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.

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.

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&quot. 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.

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

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.

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 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

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.

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

Martin C

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

Mark

There you go Jed, courtesy Photoshop.

Sam