…….

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.

But 90 tpi should be possible, and it would be near enough for for a rough mechanic like me.

If I had to do this by hand, I’d make  a holder that held the normal die and in front of it a guide with a clearance hole. I seem to remember GHT describing something along those lines

I was a bit concerned about not using a floating die holder. Using a proper tailstock based holder would cure yoour problem.Time taken to change tools is minuscule compared to the whole job I find. The image below was made on my Petal with tailstock die holder and 0.375 dia die which is non adjustable. 12ba.The threads appear quite straight so hopefully thats your main problem.

Using a Tailstock Sliding Die Holder should allow the Die to follow the work, so that does not profuce a drunken thread.

And since the load on the newly cut thread is minimal, it should be good.

Obviously lubricate; Trefolex or Rocol RTD

But put a chamfer on the end of the material to allow the Die to centre itself on the workpiece (Even if this means holding the work close to the chuck to cut the chamfer, and then pull it out to cut the full length of thread)

Also, don’t be too ambitious with the speed; even using a mandrel handle might be appropriate.

Howard

On 25 September 2024 at 18:17 bernard towers Said:

Howard says it all thats how I made the one in the above photo took less than 3 mins.

So that would be five hours to make 100

Graham (OP) mentioned quantities of hundreds, methinks some job specific tooling is called for.

So far this thread has only discussed the threaded portion of the coachbolt, goodness knows how the squared section under the head is done-:) but I could see doing the domed top with a combined parting and form tool.

Ian P

On 24 September 2024 at 21:11 bernard towers Said:

This would fit your U3

Bernard you have an interesting looking lathe and accessories. Are the attachments commercially available or have you made them?

The ‘tailstock’ is definitely unusual.

Ian P

Graham –

With great respect, your original question was not about production rate but thread accuracy.

You complained that the only dies you could buy are somehow too short. I am afraid you have missed the point many here have tried to explain.

They are not too short!

It is not the number of threads in the die that affects the truth of the thread, but the die’s alignment and the end condition of the material (they will inter-act).

Even with a deep die, you cannot hope to make clean, accurate threads with a hand-held die-holder without some means to guide that, and having faced and chamfered (by turning, not filing!) the rod end.

The simplest way with just the tools available, and one I use even when using a hand-held die to finish-profile a screw-cut thread, is to prepare the rod end as above, and gently to press the end of the tailstock barrel or something held in it, against the back of the die-holder to hold it square. I can’t guarantee it would work consistently in such small sizes as 12BA (a small thread to me, is 6BA!), but at least it would help.

Your complaint about alternative tools seems based on wanting to avoid two tailstock tool-changes per bolt. With hundreds to make that is understandable, but you have been offered various suggestions to solve that problem.

I don’t think you have told us your lathe’s tool-holding arrangements, but if you have a quick-change or four-way tool-post, could you mount the running-down tool in that, having arranged some means for repeatable concentricity and squareness? Its consistently accurate setting is critical; but it would allow the tailstock to hold a proper die-holder throughout the production run.

One or two have asked about screw-cutting. Well, that would depend on your workshop being equipped to grind a suitable tool, and your lathe being able to generate such a fine thread.

Proof of pudding or concept as the engineering types here may call it.

Well used jig piece that was milled to a sliding fit in the U3’s topslide approx 8mm wide. I then held a spotting drill in the U3 chuck and slid the block onto it which marked the position. It could be clamped to the topslid eand actually drilled and counter bored in situ but I moved to my mill. Located spot mark, drilled 4mm hole and then use d a couple of end mills to plunge 6mm deep x 5/8 to suit the die. Luckily the hole split a tapped My hole so I put a grub screw in there to stop the die turning. Die is 1/4″ nominal thickness and about half the back is hollowed out so approx 1/8″ length of tooth. 20 mins work (bit longer if all done on the U3)

Turned a bit of 1/8″ brass to 1.25mm and gave it a go just turning the pully by hand. Light downwards pressure and gentle towards the work and off it goes. Also did a longer one. No facing or chamfering of the bar, just the sawn end but at that size a quick lick with a file will do.

Phil You may have missed my video while you were typing

You just need to slide the tailstock to the end of the bed to use my idea of a cross slide block just as he is doing already in the PDF

The idea of the topslide block is so the box tool DOES not need removing or the whole tailstock changing. By using the slotted cross slide as the guide it means there is no messing about with the tailstock apart from simply moving it.

Even easier if the other side of the block were the box tool as per my sketches.

Box tool in use

Plenty of room when box tool and tailstock moved to end of bed

Having proven the concept I’ve tarted up the 3D model.

A bit more finger friendly and the flair at the bottom gives something to push down against which will help if you have oily fingers as will the grooves in the side.

Further options would be to make the key at the bottom from a separate piece held with two CSK screws. You could then remove it and if a couple of extra tapped holes were added to the top then a piece of round HSS could be held in the body enabling it to be used a bit like the Eccentric Turnado for free hand work.

As for repositioning the cross slide use could be made of the through hole if a piece of suitable bar were held in the chuck then just mve the slide in and out until the block slides over the bar. Or to save removing the die for each session make a spacer block that can drop in between the front of the slide where the carriage where the cross slide screw goes and the actual moving part of the cross slide.

I think it was Radford who designed a quick change holder for the tailstock. A sort of dovetail slot on the end of a 2mt shank. With 2 holders this could turn and thread, with parting tool on cross slide. I find if you’re careful you can follow up a die using the tailstock handwheel, but not tried on anything as small as 12ba.

Seeing Jason’s brilliant suggestion for a ‘turnroundable’ block, hand held, that accomplishes a lot of the machining operations needed by Graham has prompted me to possibly throw a smallish spanner in the works.

For the threading to work the die would have to be an unsplit type and very very accurately on centre, or alternatively in a floating holder.

As Jason showed there is no difficulty accurately spotting and drilling (and even machining the die recess) in the lathe itself but to my mind

When I bought my lathe it came with a MT2 tailstock sliding dieholder nicely made by the previous owner. I use it with a reducer in the MT3 barrel and have produced hundreds of parts with M10, M8, 5/16 threaded parts with no problems. A couple of weeks ago I need to make some M6 screws 8mm (fairly short for me) and the first few I made were just rubbish with the threads being very loose fitting in tapped holes. I traced the reason to the cutting tips of the split die being off centre even though it was only slightly closed down. With the dieholder being well made and the tailstock barrel (and lathe) being quite rigid, one part of the die was acting a bit like single point threading tool.

Ian P