Acme thread

[Shaun BelcherParticipant @shaunbelcher81617]

Posted by Hopper on 04/08/2016 04:13:15:

You have a lathe. Make one. Lock the cross slide in position and set the topslide at 90 degrees to the lathe axis and use it to feed the cut in. A moving steady will probably be required, but is easy enough to make up too.

I am needing to machine some new feed screws and nuts on my ML7 too.

Just stumbled across this thread so thought that I would ask a question.

Myford claim its 10TPI for the imperial feedscrews, but only say the metric ones are 3/8" 2mm pitch.

I'm assuming that they are 8TPI?

Either way, I thought that it would be a good time to convert the lathe to metric when replacing the feedscrews, just need to get the dials from myford.

What changewheel do I need for cutting such threads?

I don't have any that came with my lathe but I've got a friend who is an engineer and can probably machine me one on his lathe if I know the right specs. I also have no idea what type of tap to get for cutting out the feedscrew nuts.

[not done it yetParticipant @notdoneityet]

8 tpi would be close to 3mm pitch (actually 3.18). Please do not assume anything – you may well be wrong, so always check it out properly. 2mm pitch is just that – there is no need to try to convert to imperial (it would actually be 12.7 tpi!).

While not standard myford, I would make them in square thread, not acme, although at only 3/8” diameter that may be a little more awkward. Cutters are easily ground in the home workshop for both inside and outside threads.

Myford’s competitor (‘just down the road&rsquo used square threads for feed screws – but 1/2” in diameter. Nothing at all wrong with their feed screws (their lathes were of much higher quality than those of myford). I am currently cutting a new nut in square format for one of those. One only really needs ACME thread for lead screws, that need to be engaged and disengaged with half nuts.

One could always cut the screw easily enough and then make the feed screw nut in acetal. Probably better to cut a piece of extra screw for that, including clearance, as acetal will leave no thread clearance when moulded by the ‘heat and squeeze’ method.

[DC31kParticipant @dc31k]

Posted by Shaun Belcher on 24/11/2019 00:17:30:

Myford claim its 10TPI for the imperial feedscrews, but only say the metric ones are 3/8" 2mm pitch.

I think what this means is that the outside diameter of the screw is 3/8" but the pitch that is cut on it is 2mm.

Basically a metricated version of the imperial screw, the only change being the distance between the thread crests. It means all the stuff at the handle end will fit unchanged.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by not done it yet on 24/11/2019 06:30:09:

8 tpi would be close to 3mm pitch (actually 3.18).

.

[not done it yetParticipant @notdoneityet]

Posted by Michael Gilligan on 24/11/2019 09:16:49:

Posted by not done it yet on 24/11/2019 06:30:09:

8 tpi would be close to 3mm pitch (actually 3.18).

.

25.4/8 = ?

Most certainly not 2mm.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by not done it yet on 24/11/2019 09:57:41:

.

8 tpi would be close to 3mm pitch (actually 3.18).

.

By internationally agreed definition … actually 3.175

MichaelG.

.

P.S. … Yes, there’s a lot of detail in 5 microns …

https://www.nature.com/articles/s41524-019-0202-3/figures/1

 

Edited By Michael Gilligan on 24/11/2019 10:29:12

[Shaun BelcherParticipant @shaunbelcher81617]

Posted by not done it yet on 24/11/2019 06:30:09:

8 tpi would be close to 3mm pitch (actually 3.18). Please do not assume anything – you may well be wrong, so always check it out properly. 2mm pitch is just that – there is no need to try to convert to imperial (it would actually be 12.7 tpi!).

While not standard myford, I would make them in square thread, not acme, although at only 3/8” diameter that may be a little more awkward. Cutters are easily ground in the home workshop for both inside and outside threads.

Myford’s competitor (‘just down the road&rsquo used square threads for feed screws – but 1/2” in diameter. Nothing at all wrong with their feed screws (their lathes were of much higher quality than those of myford). I am currently cutting a new nut in square format for one of those. One only really needs ACME thread for lead screws, that need to be engaged and disengaged with half nuts.

One could always cut the screw easily enough and then make the feed screw nut in acetal. Probably better to cut a piece of extra screw for that, including clearance, as acetal will leave no thread clearance when moulded by the ‘heat and squeeze’ method.

I got the 8tpi figure somwhere else on another myford thread in a forum (excuse the pun lol)

I didnt think it sounded right.

