ACME thread identification question.

Mystery is solved!

I carefully read all your very useful suggestions and information.

I drilled and tapped a piece of scrap aluminium and it fitted comfortably on my spare lead screw which confirms that the spare is imperial. I am still a little puzzled that my 7/16" x 10 ACME tap doesn't seem to fit the existing brass nut on this leadscrew.

Using the tip above from Pete Rimmer the measurements he gave confirmed the spare leadscrew as imperial and the one on my lathe with the worn brass nut as metric. This also explains the graduations on the cross slide dial which are numbered 0, 0.5, 1, 1.5, 2.0, 0 with 50 graduations between each number (a total of 250 graduations). Unfortunately I do not have the imperial cross slide dial, only the leadscrew and nut.

What tap do I need to cut a metric thread to match the metric leadscrew? I presume it is a trapezoidal tap but the sizes confuse me and they also seem quite expensive compared with the imperial ACME tap I have already purchased.

Imperial leadscrew (my spare):

Metric leadscrew (in use and worn nut):

Cross slide dial:

What tap you need depends on the diameter of the screw. Hopefully it'll be 12mm then you'd need a TR12 x 2.5 LH (Trapezoidal, 12mm diameter, 2.5mm pitch, Left Hand) tap.

However, there's no rule to say it has to be an ACME thread. For instance the Denford lathe, a close relative of the Boxford range (they were once the same company) uses a cross slide screw which is 1/2" diameter, 2.5mm pitch and neither ACME Nor Trapezoidal thread form but "modified square section" thread.

I'd try to find a drawing for the screw or the nut.

Hi Nick

Hope that this doesn't read like there is an "granny egg-sucking" element here - apologies if so..

If it's a standard LH Metric trapezoidal thread with a pitch of 2.5mm, then the only other dimension that you need to establish is the major diameter - I'd expect it to me a whole number integer (perhaps 11 or 12mm?) - measure across the widest part of the least-worn part of the thread.

The designation of the tap would then be something like "TR LH 11x2.5" or "TR11x2.5LH", where the "11" represents the major diameter of the thread and the "2.5" the pitch.

IF the diameter is not a whole number in millimetres, then it is possible that the screw is a "bastard" size and some checking with Boxford owners will be necessary.

To throw in couple of general observations, suitable taps from UK suppliers do seem inordinately expensive if you only intend to do this one job with them. I'm sure that something direct from the Far East will be available at a fraction of the price, and I, personally, wouldn't completely discount their usefulness for a one-off job in bronze. As you already did, I would also drill & tap a test-piece in order to check the fit first, whatever the source.

Having said all of that, I note that in the link that Michael Gilligan provided in the first reply you had to the original post, a Metric feedscrew nut seems to be available for about £60 - (is this the correct part? ..I've never been that intimate with a Boxford) ..considering the costs in tooling. materials, and time, I wouldn't hesitate to buy the spare part - and there's certainly no shame in buying what is, after all, a consumable service item. On the other hand I do recognise the challenge & satisfaction of a completely owner-executed repair, in which case I hope it goes well.

The Reltub guy who sells them on eBay quotes the size as 7/16" x 2.5mm ACME which is a bit unfortunate but not surprising IF he's correct about the ACME thread form. As I said above Denford used ACME for the imperial screws and modified square for the metric.

I'd be tempted to use the imperial one and suffer the 0.04mm-per-turn error for now until a metric replacement pops up. It's no worse than having the tool off centre height on a small diameter and much less than a badly worn screw.

Hi Diogenes,

Far from reading as egg sucking instructions I appreciate the detail and the time taken to reply.

Interestingly the measurement of the major diameter is 11mm making the thread 11 x 2.5. Nobody seems to make one of that size!

The replacement part mentioned in the link from Michael Gilligan does appear to be the correct part and I agree it would be cost effective. Having said that I enjoy the challenge and like making things!

Hi Pete,

The size quote in your post above from the ebay seller might just be what I am looking for. I cannot find that seller on ebay, do you have a link I could use?

https://www.ebay.co.uk/usr/reltub34

Edited By Pete Rimmer on 30/08/2019 11:58:47

I contacted reltub34 via ebay and he quickly replied stating that, as Pete stated, the size is a 7/16" x 2.5mm ACME thread.

My heartfelt thanks for all of the valuable input to help me understand what I had actually got.

