Introduction: Thread Cutting - Internal and External on a Myford ML10 Lathe
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.
- 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
Step 1: Thread Cutting - Choosing a Pitch
I had some aluminium tube and some similar-sized aluminium bar.
The thread size comes in two dimensions, thread pitch and thread diameter. I decided to cut a thread with a diameter of M23.6 and a pitch of 1mm. This is why I chose 1mm pitch.
On a Myford ML10 with a metric leadscrew (like many lathes with metric leadscrews), some pitches are painful, because the thread indicator is virtually useless. I shan't detail the explanation as to why this is, but the table above, shows which threads can make some use of the thread indicator (highlighted in yellow) and which can't. The column labelled "Numerator" is the key to this. If the numerator has a value of "1" then that pitch is super-easy because you can engage the leadscrew at any time without worrying about the thread indicator at all. (Conversely, if the numerator is "4" or more, the thread indicator is so hard to interpret, that you have to keep the leadscrew engaged permanently and return the cutting tool back to the start by reversing the whole lathe).
So, I looked at the table and saw that a pitch of 1mm was one of the super-easy pitches. In addition, the mathematics is nearly all based on the pitch, so a value of 1mm was brilliant from that point of view (less maths to do! - see next step).
A pitch of 1mm is decided!
Step 2: Mathematics of Thread Cutting
The following maths seems unavoidable to me, once you stray away from the 'standard' threads. I have not seen a 'Machinists Handbook', perhaps they have tables for virtually every diameter? No doubt there are websites, which would do the maths for you.
Skip the rest of this step if you are bored by the maths - the next step does just the same, without any theory!
The pitch (1.0mm for me) dictates the 'main' thread triangle height (0.866 x the pitch for a 60 degree thread). For me this dimension is 0.866mm and most of the rest of the maths is based on that height.
I did not cut a radius/flat at the bottom of either the external or internal threads. In theory, a specific HSS cutting tool would be made for each thread pitch to achieve the proper profile - both my internal and external cutting tools were as sharp as I could make them, so the 60 degree angle went right to the bottom of the threads. This does not affect the maths.
In one respect, the thread diameter is not as obvious as it seems! Take an M10 thread as an example:
- You might expect an M10 bolt to be 10.0mm across the tips of the triangles which form the thread. This is not true. It is 10.0mm across the flats which exist at the tips of the threads. The thread profile is not a triangle. It is a flat-topped triangle where the triangle has been chopped off by 1/8 of its height. (The width of the flat is therefore 1/8 of the thread pitch).
- You might expect an M10 nut to 10.0mm to the base of the triangle which forms the thread. This is not true. It is 10.0mm across the flats which are (or should be) at the bottom of the threads. They should have a flat base, 1/8 of the thread height up from the point of the triangle.
- You might expect the triangle which forms an M10 nut to have a pointed peak, but is heavily flattened by 1/4 of the triangle height. So the actual height of the metal thread in a properly formed nut is 5/8 of the full triangle height (1/8 'lost' due to the flat at the bottom of the thread and 1/4 'lost' due to the flat-top of the thread).
What does all this mean for me? I settled on a thread size of M23.6 Doing the maths (see below) it meant that I had plenty of meat left in the tube walls.
Here is the maths:
1/8 of 0.866 = 0.108mm
5/8 of 0.866 = 0.541mm
7/8 of 0.866 = 0.758mm
Tube deepest cut (bottom of the triangle) = 23.6 + 0.108 + 0.108 = 23.82mm
Tube internal diameter (to the flat top) = 23.6 - 0.541 - 0.541 = 22.52mm
Bar biggest diameter (to the flat top) = 23.60mm
Bar smallest diameter (to the bottom of the triangle) = 23.6 - 0.758 - 0.758 = 22.08mm
The above are the sizes needed for preparing the bar and tube for threading. (The additions/subtractions are done twice because the diameter is made up of threads on BOTH sides of the tube/bar)
Finally. With all this geometry we know exactly how deep to cut our threads, which is useful for moving the tool in/out by the correct amount.
The top slide is angled by 29.5 degrees when cutting the threads, so that the tool only cuts on one of its edges (rather than plunging in). This enables the cut material to clear the tool more easily (and has less strain on the tool). Due to this angle being parallel to the triangle's sides, the distances to move the top slide are related to the length of the triangle side (1mm in my case) and not to the height of the triangle (0.866mm) which was used in the other calculations above.
