Cutting threads

I am hoping for some help and advice, I make a lot of small BA screws and never had this problem before, I have a Victorian skeleton clock that had fallen off a shelf some years ago and was packed away due to bent plates, dial , and broken mounting rods, anyway it’s my clock now I have straightened front and backplate, straightened dial and sent off to be re enamelled, upon making four new rods to mount clock on replacement base I have come upon something I haven’t encountered before, one end of the rod has 8BA thread went fine and the other end has 3BA , the first was fine but the second one is terrible see picture has anyone got any ideas as to the cause.

the chart said major diameter 4.1mm but I always turn slightly smaller 4.05mm use thread cutting grease and the material is silver steel the same I use for all my screw making.

Thanks Dell

Hello Dell, what method were you using to thread the rod? i.e. hand held die, die held in tailstock?

David

I have also had this problem recently. I needed to thread some silver steel 2BA. I used a die holder in the tailstock. First three were fine,then one like the OP's photo. The following four were just fine.

I can only think that the material was somehow faulty, accounting for the poor threading. A bit of a mystery!

Andrew.

The extreme ends of silver steel rods can be very hard depending on how it is cut

Roy

My theory (and it is only a theory) is that swarf from the thread cutting gets dragged back into the threads, with this result. Some dies are more prone than others with new cheaper offerings seeming to be the worst offenders.

To help overcome this I frequently blow the swarf away and avoid using thick cutting oils that can keep the detritus hanging around. Be extra careful when unscrewing the die afterwards.

Again this is only my theory and I am interested to hear the thoughts of others, having had my fair share of this scourge.

Regards,

David.

Edited By David Colwill on 05/06/2022 10:54:07

Not impossible the material was faulty, but unlikely! I've noticed most of the 'poor materials' reported on the web are found in hobby workshops, not on production lines churning out thousands of identical items!

I suspect trapped swarf wedging into the die. As the die cuts down the rod, we hope the swarf escapes through the slots provided, but they aren't very big and there's always a risk swarf is going to get minced and pulled into the cutting edges. Enough of that will cause the die to blunt and jamb, and - because the screw thread provides a massive mechanical advantage - the turning force is enough to gouge the rod as it enters the die.

Not enough to be conclusive, but in my workshop:

• Brass never mangles.
• Soft Aluminium often strips,
• EN1A-Pb rarely strips
• Ordinary mild steel sometimes jambs and strips
• Some stainless steels are vile!
• Much more likely to happen cutting by hand when nothing apart from me is keeping the die straight. I put this down to the wobbly operator skewing the die into the rod.
• Power cutting with a die on the lathe rarely strips. I think this is all to do with steadiness; the die goes straight down the rod at a steady speed with no jerks or twists. I suspect the swarf comes off cleanly with enough energy to eject away from the cutting edges.

Assuming the rod is the correct diameter and the die in good condition, I think the answer is plenty of lube, periodically winding back the die to clear swarf, maintaining a steady cut-rate, and keeping the die absolutely straight.

Dave

I note that the OP stated that he used thread cutting grease. Could this be the reason that swarf is being trapped in the die and not falling away? Oil might be better.

Turn the die by hand with very frequent reversal and use plenty of cutting oil. Turning too fast and too far without backing off will twist the part. As the swarf length builds up it will create a higher force on the part. Frequent backing off keeps the chips small and lowers the resultant force on the part. When cutting small threads I back off at least each half turn often less. Chips will also build up between the die cutting edges and can jam into the die causing stripping of the treads.

If using Taps or Dies in the lathe, particularly small such as 8 BA, using a Tailstock Sliding Die or Tap holider is necessary

A fine thread is too shallow and too weak to pull a Tailstock along the bed, and will be damaged, if not completely stripped. I found the hard way, some time ago, so bought a Sliding Die Holder, and made up a sliding Tap Holder, using ER Collets.

The holder does not need to be a snug fit on the arbor. Larger than normal clearance allows the tap or mDie to follow the workpiece and cut concentric.

Needless to say, use a lubricant such as Rocol RTD or Trfolex. If any doubt about swarf build up, stop, back out and clean before resuming.

Howard

Thanks for replies

I start the die using the tailstock to get a straight thread but once started I remove it and do it by hand in the vice, I always do one turn then back half a turn and so on, I must admit I hadn’t thought of using oil instead of the proper thread cutting grease, I have some cutting oil I use on the mill so will give that a try, my BA tap and die set are good quality and the dies are split dies and they are from London tap & die, I purchase all my steel and brass from Rapid metal so I would hope it’s not of poor quality.

All the 8BA’s came out fine it’s just the 3 BA’s I have had problems with I even checked the die with a loope to see if it had got damaged.

Is there some possible connection with the fact (?) that 3BA is a non-preferred size?

PS I have just run a 5/16 BSW thread and, not for the first time found that the diameter of the work piece needs to well under size in order for the die to start and produce a decent thread. I cut these under power in one go using Rocol RTD and have no problems with swarf jamming the cut.

Edited By ega on 05/06/2022 16:58:15

I have to agree that it’s likely that it’s dwarf in the die and as it’s silver steel the swarf is slightly hardened causing your damage. I just wonder why you use silver steel to make screws?. From my own perspective it’s a horrible material for that sort of work. Most of the small screws that you buy are from an easily machined steel and if really necessary machine from a material that readily hardened EN8 , EN 16 etc. The other possibility is to use 303 or 304 st st.

Assuming the nut holding the diestock is working correctly, it is either the die or the material.

So try the die on some different material. And try a different die on your silver steel. Compare results.

It looks to me that the die was run into the shoulder. Sometimes this can cause the thread to be stripped or be damaged enough that when the die is reversed , it unwinds and the damaged section comes out. The damaged section looks to be the length of a die. Should always stop before a shoulder, and then very carefully come up to the shoulder if it does require the full length to be threaded. Often designs can be modified is such a way that the thread is only required to be within 1 pitch away from a shoulder. Allows room for a nice radius transition at the shoulder and for an easier part to make. Silver steel can actually be quite tough on dies, especially if the dies is a High Carbon Steel die. HSS dies will last a lot longer cutting threads on Silver steel stock. An advantage to threading and then backing off, and then threading again, is that it forces the swarf to be broken, and not become a long spiral. It also allows for you see the progress of the thread and if anything is going not right can be detected very early on. Tool life can be greatly improved with proper cutting fluids as well.