For a thread like that there must have been a factory drawing showing the departure from spec for thread cutting depth on both the nut and the screw although the screws were rolled which begs the question how did they roll an ACME thread at a non-standard pitch or why would they even do that when you could roll a trapezoidal thread on the non-standard diameter without having to have non-standard rolls made.

Somebody somewhere must have that drawing.

Pete

ACME thread of metric pitch on imperial stock size is / was an "industry standard" whatever the books may say.

This was the normal method of producing metric machines used by British, American and, perhaps surprisingly, many continental makers back in the day. Metric trapezoidal is historically less common than you'd think. For all practical purposes the specifications are as per the nearest imperial size full depth ACME with a pitch adjustment.

Even if the thread was worked out from first principles the difference would almost certainly be too small for the likes of us to notice. Even without wear.

Clive

Well yes I realise that it would take nothing more than a small adjustment in half depth or tip width and it IS very small but still there must be a drawing that says more than 7/16" ACME 2.5mm pitch LH. They specified the OD for the blank for rolling the thread on the imperial cross slide screw to 4 decimals.

But could that be because the thread forming process (rolling) is quite fussy in that respect? Maybe if checking using three wires, this is the diameter that gave correct results.

It may be similar in concept to thread forming taps: for success, you need to be a lot more careful in your tapping drill selection.

Tracey Tools do not list a 2.5 X 11 trapezoidal tap, but it is worth phoning them just in case. I would think about using the imperial leadscrew, at least you have a tap to make a new nut if needed. The scale is a bother if you don't have a mill and rotary table to make a new one. A lot of lathe owners have fitted a DRO to the cross slide, if you had one, the numbers on the scale could be covered up with tape.

Edited By old mart on 03/09/2019 19:27:18

What a terrific company Tracy Tools are.

I rang them asking about a tap for my oddball size. They would not commit themselves over the telephone and clearly wanted to measure the item themselves. I sent them both the leadscrew and nut on Thursday last week. I received it back on Tuesday and a telephone call shortly thereafter confirmed the size as 7/16" x 2.5mm ACME. The bad news a tap was going to cost around £300 i.e. a one of specially manufactured tap.

I cannot emphasis how helpful and efficient they have been despite them being unsurprised in not making a sale.

Edited By Nick Edgley on 13/09/2019 00:10:43

So what is the plan now?

Plenty of choices - well a few.

Make a new nut.

Make a screw and nut.

Buy in some trapezoidal rod plus a nut (could make a nut).

Make an acetal insert for your present nut.

Probably others.

I would probably make a new square thread screw and nut if the screw is worn in the middle. If there is enough metal in the worn nut, I would use that - bore, bush and cut the new thread.

First thing is exactly how worn the existing nut/screw combo is. They will work perfectly adequately right up until they fail. Backlash is neither here nor there with a lathe - once the backlash is taken up. Remember, there will always be some backlash with cut threads (acetal, by the ‘heat and squeeze’ process will always be a very tight fit, needing some relief after moulding).

Duplicate.

Edited By not done it yet on 13/09/2019 01:42:30

It would be interesting [albeit not particularly useful] to know how Boxford went about making these 'bastard' components.

Presumably; being tooled-up to make make male screws with an ACME thread-form [whether by screw-cutting or by grinding] the easy option would have been to make their own taps to finish the nuts.

If anyone knows how Boxford produced these items, do please tell us.

MichaelG.

The Boxford drawing specifies that the threads are plunge rolled - for the imperial leadscrews at least.

Full marks to Tracy tools for an excellent customer service. Many would be happy to just sell an unwitting customer anything they asked for even if it was clear that it was probably not what they required. Clearly, Tracy tools places more value on their customer's satisfaction than your average supplier. Good show!

Edited By Pete Rimmer on 13/09/2019 11:21:19

You said you had another leadscrew and nut, could they be used?

If all else fails, just get a 12 X 3 trapezoidal leadscrew and nut and graft it onto one of your existing ones?

**LINK**

I was considering using my imperial leadscrew and nut but discovered the leadscrew is not the correct one, the length is different. It is most likely a Boxford part but from a different machine.

I have decided to buy a replacement nut from Boxford which is a little irritating having made one but been stymied by my inability to cut the appropriate thread in it and the fact that with postage and VAT it is £83!