Threading stainless 8mm on lathe 7x14

The last threads I cut on the lathe were 32 diameter and 1.5 pitch. Do you have a tap and die for this thread because I certainly don't?

Martin

Screw-cutting will almost always produce a more axial thread. Even if it's for a thread for which I have a die I'll often 90% screw-cut it using the die just to finish to size.

+1 for what Brian says.

I have a very, very good selection of taps and dies but I still tend to screwcut to relieve most of the material, then finish with a die or use one insert out of a Coventry die head as the tool to get all the right form.

Another reason for single point threading rather than using a die is that over a long thread the die can start to wander off centre. The thread cut with a single point will be more constant in pitch and centralisation on the part will be better. The length of your thread may be short but may still be off centre on the part if you use a die. Of course what you are doing may be fine for your needs but single point threading is a good skill to master.

Martin

I see others have made more or less the same comments as I was replying. 

Edited By Martin Connelly on 09/02/2017 19:11:23

As I see it, and I am no expert, threading stainless can be a bear. Grades might cut reasonably easily or just work harden. A good finish may be difficult to achieve. Carbide likes higher speeds and loadings, so may not be a better option. Perhaps better to rough cut and finish with a die? But try it and see is the best way to find out!

Possibly a case for a four jaw chuck (more grip) or even a collet to hold the workpiece. It may depend on the quality and size of your machine. There is a big difference between getting to grips with lathe threading methodology and then changing the materials to something more difficult.

One advantage with using a cutter on the lathe is that you can at least resharpen your tool! Not all dies on the market are of high quality. A lathe cutter is guaranteed to cut a concentric thread, too.

I recently had an example of an old imperial die, which cut a thread (fairly heavy cutting on a hard bar), but a new die of chinese origin simply would not cut a similar metric thread on the other end. The bar did not machine easily, but that old british die was in a different quality band to the brand new imported item. I sourced the die because I thought it was the easy option at the time. Not an expensive mistake, but I won't make it again, hopefully.

Edited to add that others got there before me

Edited By not done it yet on 09/02/2017 19:22:56

Edited By Ian Flannery on 09/02/2017 19:35:30

I find it very difficult to satisfactorily die cut a thread in mild steel above 3/8" diameter. I am happy screw cutting in the lathe so I end up using a die as a chaser. I have not tried this yet with stainless steel.

JA

You will find that a quality die will cut with far less effort than a cheap one therfore much less likely to turn the stock in your chuck.

I have done a couple of threading jobs in 316 recently - 6 and 12 mm rod. In neither case could I get sufficient grip in my 100 mm 3 jaw to thread using a die in a holder. Possibly because I wasn't tightening it enough - I have old ice skating injuries to both which cause extreme pain when I apply pressure in some directions, lathe chuck tightening being one of these, or because the rod was polished and just hard to grip (must remember to try a wrap round with emery next time to see if that will make the difference).

I ended up tapering the end in the lathe and just getting a square start with the die before transferring the work to the bench vice to which I could apply some muscle. Still difficult stuff to get a grip on. On the 6 mm rod I applied some Trefolex and proceeded as an experiment to cut the full length of the thread (15 mm) in one shot (no backing-off etc.). Possibly the best thread I have ever cut - very smooth no chipping or other marks. I carried on to do the rest in the same way.

Same problem with the 12 mm in the lathe so I approached it in the same way, also using a 1" button die in a short handled die holder. This is where you learn just how much force is required to cut a thread. I did complete the 50 mm of thread required (with much backing off and frequent applications of cutting paste) and straight enough to meet requirements. However on test fitting I discovered that the downward pressure I had been applying had very slightly stretched the thread pitch and it was jamming in the 'matching' female thread. Use of thread gauge and caliper confirmed that it was me at fault.

Two important findings - I need to set up the Myford to cut metric threads and learn the technique as recommended above. Buy larger button dies and a bigger holder for larger diameter threads (now in hand).

A long-winded way of saying that you can cut very nice threads in 316 but you need to be set up appropriately to do so.

Pero

I bought some ER 32 collets and the hexagon block from Arc Euro,18 months ago ,to see if they would cure the problem of holding bar in the lathe chuck without slipping,these collets can grip really tight and I can now cut the threads with a die without any slippage,recently cut some some 1/2 inch whit worth studs with only 5/8 of plain dia in the centre, though they did require a really good pull on the wrench, the bar must be clean and dry,the black gungy muck that the steel suppliers apply to prevent rust will cause slippage. The hex block can also be used in the vice,to die down threads as it is better to use a collet rather than leave the serrations from vice jaws on the work.I make a lot of coarse thread studs as I restore stationary engines, and some commercial repairs on full size steam and the collets were certainly a "best buy"two of my fellow engine restorers now use ER collets and swear by them. Last week I had to tap a quantity of nuts 1 1/8 Whit, to hold the tap I mounted it in a ER 40 23 mm collet,the collet was held in the morse tapered holder which I mounted in my home made saddle mounted 4mt holder, on the Master lathe. the collet needed a good pull on the long wrench and it worked, my recently built saddle attachment also allowed me to power feed the tapping drill , one might say a bit brutal,but I did mount up an old 3 jaw chuck and the cost of the collet was lost in the job and the customers happy.I also mounted up a 46 mm spanner to locate on the flats of the collet holder with the other end supported in the tool post,this "torque "arm took the strain off the morse taper socket.

+2 on what Brian and JS said. I find it quickest and easiest in most cases to screwcut the thread then get the final finish with a die. On larger sizes where I don't have big enough dies, a couple of old thread files come in right handy for final fit and finish. I just hate struggling along trying to cut the whole thread with a die. So slow going back and forth, bringing up the tailstock to hold the die square, backing it off to break the chip over and over. Work turning in the chuck and so on. It really is quicker to screwcut and finish with a die.

Another vote for screw cutting and finishing with a die. Dead straight and no pitch errors or wandering off centre.

The next MEW (252, March, in the shops in about a fortnight) will a have a cross-slide mounted die holder for cutting long threads in the lathe in it.

Neil