Why ACME threads on leadscrews?

[Andrew TinsleyParticipant @andrewtinsley63637]

I was looking at a cheap and cheerful Chinese 920 lathe recently. The cross slide and top slide were dismantled for maintenance. Much to my surprise, the lead screws in these positions were, I think, standard 10 tpi threads of what appeared to be either 60 or 55 degree form. Certainly NOT ACME threads!

I was a little disgusted by this cheap use of standard threads. I then thought "well why not"? What is so special about ACME threads for slide ways. Having asked the question, I could not think of a decent answer! So why are ACME threads so widely used?

Andrew.

[Andrew TinsleyParticipant @andrewtinsley63637]

[Brian WoodParticipant @brianwood45127]

Andrew,

ACME form leadscrews were first proposed in 1895 in the USA as superior precision power threads over the square form versions used beforehand. The rationale was that they were easier to engage and disengage, the base of the thread, being tapered, was therefore stronger and they were easier to make into the bargain. Why the flank half angle of 14.5 degrees was chosen I can't explain.

Manufactures in the USA were quick to adopt the new standard, Hendey in 1897 and others closely followed suit.

As to the 'strange' flank angles you mention, the metric versions [trapezoidal screws] have a flank angle of 15 degrees, I think therefore you have the metric form but cut to give TPI threads.

Regards Brian

Edited By Brian Wood on 03/12/2017 18:54:15

Edited By Brian Wood on 03/12/2017 18:54:37

Edited By Brian Wood on 03/12/2017 18:55:40

Edited By Brian Wood on 03/12/2017 19:03:22

[Jeff DaymanParticipant @jeffdayman43397]

Acme thread has thicker "tooth" so has more strength in shear in the thread than std 60 deg thread tooth form. The 29.5 deg thread flank should in theory allow more thrust to be transmitted than a 60 deg thread. (lower angle means more thrust gets vectored down the axis than out sideways. If you look at presses and force testing machines (and some reverser screws on locomotives) they have square threads, which further increase attributes mentioned above.

[LathejackParticipant @lathejack]

The Chinese 920 & 918 lathes are copies of the Austrian made Emco Compact 8 lathe, so it would be interesting to know what thread form was used on the Emco original.

I don't think the Chinese made machines are particularly at fault for using a standard type thread form on some models. Some years ago I was examining a new Warco WM280 lathe, these are based on the much more expensive Wabeco D6000 lathe. Looking under the Chinese made 280 machines topslide I noticed it did not use an Acme or square type thread, just a normal thread form. So I immediately walked over to the Wabeco D6000 and saw that it also used a normal thread form, the same as that used on the Chinese copy.

[Andrew TinsleyParticipant @andrewtinsley63637]

Hello,

Yes I am aware that the Chinese 918 / 920 lathes are copies of the Emco compact 8 . I have seen the equivalent lead screws on a compact 8e and these were ACME threads to the best of my recollection.

Andrew.

[not done it yetParticipant @notdoneityet]

Square threads, when cut, are stronger than other cut varieties (rolled threads are likely better than the cut versions), but are not so easiy engaged for lead screws and half nuts. The 14 1/2 degrees may have come from involute gears, I don't know, Maybe from the usual angle setting for thead cutting of imperial 60 degree threads – simply standardisation at the time?

Cheap leadscrews, if of V form – simple as that, whatever origin.

[JasonBModerator @jasonb]

The 918 and 920 are actually based on the Emcomat 8.4 and 8.6 machines which had a geared head, the compact 8 came later and had a belt head to reduce costs. The 8.6 that I had did not use ACME or more likely being Austrian trapizoidal threads on the cross and top slides but 60 deg metric or imperial if I remember rightly

Edited By JasonB on 03/12/2017 19:51:32

[colin hawesParticipant @colinhawes85982]

Acme threads facilitate engagement of half-nuts of the rotating leadscrew. Colin

[LathejackParticipant @lathejack]

The 918 and 920 are definitely copies of the belt drive Emco Compact 8. Apart from minor details they have the same looking headstocks, bed castings, tailstocks, saddles, cross slides and topslides. They also have the same long lever behind the headstock for slackening and tensioning the drive belt. The Far Eastern machines also have the addition of the screw cutting gearbox and a longitudinal power feed built into the apron. 

Andrew mentioned the Emco Compact 8e does have Acme feedscrews. The Compact 8e is made in the Far East and is a little different to the original Austrian Compact 8 and the 918 & 920's.

Edited By Lathejack on 03/12/2017 21:26:06

Edited By Lathejack on 03/12/2017 21:27:49

[Mick B1Participant @mickb1]

Posted by Lathejack on 03/12/2017 19:17:39:

…

Some years ago I was examining a new Warco WM280 lathe, these are based on the much more expensive Wabeco D6000 lathe. Looking under the Chinese made 280 machines topslide I noticed it did not use an Acme or square type thread, just a normal thread form.

