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Thread Cutting - Internal and External on a Myford ML10 Lathe

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This Instructable shows my first effort at thread cutting. It seemed to be reasonably successful and I didn't want to forget what I did, so having a long-term record should be useful for me. I don't expect to do too much thread cutting in the future???

The Instructable gives details about making a pair of internal/external metric threads in 1" (25mm) diameter aluminium tube/bar. It covers the calculations, the preparation of the cutting tools and doing the whole job. I have a Myford ML10 with a metric leadscrew, however only a couple of steps will be specific to that lathe.

Step 3 explains exactly how to prepare the bar and tube with virtually no mathematics involved. One problem, I am a complete novice, so there are probably errors and examples of poor practice - be warned! I hope that comments from other people, will highlight these so I can correct them.

Steps:
  • Choosing a pitch
  • Mathematics - deciding on the thread size and exactly how to move the cutting tools, etc
  • Thread cutting calculations without mathematics!
  • Preparing the bar for external threading
  • Grinding an external HSS threading tool
  • Setting up the lathe leadscrew and backgear
  • Cutting the external thread
  • Grinding an internal HSS threading tool
  • Preparing the tube for internal threading
  • Cutting the internal thread
  • Final fitting
Hope this might help someone else?


 
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mangelo4819 days ago
Well done. I've yet to try thread cutting myself, but your explanations are very clear and I particularly like your comments about what was actually happening at each moment.
qthurtle (author)  mangelo4818 days ago

Many thanks for your comment. Have a go! If you buy a really cheap, plastic cooking spoon from a discount store, you end up with 30cm of polythene rod which is over 1cm in diameter. Cut a short length, chuck it up and get threading. This is brilliant for playing with external thread cutting - you can't damage anything because the material is soft, but it holds a thread nicely.

Best of luck.

zizipo5 months ago

hi guys I am doing my level 1 apprentice as a toolmaker so I want to know how can I make M12 can you guys help me with all the calculations and preparation. this is a turning between centres workpiece.

qthurtle (author)  zizipo5 months ago

Hi there. Use the info in Step 3 to work out the dimensions you need.

D = 12mm and P=1.75mm

skrubol1 year ago
You can't rotate the tool holder on your lathe?
qthurtle (author)  skrubol1 year ago
Hi there. Thanks for your interest.

I'm not quite sure what your question refers to. The top slide rotates to any angle I want (with respect to the cross slide). So, as you can probably see in the photos, I can rotate it to 29.5 degrees in both directions (there is a graduated scale of degrees engraved into the lathe, which shows me what angle I have set).

Also, I can rotate the tool holder to any angle (with respect to the top slide), just by slackening off the large clamping nut in the centre of the four-way toolpost.

To make everything work, I have to do both rotations. First I set the 29.5 degree angle (so that when I advance the top slide, it only cuts on one side of the thread); then I adjust the angle of the four-way toolpost to bring the cutting tool back to be 90 degrees to the work.

Does this answer your question?

Best wishes
After posting that and reading the rest of the instructable, I can see that you can rotate the toolholder. I was thinking the offset angle on the tool was to compensate for the 29.5 deg on the compound (didn't do the math to see that 10 != 29.5..)
I guess my question is, why the angle offset on the tool? Why not just a straight on 60 deg (30 each way) cutter?
qthurtle (author)  skrubol1 year ago
Ah - I understand the question. The reason for offsetting the 60 degree tool by an extra 10 degrees as I ground it, was so that I could keep the workpiece very close in to the chuck without any danger of the top slide crashing into the rotating chuck. To explain: on my small lathe, I try to work very close to the chuck for maximum rigidity, this means that I might be working the cutting tool as close a 2mm or 3mm from the chuck. When I clamp the cutting tool into my 4-way toolpost, there is a little bit of overhang, where the toolpost is slightly bigger than the tool and overhangs the tool in the direction towards the chuck. If the 4-way toolpost and tool are exactly at 90 degrees to the axis, and I try to drive the tool very close to the chuck, then the (oversized) toolpost can foul the chuck. If however, I grind the tool to have a 10 degree offset (as described), then the toolpost has to be rotated by 10 degrees as well (so that the tool is now at 90 degrees to the axis). This means that the head of the cutting tool is now the first thing which would hit the chuck - ie I can work the cutting tool right up to the chuck and not worry that the tool post (or the top slide) might foul the chuck.