Anyway, ive done a bit more research and yes, its supposed to be a square thread, and I believe this is what Myford use.

I also found a blog where someone machined their own feedscrews and nuts. You need a moving steady and the right screwcutting gears and ground cutting tooling etc.

Quite a lot of work and hard to do a nice job, he had to do many slow passes on the lathe.

He made his shaft 9mm but 3.8" is closer to 9.5mm

Does shaft diameter affect TPI?

If not, I think ive found the answer.

I can find many retailers who stock square threaded shaft in 10mm diameter, 2mm pitch.

Should be able to find the correct tap to make my own nuts also.

If you look at the aftermarket feedscrews that myford solutions sell, they look simply like they are made from cut down prethreaded rod, with the end machined down and threaded to accept the handle.

They differ in that they dont have the unthreaded "lip" at the end of the shaft, I thought this may have been required, so not sure why they machined them like this, unless they had to do away with it by using prethreaded metric rod.

See photos:

Their feedscrews also are bronze, not the crappy zamac alloy that myford uses, but I will machine my own if this will work with 10mm rod if it does not affect the dial calibration.

[HopperParticipant @hopper]

I think you will find Myford uses Acme thread (or maybe trapezoidal for metric?) and not square. But square will certainly work for you if that is what you can buy in threaded rod.

Tracy Tools and others can supply square and Acme thread taps. Many on eBay too. Can be a bit hard to tap large chunky threads like that from scratch so it is common to rough it out by screwcutting in the lathe, even with a steep-sided V tool and finish with the tap. Or buy a set of progressive taps.

I make all my leadscrew nuts from brass, not bronze. Less wear on the leadscrew that way. And easier to machine.

[Shaun BelcherParticipant @shaunbelcher81617]

Posted by Hopper on 27/11/2019 07:01:25:

I think you will find Myford uses Acme thread (or maybe trapezoidal for metric?) and not square. But square will certainly work for you if that is what you can buy in threaded rod.

Tracy Tools and others can supply square and Acme thread taps. Many on eBay too. Can be a bit hard to tap large chunky threads like that from scratch so it is common to rough it out by screwcutting in the lathe, even with a steep-sided V tool and finish with the tap. Or buy a set of progressive taps.

I make all my leadscrew nuts from brass, not bronze. Less wear on the leadscrew that way. And easier to machine.

Im quite lost to what thread Myford uses, but most sources ive seen say that myford used square thread on all their lathes.

Anyway, from what i understand, acme was the term used for trapezoidal thread in the days of imperial?

As far as bronze goes, I think alot of people call brass bronze and vice versa. Bronze is supposed to look more copper in colour isint it?

The nuts in the photo certainly look brass in colour than the coppery bronze colour.

Anyway, would a 0.5mm increase in diameter over the original myford imperial feedscrew change the TPI and affect the dial readings? Or does it stay constant irrespective of the diameter if the thread pitch is the same?

[Michael GilliganParticipant @michaelgilligan61133]

For the sake of clarity [hopefully not for further confusion] regarding the metric screw threads

The aftermarket versions supplied by MyfordSolutions are very clearly stated to be 2mm pitch.

**LINK**

https://myfordsolutions.com/collections/ml7-carriage-assembly

Click the thumbnails for individual descriptions:

” Each rotation of the spindle gives 2 mm displacement. “

MichaelG.

[Shaun BelcherParticipant @shaunbelcher81617]

Posted by Michael Gilligan on 27/11/2019 08:47:55:

For the sake of clarity [hopefully not for further confusion] regarding the metric screw threads

The aftermarket versions supplied by MyfordSolutions are very clearly stated to be 2mm pitch.

**LINK**

https://myfordsolutions.com/collections/ml7-carriage-assembly

Click the thumbnails for individual descriptions:

” Each rotation of the spindle gives 2 mm displacement. “

MichaelG.

Myford also state 2mm pitch for their metric feedscrews.

What im saying is I can buy the prethreaded rod 10mm in diameter with a 2mm pitch.

Myford say theirs is 3/8" diameter which is about 9.5mm diameter.

Is there any issues with accuracy if i use a slightly larger diameter feedscrew? e.g, change the TPI rate?

Please note, that I also would be machining my own nut to match.

Edited By Shaun Belcher on 27/11/2019 08:52:51

Edited By Shaun Belcher on 27/11/2019 08:53:10

Edited By Shaun Belcher on 27/11/2019 08:54:25

[not done it yetParticipant @notdoneityet]

Hi Hopper,

I must have missed the square taps. Can you supply a detailed route to them, please?

