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ACME thread identification question.

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Nick Edgley25/08/2019 12:35:44
7 forum posts
3 photos

I have a Boxford BUD lathe. I have a considerable amount of wear in the brass nut driven by the crossfeed leadscrew so decided to make a new one.

All went well until it came to identifying the ACME thread size. Bear in mind that I am a complete novice when it comes to threads.

I measured the thread pitch at 10 tpi. I measured the major diameter as 7/16". I ordered a 7/16" x 10 LH ACME tap. In my ignorance thinking this must be correct.

When I tried the new tap in the existing brass nut to check that it was correct it quickly locks up, it is the correct diameter but clearly not the correct tap.

I have a spare cross feed leadscrew, which is similar in diameter but the brass nut that is with it neither fits my original leadscrew nor does the new tap fit that either.

I have spent many hours searching the internet and only learned that threads are an incredibly complex subject!

I have learned that ACME threads have a 29° thread angle but that metric trapezoidal threads have a 30° thread angle. Given the age of my lathe I assumed the cross feed leadscrew would be imperial. My spare leadscrew may be metric. Either way my 7/16" x 10 tap fits neither!

I am throughly confused and at a loss to identify the thread and order the correct tap.

I would be grateful for any advice and suggestions.

Thank you.

Michael Gilligan25/08/2019 13:11:54
15853 forum posts
693 photos

Welcome, Nick

You may find some clues here: **LINK**

But hopefully one of the Boxford users will be along soon, to advise.


old mart25/08/2019 13:20:05
1819 forum posts
148 photos

Are you sure that the leadscrew is left hand? If it is left hand, looking at it, the threads will tilt slightly clockwise from vertical. Measuring the pitch can also catch beginners out, measure an inch of thread. There should be ten pitches from the start of the first to the start of the tenth. I would think that Boxford would either use 10tpi or 8tpi, assuming the lathe is imperial and not metric.

 Don't worry about the ACME being 29 degrees and the Trapezoidal being 30 degrees, you will not be able to tell the difference.  

Edited By old mart on 25/08/2019 13:25:02

not done it yet25/08/2019 13:48:36
4728 forum posts
16 photos

As you noted, trapezoidal (a metric thread) is quite different to an ACME (imperial thread). While your lathe may well have an imperial dial on the cross feed, you need to check the travel, for a few turns, with a dial gauge. 2.5mm is quite close to 10tpi and one can never be sure if the lathe has been modified. Home turned feed screws are most definitely not unheard of.

Michael Gilligan25/08/2019 13:59:40
15853 forum posts
693 photos


May I suggest that you post a few photos of the items in question ?

You may be sufficiently enlightened to instinctively know how to do that on this forum ... But if, like many newcomers, the process eludes you: Try reading this **LINK**


old mart25/08/2019 14:11:06
1819 forum posts
148 photos

When the exact pitch is in question, then using a longer reference length will give more accurate results. For instance, the difference between 8tpi and 3mm pitch is only about 0.007", which would not be easy to measure over one pitch. Not so difficult if measured over 300mm, (100 X 3mm), or 12", (96 X 8tpi).

Neil Wyatt25/08/2019 14:37:08
17970 forum posts
709 photos
77 articles

Welcome to the forum Nick,

Might be some helpful hints here:

Which suggests your measurements are correct.


Pete Rimmer25/08/2019 14:59:15
729 forum posts
49 photos

Your problem might simply be a matter of the wear in the nut having caused burrs that the worn screw rides past but the new tap does not.

Measure twenty threads on the screw. If it's 50mm you have a metric screw, if it's 50.8 (or 51 to a steel rule) then it's an inch screw. Inch screws are 7/16" x 10TPI LH

old mart25/08/2019 15:02:56
1819 forum posts
148 photos

I have looked up the thread calculator, and the standard bore through an ACME nut of 7/16" by 10tpi, is 0.343". The tip of your tap should be this diameter or a little less. To verify your pitch measurements, the scale on the leadscrew handle should be 0-100 in thousandths of an inch. Holding the two leadscrews alongside each other will confirm whether the pitches match.

I don't know how to get a link to work on this forum.

Edited By old mart on 25/08/2019 15:04:46

Clive Foster25/08/2019 15:09:20
2240 forum posts
73 photos

Further to the suggestion from old mart about measuring a longer length of thread to get more accurate results. Probably essential when dealign with unknown amounts of wear and unknown tolerances.

Given that such imperial / metric feed screw confusion is only likely where the pitches are very similar the best method is to use a length corresponding to half a pitch difference. Doing things that way it becomes a simple ruler job. On one thread the specified length will be between the same sides of the thread whilst on the other it will be between opposite sides. Easily seen. Distance doesn't have to be terribly exact. Within three or four threads either way will usually do.