My lathe was once owned by an educational establishment and the top slide shows much evidence of chuck-crashing!!!

I hope all of the above makes sense - sorry to be so wordy. If you need more, let me know - I will try to get a photo to show the benefit of the 10 degree offset.
Yes, that makes sense. Didn't realize quite how close you were to the chuck and that your toolpost was a bit big.
If I were to add anything to your instructable I would say that you should consider a Positive Rake Cutting tool. This will help "cut" the threads instead of smashing/ forming them (like in a zero rake tool), especially in aluminum. When making your HSS threading tool you will also want to "hone" a slight radius on the cutting edge with a fine grinding stone. This will allow you to achieve a sharper cutting tool, thus a better finish.
Also, don't forget to use some sort of cutting oil (way oil, synthetic, etc) during the cutting process.

**The reason a 10mm bolt must have a smaller major diameter is because of Thread Class Engagement. This means that the best strength-to-fit ratio is around 75%. If the threads fit better than this -say 90%- you would not be able to spin the two together and, if the threads were around 20% they could easily pull out.

Now when cutting threads again, always check them with a hardware store nut/ bolt.

And YES, it is possible to re-chuck a piece of work to "chase" the threads after it has been taken out of the chuck.

ALWAYS trust your math!!
qthurtle (author)  sw.machinist1 year ago
Many thanks for the comments. The finish I achieved was not brilliant. I have not really got the 'hang' of honing my HSS tools; and in addition, I don't have a particularly fine grindwheel on my grinder. Hence I think your thoughts are probably spot on!

When I try honing, I am never convinced that I am actually IMPROVING the finish. There are virtually no YouTube videos on honing lathe tools (a good number on honing knife blades, but they are quite different). I don't really know how to judge whether the edge is sharper or when to stop honing. I don't know what a typical number of honing strokes people use (1, 5, 10, 20???). I have a proper 'fine' oil stone and three (fairly cheap)diamond impregnated stones (medium, fine and very fine) but I'm not really sure which would be best to use. Finally, I'm not very sure about the best way to hold the cutting tool, or the best order for honing the edges. Apart from that, I think I understand it fully!

Best wishes and thanks for the advice on lubrication, etc
I hope you know that we are just being picky!! As long as your threads fit they are fine. As you "hone" your threading skills you will get more comfortable and familiar with how your lathe reacts to the tools that you present and eventually you can worry about the smaller things like thread measuring and centricity.
clockworld1 year ago
Have you shaved off some of the topslide casting? If so, by how much. I was thinking of doing this in order to use my existing quick change tooling. Matt
qthurtle (author)  clockworld1 year ago
No, I put my 4-way toolpost in the 4-jaw chuck and took a carefully calculated amount off the bottom of it. This bought the top face of the HSS tools to their correct place (just below the centre line of the lathe). I took the bottom of the toolpost down to about 3.5mm thick. This is as thin as you would want but I think it was calculated to suit 9mm (or10mm??) tooling to fit. I currently use 5/16" tools which need packing underneath them, which is ideal.

The 3.5mm thickness is not as bad as it seems, remember that the TOP of the toolpost is very thick (10mm+) and its screws press the HSS tools, the packing beneath them and the bottom of the toolpost VERY firmly onto the top slide. There is not a hint of instability or movement in the setup.

Hope this helps.
qthurtle (author)  qthurtle1 year ago
I don't have a brilliant photo of the toolpost, but you can see how it fits in the top bit of this shot.
DSCF3166.JPG
One little trick if you don't like running the tool towards the chuck and would rather run away from it. Flip the tool over and run the lathe in reverse. This will allow you to cut right hand threads from left to right. Be sure to adjust the tool so it is ABOVE center, not below, as everything is upside down now.


You don't have to feed in with the compound slide (top slide as you put it). You can simplify things and feed in with the cross slide. The only difference is you will be cutting on both sides of the thread instead of one side. If your tooling is sharp, this won't be a problem. Saves a lot of time for production work.


