On 24 September 2024 at 05:08 Graham Green 3 Said:

…they tend to wander all over the place

Could we apply some thought to that statement please, as possible mitigation/solution if you are ultimately unsuccessful in your quest.

Consider a single point screwcutting tool. That does not have even a couple of threads cut into it. We might be generous and say it has half of one thread cut into it.

Single point screwcutting tools do not ‘wander all over the place’ because they are rigidly held and guided along a definitive path by the machine in which they are used. And on slender stock, we would use a travelling steady to stop the stock itself wandering away from the screwcutting tool.

Similarly, a die can be prevented from going astray from its required path by appropriate workholding (of both stock and tool) techniques.

You seem to be trying to solve a batch-production problem with single-operation tools. Large batches, too.

The thickness of the die is no guarantee of concentric cutting. The tool alignment is what matters, not its type. If the die starts off-centre or at angle to the axis, it will continue so irrespective of its number of internal thread turns.

One thing that affects the risk of a wonky thread is the condition of the end of the material. If if that is not faced square with a chamfer, it is very likely to push the die out of line right at the start.

If possible, turning a little spigot to core diameter for a couple of pitch lengths can help start the die accurately; but it is an extra operation I would guess you’d rather avoid! Whether you need face that off afterwards, obviously depends on the specific use of the finished screw. Left on, it can help subsequent assembly and gives some protection to the thread start; and this was sometimes used for applications like very large marine engines.

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There only two ways to ensure an accurately-aligned thread with an ordinary lathe and tools.

A) Use some form of guide. Either a tailstock die-holder / die-head or (if the work length permits) a hand die-holder with a fitted guide – a leading bush with a hole that is a sliding fit on the unthreaded work.

OR

B) Screw-cut the thread, if necessary cutting to near-depth then finishing with a die or chaser.

Unfortunately the only commercially-manufactured die-holders I have seen with a proper guide, were for conduit-dies, and the curious little (6BA?) dies supplied with an early construction-kit branded Juneero.

The conventional hand die holder is made to die-thickness, but it would not be difficult to make a deeper form to hold a flanged guide-bush as well as the die.

You could investigate – and would need make – a sliding die-holder akin to a tailstock die-holder but held in the tool-post. This though come with a cost: it assumes both a Quick-change tool-post and some way to ensure the die-holder is repeatably, truly on and concentric with the lathe axis at every tool change.

If you need make hundreds of the things, consider a capstan or tailstock-turret attachment, though I would not expect these to allow cutting very long lengths at a time.

…..

Bernard –

Do you not mean Grub-screws? A Set-screw is headed like a bolt but has a full-length thread.

…….

I recall one bad, and totally avoidable, result of a wonky thread. We were presented with a miniature overtype steam-wagon loosely resembling the Foden ‘C’, built from a kit sold by a certain company that soon gained a reputation for introducing new models without addressing QC errors in the previous ones.

On raising steam, we soon repeated the reported fault that had bewildered the unfortunate owner: there was no way of stopping it.

Investigation revealed the regulator slide-valve was moved by screw-thread, not lever. The vavle was tapped directly – no bridle or cross-nut, so no float action. The control-rod’s thread was visibly eccentric, so rotating it lifted the valve, and there it stayed as the steam used the thread clearance to hold it above the port-face.

A tailstock mounted Coventry die head with 12Ba dies ? A 1/4″ die head is not so big,12 Ba dies should be available and can be sharpened if need be. Myford made 2 types of capstan attachment for the 7 series, but these are expensive. Noel.

If you want to cut the thread by hand-held die stock without wandering, you might have to make a die stock with a guide on it the same diameter as the thread OD. It could stick out ahead of the die and hold the tool in alignment. Or it could stick out behind the die if holes were provided for chip clearance perhaps.

Or you could make an insert to go on your 1″ die holder that stepped down to 13/16 and also had a guide protruding with guide hole in it to match thread OD.

My thoughts on a simple 2 tool turret

Something to locate in or onto the tailstock barrel. Plate with an offset pivot. Second plate with a central pivot hole and two location holes for the tooling. One tool is a running down box fixed solid. Other tool is like a small tailstock die holder and free to move along it’s shaft. A simple detent of some sort to locate each tool whan rotated could be a sprung ball a sboth tools can be self aligning.

Running down mode

Change by rotating the “turret”

Threading mode just slide the die up towards the rotating work and let go when the required length of thread has been reached, back it off and you are ready to do it all again

Graham, I appreciate this isn’t a direct answer to your question, which you’ve obviously thought through.
It might be worth giving this outfit a bell with your query, as they are a cutting tool manufacturer, who also took over the stock from the Tap and Die company.
The ones they list here are 13/16″, but I don’t know if they are made in house, or they may have older non-listed  stock
https://crtoolsuk.com/product-category/dies-die-nuts-die-holders/dies/ba-dies/

Personally I’m fortunate in having a ¼” Herbert head but only chasers down to 10BA.

Bill

On 24 September 2024 at 07:42 not done it yet Said:

…feed the carriage at the required feed rate?

Of all the various thread forms, BA was lauded as being the most ‘scientific’ because there is a mathematical relationship between thread number and thread pitch.

The relationship is pitch = 0.9^n where n is the BA number and the ^ symbol means ‘raised to the power of’.

Your calculator will tell you that 0.9^12 = 0.282mm or 89.9 tpi. That would be a challenge to produce with change gears.

You can use a pair of dies in a holder that you will need to make. Using spit dies will allow you cut a test piece. Using the bottom die first, with only the bottom screws for setting that die, cut a test piece at the correct thread diameter. Then you can wind on the die that will become the 1st die and then wind in the second die. Radially align the bottom die and then mark so you can set the holes for the front die. The other option is to use a nut as the guide behind the front die.

This would fit your U3