For example comparing 10 TPI with the very similar 2.5 mm pitch we find that 3.1" of thread covers 31 pitches of 10 TPI and 31.5 pitches of 2.5 mm. Going the other way and comparing 2.5 mm pitch with 10 TPI we find that 80 mm covers 32 pitches of 2.5 mm and 31.5 pitches of 10 TPI for all practical purposes. Actually 80.010 mm but the odd 0.010 mm is hardly relevant.

Comparing 8 TPI and 3 mm as suggested by old mart we find that 1" of thread covers 8 pitches of 8 TPI and 8.5 pitches of 3 mm, (1.004" actually). Going the other way 27 mm covers 9 pitches of 3 mm and 8.5 pitches of 8 TPI (26.988 mm actually).

Fairly easy spreadsheet calculation if you are computer savvy. I've done the calculation and comparison exercise three time now. If I ever have to do it again I shall spend some time drawing out the thread pitches as a set of side by side bars giving me an instant comparison ruler. Obviously only good for feed screws and similar coarse pitches but they are the ones where the ordinary inch or so long thread gauges run out of resolution.


Edited By Clive Foster on 25/08/2019 15:10:11

Edited By Clive Foster on 25/08/2019 15:11:13

Edited By Clive Foster on 25/08/2019 15:11:47

Edited By Clive Foster on 25/08/2019 15:30:20

Michael Gilligan25/08/2019 15:12:15
15853 forum posts
693 photos
Posted by old mart on 25/08/2019 15:02:56:

[ ... ]

I don't know how to get a link to work on this forum.



old mart25/08/2019 16:25:20
1819 forum posts
148 photos

I needed a crosslide leadscrew and nut for the Smart & Brown model A at the museum, and bought a new leadscrew with two nuts which was long enough to modify for the lathe. It was advertised as 8tpi, but turned out to be 3mm pitch. I told the vendor, who immediately refunded my money and said keep it. It was one of those times when hard choices have to be made. Keep looking for a hard to get item, or make the most of what you have got. I used the leadscrew, and also used both nuts for an adjustable backlash feature. On an old imperial lathe, a metric leadscrew is not a good idea, so I used the rotary table on the mill to make a scale for the crosslide with 118 divisions instead of 125. The actual error is tiny, especially if you creep up on the last bit of the size required. The backlash is set at 0.001", which makes for smooth running.

Gary Wooding25/08/2019 17:22:19
729 forum posts
192 photos

Here's how I insert a link into a posting.

Copy the URL of the link you want, either by writing it down or using the Windows COPY facility (ctl+C).

Then click the LINK icon above to bring up link window and enter the information required. When you click OK the link data is saved t the cursor position.


add a link1.jpg

add a link2.jpg

add a link3.jpg

old mart25/08/2019 18:19:55
1819 forum posts
148 photos

Not quite the exact topic, but worth bookmarking:


Thanks for the the instructions on how to post links, I will try to remember how.

Edited By old mart on 25/08/2019 18:21:30

DC31k25/08/2019 18:50:11
210 forum posts

Can you describe the graduations on the cross-slide dial please. How far does the cross-slide move when you rotate the wheel one turn? Take a marker pen and trace round the crest of the thread and tell us what you see.

There is a possibility it could be 5tpi two-start.

Pete Rimmer25/08/2019 20:33:06
729 forum posts
49 photos

The drawing calls for 7/16"-10 LH so that's surely what it will be.

Nick Edgley25/08/2019 20:39:55
7 forum posts
3 photos

Thank you all very much for your rapid and comprehensive replies.

I will need to spend some time investigating all of your suggestions and links. I am away from home until Tuesday but when I get back I will post some comprehensive photographs of the two leadscrews in my possession.

Clive Brown 125/08/2019 21:03:23
444 forum posts
14 photos

I've owned 2 Boxfords, both imperial, one of them from new. Both have 7/16" x 10 tpi LH single start feedscrews.

old mart25/08/2019 22:34:43
1819 forum posts
148 photos

What condition is your spare leadscrew and nut? Could it be used? I recently made a new 3/4" x 5 Acme nut for a mill, and bought a tap from Tracy Tools. When I tried a test thread in aluminium, I realised there was very little chance of tapping the thread without a tap wrench with a 3 foot long handle. The tap was about 8" long with a gradual taper, and I had to shoehorn a smaller acme insert on a special threading tool just to partially produce the thread for the tap to finish, and then it was hard work. This was in leaded bronze (gunmetal).

Do try a test piece before the actual nut, you may have to tap into a slightly oversize hole. Keeping the tap straight could be made easier using a dead centre in the end of the tap, with the nut in the chuck, preferably a 4 jaw independent.

thaiguzzi26/08/2019 06:14:32
698 forum posts
131 photos

Metric threads were 2,5mm pitch, imperial were indeed 10TPI.

My Boxford, a 69 VSL, has an imperial leadscrew and gearbox but metric cross and top slide screws and dials.

Stock, from the factory....

That is another give away - what dial do you have?

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