Try and learn to know what good threads will look like. You mentioned your threads were pointier than you liked. You don't have to feed in the calculated amount, as long as the threads are cut correctly. Many variables go into how deep you need to go. The calculated depth is more of a reference for about how far you need to go. You might consider buying a magnifying loop to inspect the threads while cutting. It's amazing how much more you can see when things are magnified.
qthurtle (author)  NitroRustlerDriver1 year ago
Many thanks for your comments. It would be good to cut away from the chuck! Some machines like mine have screwed-on chucks, so cutting anything with the machine in reverse is a big no-no for me (the chuck can/will unwind and fall off!!). Looks like a real possibility for other machines??

I'm sure you are right about the depth of cut. In many cases, people have a 'target' nut or bolt which they test when they are near the end. Then they know when to stop.

Thanks for your interest.
Smokio1 year ago
Nice instructable! I would like to give you one pointer regarding the tool chatter as you get deeper. This is because the screw cutting tool is cutting on both faces. For the best results keep your compound slide parallel to the the axis of the thread. As you get deeper in the cut, wind the compound slide forwards a bit. If you follow me, the thread is then opened up, and the thread cutting tool is only cutting on one face. You get a nice shiny thread (especially with cutting oil). As for the little dial, I never use that, I just leave the screw engaged and reverse back to my start point. Another somewhat scarier tip is to do the thread with a faster spindle speed, usually your reactions are the limit, when you see a CNC doing a small screw thread it is actually quite horrifying! :)
qthurtle (author)  Smokio1 year ago
Thanks for that. I think the offset of 29.5 degrees forces you to cut on one side of the thread only???

I found that (as a novice) I had a very high 'workload' during the whole screw cutting process (operating levers, turning handles, and thinking!!). The prospect of having a faster spindle speed frightens me!!
I know the feeling. I started out the same.
It becomes like riding a bike. it all just falls into place, the more you do it.
You are correct the 29.5 degree angle, cuts on one edge.
Can I ask what speed you were using to cut the thread. I thread to 125 on my M300. Any faster I would have a nervous breakdown. It is hands and eye coordination and brain trying to keep everything in going correctly. The more you do, the easier it gets.
Doing work on Rifles I thread imperial. My Lathe is metric. I need to keep the lead screw engaged whilst doing imperial. I did this for so many years. I cut my first Metric thread and thought this is so much easier disengaging and re engaging. It is all a learning curve. The more you stand at the lathe cutting metal the faster the learning curve is.
qthurtle (author)  BordersSika1 year ago
I only have six speeds on my lathe. There are three different pulley pairs - adjusted by moving a belt to join the required pair; and the three, very slow corresponding backgear combinations.I was using the slowest possible combinations. I can't remember what rotational speed this is, I'm afraid. If you specifically need the RPM, then I can look it up.

To be honest, I think aluminium should be cut at a much faster speed than I was doing. I really don't know how the correct balance is achieved - the best possible finish or a managable work rate!

Best wishes
I think you did a fine job with this. While it does cool quickly, aluminum is an easily deformed metal, and could easily become stretched/expanded when threading the inside of a pipe.
Ah, I'm sorry. I just re-read that bit, sorry it's been a long day. Indeed you are right. Slight difference in technique :) I use the cross-slide for my dimensions and the compound slide to open up. I guess its the usual then, maybe try the usual... less tool sticking out, a slightly more positive cutting angle, cutting oil, higher spindle speed, smaller cuts, etc. :)
Smokio Smokio1 year ago
BTW by wind the compound slide forward a bit, I mean like 0.05mm at a time, you can also do it while it is cutting on a longer thread, you can see immediately when the tip is only cutting on one face.
Looks like some kind of counterfeit money in that picture. Who is that lady???????
qthurtle (author)  Dickerson8881 year ago
The lady is HM The Queen! It is a UK 10 pence piece. Ten of them make a GB Pound. It is just under 25mm in diameter.

I have recently been trying to make a double-headed 'copper' coin on my lathe using a UK 2 pence piece. Unfortunately, our copper coins are mainly steel with a thickish copper-coloured metal coating (the coins are fully attracted by a magnet). When you cut into a face, it rips up the copper coating which makes it very hard to hide the join when adding the second headed face back. I might try again with a 10 pence piece shown in the photo - I think these are solid. In the past, our copper coins were some sort of solid copper alloy - not coated.
Use the "rim" of the coin to hide the work. Think of a soda bottle cap, and the second heads as an "insert" into it. Drill/lathe into the first head, leaving the rim. Grind down the second head to the proper thickness/depth, and trim off the rim.

