Metric ML10 lead screw threadform

[Pete RimmerParticipant @peterimmer30576]

Evening!

Does anyone know if the metric lead screw for the ML10 is trapezoidal thread or ACME? OD is 16mm with 3mm pitch but I don't know the form or if it's full form or stub form.

If someone had a factory print that would make my day!

Pete.

[Pete RimmerParticipant @peterimmer30576]

[John HaineParticipant @johnhaine32865]

If the ML10 follows the practice of the 7 series lathes, the leadscrew will not be metric though the x slide and topslide will be. The LS is probably 8 tpi though I don't know the threadform.

[HopperParticipant @hopper]

Posted by John Haine on 24/04/2018 22:06:14:

If the ML10 follows the practice of the 7 series lathes, the leadscrew will not be metric though the x slide and topslide will be. The LS is probably 8 tpi though I don't know the threadform.

Again, if it follows 7 series practice, the 8tpi leadscrew will be Acme threadform.

[Pete RimmerParticipant @peterimmer30576]

I thought the same thing about them all being 8tpi because of the ML7 being so but the screw is certainly 3mm pitch. It's confirmed on the lathes page that they did make metric ones.

I can't think why they would make a metric pitch screw with ACME form but you just never know.

[John HaineParticipant @johnhaine32865]

I found the lathes site ambiguous on this.

"Available in both full metric and imperial versions each had compound-slide feed screws, micrometer dials and the leadscrew to the correct specification. To convert an imperial machine to metric screwcutting required only two 21t changewheels in addition to the normal set (it was not necessary to change the leadscrew) – while to convert the rest of the lathe to a metric specification (or the other way round) needed only the substitution of the correct cross and top-slide feed screws, nuts and micrometer dials. The lack of a tumble…".

Seemed to imply that converting either way didn't need a different leadscrew. I guess you have a metric ml10 and have checked? If they actually made the 10 with a metric LS, it just confirms how peculiar Myford's approach to product engineering is that they didn't do the same for the 7 series.

As far as I can see fro Wikipedia the main difference between Acme and the metric trapezoidal form is 1 degree on the flank angle. Assuming the screws were made in house, they could easily have used the same tooling for both.

[Michael GilliganParticipant @michaelgilligan61133]

Neither of these answers the opening question, but, here is evidence from 'Myford' that the leadscrew pitch of a 'Metric ML10' was 3mm:

https://www.myford.co.uk/acatalog/ML10-LEADSCREW-NUT-ASSEMBLY-METRIC–10295-ACC-192.html

https://www.myford.co.uk/acatalog/ML10-METRIC-LEADSCREW-MICROMETER–30-055-1429.html

.

That said: Thread profile gauges [to the standard required for this check] are not particularly expensive, or difficult to make

Useful comparison of profiles here:

http://bakergauges.com/product-detail/thread-gauge-profile/ 

MichaelG.

Edited By Michael Gilligan on 25/04/2018 08:04:12

[John HaineParticipant @johnhaine32865]

That seems to nail it Michael.

[Pete RimmerParticipant @peterimmer30576]

It's certainly 3mm pitch.

 Assuming the screws were made in house, they could easily have used the same tooling for both.

That's what concerns me, or rather that they might have used a 8tpi insert and set the lead to 3mm.

 

 

Edited By Pete Rimmer on 25/04/2018 08:53:27

[Mick B1Participant @mickb1]

+1 for the 3mm pitch. I'd've been nearly 7 thou per turn out on many things I made on my metric Speed 10 if it'd been 8 TPI.

I always assumed it was Acme, but I'd never've picked up a 1 degree flank angle difference.

[Pete RimmerParticipant @peterimmer30576]

Pretty easy to get the angle with a gear tooth caliper and some basic trig. I guess I'll grind a 30 degree tool in preparation and check the original screw when it gets here.

[not done it yetParticipant @notdoneityet]

ACME is TPI and metric trapezoidal is pitch and never the twain shall meet – or at least they shouldn’t!

[Pete RimmerParticipant @peterimmer30576]

Posted by not done it yet on 25/04/2018 09:30:22:

ACME is TPI and metric trapezoidal is pitch and never the twain shall meet – or at least they shouldn’t!

True that, but then my Denford lathe had a cross slide screw that was 1/2" OD x 2.5mm pitch. They just used the same material and cut a 2.5mm thread in place of 10tpi. Weird things happen.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by not done it yet on 25/04/2018 09:30:22:

ACME is TPI and metric trapezoidal is pitch and never the twain shall meet – or at least they shouldn’t!

.

A very sound comment, in terms of finished screw-threads … But:

Thread forms are, by convention, [always?] specified in terms of the proportion of their various features to a basline reference dimension … the pitch.

MichaelG.

[John HaineParticipant @johnhaine32865]

Posted by Michael Gilligan on 25/04/2018 10:20:21:

Posted by not done it yet on 25/04/2018 09:30:22:

ACME is TPI and metric trapezoidal is pitch and never the twain shall meet – or at least they shouldn’t!

.

A very sound comment, in terms of finished screw-threads … But:

Thread forms are, by convention, [always?] specified in terms of the proportion of their various features to a basline reference dimension … the pitch.

MichaelG.

If you look at the Wikipedia page on trapezoidal thread forms it shows that the basic definition is identical, given in terms of the pitch, except for the included flank angle.