Cutting external square threads is a relative doddle – even if the cutters may be a little fragile. It’s not like a lead screw in length and the width of the thread is relatively easy to adjust (as long as the cut is too narrow).

I have used cutters ground from 4mm HSS rod held in a home made holder (usually a length of silver steel rod). Clearly more difficult as the diameter decreases and the length increases!

A square HSS cutter in a square hole would be preferable, but I have got by with round ones so far.

[Shaun BelcherParticipant @shaunbelcher81617]

Here is what I can source at a fraction of the price and make my own replacements.

Exact same 2mm pitch as specified, only difference is the diameter is 0.5mm larger than myford's.

https://www.mooreinternational.co.uk/ProductGrp/10×2-quality-rh

making the nut is a piece of cake too.

What complicates things is if you need to machine it to the exact 3/8" (9.5mm) diameter that myford use.

If the feedscrew diameter is not an issue, then this is a simple and cost effective replacement.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Shaun Belcher on 27/11/2019 08:45:24:

[…]

Anyway, from what i understand, acme was the term used for trapezoidal thread in the days of imperial?

[…]

.

Shaun

This is important … although sellers all over the world seem to ignore the facts these days:

The ACME thread form is very specific, and has 29° flanks

The Metric equivalent is Trapezoidal, and has 30° flanks

Details of both are readily available.

[sorry for preaching from my soapbox]

MichaelG.

[Shaun BelcherParticipant @shaunbelcher81617]

Posted by Michael Gilligan on 27/11/2019 09:09:04:

Posted by Shaun Belcher on 27/11/2019 08:45:24:

[…]

Anyway, from what i understand, acme was the term used for trapezoidal thread in the days of imperial?

[…]

.

Shaun

This is important … although sellers all over the world seem to ignore the facts these days:

The ACME thread form is very specific, and has 29° flanks

The Metric equivalent is Trapezoidal, and has 30° flanks

Details of both are readily available.

[sorry for preaching from my soapbox]

MichaelG.

Either way, I dont think its critical for operation but may affect how smooth the handles turn?

May also affect the rate of wear? Seems trapezoidal is easier to find than square thread anyway.

The supplier in the link i posted also stocks rolled ballscrew, ground ballscrew and satellite roller screw.

I have no idea what the difference is between them.

[Michael GilliganParticipant @michaelgilligan61133]

You are, of course, at liberty to make whatever you want, Shaun

… and MooreInternational does look a very good place to source ‘make-from’ materials.

MichaelG.

[mark smith 20Participant @marksmith20]

Shaun , a slightly larger diameter screw would still work the same in regard to the dials if the pitch TPI is the same .(as long as everything else is machined to fit (the nut, dials ,handles etc…)

Edited By mark smith 20 on 27/11/2019 09:37:59

[IanTParticipant @iant]

Posted by Shaun Belcher on 27/11/2019 09:05:21:

"making the nut is a piece of cake too."

I'm not going to jump into any Square/Acme/Size of Material discussion here Shaun – just wanted to support Hoppers statement (warning) that 'tapping' an Acme tread is not as simple as just buying a single Acme tap and drilling a suitable hole. I think you will find it is very hard (if not impossible) to turn it.

It's either a staged tap (e.g. more than one tap = quite expensive) or an interior thread cutting operation, perhaps using a tap as a finishing touch, if you already have a suitable tap. I've seen YouTubes of folk going straight in with a single Acme tap but I don't know how they do it – because I certainly couldn't when I tried it, albeit on a slightly larger (1/2" x 10tpi) nut.

Of course, if you can purchase a screw & matching nut and modify them to suit your application, then this need not worry you.

Regards,

IanT

Edited By IanT on 27/11/2019 10:22:46

[Shaun BelcherParticipant @shaunbelcher81617]

Posted by mark smith 20 on 27/11/2019 09:37:14:

Shaun , a slightly larger diameter screw would still work the same in regard to the dials if the pitch TPI is the same .(as long as everything else is machined to fit (the nut, dials ,handles etc…)

Edited By mark smith 20 on 27/11/2019 09:37:59

Thanks, thats all I need to know.

I would hate to find the dials were not accurate if i used this feedscrew.

