Metric internal thread cutting dimensions

I am attempting (again) to make an attachment to fit the Unimat 3 nose, which is threaded M14 x 1. I have a nice small internal insert threading tool from JB Tools. I've had a couple of attempts using Mach 3 both of which seemed to turn away most of the thread already cut! So I wanted to check the dimensions if anyone can help please.

Nominal tapping size for this thread is 13mm, minimum minor dia 12.917 and max 13.153. Major dia is correspondingly 14 to 14.34 mm.

For the first try I bored the hole to 12.92, and set the lathe to thread out to 14.21 (which is the maximum corresponding size according to a thread milling program I have). This didn't work so I'll try again. Seem to me that boring the hole to 13mm and setting the tool to cut from this diameter out to 14.1 mm should be about right.

Yes, I know Mach3 has a poor reputation for threading but I've done quite a few external threads with no problem so I don't think it's M3's "fault".

I think your problem may be with the "Truncating" of the thread form due to the insert not being full form.

When I screwcut I always end up going a few thou deeper than the book size. Usually get to fit and function rather than measure over wires. Internal threading really requires a gauge that I do not possess.

Internal thread cutting tools inevitably suffer much more from spring due to the actual cutting tip being at the end of a somewhat slender shaft. Getting a first cut going can be an issue. I recall one that just wouldn't start until I wound on about half the thread depth. Cleaning up ready to make sense of finishing was, ahem, troublesome.

Insert tooling is generally worse for spring than boring bar style because the length of the shaft is what it is, in my experience always too long for the job I'm doing. A boring bar style carrying an HSS bit can be pulled right back into its mount so only the very minimum of unsupported shaft sicks out.

Getting the tip cutting exactly in centre height is harder when working inside a bore.

Heel clearance is often a issue too. I've codged up more than a few smallish bores due to not verifying that the heel of the tool under the tip has actually been ground back far enough and at a sufficiently steep angle to clear the bore proper allowing the tip to cut. Got so fed up that I made a set of drawings to show me the angles and tool depths needed for clearance. Thread cutting tools have exactly the same issue. Both holder and inserts should have specifications for minimum size bore. 13 mm sounds close to bottom limits for anything approaching normal sizes of tooling.

If you have CAD it may be worth spenigh a few minutes drawing help visualise the thread form. Basically draw a triangle on top of a rectangle representing the initila shaft size and add lines for the tip of the tool and the tip of the thread across it. Then insert dimensions to get the appropriate radii. Zeus, or other references will have the base diagram and (far too many!) dimensions. Like most folk I sort of picked up how to cut a thread that fitted decently. I suspect all would have gone much more smoothly if I'd been sat a a drawing board and made to draw out the profiles and what the dimensions all mean thereby getting a solid visualisation of things.

Clive

Edited By Clive Foster on 16/02/2021 18:23:41

With any internal boring operation (Which is what an internal thread is, but with a high feed rate to,produce a spiral cut ) spring always comes into play, As the thread becomes deeper, the cutting forces increase so the spring becomes greater...So one or more spring cuts will still cut metal.

Howard

I think your first step should be making a dummy spindle nose to act as a gauge. You don't want to make the thread to the theoretical size then have to take the set-up apart to check it followed by re-assembling to cut deeper. I've got one I made for my lathe that was a piece of scrap aluminium alloy. Good enough for home use where it only gets very occasional use.

Martin C

Apologies to everyone who kindly responded here. I've just successfully made the thread using Mach3. The problem I was having was that I had the right bore diameter, 12.92, but was using (in effect) much too large a thread depth. Basically I looked it up in Martin Cleeves' book "Threading in the Lathe" and used his numbers - for Mach 3 you define the bore and the thread depth by the crest diameter - putting this at 14.04 mm (from his data) worked just fine.

I find the trick with Mach 3 for boring is to start quite a bit smaller than final size and come up to size in quite large bites with an insert tool, with the last cut at finishing size, not pussyfooting round with "finish" cuts trying to take off gnats whiskers.