Thread cutting tolerances ?

[pierre_wilmotParticipant @pierre_wilmot]

Hi all,

I recently aquired a lathe (MyFord ML10), and I’m reaching the point where I need to cut some thread.

I’m looking to cut a 1.25mm pitch thread. According to the chart on my lathe, this can be done with a 25, 50, 63, 40, 35& 70 gear. If my math is correct, this means a reduction of (25 / 50) * (63 / 40) * (35 / 70) = 0.39375. That is the lead screw does 0.39375 rotation for each turn of the driver.

I do not have a 63T gear, and I was looking into ordering one, I syumbled on this other chart on the Myford website : https://www.myford.co.uk/acatalog/D-10428METRICSCREW.html#SID=291

It indicates that the same 1.25 pitch can be achieved with a 25, 40, 50 & 60 gear (which I all have).

But doing the math : (25 / 40) * (40 / 50) * (50 / 60) = 0.41666666666, I get a reduction that is close, but not the same as the previous one.

Is that to be expected ? Are threading reduction giving an acceptable pitch with some tolerance? Or is there some difference between older and newer lathe in the leadscrew or spindle to driver reduction?

Or did I make a mistake in my math (can’t rule this out) ?

For context, I am trying to build a worm gear for a robotic project, I’m using an aluminium lazy suzan from amazon in which I’ve cut teeth using a M8 tap (that where I got the 1.25 originally). I was planing to drive the contraption with a M8 threaded rod, but every threaded rob I’m getting from the hardware shop is bent to some degree, making the system too hard to turn for the small Nema17 motors I’m using. So my next step is to cut my own threaded rod from a piece of brass.

[DC31kParticipant @dc31k]

The chart you have found is for a machine with a metric leadscrew.

If you look lower down that chart, at the imperial pitches, they all use a 63t gear as the driven gear.

On your own machine, which has an imperial leadscrew, and on your own chart, the 63t gear is a driver.

So, no. The numbers on that chart will not help you.

Buy yourself a copy of Martin Cleeve’s ‘Screwcutting in the lathe’. It is well worth the cost and is has not been bettered by the two hobby screwcutting books that came after it.

 

[Brian WoodParticipant @brianwood45127]

Hello Pierre,

You can get a thread pitch of nearly 1.25 mm with a simpler gearing set up on an ML10.

The choices are reduced by having a fixed 25 T driver on the spindle, but this combination will give you a pitch of 1.247 mm, close enough I think.

25/50 x 55/70 with an idler on the 2nd stud

Regards  Brian

[BazyleParticipant @bazyle]

There are several excel spreadsheets around that you put in the gears you have and it calculates all the combinations that can be used for a particular thread and the error. Then you can select the best fit.

If you have or know someone with a 3D printer there are gears on thingiverse for printing, including one in openscad that will print any number of teeth. Myford wheels are 20 DP.

There are also other charts for metric approximations. Early Myfords used the 46/73 combination.

[Howard LewisParticipant @howardlewis46836]

If the chart calls for a 63T gear, that suggests that it has an Imperial Leadscrew.

Have you carefully checked the pitch of the Leadscrew? (What are the graduations on the Cross Slide and Top Slide handwheels? If they are Imperial, the chances are that the Leadscreww is also Imperial; possibly 16 tpi.  If need be, check, Engage the half nuts, Take out the backlash, and them set a DTI to zero and measure how far the Saddle moves for 5 or 10 revolutions of the Leadscrew. A simple calculation will tell you the pitch of the Leadscrew. The answer is likely to be 0.0625″ if Imperial, 1.5 mm if Metric.

If the graduations are Metric, than the leadscrew might well be 1.5 mm pitch.

If your machine had a 1.5 mm pitch Metric Leadscrew, to cut a 1,25 mm pitch thread, you would need to set up a gear train where the Leadscrew rotates 1.2 times slower than the chuck. (According to the chart for my mini lathe, a possible changewheel set up would be DRIVER 50T, IDLER 40T, LEADSCREW 60T.

IF your machine has an Imperial, 16 tpi Leadscrew, you would need to compound the 63T gear with the 40T IDLER, using spacers to bring all the meshes into line.

If this IS the case, and you need a 63T gear, you should be able to buy one, in Aluminium alloy, from Arc Eurotrade

Carefully measure before doing anything!

Martin Ckeeve’s book,”Screwcutting in the Lathe” is No. 3 in the Workshop Practice Series.

