Thread cutting problem

Hi I am after a bit of advice about a thread cutting problem. I am trying to cut a 3/16 x40 thread with the die held in a tail stock holder on the lathe. 4 times now this has happened and I can't work out why. The die looks to be in good condition but I am thinking it might be no good. Does anyone have any ideas?

Thanks

Does your die have a long taper lead on one or both faces?

You could, of course, screwcut the thread.

Edited By ega on 28/11/2020 15:15:09

It has a lead on one face which I use to start the cut

Is it a solid or split die? What diameter is the starting point? How and how long is the lead-in chamfer? Is the die sufficiently concentric? Are you providing any force to drive the tailstock? What is the material?(die and workpiece)?

Any and all these points may help you get a result for your failed attempts.

edited to add: make the threaded part longer and reduce it later would be one way to achieve a decent thread?

Edited By not done it yet on 28/11/2020 15:30:33

It looks as though your die is also thinning the thread. Depending on your equipment, you could mount the die on the lathe saddle and advance it via the leadscrew at the appropriate pitch.

PS Good points by NDIY

Edited By ega on 28/11/2020 15:33:25

Can you explain more about how the thread was cut? That sized thread I'd cut by hand not with the lathe under power. Is the die free to move along as the thread cuts; a rigidly held die will strip the thread, including if it jambs on the face.

What sort of metal is it? Mild-steel can be difficult, and it's not the only metal that tears. Lubricate.

Dies tend to squeeze metal and fatten the diameter as the cut proceeds, making the final turns stiff or even impossible. If the metal is plastic cut a shallow groove at the end to accommodate fattening and keep the die clear.

Stating the obvious, but reverse the die a quarter turn every turn or so to make sure the swarf breaks clear. Swaft getting caught in cuts causes endless bovver.

Nothing to do with the main problem, but die cut threads can be finished by reversing the die in the holder and cutting the last few turns with the un-chamfered end, which can get closer to the chuck. Don't start threads this way round, the tapered lead is important for that!

Dave

I bought a tail stock die holder and it produced results like yours, never used it since then, its destined for the bin unless I can make something else out of it!

They tend to firmly hold the die off centre which results in the thread being cut off centre.

I suggest you use a normal die stock instead, and let the die align itself onto the bar, it will naturally follow the diameter. Just make sure you press something up against the die stock with the tail stock, I use a drill chuck or a Morse taper shank to keep the die true to the bar I'm threading. Rotate the lathe chuck by hand and keep the tail stock pressure evenly applied.

IanJ

Its leaded steel that cuts very nicely. Don't know what the die is made of but it is split. I assume its concentric but don't know how to check. Its held in a tailstock die holder that is free to slide along a shaft. I am not cutting it under power but turning the die holder by hand. I cut the bar down to 0.18" and i put a chamfer on the end that was less than 45 degrres probably about 30. I used a cutting oil to lubricate. I did apply some pressure by hand to get it to start and then it pulled itself along. I did break the swarf off regularly by going backwards. I have cut lots of threads on the lathe this way but for some reason it is stripping the threads off as it goes along and its got me really puzzled.

Has your tail stock holder got a big enough clearance hole after the die?

Ian P yes it has.

Ian J thanks I will give that a go.

The ME 40 tpi thread is only 0.016" deep, so will strip if asked to drag a Tailstock along the lathe bed.

You need a Tailstock Die Holder, where the Die Holder can slide along an arbor.

I did have a problem, where the Die had been marked on the wrong side, so what seemed to be the front had no lead in chamfer.

For the very same reason, I made a similar device, using ER25 Collets, to hold Taps. It has the advantage that the Tap slips in the Collet rather than breaking if the going gets tough!

It goes without saying that a lubricant such as Rocol RTD, Trefolex or something similar should be used.

Unless you make several passes, tightening the Die each time, cutting a thread in one pass is a pretty hard task for the tooling. After all, we use three Taps, 1st, 2nd and Bottoming, to cut an internal thread.

Howard

Why does predictive spelling think that it knows better?

Edited By Howard Lewis on 28/11/2020 17:02:09

Got me puzzled too - that all sounds right, except perhaps the 0.18" diameter?

I've probably got it wrong due to being metric but my Model Engineer's Handbook says 40tpi requires a ⅛" tap drill. If the thread depth is 0.64xpitch, I reckon the rod should be ⅛" plus twice the depth of thread :

Depth of thread = 0.64 x 1/40 tpi = 0.016"

so both sides is 0.032",

making the total 0.125 + 0.032" = 0.157" diameter

If that's right, 0.18" is 20thou oversize, which might explain the chewing.

Beware - my maths is awful, and I've never cut a 40tpi thread. Worth trying a reduced diameter though.

Dave

Others type faster than I do!

If there is a problem with a Tailstock Die Holder holding the Die off centre,

1 ) Turn a chamfer, deeper than the thread on the end of the job.

If a flat end is imperative, make the job a little longer, to compensate for the chamfer, and face the end flat after the tread has been cut satisfactorily, and just clean up then thread again with a final pass of the Die..

2 ) Turn a few thou off the diameter of the arbor for the holder, so that the die can centre it self on the pre-mentioned chamfer, as you press the die to start cutting.

Howard

Three taps might be necessary for hand tapping but a single spiral flute tap can be used at a single pass in the lathe or mill/drill and produces an acceptable thread. When using a split die it should ideally be set correctly at the outset and the thread produced at a single pass even if it is necessary to reverse periodically to break the chip.

I notice that Arc now sell solid dies.

I have been through this, quality die and trefolex were the answer for me. Tailstock die holder should be fine. it does need a loose enough fit between the holder and Arbor to self align any slight offset of the die in the holder.

Possibly being brutal, I usually adjust the Die to preferably, a commercially made bolt, setscrew or stud, (Or use the mating thread as a gauge ) and then cut the thread in one pass, reversing occasionally to break the swarf.

But making sure that there is plenty of lubrication!

Howard

I would also add that in my experience the lead in chamfer is not required and can make things worse as it weakens the first few threads and makes it more likely to strip. There is already a chamfer on the die, which should be enough for a start.

you've got mixed somewhere Dave, for 3/16" * 40 thread I'd turn it to 0.185"

I suspect the die, try running it down a bit of 3/16" bar by hand with the bar held in the vice

If the die is not split, it is only good for cleaning up an existing thread. If the die is split, the die holder has to have a big enough bore to allow the die to be expanded by the central screw, for the first pass. Then the first screw is backed off a bit and the other two screws done up lightly on the die for the second pass. The quality of dies varies from excellent to christmas cracker, you may have the latter and could be trying to accomplish the impossible.

I have a couple of Whirlwind brand dies which are particularly effective. They are the solid type but with inserted brazed-in cutters of, presumably, ground HSS.