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oxford
11-07-2012, 05:18 PM
I am making an acme tap out of a piece of threaded rod. I am going to be tapping delrin and using the same rod that the tap is made out of for the lead screw. The thread is 5/8-10 LH. The book I have says the hole size should be pitch minus bolt dia plus .005, which should be a hole size of .530". The rod I am using has a thread depth around .060-.065". Should I use the .530" hole size or go down some? When using Evans delrin nut method it would have no thread clearance, so it got me thinking.

I also read some where that acme threads can be hard to start and keep straight and a guide bushing would be better if made on the end of the tap. Any truth to this or is it needed? I will be starting the tap in the mill to get it straight then finishing by hand.

rohart
11-07-2012, 06:31 PM
I am about to make a tap and rod for a trapezoidal thread. The way I read it, I should make the tap slightly thicker/wider that the screw I cut to leave clearance in the root of the nut thread. Clearance in the root of the screw thread is covered by making the hold in the nut slightly larger than the core of the screw.

I suspect that this also applies to acme, but I haven't looked this up. I'm going by the excellent roymech site.

I do admit that I have read in some sources that there should be contact between the crest of the screw thread and the root of the nut, but since all adjustments use the flank this seems strange to me. That given, delrin will give you the best chance of succeeding with contact there.

But to come to your question, if you have no clearance in the root of the nut thread because your tap has the same diameter as your screw, I would definitely provide a little clearance in the root of the screw thread.

I've found tapping with a sequence of two taps, 8mm diameter, 2mm pitch in brass to be a hard job. I tapped in situ, so the tap was supported by the bush that would support the screw and the dial later. My next attempt will use a sequence of three or more taps.

I hope this helps.

oldtiffie
11-07-2012, 06:55 PM
http://s200.photobucket.com/albums/aa294/oldtiffie/Screw_threads/?action=view&current=Screw-thread_form1.jpg

and Machinery's Hand Book 27 page 1825 - "Acme Screw Threads" - see Figure 1 (1826) for the general form.

oxford
11-07-2012, 08:30 PM
oldtiffe, the link you gave doesn't have acme in it, and I do not have a copy of the machinery's hand book. I only have access to some old machine shop practice books.

oldtiffie
11-07-2012, 08:39 PM
Sorry.

It does now - see acme form - bottom left - allow extra depth for "clearance".

http://i200.photobucket.com/albums/aa294/oldtiffie/Screw_threads/Screw-thread_form1.jpg

oxford
11-07-2012, 08:48 PM
What kind of numbers does "wider and deeper" mean? Is going bigger in diameter on the hole and using a tap the same as the rod going in the same as cutting deeper with a thread tool?

oldtiffie
11-07-2012, 09:29 PM
You will need some "slop/play" ie "clearance" over and above the threaded rod you have. Its a matter of "feel" and "judgement" as the rod will not be quite to "standards" so you will need to make the thread a bit wider and deeper and when its "near enough" its "good enough" - a bit "Shade-Tree Mechanic" or "Red Neck" I suppose but it will work - as a lot of STM and RN stuff does!!!

If it were me, I'd turn down a short spigot on the threaded rod down to the root of the thread, measure it, take and make the "hole" say 0.010/0.020" less than the spigot to allow for "root clearance".

oxford
11-07-2012, 10:18 PM
If it were me, I'd turn down a short spigot on the threaded rod down to the root of the thread, measure it, take and make the "hole" say 0.010/0.020" less than the spigot to allow for "root clearance".

Make the hole .010/.020 less than or more than what the rod ends up at when there are no threads left after machining down?