I never said it was easy work. Alignment alone will take some time, and mounting it to do all of this will also be some effort.

Have fun!
espdp21 year ago
Very cool project. I would love to be able to just watch over your shoulder. Can you post a video of some of the math being worked out and the cutting being done?
Cheers,
Patrick
qthurtle (author)  espdp21 year ago
Many thanks for your comment. I'm not good for video as I don't have the necessary set-up. There are lots of good videos on screw-cutting YouTube.

Step 3 shows the 'easy' way to calculate the three calculated pre-form diameters (three, because the main thread diameter does not need calculating!). They are based on the values of diameter and pitch and just need a calculator.

I just hope that they are correct! I have not seen them presented in this way before, although I'm sure its not new stuff!

Best wishes
Hi
This is going to sound stupid but where do you take the 29.5 degree reading from.
The notch underneath the top slide, on the front of the Apron. For years I did it this way. Till I found out that was not the correct procedure.
The 29.5 degrees is slightly less than half the thread angle - 60 degrees, so you're always cutting, as you move the tool down the thread flank.
Steve
I know all about threading and angles.

When I first got my Harrison M300. I took the 29.5 degree reading from the front of the apron. I could make a thread. that a nut screwed onto. Was not in my opinion a good thread. I found a very interesting post on how to set up to do a 60 degree thread. I think have saved it. I will try locate on my computer and post a link. Everything will fall into place once you read it. When I read it. I went to the lathe and set up as described. My threading jobs just became real threading jobs. A thread to0 be proud of.

Imagine standing facing the lathe in front of the apron. The mark on the metal that you put the 29.5 degree setting at. Ignore that. Imagine that notch is 6 o'clock. go to 9 o'clock. now set the top slide so it is running parallel with the centres of the lathe. Look down at the 9 o'clock position where all your numbers of degrees are etched. you will see the zero. scribe a line directly under the zero. now position the top slide at 19.5 degrees to this zero not the scribed line at the front of the apron. Cut a test thread and take a look at it compared to a previously thread. You will see a vast improvement in the quality of the thread.

I work on Rifles as well as making infra red illuminators. I get comments regarding the quality of my threads from customers all the time.

Forget using brasso on your aluminium. Get a gallon of Hydraulic oil for tractors, log splitters and the like. I buy good quality oil for my threading jobs. I mistakenly filled my oil bottle with hydraulic oil. I thought to myself at the time this oil looks a bit thin. It done the job every bit as good as the expensive stuff. I get it for free. I pay a fortune for 5 gallon drum of the right Oil.

I will try find the link to the description on how to do it the way I have mentioned. It has pictures showing the process. I am not a trained machinist. self taught. I am only trying to help others here see the light. for thread cutting.
I know about threading too, since I routinely make threads with a big CNC lathe. I thought the question was more about why its 29.5 degrees for a 60 degree thread than the convention of where you measure the angles from....
Steve
Please do not take offence my friend. Making threads on a CNC lathe and on a manual Lathe are two different things. I can not scratch my arse when I am making a thread on a manual Lathe. You have already programmed in what thread you want to do and can stand and look in the window whilst this is being done. Please do not think I am saying you are less skilled. On the contrary I have the greatest respect and envy at the skills you possess to programme the CNC to do its magic. I would love to be able to do the same and be able to afford a machine to cary out the programmed tasks.
I think we need to clarify the reasons for the "magic angle" - and that for Whit forms, it would not be 29.5 deg, is all I'm trying to get at.
qthurtle (author)  BordersSika1 year ago
Thanks for that - if you find the link, post it.

Best wishes
I am still trying to locate the more in depth link. Found this for just now. Hope the pics in the link show a visual as to how it should be set up.
http://www.modelenginemaker.com/index.php?topic=1828.15

This getting old is tough. Forever forgetting to post the link. Apologies.
qthurtle (author)  BordersSika1 year ago
I think you basically have the correct idea. However, my topslide has a zero mark when it is at 90 degrees to the apron, so you have to take that into account.Hence, you rotate it until it reads 60.5 degrees, which puts the cross slide at the correct 29.5 degrees
DSCF1531.JPG
bmossberg1 year ago
Ok, maybe this sounds stupid but did you ever consider buying a tap and dye kit? not very expensive and would have saved a TON of time.
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