How do you thread a screw on the lathe without shearing it off?

[GregG]

I've been trying to create a dial hip screw, and I'm running into an issue when threading the screw with an M0.7 thread.  I can create the rough form of the screw from O1 steel with little issue, but creating the thread is where things can go wrong really quickly.  I am using a thread plate, and I've seen tutorials which say to hold the thread plate square against the screw by using the tailstock as a brace.  I use tapping fluid, apply gentle pressure to the tailstock lever and I'm very generous with regards to advancing the headstock to cut the thread, and then backing off to help break chips.  But each time I get nearer to the base of the screw near the head, I end up shearing the screw off completely from the rest of the body.  With a screw that small, you have almost no feedback from the screw if you're going too hard, which ends up breaking the screw.  I turn the screw shank to be a little longer than the width of the die plate, so that there's something to grab with a pin vice if it breaks off.  What can I do to better my chances of success?

[caseback]

My first suspect would be (the quality of) the thread plate itself..

[AndyGSi]

What size are you machining the blank before you start threading.

[GregG]

Just now, AndyGSi said:

What size are you machining the blank before you start threading.

About 0.67 mm.

[AndyGSi]

In that case as @caseback said I'd put it down to the plate.

[Neverenoughwatches]

You won't get any feedback offering the plate to the screw by lathe, at either the pmate or headstock where the screw is mounted. I'd go for offering the screw to the plate, by hand in a thin pin vice, you'll feel much more of the process.

How good is the cutter ?

[caseback]

My second suspect would be the O1 tool steel. Is it still "soft" enough after the turning operation? If the part got hot, it might be work hardened, making it more difficult to thread and easier to break.

[nickelsilver]

What kind of screwplate are you using? The old Martin ones commonly came in B and L variants. The thread pitch is slightly different between the two, and the full thread diameters tend to be a little off from metric nominal, but the B plates are close enough to metric that they work most of the time. The numbering follows (pretty much) the numbering for stems- at least how stems threads are commonly called out. So a "tap 12" would be 0.70mm. (I can only guess the "tap" size for stems comes from these old plates)

 

These old plates tend to cut some and also form the thread by displacing metal. I think in part because they are low-tech, and also generally a bit worn. So it's good to start quite undersize with your initial diameter, and work up to something that gives a full thread form. There are variances plate to plate too, most likely a combination of manufacturing differences and wear.

 

I haven't tried any modern screw plates, I think they are of questionable origin, and even if marked in metric, probably merit some experimentation like the antique ones to find a good starting diameter.

 

 

Edited 2 hours ago by nickelsilver