Single point Whitworth Threads

Thanks Clive. I think I could probably do the widening that way although my old ML7 is so crude it only has fixed dials on the cross-feed and topslide.

...but when I think about it though, the 12 TPI roots only require taking just over 13 thou off the point of the 55 degree point tool. So it's probably not too bad even if I have to regrind it again afterwards for other thread sizes.

So the way to go for this "bodge" would seem to be to take off the 0.1478p from the major diameter and then with a rounded off point tool (taken off by 0.1600p) go in 0.5664p or maybe a smidge further until it fits?

I'll give it a try over the w/e

Thanks again

Jon

Hello all

When single-point cutting threads, I find that the crests can be rounded off using one of my thread files.

They are designed to be used manually to clean up bruised threads, but I use them in the lathe with the lathe rotating under power at slowest back gear speed.

The file is applied to the threads and not moved, but with the work located on one of the cross-grooves in the file.

The file then acts just like a chaser. (I am a grown-up so please don't anyone tell me its dangerous. I know it's frowned upon, but many time-served machinists used files in the lathe, they just didn't boast about it. If you are not sure about it don't do it.)

The advantage is that each file has 8 different pitches. I have three files, BSF,Whitworth and BSP so 24 chasers although there is overlapping of pitches, all being 55 degree thread angle.

Works for me.

Jim

Though I am reluctant to exhume a thread which died eight years ago, I feel it's important to correct some incorrect information within it to avoid misleading future readers of this thread

The original diagram comes from the website of the American firm Gage Crib Worldwide and is based on a lot of incorrect assumptions, not least of which is that the truncated Whitworth thread didn't comply with any standards. In fact it was fully specified in B.S. 84-1940 (Amendment No. 3, August 1945).

In particular, the thread root was not a V, but had the same profile as the 'normal' Whitworth thread.

I have emailed Gage Crib to offer them corrected information. It will be interesting to see if they respond.

George

I have made several back-plates for my Myford, both from noggins of mild steel and un-machined cast iron blanks. I have cut the threads on my Colchester. I can't say that, apart from the major and minor diameters of the thread and a slight rounding of the point of the screw-cutting tool that I have paid much attention to the precise details of the thread, except obviously for depth, but I have paid a little more attention to the registers. The back-plates screw on and off the lathe freely and the chucks appear to remain concentric. I always cut internal threads on the rear of the hole with the screw-cutting tool inverted in the tool-post and the top-slide set over to half the included angle. Using this method means you don't have to grind a tool to the precise angle because the rear flank is formed by its advance into the work and you don't have to think about which way to withdraw the tool at the end of the cut as it is withdrawn, as of habit, in the same direction as cutting an external thread, you also have a better view of the thread forming as the tool enters the work.

+1 . Seems like much ado about nothing. Rub the point of the tool on the bench oilstone until there is a bit of a radius and carry on. Put the radius on the peaks with a file at the end of the job. Make the OD a bit undersize to start with. Final fit by trial and fit to a sample nut or chuck or faceplate thread etc. All the rest is superfluous for most practical purposes.

.

Well done, George … I previously had no idea where that sketch originated.

I hope you don’t mind, but I am adding a link to the relevant page: **LINK**

https://www.ring-plug-thread-gages.com/ti-bs-Full-Form-vs-Truncated-Modified.htm

MichaelG.

.

Despite his not referencing B.S. 84-1940 (Amendment No. 3, August 1945). I think ‘Wayne’ makes a pretty fair job of describing the truncation … if the source had been referenced in the opening post, this discussion might have been simpler.

The root and crest radii on any thread are only to remove sharp edges and stress-raisers, and provide a small clearance. They are not usually critical, within reason.

Generally I find it necessary to finish the thread with a die, and that will give the right roundings.

'

Jon -

Fixed dials. I know - I have a Myford ML7 and the dials are not only fixed but also small enough to be difficult for me to read. So a tip applicable to setting any tool for set depth:

Having set the tool in height and azimuth -

- Keep the screws clamping the tool itself, slightly slack so the tool can slide under them.

- Push the tool forwards to just touch the bar.

- Now keep that light pressure on the back of the tool, to maintain its contact, while winding the slide in until the dial either reads 0 now, or will read 0 when the cut reaches depth. (The latter way had not occurred to me until I read someone elsewhere here, saying that's his method.)

- Still keeping the tool gently pressed against the surface, now tighten the clamp-screws.

.

How to set a thread-cutting tool in azimuth?

That's what the appropriate Vee-notch in a thread-setting gauge is for. Set it to a piece of any straight, good-quality rod in the chuck or collet, if the work-piece's own shape makes the gauge impractical on that.

However, and perhaps more easily, the following works if the tool it is either an HSS one ground accurately with respect to the sides of its shank, as ideally it is; or is an insert type whose tip-holder is manufactured anyway to high orthogonal accuracy.

I abut the flank of the tool (or of a QCTP block) against the face of the chuck-jaws or faceplate, under very gentle hand-pressure applied via the saddle hand-wheel, while tightening the central tool-post stud.

This method is irrespective of the top-slide's own angle you may have set for diagonal-feed screw-cutting or to cut a taper on the work - or simply to clear the tailstock.

Or make your own larger resettable dials. Best thing I ever did for my ML7! No more felt pen marks and tortured mental gymnastics. All done in the lathe with no dividing head needed. Drawings and details coming up in MEW at the editor's convenience.

To be quite honest, I think you are over complicating this task. As long as the thread angle and pitch are correct the rest just follows on - if important. The outside diameter may be touchy.

I you are using this piece for mounting something else, it may well be positioned by the register surfaces - so the thread only tightens the part, not the position.