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ACME versus Square thread profiles

Undercuts revisited

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Sam Stones10/11/2020 00:41:15
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835 forum posts
315 photos

Referring to Andrew Tinsley’s thread …

Why ACME threads on leadscrews?

**LINK**

At the time, a CAD image seemed an appropriate method of displaying how the helix angle presents an undercut. It also offered another explanation why lathe leadscrews have ACME profiles.

To digress slightly, helix undercutting can be an issue with moulded external threads, and often influences the choice of thread profile. The angle of the pressure face of a buttress thread is one such instance.

Unfortunately, try as I might, I failed to discover how to manipulate the solids, i.e. subtract the yellow blocks from the helix. I left it looking like this ...

square-thread-undercuts.jpg

That was back in 2017.

The other day a friend of mine mentioned that he had made some cast iron half-nuts to fit a new 12x2mm ACME leadscrew for the lathe he was refurbishing. The nut halves were a tight fit on the leadscrew so he decided to lap them using a leadscrew offcut. After lapping the thread of the half-nuts he discovered ‘… the fit was too good and was making it difficult to engage and almost impossible to disconnect under load.’

It was time for me to fire up the seventeen year old CAD package and search again for a way of generating a solid that I could manipulate. After squinting at and dabbing icons for a while, I suddenly spotted one I’ve used many times.

Create Swept Solids.

This was one of several dropdown windows exposing blocks of icons. There, amongst the block of six was …

‘Create a solid by Sweeping a Profile along a Helix’

We were in business, and soon I had four images showing the extent of interference, bearing in mind that I was using size for size, i.e. no clearances and no radii etc.

thread-undercut-evaluation---03.jpg

Travelling perpendicular to the thread axis, you can see how the undercuts interfere.

This issue has clearly been known since the late 1800's, yet I can't find a link. What do you people think?

Sam

Footnote: Usually, half nuts are less than half. Is that intentional?

mgnbuk10/11/2020 08:31:14
1032 forum posts
69 photos

Footnote: Usually, half nuts are less than half. Is that intentional?

Just the way they are made ?

Boxford half nuts were a one piece casting that was drilled, tapped & then split into two along the center of the thread. My Super 7 half nuts appear to have been made the same way. Each half nut will be less than half the diameter due to the width on the slitting cutter.

Can't offer any suggestions on the other issue.

Nigel B.

Michael Gilligan10/11/2020 09:11:18
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18923 forum posts
941 photos

There are some useful historic references available at Note_1 on this Wikipedia page: **LINK**

https://en.wikipedia.org/wiki/Trapezoidal_thread_form

MichaelG.

old mart10/11/2020 14:15:11
3345 forum posts
208 photos

Because the ACME, and trapezoidal threads have sloping sides, they are easier to produce. In the lathe leadscrew application, the nut is engaged radially and the sides of the threads should touch before the threads bottom out. This allows adjustment for minimum backlash and wear. The ammount of engagement of a square thread will not change its fit, and it can only get looser with wear. Square threads have the lowest axial friction of any thread but that is their only advantage as far as I can see.

Andrew Johnston10/11/2020 15:23:13
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6264 forum posts
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A square thread doesn't convert any of the axial force to a radial force, which could be useful in shock loading as one sees on a flypress.

The main problem with a square thread is that interference can be caused on the flanks, on the OD and on the ID, and they're all independent of each other. Whereas for an Acme thread cutting the full thread form a bit deeper will automatically take care of all three possible interferences.

Andrew

Michael Gilligan10/11/2020 16:35:56
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18923 forum posts
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It is well worth reading both the Letter and the Paper by Mr Powell, and of course the Editorial, in the ‘American Machinist’ links that I referenced earlier.

MichaelG.

Edited By Michael Gilligan on 10/11/2020 16:46:56

blowlamp10/11/2020 16:42:36
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1516 forum posts
98 photos

Trapezoidal threads allow half-nuts to engage whilst not fully in alignment along their length.

Martin.

old mart10/11/2020 20:10:14
3345 forum posts
208 photos

Good point, so do ACME, but square threads would not work very well with that sort of nut.

Michael Gilligan10/11/2020 20:24:01
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18923 forum posts
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Posted by old mart on 10/11/2020 20:10:14:

Good point, so do ACME, […]

.

Strictly speaking, of course ... both thread profiles are trapezoidal, and therefore work similarly.

Although modern usage dignifies the Metric form by giving it a capital T

angel MichaelG.

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