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 Wednesday at 07:29 AM by nickelsilver

[GregG]

Sorry @nickelsilver, I'm just seeing this now.  It is a standard metric screw plate.

I followed the suggestion of doing the thread cutting in a pin vise.  It took me forever because the piece is so delicate that I cut and cleared chips very frequently.  But eventually I did get it.  Not pretty, but I got it; the first thing I ever successfully made on the lathe. I cut the screw slot with a jewelers saw.  How can I ensure that the slot is centered on the screw head?

[caseback]

You could make a guide/jig. Cut a slot in a piece of metal approx the same thickness as the screw head, then drill a hole (screw head diameter) in that slot.

[nevenbekriev]

I would repeat what @nickelsilver says, and even more.

I have at leas 10 old thread plates and I have never seen one that will cut like a normal die. I don't know if they are completely worn or have never been sharp enough, but I find them completely useless. And, for the new Indian ones, I have heard only bad things. For sure, if one tries to cut more thread on a winding stem, this hole of the plate is ruined.

Further more, I have bad experience with all watch size thread dies that I could buy from the available machine  tool shops here. They could not cut normal thread even on brass rod. The size was significantly smaller and the pitch is like it should be for the claimed size... The thread shape was bad too.

I will explain how I do threads.

I use lathe that can cut threads and thus I can prepare perfect taps. I use milling attachment instead cutter in the tool post and so I can cut thread even on hardened rod. But I prefer to harden the rod after thread cutting and temper it only to light color change as to have the maximum hardness. Then I make a tap out of it by grinding.

I prepare my dies for screw thread cutting with the taps I have prepared as described. In watchmaking, dies exist and are often used that are not cutting, but rather forming the thread. They are like nuts and don't have cutting holes and edges. This kind of dies I prepare and use. You will need soft steel plate , drill a hole in it, then cut thread, then make chamfer on one side, repeat the thread, harden. You can temper just a little or leave untempered, depending of the steel You use.

Using such die is with soft steel rod (or completely annealed). Good oil is to be generously applied. The rod has to be with smaller diam than the thread size (about 0.8 - 0.9 x ), the diam is better determined by tries. The rod elongates by the thread forming too.

When I have to make screws from harder material like SS, I just cut them on the lathe.

You screw needs hardening and tempering to be finished as real screw.

 

[Dell]

I have only made screws down to M1 & 14BA & I had trouble getting the slot in the centre of the head so I made a jig to help me be more accurate when cutting the slot, here is my video.