Acme Tap Heads-Up

This topic has 7 replies, 5 voices, and was last updated 30 August 2017 at 21:59 by Robbo.

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28 August 2017 at 01:48 #314238
Enough!
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@enough
I bought a new ML7 top-slide leadscrew and nut from Myford. When received the leadscrew was very tight at one end of the nut to the extent that I couldn't turn the leadscrew by hand (parts unassembled to machine, just lying in my hand without knob).

I'm taking this up with Myford but I checked the old nut on the new leadscrew and vice versa and determined that the problem was the nut. I remembered that I'd bought a 3/8 x 10 ACME tap ("For Myford Top-Slide" ) from RDG** some time ago and never used it.I was loath to use it on a new part for fear of loosening the basic fit but thought it would be OK to lightly thread through by hand (no wrench) and perhaps clear any burrs

The tap wouldn't even start – not a prayer. When I measured the tap OD I got 0.392" (i.e. 10mm) rather than 3/8'. The tap was clearly marked 3/8-10 ACME and the pitch appeared correct. Presumably whoever produced it got a little mixed up with their material stock.

So anyone considering buying one of these taps might want to keep this in mind.

** I'm aware that Myford RDG are essentially the same company.

(Edited to remove the inevitable phantom winkie)

Edited By Bandersnatch on 28/08/2017 01:49:46
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28 August 2017 at 01:48 #18630
Enough!
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@enough
28 August 2017 at 03:45 #314239
D Hanna
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@dhanna35823
The acme tap, as with all taps will measure larger than nominal as there is clearance for the OD of the screw. The flanks are where the correct axial clearance is controlled. I have one of those taps and have made numerous replacement 3/8″ x10 nuts without any excess backlash. Hope that helps.

Edited By D Hanna on 28/08/2017 03:54:05
28 August 2017 at 03:55 #314240
Enough!
Participant
@enough
I did not know that, thanks D Hanna.

(OTOH the tap will not even attempt to start in either my old (worn) or new nut)
28 August 2017 at 03:57 #314241
D Hanna
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@dhanna35823
Are you in the UK!! You should be still in Bed!! I’ll walk up to the workshop and measure my tap.

Tap by HBM measures OD 0.392″ Using shanks of 1/16″ drill (0.061″) over three wires measures 0.430″. Note that these over wire measurements are for comparison only. I didn’t work out any correct dia. for this thread.

Edited By D Hanna on 28/08/2017 04:18:28
28 August 2017 at 04:26 #314242
Hopper
Participant
@hopper
Sounds like maybe the manufacturer made the OD oversize to provide clearance, but then forgot to add the extra on to the thread depth when cutting the thread? IE, if you make the tap ten thou oversize diameter, cut the thread an extra ten thou deep (total) etc.

Edited By Hopper on 28/08/2017 04:26:54
28 August 2017 at 17:12 #314307
ega
Participant
@ega
This reminds me of something which is not, I hope, too far OT.

Some years ago I bought from Myford (Nottingham) a new nut for my Super 7 tailstock screw. This is a triple start thread which I felt would be well beyond my ability to screwcut.

On receipt of the new nut I found the old screw would only just enter and then refused to go any further. Whilst I pondered what to do about this I decided to fit the lever feed tailstock attachment which came with the lathe but which I had never used. This has proved so much better than the screw feed that it has remained in place to this day.

The expensive answer, of course, would have been to buy a nut and screw together.
30 August 2017 at 21:59 #314631
Robbo
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@robbo
I have one of those Acme taps, which I have used many times for cleaning out old nuts, which it did very well without cutting any metal as well.

It will also not enter a new feed nut without a deal of force.
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