For a 1.25 mm pitch thread, he quotes FOR a 8 tpi LEADSCREW

35 – 50

45 – 40

30 – 60  He says that this gives an error of   +1 in 8000

Or alternatively

45 – 55

50 – 40

25 – 65  He says that this gives an error of -1 in 1144

So you would need to change at least one of the wheels to suit a 16 tpi Leadscrew, so that it rotates twice as fast, relative to the chuck, as it would for a 8 tpi Leadscrew.

Change the 40T to a 80T driving the 30T, in the first example?  Check my arithmetic!

Another book that may also help you is Brian Wood’s “Gearing of Lathes for Screwcutting”

HTH

Howard

[Brian WoodParticipant @brianwood45127]

Howard,

Thank you for recommending my book but it will not help Pierre; it was written mostly for gearbox lathes.

And if I may correct you on one or two more points. The ML10 was an 8 tpi leadscrew lathe with a fixed 25 T gear on the spindle. Hence, if you look at his chart supplied with the machine, all the gearing set ups start with 25 T as the first driver.

Sorry!!   Brian

[pierre_wilmotParticipant @pierre_wilmot]

Thanks all, didn’t expect to recieve that much help that quickly 🙂

As many of you pointed out, my lathe has a imperial leadscrew, and the myford website chart is for a metric one. This was the source of my confusion.

I’ll try out one of the alternative gear configuration with a “close enough” to 1.25 result and see how that mesh with my gear. If that is no good I’ll just order the 63T.

As it turns out, I already have the “Screwcutting in the Lathe” book, just haven’t read it yet.

 

[Grindstone CowboyParticipant @grindstonecowboy]

Evan Lewis, who is a regular on the Boxford group, has an online program called Ride The Gear Train, which works out the best combinations of gears if you input a few parameters, such as leadscrew pitch and what gears you have.

Rob

Links to https://ridethegeartrain.com/

[old martParticipant @oldmart]

A slight pitch inaccuracy will not be a problem unless you are making very long nuts that match a long male thread. Your brass would only need thread in the centre 20mm. Alternatively you could get your studding and hold the thread in a lathe chuck and turn true either end leaving a short length of thread in the middle. Then make bushes to fit the bearings.

[Howard LewisParticipant @howardlewis46836]

With an Imperial Leadscrew, you will definitely need a 63T gear, to cut Metric threads. (ideally it should be a 127, but a 20 DP gear would be big! 6.45″ OD)

The 63T gear will produce small inaccuracies, but not enough to cause you problems, unless you are poposing to cut long lengths and use them for accurate measurement.

The problem may be finding a 63T 20 DP gear with a 14.5 degree pressure angle.

The ones sold by folk such as Arc Euro will be Module, with a 20 degree Pressure Angle, so will not mesh properly with your Myford gears. (And would need a special stud to match the Metric bore of the gear.)

Lacking one, you may need to find some kind soul to cut one for you.

Howard

[Howard Lewis]

On Howard Lewis Said:

…
The problem may be finding a 63T 20 DP gear with a 14.5 degree pressure angle.

…

That does seem to be gradually becoming a problem.  However, RDG have them for a mere £10.96.   They aren’t a standard off-the-shelf gear, so availability is likely to drop over time.

If I were an Imperial Myford owner who didn’t have a 63T already, I’d flash the cash now as a future proofing measure.   In 20 years time an Imperial Myford that can’t do metric will be unattractive.    Adding a 63 tooth to the gear set now is an easy fix, likely to get difficult in the future.

Dave

[DC31kParticipant @dc31k]

We should pay good heed here to Brian Wood’s observation that the lathe has to use a 25t gear as the first driver.

There have been two brute force methods suggested: a spreadsheet and the Ride the GearTrain software.

For those who have suggested this approach, please could you confirm that the methods have the ability to specify the first driver gear.

If not, perhaps it would be a good service to contact the authors of those tools and suggest that ability be added as an additional feature.

 

[Grindstone CowboyParticipant @grindstonecowboy]

You can specify the spindle gear in Ride the Gear Train, under Optional Entries.

Rob

[bernard towersParticipant @bernardtowers37738]

Isn’t there a method of using 21t gears to do this?

[Brian WoodParticipant @brianwood45127]

Bernard,

Not realistic with the fixed 25 T gear driver on the spindle. I believe these are also getting difficult to source anyway.

I have proposed a working gear train that Pierre can use with the gears he does have, the error at 1.247 mm pitch is negligible and the working gears are only 4 in number. There will be an idler needed to complete the gear train. He can then buy a 63 T gear at his leisure. As DC3 says, now would be a good time to add one to the collection.

