelvin, at 20 C.) Anyhow, it works out to roughly half a thou (.00048") for each 100 degrees F. Bad things happen to aluminium alloys when overheated, but 300 F in an oven is not a serious problem for it. Which, according to calculation, would make the drilled hole temporarily 1.5 thous larger (assuming the plug remains at ambient temperature, aka around 70 degrees).

The downside is that aluminium is a great radiator (which is one of the reasons you find it in heat sinks). The piece will come to thermal equilibrium with the environment quickly after removing from the heater (aka it will cool down). You have to work fast, and if you hesitate, you could end up with a plug forever stuck half-way in.

Which is why I recommend the arbor press instead.

Step 21: Shotshell Rim

Assuming the plug seated correctly, you can now treat the grenade as a solid block of metal. Chuck it up again, and face off right across the original base and across the plug.

After measuring the actual width of the base using calipers, and zeroing the DRO to that, face down to the correct rim thickness of .10" -- if you've done everything else right, the grenade will now be 2.3" overall (minus the button). I used an RPM of 910 and a slow powered cross-feed to get the cleanest surface achievable.

Next is turning down the rim to .886". For this, don't bother with power feed.

To achieve the look of a proper shot-shell, you need to cut a gentle angle into the base. There are various ways to do this. For my first grenade, I lined up a parting tool at the correct angle and used the compound slide to drive it in (similarly to how the angle under the knurl was achieved.) For the grenade I recorded in this instructable, I set up a left-hand turning bit and drove it straight in using the cross-feed slide until it touched the grenade body.

At the moment the final dressing of the rim is arbitrary; a couple of very gentle chamfers to break the edge a little and round it slightly. You can do better if you wish -- you could even chose to do a proper radius.

(I think I undershot on this my second grenade; the first had a thinner rim and I think that looks better).

Step 22: Primed...

The last machining to be done to the grenade body is drilling the primer hole and chamfering the edge of the primer well.

This is, on paper, straightforward enough; chuck an 11/64" drill bit in a tailstock chuck, zero the quill, run it in to .30" deep, pop a threading bit or similar into the quick-change tool post and lightly open up the edge at about a 35' angle, going only .02 or so further out than the original hole.

Except. Except you won't always have the right drill bits in stock. And they won't always be straight. And the bits may clog during drilling leaving you with a less-than-clean hole. 

At a great many of the stages of this grenade build, you must measure what it is you actually cut, and record that measurement. If you've drilled the primer well too deep, for instance, you need to modify the length of the primer.

In the case of the grenade recorded in this instructable, I couldn't find the 11/64" bit. The available size was just a hair too small to look right. Now, you can save yourself a lot of trouble in drilling if you purchase some boring tools. Those allow you to make much more accurate holes, and make them whatever diameter you wish.

Lacking those tools, I tool a scrap of mild steel (not even tool steel) from the scrap bin and ground it in to an impromptu boring tool. It worked -- at least in that I was able to open up the hole by the few thous required. It also left a rather messy hole, but this is not really a problem since the primer hides it.

My improvised tool also pushed out a little metal on the back of the grenade, requiring it to be re-faced. Fortunately, the damage was under a thou; not enough to harm the final dimension.

Step 23: ...for Action

The four-jaw chuck won't fit something as small as 1/4" brass rod. Change chucks again, but don't worry; the stock is .250 and we are turning it down to .234, so the run-out of a three-jaw chuck is not an issue.

Once again, use the dimension you actually cut the hole too, not the dimension given in the plans.

The primer is faced, turned down, and chamfered lightly. Then parted. If you want to be really clever, cut a chamfer into the inside end using a thread-cutting bit before you part off.

The smart way to set the primer is to make it enough undersized (tuning down to .233 is sufficient) that it drops right in. Once you are sure it fits correctly, put a drop of superglue in the hole then drop the primer in.

I cut mine a wee bit large, and as I was testing the fit it got semi-stuck. I could have wrenched it out, but since it was close enough anyhow I went ahead and used the arbor press on it, too.

Step 24: Red Caps

One of the elements that seems to have the most individual interpretation is of the plastic caps that act as a safety for the button. These are polyethylene -- soft enough to be thumbed off the front of the grenade when it is desirable to deploy it manually.

The most common option is a 5/8" pipe cap made by Stockcap; the Stockcap C-5/8. Unfortunately, the minimum order from Stockcap is 500 units; about a hundred bucks worth of caps. The option I went with is the Mocap S-11/16. It ships a darker red than expected, but I think it looks okay.