When cutting the external thread in the bar, the top slide needs to move in by 7/8 of the 1mm pitch (ie 0.875mm). On the ML10, that is 44 divisions on the top slide dial.
When cutting the internal thread in the tube, the top slide needs to be moved out by 3/4 of the 1mm pitch (ie 0.75mm). On the ML 10, this is 38 divisions on the dial).
Step 3: Thread Cutting Calculations Without Mathematics!
Ignoring all the mathematics details. Choose a pitch and diameter, get a calculator and plug the two values into the formulae in the diagram above. To avoid stupid results, make sure you do the multiply bit first (eg 1.5155 x P) then do the addition/subtraction!
Step 4: Prepare the Bar for Threading
With the diameters known, turn the outer diameter. Leave a nice finish - it will form the 'flats' at the top of the thread.
I used a parting tool to form the lead-in and lead-out areas. I'm not sure what is the best practice for this. I know that some sort of chamfer is sometimes used - not sure about this.
Step 5: Grinding an External HSS Cutting Tool
I wanted to be able to have the tool cutting threads close to the chuck, so I set the 60 degree cutting triangle on a base which was sloping at 10 degrees.
To get an idea of what my threading tool should look like, I used a DTP program to mock-up the shape. The lines in the DTP file are rotated to 70 degrees and 130 degrees, then lined up to touch the rectangle.
After much mental torture, I worked out what angle the HSS tool steel had to be held at, on the grinder, to create the desired shape. Then I used the DTP software to produce lots of parallel lines at the two necessary angles (see the photographs). I stuck these onto the grinder tool rest and then worked hard to keep the tool lined up at all times. The tool rest is angled in a way to produce some relief, when the tool is held firmly on the (tiny) tool rest (5 to 10 degrees?). I must produce a bigger platform for the grinder!
The shape seemed ideal, with a skew which meant that I could set the tool post at the perfect angle to cut close to the chuck (see "Cutting the external thread" step).
Step 6: Setting Up the Lathe Leadscrew and Backgear
The ML10 manual shows the change gears needed on a metric leadscrew to get a pitch of 1mm.
60T and 40T on the first stud, 45T and any small spacer gear on the second stud, and 50T on the leadscrew.
The use of the small spacer gear, and the position of the leadscrew spacer can be seen in the photos.
I don't take the stud bolts off the banjo, just undo the screws on the studs and slide off the studs. I slacken off the stud bolts and fiddle about until everything is meshed properly. I make sure that there is a little slack in the meshing of all the gears (I guess this could be achieved by putting paper or foil between the gears when you tighten up the stud gears - I just make sure there is a little 'play'). Finally, I rotate the banjo to engage the 25T spindle gear with the 60T gear on stud 1 - then tighten up the banjo clamping gear.
Some grease can be used on the gear teeth to quieten them a bit.
To make the machine cut more slowly, the backgear system is used. To make this happen, the following steps are done:
- The 'special' short allen key is used to undo the screw on the bull wheel
- The teeth which engage with the bronze gear are retracted and the screw is tightened again.
- The backgear idler shaft nut is loosened and brought into engagement with the bull wheel and bronze gear - then tightened again
- The backgear idler shaft is lubricated
- The pulley is lubricated via its oil nipple (note this is OIL not grease)
- Finally, the whole arrangement is tested by hand rotation, to make sure everything is right, before switching on.
Switching on the lathe will produce a different sound from normal. Lots more clinking from the sets of gears which are now in the drive train.
When the half-nut lever on the apron is engaged, the apron/cross slide will travel slowly towards the chuck.
Step 7: Cutting the External Thread
I can adjust the zero of my cross slide, so I zeroed the top slide, brought the cross slide in until the tip of the tool was touching the outside of the workpiece - and zeroed the cross slide dial.
Then it is fairly straightforward.
- Move the apron towards the tailstock a bit
- Make sure the cross slide is zeroed
- Move the top slide in to the desired cutting depth
- Engage the half-nut lever (mine did not engage fully sometimes, so I made sure I had plenty of distance before the cutting started, just to give me a chance to re-engage it if necessary). I did not have to worry about the thread indicator for this chosen pitch of 1mm.
- Watch the tool as it cuts (should only cut on the left-hand edge due to the angled top slide)
- When the tool stops cutting at the end of the threaded section, dis-engage the half nut lever.
