problem with ACME tap

[New chips]

I am trying to make an Acme tap for a 7/8 D 9 tpi LH class 3 thread but I cant figure out the dimentions I need to make the cutter from the Machinest handbook. Do I add the .259 x the major diam allowance to the get the fit or do I subtract it like normal. and change my major and minor diam of the tap to get the fit?

[Evan]

Here are the specs.

[IOWOLF]

Do your self a favor, and make a starter tap on it.Or just buy one, you will have more uses for it in the future.

[New chips]

Yes i have that info. I guess I am asking on a tap do I use the external thread dimensions or the internal thread dimensions. I would think sence it will be cutting an internal thread you would us the internal thread dimensions.

[Evan]

The tap should be sized to cut the nominal OD of the acme screw plus .1295 * pitch. That means the measured OD of the tap should be nominal plus .259 * pitch.

[rkepler]

Don't forget that the root diameter of the tap is undersize by the same amount that the outside is oversize. This means that the nose of the threading tool isn't standard and just a little bit less than you'd normally have for the thread pitch.

Below is a tap I made to cut 1/2-8 LH to make tailstock nuts for my 10EE. A2 so it wouldn't warp too bad. I didn't get enough relief or enough gullet in front of the teeth so it's a real PITA to use, but for occasional use it's fine.

[Rich Carlstedt]

The Guiding principle here, is that you do not want the threads on the screw to
bottom on the nut threads, or in other words, the flanks ( both) of the screw thread profile should not mate with both flank profiles of the nut. This causes a wedging problem and the screw can lockup.
We all know that a 7 degree angle( or less) is a "Locking Angle" and although the angle of a Acme is more than twice that, it does create problems in some applications. Because of that, the concept is to make the width of the nut root (internal now !) slightly wider than the screw thread tooth crest width.
This is generally about .001 or larger depending on thread size and class.
Using standard formulas like Evan's will yield about .015 "Allowance" total for Pitch Allowance.
My Tool Engineers Handbook, stipulates .011 total for a .875 Acme class 3 thread.
This of course will give you .0055 PER SIDE (radial ) and a projected "backlash" of .0014 in the central neutral position.

After 10 years of making well over 2,000 threaded Aluminum rings up to 84 inch diameter with 6 and 8 pitch threads, I find the standards somewhat lacking in clarity and definition. This was confirmed by the numerous vendor problems and interpretations on the threads which our Engineers spec'd out.

What I am alluding to is that there are reasonable differences of opinion and I admire the question asked on this thread. Just be prepared for a few variations.
An example is the radius's and bevels, shown on various Standard prints.
If I had the total answer, I would give it

In Sumation, you may want to make a second tap after seeing the results of the first attempt.. I hope I am wrong

rich
Remember ..the crest of the internal thread, which is the Diameter of the drilled hole through the nut, is the easiest to control, and also the least important.

[DR]

FWIW, I have a set of three taps used in production to cut 1"-8 LH Acme threads. Until reading this thread i hadn't paid much attention to them (a scrounging find, I haven't had a need to use them). They came from a company that built heavy equipment for logging and mining.

The taps are marked number 1 through 3 for use in that order.

An interesting feature is the taps are tapered their whole cutting length, 4-3/4", as opposed to "regular" taps that cut only on the ends. The taper is more like a tapered pipe tap only not nearly as much taper, the taper is not enough to be visible by eyeballing.

The final tap, #3, has a major diameter of 1.021" at the big end.