Im actually considering making my own resettable dials anyway, but would need to CNC engrave the markings on the outside accurate enough

[Shaun BelcherParticipant @shaunbelcher81617]

Posted by IanT on 27/11/2019 10:20:08:

Posted by Shaun Belcher on 27/11/2019 09:05:21:

"making the nut is a piece of cake too."

I'm not going to jump into any Square/Acme/Size of Material discussion here Shaun – just wanted to support Hoppers statement (warning) that 'tapping' an Acme tread is not as simple as just buying a single Acme tap and drilling a suitable hole. I think you will find it is very hard (if not impossible) to turn it.

It's either a staged tap (e.g. more than one tap = quite expensive) or an interior thread cutting operation, perhaps using a tap as a finishing touch, if you already have a suitable tap. I've seen YouTubes of folk going straight in with a single Acme tap but I don't know how they do it – because I certainly couldn't when I tried it, albeit on a slightly larger (1/2" x 10tpi) nut.

Of course, if you can purchase a screw & matching nut and modify them to suit your application, then this need not worry you.

Regards,

IanT

Edited By IanT on 27/11/2019 10:22:46

OK, ill take note of this. I never realised it was that difficult. I see Geffoery Crockers restoration video on youtube he is simply tapping the holes after drilling and looked fairly straightforward to me.

Either way, i have a friend who can probably do this easily enough on his machine at work.

[Shaun BelcherParticipant @shaunbelcher81617]

Also regarding material of choice, i see I can choose the option of stainless or just (mild?) steel.

What is the best option to go for regarding wear?

Stainless has the added benefit that it wont rust.

I think its pretty clear to me now that the metric feedscrews myford solutions sell are simply made from standard 10mm  2mm pitch trapezoidal prethreaded rod.

Edited By Shaun Belcher on 27/11/2019 11:03:04

[mark smith 20Participant @marksmith20]

Posted by Shaun Belcher on 27/11/2019 10:39:37:

Also regarding material of choice, i see I can choose the option of stainless or just (mild?) steel.

What is the best option to go for regarding wear?

Stainless has the added benefit that it wont rust.

I think its pretty clear to me now that the metric feedscrews myford solutions sell are simply made from standard 10mm 2mm pitch trapezoidal prethreaded rod.

Edited By Shaun Belcher on 27/11/2019 11:03:04

I made some here for my southbend when i got it. This was the result of only my third attempt ever at screw cutting. I just used leaded steel and used the original nuts . They were a large improvement in back lash etc on the original screws . Still working fine after a few years. I grafted the threaded section into the original parts using loctite and a pin.

Edited By mark smith 20 on 27/11/2019 11:11:36

[BazyleParticipant @bazyle]

There is an important difference between ACME/Trapezoidal thread and square thread when used in a cross or topslide as the form can be used to take up wear. If the nut is made with thinner slots & thicker webs so that it won't actually go on the screw, then slit lengthways the two halves can be fitted round the screw but not quite meet together. The section of the web of the nut is a wedge and is gong into the angled slot of the screw thread, as far as it can. When it wears it can be pushed further into the slot of the thread taking up the slack. Some lathes do make use of this feature which although not ideal may be advantageous if you want a minimal backlash setup.

[egaParticipant @ega]

Posted by IanT on 27/11/2019 10:20:08:

I'm not going to jump into any Square/Acme/Size of Material discussion here Shaun – just wanted to support Hoppers statement (warning) that 'tapping' an Acme tread is not as simple as just buying a single Acme tap and drilling a suitable hole. I think you will find it is very hard (if not impossible) to turn it.

I was surprised by this as I recently had no difficulty in tapping a length of cast iron 1/2" x 10TPI acme. I should add that this was done under power and the tapping hole was slightly oversize. Other materials may well be problematic, of course.

[Shaun BelcherParticipant @shaunbelcher81617]

Posted by ega on 27/11/2019 14:29:18:

Posted by IanT on 27/11/2019 10:20:08:

I'm not going to jump into any Square/Acme/Size of Material discussion here Shaun – just wanted to support Hoppers statement (warning) that 'tapping' an Acme tread is not as simple as just buying a single Acme tap and drilling a suitable hole. I think you will find it is very hard (if not impossible) to turn it.

I was surprised by this as I recently had no difficulty in tapping a length of cast iron 1/2" x 10TPI acme. I should add that this was done under power and the tapping hole was slightly oversize. Other materials may well be problematic, of course.

Its interesting to know anyway. Im not sure if its as difficult with brass or bronze?

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