- Pull back the cross slide to clear the work, then wind the apron back to the tailstock end of the job (don't touch the top slide handle)
- Repeat until finished
Being a total novice, I was surprised at how severe the cutting got, as the thread got deeper. This is pretty obvious due to the increased width of the cutting surface! I did not really adjust the depth of cut much, but realise that I could have started with a deeper cut and then reduced it considerably as I got near the end of the job. Somewhere I read that you should leave the last normal pass a thou or so short, then do a final plunge cut with the cross slide, to leave a nice finish.
I used the calculated dimension to know when to stop the top slide. I'm not convinced I got everything right, because the threads seemed very sharp with no real evidence of a flat top (I did clean the thread up later). However, the final job is nice and close-fitting so I think the basics are right?
Step 8: Grinding an Internal Thread-cutting Tool
I didn't have an internal thread cutting tool, so I adapted a home-made tool-holding bar (which was made for undercutting).
This tool-holder has a 3/16" hole through it, so I used a broken 3/16" drill bit to make a 60 degree cutting tool.
It ground down rather quickly, so I guessed that it was probably a high carbon drill rather than HSS. I'm not quite sure about all this, so I thought I would harden and temper the tip.
I heated it up to cherry-red and dunked it in water - no problem. To temper it, I wanted to judge the colour change with the tool starting from a high polish (I gather this is what you have to do - heat it up until the surface colour changes). However, my hardened cutting tool was pretty black and I was not sure how to make it shiny! I did not bother in the end, I just used it in its hardened state - the aluminium would not tax the cutting tool too much.
Step 9: Preparing the Tube for Threading
I first bored out the tube to the correct dimension using the boring bar, then used it to 'relieve' the starting 3mm and also produce an internal groove between 15mm and 20mm inside.
I used an adapted digital tyre gauge (magnetic attachment) which made the job easy - especially as the dimensions were not critical:
- Before starting, zero the gauge by eye when the boring bar tip is just at the edge of the workpiece
- Use the top slide to bring the bar-tip to the required cutting depth
- Move the apron to cut the 3mm for the 'start-groove'
- Move the top slide so that it clears the workpiece
- Move the apron until the digital gauge reads 15mm
- Move the top slide back to the cutting depth
- Move the apron along to 20mm
- Move the top slide to clear the workpiece and wind the apron towards the tailstock until it is out of the workpiece
- ...repeat until the desired depth has been reached
Step 10: Cut the Internal Thread
Zeroed both the cross slide and top slide to touch the interior surface of the tube.
- Make sure the apron was well clear of the workpiece - towards the tailstock
- Zero the cross slide
- Adjust the top slide to give the required cutting depth
- Engage the half-nut lever - making sure it was properly engaged
- Watch the depth gauge go from a negative value, through 0mm and start thread cutting after 3mm. The leading edge of the cutting tool should be making the cut.
- Keep cutting until 15mm has been reached - disengage the half nuts
- Wind the cross slide in to make sure the cutting tool clears the workpiece
- Move the apron back towards the start
Again, the later cuts were much more severe, due to the increased width of cut as the threading tool cut deeper into the workpiece. I probably should have reduced the depth of cut at this later stage more than I did.
Step 11: Final Fitting
- Measuring across the external threads gave me a reading which was too large by 0.1mm or so. The threads also looked to be without a flat top. Not quite sure how this happened! I re-chucked the bar (not good) and took the threads down by about 0.1mm.
- I had messed up engaging the half nuts on a couple of occasions which caused some grief at the start of the internal thread. To help the fitting, I parted off the 3mm relief cut from the start of the internal thread - and with it, the poor thread-start which the half-nut errors caused.
This did enough to enable me to screw the parts together. Not particularly smoothly, but well enough.
In general, I was not overjoyed with the quality of the finish. I didn't take a tiny 'polishing cut' at the end of the thread cutting, and some of my depths of cut were too big, I'm sure. This meant that the aluminium had some burring and a poorer finish than I would have liked.
To improve the finish, I 'lubricated' the thread with Brasso (metal polish - slight abrasive) and screwed/unscrewed the threads a few times.
Cleaning and oiling the threads then left a silky-smooth fit on the threads.
In general, I think the basics are sound, but (as ever) several learning points for my next attempt!