Acme Tap Heads-Up

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

Edited By D Hanna on 28/08/2017 03:54:05

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)

Edited By D Hanna on 28/08/2017 04:18:28

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

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.

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.