Single point threading

I appreciate this is a common topic. I've read over many threads but I'm still messing up in the same way. Allow me to describe things in detail, and hopefully the answer is obvious to those with more experience...

I want to make a fixing/bolt with 25mm of M6 thread on one end, a shoulder, and a knurled knob on the other. The material is 303 stainless, from a 20mm round bar. I'm using a full profile 1mm pitch threading insert, and a fresh edge at that.

I start by turning down 25mm of bar to a shade under 6mm...

I then use a parting blade to cut a gutter on the left. I also add a chamfer on the right end of the rod.

Running at 80rpm (as fast as I dare) I cut the thread, taking 0.1mm off the diameter each time (0.05mm DOC). I use cutting fluid. The insert peels off a clean chip. Note that I am plunging straight in, not using the compound at an angle.

On two attempts, after around 7 or 8 passes, this happens...

In some ways it isn't surprising, as 25mm is a lot of stick-out for a 6mm rod, and the gutter creates an obvious point of weakness. But I'm still surprised that ~4mm of stainless can be snapped by the cutting force of that tiny little triangle on the end of the cutting insert. It occurred to me that tail support would help, but I can't get my centre and threading insert to occupy the same position in space at the same time.

After the part broke, I examined the cutting insert under a microscope, and the edge looks pristine, so I don't think the tool has snagged or degraded.

I appreciate that this would best be done with an M6 die, but I'm practising my single point threading. Am I making a basic mistake here? Or is thread-cutting long thin bolts not generally possible?

Jed

Three suggestions from a beginner:

Grind an HSS tool with a narrow nose that you can use in with the centre.

Turn the blank long enough to get the centre in to support it then cut to length after threading.

Also maybe skip the gutter or cut it with a radiussed tool to reduce the stress riser.

Definitely use a centre, also you DOC was very small.

Tony

PS Seen this many times when the work climbs over the tool

Edited By Tony Pratt 1 on 26/05/2020 13:24:46

6mm is quite small for single pointing but certainly doable. I generally use a die for anything under 10mm. Like the others have suggested a centre at the unsupported end would help, it would probably have to be a half centre in order to still get the tool in. Using a radius grooving tool would also strengthen the undercut as it would eliminate sharp corners which are a weak point. The undercut need only be a shade under the depth of the thread, 0.5mm per side in this case.

It's pushing it a bit. The single biggest error is forming the gutter before screwcutting. It seriously reduces the resistance to bending of the part. Ideally one would add also a centre. It doesn't need to be fancy; a bit of silver steel with an appropriate chamfer and held in a drill chuck will be fine. It's not taking significant loads, just enough to stiffen the work in respect of bending. Even at small depths of cut as you approach the final thread size the insert will be cutting over quite a length so the forces will be correspondingly higher than the first few passes.

Andrew

The diameter verses length ratio is not in your favour, as already mentioned, a centre is needed. For this small diameter a thin extended live centre would have the best chance of not fouling the tooling. Every cut you make removes a larger ammount of metal, so the depth of cut should get less each pass, the last cuts should be as low as 0.025mm (0.001". To give more clearance for the centre with such small diameter threading, the right side of the tool can be ground back quite a lot. You could also add 6mm to the length and take that down to the core diameter and cut it off when the thread is complete. You will need some small centre drills as well.

Keep practicing, you are nearly there.

to add to the advice on using a centre (more likely a half centre) you should swing the topslide round. Not necessarily to the thread angle, just to get it out of the way to allow the tailstock to get closer to the job.

Angled feed will also reduce the load as it's only cutting on one side.

I'd use a die to tap something as small as M6. However:

I suspect the gullet is causing a fair amount of grief: it reduces the diameter to about 4mm, and is made with a parting tool that leaves sharp stress raising edges. Highly likely it also work-hardens the 303, further weakening it.

If the thread is cut conventionally towards the chuck, the tool will apply considerable leverage just where the gullet has most seriously weakened the part. It's also likely that the threading tool is work-hardening as well and causing  cutting stress on the work to increase steadily as the thread deepens.

The geometry is adverse too. When cutting towards the chuck the rod compresses as a column, and columns buckle easily.

I suggest:

• Make the relieving groove with a rounded bottom.
• Lubricate well to discourage work-hardening
• Cut the thread in reverse, ie from gullet out. This will tension the rod so it can't buckle, hence much less likely to fail.
• With a carbide insert and in reverse, cut the thread with fewer passes. For M6x1.0 I might try 0.2,0.2, 0.15, 0.15, 0.15 and then test for fit.

Dave

Edited By SillyOldDuffer on 26/05/2020 14:20:10

Gentlemen, thank you very much for such comprehensive and prompt advise. It has been digested and acted upon...

The setup...

the result...

This was still less then ideal, as I don't have any means of cutting a groove for the gullet, instead of using my parting tool. And I only have a big chunky dead centre. If I do more of this I'll be looking at some grooving tools and smaller centres. But it worked with what I had to had - thanks again.

Jed

The main advantage to screw-cutting a thread of this size is concentricity. For jobs like this I often start by establishing a straight thread using the lathe then finish with a die which finishes the thread form.

Finished part, thanks all...

I have a terrible time threading up to a shoulder as I can't seem to stop the carriage just at the right time with the motor turning the spindle so I made up a crank that would turn the spindle. That lets me turn the spindle very slowly, much slower than the motor will go and I can then stop the turning just where I want.

I have cut 6mm with HSS tool bits [at 50 revs ] without any trouble. I never bother with tailstock die holders, not much use for anything over 6mm , I found.

Support from the tailstock does help, but threading right up to the shoulder ?, How often do you need to ?

I tried that on my lathe but it is too large to really use a hand crank Even if you get the spindle to turn smoothly you can't see the work from where you are standing. I then fitted a three phase motor and inverter which allows you to dial down the speed to <10rpm when you apprach the shoulder. Works very well.

Edited By lfoggy on 26/05/2020 17:14:16

Your shade under 6mm relief groove looks to me like it was far too deep. The tapping size for M6x1 is 5mm and this is what the diameter should be at the relief. Anything deeper reduces the strength. Also some stainless work hardens and trying to take small cuts runs the risk of the tool rubbing and not cutting. Once rubbing starts it can cause this work hardening and then you will have issues.

Martin C

Having now seen the finished product I'd have cheated, make the knurled bit with the plain diameter sticking out, drill a hole up it, find a bolt (cap head?) with enough thread and loctite the plain shank of the bolt into the turned bit.

However, you now know how to screwcut, so happy days.

Well done! before going any further ou should make yourself a mandrel handle. that way you can feel the resistance as it starts to jam.

If you can't use a centre in the end consider a travelling steady.

Martin C

ARC sell half centres to allow more clearance. I would also have cheated the same way as Duncan.  

**LINK**

Edited By old mart on 27/05/2020 15:38:46

Good point Martin, I actually have one of those and would have never even considered it.

Mark