[Pete RimmerParticipant @peterimmer30576]

I just found a NOS set of half-nuts on eBay and the thread is quite roughly cast into the metal (zamak, possibly) half-nut. I guess that makes the half a degree half-angle difference immaterial.

https://www.ebay.co.uk/itm/MYFORD-ML10-HALF-NUTS-3-0-PITCH-FOR-METRIC-LEADSCREW-/352300339399

[Mick B1Participant @mickb1]

Posted by Pete Rimmer on 25/04/2018 10:51:23:

I just found a NOS set of half-nuts on eBay and the thread is quite roughly cast into the metal (zamak, possibly) half-nut. I guess that makes the half a degree half-angle difference immaterial.

https://www.ebay.co.uk/itm/MYFORD-ML10-HALF-NUTS-3-0-PITCH-FOR-METRIC-LEADSCREW-/352300339399

 

I think so too. When I put that pic up on a largish screen and looked at the flank angle with my Lidl digital protractor, it came out a gnat's over 25 degrees included, so I reckon the flanks are in wide clearance and the halfnuts are riding on leadscrew crest to nut trough – I'm guessing to minimise the effects of oily crap on the leadscrew.

During the 15 years I had my Speed 10, I found the engagement gritty and uncertain and took the halfnuts out, expecting to have to replace them. It wasn't necessary – once I'd poked out all the impacted crud, the inside thread came up shiny and unscored, so I put 'em back and had no more trouble

Edited By Mick B1 on 25/04/2018 11:51:51

[Neil WyattModerator @neilwyatt]

Martin Cleeve recommended fitting a long grub screw to stop the half-nuts completely closing on the leadscrew as this give greater tolerance for gunk and greater repeat accuracy.

Of course this is already a standard fitting on better class machines, like mini-lathes

Neil

[Clive FosterParticipant @clivefoster55965]

Metric pitch ACME on imperial size bar stock doesn't seem uncommon. Pretty sure I measured my metric Bridgeport screws as being ACME form, not trapezoidal, when I gave it the post purchase scrub down, de-crud and spruce up treatment.

Clive.

[Mick B1Participant @mickb1]

Posted by Neil Wyatt on 25/04/2018 12:22:38:

Martin Cleeve recommended fitting a long grub screw to stop the half-nuts completely closing on the leadscrew as this give greater tolerance for gunk and greater repeat accuracy.

Of course this is already a standard fitting on better class machines, like mini-lathes

Neil

Pete R's Ebay item already has that, and I *think* my Speed 10 did too. I might've wound it in a turn or two to minimise backlash – can't remember for certain.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Neil Wyatt on 25/04/2018 12:22:38:

Martin Cleeve recommended fitting a long grub screw to stop the half-nuts completely closing on the leadscrew as this give greater tolerance for gunk and greater repeat accuracy.

.

Interesting concept, Neil … Could you please share the reference ?

Whilst I can certainly accept that there are benefits in not closing the [slightly less than] half nuts onto the screw; the 'explanation' fails to impress.

MichaelG.

[Neil WyattModerator @neilwyatt]

Cripes, it was one of Martin Cleeve's articles in model engineer… probably in the 50s or 60s… does that help?

[Gordon WParticipant @gordonw]

I don't cut threads very often, I use a round silver steel cutting tool with a 60 deg. point hand ground. This can be used for internal and external threads in the correct tool holder. Would this work for a leadscrew thread and nut, with the appropriate angle ground on ? Then the exact angles don't matter, so long as you don't need a spare in 10 years.

[Pete RimmerParticipant @peterimmer30576]

If you hand-grind a threading tool to within the half-degree difference between acme and trapezoidal flank angles you're some kind of superhuman. It's only half a thou difference at the tip of the thread at the pitch we're discussing.

[Mick B1Participant @mickb1]

Posted by Mick B1 on 25/04/2018 11:50:25:

Posted by Pete Rimmer on 25/04/2018 10:51:23:

I just found a NOS set of half-nuts on eBay and the thread is quite roughly cast into the metal (zamak, possibly) half-nut. I guess that makes the half a degree half-angle difference immaterial.

https://www.ebay.co.uk/itm/MYFORD-ML10-HALF-NUTS-3-0-PITCH-FOR-METRIC-LEADSCREW-/352300339399

I think so too. When I put that pic up on a largish screen and looked at the flank angle with my Lidl digital protractor, it came out a gnat's over 25 degrees included, so I reckon the flanks are in wide clearance and the halfnuts are riding on leadscrew crest to nut trough – I'm guessing to minimise the effects of oily crap on the leadscrew.

During the 15 years I had my Speed 10, I found the engagement gritty and uncertain and took the halfnuts out, expecting to have to replace them. It wasn't necessary – once I'd poked out all the impacted crud, the inside thread came up shiny and unscored, so I put 'em back and had no more trouble

Edited By Mick B1 on 25/04/2018 11:51:51

Doh! Rubbish from me on 2 counts.

Firstly, looking at the particular pitch in the Ebay photo where the camera's looking square to the thread entering the halfnut, it's a bit over 29 degrees included.

Secondly, that's correct for Acme – I'd forgotten that the 15.5 is the half-angle, not included.

I do think it's more likely Acme than trapezoidal Metric.

[Author]

Posts