…

Nevertheless both crosslide and longitudinal leadscrews on my 2015-dated WM250V show an Acme form.

Can't really see what the fuss is about – Acme's screwthread may be a worthy design, but it lacks the sparkling innovation of, say, their rocket boots used by Wile E. Coyote…

Edited By Mick B1 on 03/12/2017 22:40:49

[vintagengineerParticipant @vintagengineer]

Years ago I used work in Hydraulics and a lot of the lifting jacks had a buttress thread on the shafts to lock the jacks in position.

[Nick HulmeParticipant @nickhulme30114]

The shallower the angle the less suitable a thread is for power transmission and repeated work in opposite directions, partly due to the wedge action under load which makes 60 & 55 degree threads so good for tightening to a set torque then holding.

 

https://www.google.co.uk/search?q=threads+used+for+power+transmission&oq=threads+used+for+power+transmission+&aqs=chrome..69i57j0.19738j0j7&sourceid=chrome&ie=UTF-8

 

I covered all this at college but Google is your friend

Edited By Nick Hulme on 04/12/2017 00:36:33

[Sam StonesParticipant @samstones42903]

With a square thread, the (lathe) split nut would not engage nor disengage without interference.

This results from undercuts in both nut halves formed by the root diameter of the helix.

It’s the same with split moulds for plastics and die-casting etc.

I tried to illustrate this but I can’t work out (in my CAD package), how to subtract the yellow bits from the thread flanks. The helical section of the thread is somehow locked in as a single entity, and refuses to cooperate.

Sorry, but you'll need to imagine which part of the thread is interfering.

Buttress threads are highly recommended for plastics caps and closures. With a near perpendicular flank, they produce lower lateral forces, and improve cap/closure retention. The same interference issue applies to this face as that for a square thread.

Sam

Edited By Sam Stones on 04/12/2017 01:24:23

[HopperParticipant @hopper]

Posted by Mick B1 on 03/12/2017 22:30:21:

Can't really see what the fuss is about – Acme's screwthread may be a worthy design, but it lacks the sparkling innovation of, say, their rocket boots used by Wile E. Coyote…

Te he. Pay that one.

I think the 60 degree thread for cross and topslides would be fit for purpose on a hobby lathe. Acme thread on the main leadscrew because it actually transmits power. But cross and topslide screws mostly are moved in to increase depth of cut then used under static conditions. Two different applications. Of course top and cross slide screws transmit power when taking a facing cut or cutting a taper, but that is a distinct minority of the time for most users.

Half nuts will engage just fine on a square thread, to wit "The Flagellator" my old Drummond M-type with its square thread leadscrew. You just have to wait until the threads line up exactly and then firmly snap the halfnuts into engagement. With an Acme, the slight taper on the tooth form makes engagement easier. Also, with the Acme, as the threads wear, the halfnuts can be adjusted inwards to take up the slack, still riding firmly on both thread flanks. With the square thread, once it has worn on the flanks there is little you can do about the resulting backlash.

[Thor 🇳🇴Participant @thor]

Lathejack, my old Emco Compact 8 uses ordinary 60 deg. metric thread on the topslide feedscrew so I used the same when I made a new topslide with longer travel.

Thor

[JasonBModerator @jasonb]

Posted by Lathejack on 03/12/2017 21:21:29:

The 918 and 920 are definitely copies of the belt drive Emco Compact 8. Apart from minor details they have the same looking headstocks, bed castings, tailstocks, saddles, cross slides and topslides.

Yes they share some of the compact 8 headstock design but the rest is Emcomat which the 8 is based on, have a look here and see all the other parts are the same.

[Martin DowingParticipant @martindowing58466]

ACME and 30deg trapezoidal threads are much easier to manufacture than square version and I have red that this is main reason why they are used. Sqare threads are not difficult to engage if little backlash is permitted and they are not tending to partially disengage during heavier cuts, what leeds to thread errors.

IMO suare version is superior.

60 deg threads are even worse in this respect and they are also mechanically unsound. OK, hardened versions might be OK for some applications.

Martin

Edited By Martin Dowing on 04/12/2017 08:25:22

[Mick B1Participant @mickb1]

Posted by Hopper on 04/12/2017 02:08:54:

With an Acme, the slight taper on the tooth form makes engagement easier. Also, with the Acme, as the threads wear, the halfnuts can be adjusted inwards to take up the slack, still riding firmly on both thread flanks. With the square thread, once it has worn on the flanks there is little you can do about the resulting backlash.

I think that's possibly the key, at least where halfnuts are involved – it's less easy to do anything remedial where crosslide or topslide nuts are concerned. However I've heard opposed Acme nuts described, where backlash is taken up by adjusting the spacing between the two nuts so that they each bear on opposite flanks of the screw. Don't remember ever actually seeing such an arrangement, though.