Regards   Brian

[bernard towersParticipant @bernardtowers37738]

Brian it actually says on lathes.co.uk site that 2 21t wheels are all thats necessary to do most standard metric pitches on a ML10. Perhaps this is misleading?

[Brian WoodParticipant @brianwood45127]

I didn’t know that Bernard, thank you for pointing it out.

In the old forum there were reports of difficulties in finding 21T Myford gears, I don’t suppose things have improved

[bernard towers]

On bernard towers Said:

Brian it actually says on lathes.co.uk site that 2 21t wheels are all thats necessary to do most standard metric pitches on a ML10. Perhaps this is misleading?

I think misleading, because lathes.co.uk have more to say on the subject, my bold:

To convert an imperial machine to give basic metric screwcutting required only two 21t changewheels, though to obtain a complete range of pitches it was also necessary to have the following gears: 27t, 33t, 36t, 39t, 42t, 48t, 50t and 80t. The standard set of changewheels supplied with the lathe comprised:: 20t, 25T, 30t, 32t, 35t, 40t, 45t, 50t, 60t, 65t and 70t. …

I suspect ‘basic metric screwcutting’ means the ML10 can approximate a few pitches with a couple of 21t wheels and the 50t standard wheel.  Unfortunately, lathes.co.uk don’t say what those few pitches are, and I’m too tired to calculate them.    To get a complete metric pitch range out of an ML10 you need a lot of extra gears.   I guess buying a 63t is a cheaper option.

Dave

 

[Charles LamontParticipant @charleslamont71117]

Another excellent gear train program is Duncan Munro’s: https://metal.duncanamps.com/software.php

You can ask it to find gear trains that lie between pitch error limits using the available gears, and it is often surprising what it comes up with.

Among other extras, I have 2 x 21T, a 63T  (and a 127T which I have never needed to use).

[pierre_wilmotParticipant @pierre_wilmot]

Thanks everyone for all the information.

Happy to say that the gear train Brian suggested worked well for my use case.

I now just need to reprint the support to accomodate for a non-bent screw, and I should have a working worm gear.

[Brian WoodParticipant @brianwood45127]

Hello Pierre,

Thank you for the end of story report, it is always gratifying to know the outcome of one’s endeavours.

Regards   Brian

[DC31kParticipant @dc31k]

Slightly tangential to this thread, but hopefully useful:

There is a threading book by Atlas Press, for their 6″ lathes at vintagemachinery.org

http://vintagemachinery.org/pubs/detail.aspx?id=19942

Among other things it has metric approximation gearing using 11/13 (x 30tpi = 25.4tpi or 1mm pitch).

It also has gearing for coil winding based on various AWG sizes (effectively a very fine feed close to the diameter of the wire being wound). That is something I had not seen documented elsewhere.

[Michael GilliganParticipant @michaelgilligan61133]

That Atlas book is a gem … Thanks for the link

MichaelG.

[Brian WoodParticipant @brianwood45127]

DC31k,

Thank you for the link. I looked at the book and found it difficult to follow.

Not wishing to be critical but I think the gearing charts would have been so much easier to follow with gears labelled as Drivers and Driven and Idler [bracket size] I find the logic as laid out as in UK fashion starting with the beginning of the gear train and finishing with the leadscrew (screw) gear as the last in the train a lot easier to follow as well.

I had to work out for myself that the leadscrew pitch used in this size of Atlas lathes is 4 tpi, admittedly this wasn’t too difficult, but the fact was not stated anywhere. It was assumed that anyone using the tables would already know that.

Maybe I am just being picky as clearly a lot of work had gone into creating the material in the first place.

Regards   Brian

 

[HopperParticipant @hopper]

Glad I am not the only one baffled by the Atlas drawings and charts. But it would be a lot easier with the actual lathe in front of you I am sure. And could be adapted to 8tpi leadscrew by the incorporation of a double sized gear in the right place etc etc.

But Martin Cleeve’s book  Screwcutting in the Lathe has all the charts for cutting metric and other odd threads using a standard set of Myford etc change wheels. Very handy. (But no good on the ML10 with its weird 25T spindle gear!)

The coil winding pitches are interesting. But it sure would be tedious work, having to stop the lathe to reverse the feed direction at the end of each run. OK on long industrial coils but painful on small things like say a motorbike magneto or generator. I guess life was more relaxed and labour hourly rates a lot lower in those days!

 

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