Both of these options are slightly smaller than the body diameter of the grenade, which seems to match up with screen shots. However, there is some argument that the cap should be designed to be flush (aka the same outside dimension as the grenade body). Further confusing the issue, some screen shots, and the third-party technical manuals, seem to suggest a tapered cap instead!

Of course, you don't have to have a cap. Or button. Many people lathe or cast the grenade in one piece and simply paint the top red.

Step 25: The Thin White Line

The Mocap is slightly long, and has raised letters on the top. For the former, a careful trim with x-acto knife and sand flat. For the latter, sand down -- but it helps a lot if you carefully cut off the raised letters first with that same x-acto knife.

After sanding flat, wet-sand with 400 or above, then buff briskly to restore the shine -- I buffed mine by hand on the t-shirt I was wearing!

The white line can be painted with Rustoleum Direct-to-Plastic spray paint, but the result is a little rough and flakes off with too much handling. Pinstripe tape would be better, but a hand-cut strip from a roll of white electrician's tape looks okay. Even a spray of gloss coat didn't seem to stop the creep, however. occurs to me while writing this that you could cut vinyl sticker stock in the necessary narrow strips...using the CNC Vinyl Cutter also available at TechShop.

Step 26: Spit and Polish

The grenade is done. But it could do with a little cleaning up.

A prop this nice should expect to get a lot of handling. So you want to take down those razor-sharp edges left by the lathe operations -- especially the edges of the grooves.  Gently wiping them with a folded scrap of 200-400 grit emery paper worked for me.

400-grit emery paper also took the worse of the dings out from chuck jaws and getting dropped and other minor accidents as I worked on it for several days. Then 0000 steel wool on top of that.

To get a really nice shine, buffing compound and a Dremel buffing wheel. You can get aluminium to a chrome-like shine with that. The look of the real prop was not quite that shiny, however; a better solution for me was to rub it briskly with rubbing compou...actually, toothpaste from the tube (an old GI trick).

Wipe off the oil, rinse, dry well. A little drop of sewing machine oil in the action is probably safe, but don't overdo it; it is a closed space and oil attracts dust and grime.

The kind of dust and grime found in abundance somewhere like, oh, say, LV-426......

Step 27: Addendum

I've made a couple more since, and there's a few changes in the way I build them -- plus there are a few errors that slipped in when I did a clean re-draw of the plans.

1) Gave up on having a step for the plug; so instead it is just an 11/32 hole all the way to the back of the grenade

2) The rear slot is just a 1/8" bit, not the "9/32" called for in the plans.

3) The angle where the knurl sits is 16 degrees, actually. Haven't figured out the other angles yet.

4) Instead of trying to use a cutting tool, the chamfer around the primer well is just cut with a larger sized drill bit

5) The button should be one or two thous smaller than the hole, especially since the hole is going to be a little ragged (even with a fresh drill bit).

6) However, the plug seems to work with being from five to fifteen tens larger than the hole.

When I've one or two more built and more bugs worked out I'll edit this Instructable.

Addendum Part II:

As of grenade #10, three more changes to my production process:

1) All the regular turning is done at the same time; the nose cuts are done before the grooves are cut.

2) Instead of rotating the compound rest for the knurl, I'm doing it from the same 16' position as the cut. This does mean having to adjust with the longitudinal traverse wheel whilst driving the tool in with the cross slide wheel.

3) The grenade bodies are now turned to .790", for better compatibility with some replica guns. In addition, I turn first to .792, then do a final finishing pass after all the grooves and knurls are cut. The latter is especially important; the knurl throws up material into the diameter, increasing it too much to allow the prop to chamber correctly in a replica firearm.

Erratum Part III:

One is always learning new tricks, with this machining stuff!

I now cut the grenades to .794 with the roughing pass, and add .04 to the grooves so they will be right when the finishing pass at .790 is made.

A more important change is all the holes are drilled from the back. You see, any error in the alignment of the lathe or any bend in the drill bits ends up in a mis-alignment of the holes. And that meant having to custom-cut the buttons to fit properly. By drilling with the smallest bit all the way through to the end, then chasing it with the next larger bit, you get a cleaner hole and better alignment. And that means you can run off a bunch of buttons and plugs at the same time.

As it happens, top-loading works too. I purchased some stainless steel tubing with an ID approximately that of the button. Drill a blind hole in the top, load in the spring, the button, then a short piece of the stainless steel tubing -- which I secured with JB Weld rather than try press-fitting in.