There can be requirements for substantial force to be transmitted by crosslide leadscrews – knurling, for example.

I do suspect that Acme threads, like many things in engineering, are a compromise – in this case between strength, adjustability and ease of manufacture.

One of my first engineering jobs was operating a big thread-roller at a Nottingham factory that made jacking-screws for artic trailers. I was rolling 10 or 15 foot lengths of bar-turned 1 3/8" bar with two-start Acme threads, some left-handed. You could get a good piece-rate if you fed the start of one bar hard up against the tail of the previous one. If you failed, you had to open and re-close the roller on the next bar, which lost a minute or so, and also some inches of thread so the whole bar wasn't usable in the next machining process. I don't know whether that follow-on, or even the whole rolling process would've been as easy on square thread.

Edited By Mick B1 on 04/12/2017 11:28:11

[Andrew TinsleyParticipant @andrewtinsley63637]

Seems that I have caused a bit of discussion here! I was not knocking anything, just curious as to why an ACME thread might be better. Seems to be several answers to that! Very informative too.

I have an apology to make, I checked on the Emcomat 8e and id doesn't have an ACME form on the cross and top slides, so mea culpa.

Interestingly enough, the fix for the rather grotty system on the 920 is to use a larger diameter 10tpi thread of 60 degree form together with a ballrace and bronze nut. There seems to be a lot of info on the US forums about improving the 920s faults. A friend of mine is going to buy one and as he is a newcomer to the hobby, it might be that I have to help with the necessary improvements.

Looks like I will be considered as the "expert" go to, little does he know it will be blind leading the blind.

Andrew.

[Clive FosterParticipant @clivefoster55965]

Given the date of standardisation 14.5° was probably chosen as being a sensible round number for development of a slope from linear measurement. Its near as dammit 3.1" per foot if you work off the hypotenuse of the triangle. Error is around 1 minute of arc. Good enuf back in the day methinks. Accurate direct angle measuring systems being both uncommon and expensive in those days.

Wear and thread depth are important issues when considering feed screws.

As mentioned previously an ACME is easily closed up a little to compensate for (even) wear. Depth of ACME can also, in principle, be independent of pitch although in practice there are standards. With a 60° thread its hard to compensate for more than very limited wear by closing up and overall depth is prescribed by the pitch. Hence finer feed screw threads like 10 TPI may be shallower than desired limiting power transmission and stability under load whilst coarse ones like 4 TPI on large lathe feed screws may be deeper than desired needing an unnecessarily large feed screw diameter.

Clive

[SillyOldDufferModerator @sillyoldduffer]

Interesting : I did a little bedtime reading on this:

• Square threads are 'best' for power transmission. However, they are expensive to make and so are the nuts. Even more costly, the fit has to be first class too.
• For many purposes the ACME thread is a much better compromise. For pretty much the same performance as a square thread, it's both cheaper to make and cheaper to fit. Furthermore, when ACME is used as a lead-screw, engaging the half nut is much less critical. Despite being 'cheap' it's a better solution to this engineering problem than a square thread.
• The main purpose of square and ACME threads is power-transmission. There are other useful thread properties such as accuracy, alignment, and minimising backlash. I'm not sure use of an ordinary thread form on a lathe automatically means 'cheap', though of course it might be the motive.

What I couldn't find is how much it costs to make ACME threads compared with the ordinary form. The cost can't be prohibitive because my cheap and nasty work-stand has two of them working the vice top. It's over-cheap MDF and thin steel pressings that let the stand down, not the screw-threads.

Dave

 

Edited By SillyOldDuffer on 04/12/2017 11:40:49

[HopperParticipant @hopper]

The 29 degrees on an Acme thread makes it ideal as a worm thread to match the 14.5 degree pressure angle gears that were the norm back in the early days. The Acme thread cross section is basically rack format of the mating gear. Not sure if 29 degrees was chosen specifically for this though?

As far as cheaper to manufacture, possibly easier to roll Acme threads due to less binding on the angled flanks vs the square flanks of square threads?

[Martin KyteParticipant @martinkyte99762]

My twopence worth

For trapezoidal thread form there is a reasonably large cylindrical surface for the half nuts to slide over before engagement . With a steep flank angle you get to choose the depth of thread for any given diameter shaft and TPI. Choosing the shaft diameter you get to control the helix angle too so you can design for strong threads, large working surfaces, ability to adjust clearence by using half nuts.

regards Martin

[egaParticipant @ega]

SillyOldDuffer:

If your work-stand is a B&D WorkMate then those acme screws are probably twin start, a feature that the late Ron Hickman apparently regarded as essential to his design. In principle, the screws could have been of vee form but I wonder whether the acme stands up better to the swivelling action of the rear jaw.

The early versions actually mounted a conventional woodworking vice which in production form was replaced by the two-part top/vice; natural, then, to carry over the vice screw to the new design. I don't know whether the idea of screws of opposing hands was considered.

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