Thread wobble - single point cutitng of small diameter threads

I have been cutting a few threads using a single point indexable tool on my lathe. Recently I tried to cut an M6 but for some reason things did not go well. When I screw the bolt down the thread it tends to wobble, and not seat "flat". The thread has some (not tons) play, but it seems like the thread is not cut straight. I did some spring cuts but likely not enough. I would like to confirm if this is likely caused by deflection when cutting the thread?

I cannot use my live or dead centers as they are both too big to use with such a small diameter shaft (6mm). I assume I am meant to use a follower rest for small diameter work? What is the accepted practice here?

How long is the thread? Normally a few spring cuts should fix any flex, unless you are cutting a really long thread.

Normally for M6 and under i use a die, actually i think that would be M8 and under.

Your nut may also be "drunk" ie the hole has not been tapped straight. I have seen that plenty of times before

Ew

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Originally Posted by variant22

I have been cutting a few threads using a single point indexable tool on my lathe. Recently I tried to cut an M6 but for some reason things did not go well. When I screw the bolt down the thread it tends to wobble, and not seat "flat". The thread has some (not tons) play, but it seems like the thread is not cut straight. I did some spring cuts but likely not enough. I would like to confirm if this is likely caused by deflection when cutting the thread?

I cannot use my live or dead centers as they are both too big to use with such a small diameter shaft (6mm). I assume I am meant to use a follower rest for small diameter work? What is the accepted practice here?

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Sounds like your round stock (bolt) diameter was not big enough.

Also you should use a centre to support the shaft.

If your existing ones are too big, machine a 60 degree taper on a small rod, harden it, mount in the drill chuck in the tailstock and use as a dead centre.

Rob

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Originally Posted by Ueee

How long is the thread? Normally a few spring cuts should fix any flex, unless you are cutting a really long thread.

Normally for M6 and under i use a die, actually i think that would be M8 and under.

Your nut may also be "drunk" ie the hole has not been tapped straight. I have seen that plenty of times before

Ew

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The threads are about 50mm long to a shoulder that is ~10mm. In hindsight I should have taken it a bit easier and done a few more spring cuts.

Interesting that you use a die for smaller stuff. The bolt is unfortunately not drunk. It screws down nicely on an M6 bolt that I did not cut :~

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Originally Posted by nearnexus

Sounds like your round stock (bolt) diameter was not big enough.

Also you should use a centre to support the shaft.

If your existing ones are too big, machine a 60 degree taper on a small rod, harden it, mount in the drill chuck in the tailstock and use as a dead centre.

Rob

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Interesting that the diameter was possibly not big enough. The calipers were showing 6mm, but they can lie. I did not mic it, so I really do not know. I have seen this before however when I tried threading a bunch of studs. The studs in the middle of the bar were deflecting and ended up with floppy bolt syndrome - aka wobble. That said, it could have been a difference in diameter. I never checked the diameter in the center of the bar.

I like the idea of a 60 degree small dead center. That said, what material should I use? I have a good supply of 1020 carbon steel - but hardening is a problem(?).

I could really do with an MT3 watch makers live center.

its such a good idea that i am going to make a couple of tiny dead centers myself cos i do a few small threading jobs in the engines i am making and i never thought of that solution before .
for the job i have some silver steel rods ( drill rod ) that i can use to make them from .
thanks for the tip .

johno

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Originally Posted by variant22

Interesting that the diameter was possibly not big enough. The calipers were showing 6mm, but they can lie. I did not mic it, so I really do not know.

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It is likely that the diameter was correct (or at least good enough that it wouldn't affect things) I think it more likely that with the thin material unsupported that it was bending way from the tip or rolling itself up on the tip so that the form was not correct. As you have noted, take it easy (small DOC) and lots of spring cuts (make sure the thread is cut). I agree with Ewan, in that for small diameter threads like these, especially when they are long ones while I may start on the lathe and rough out the thread I will finish with a die to get the form correct.

Michael

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Originally Posted by variant22

I like the idea of a 60 degree small dead center. That said, what material should I use? I have a good supply of 1020 carbon steel - but hardening is a problem(?).

I could really do with an MT3 watch makers live center.

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Just get a hard bolt and anneal it, grind or machine the taper, heat red hot and quench - good to go.

As the speed is slow and load light, it will be plenty good enough.

The wobble could also be caused by a slightly oversized nut (female section) - not all taps and dies cut to the correct size, and variation is not uncommon.

Try some other nuts on your thread to see if they vary in feel.

If the female section you are wanting to use is loose/oversize, just do the job again using slightly oversized stock to compensate.

I often do this on small stuff. You can easily skim the thread crests back later to get a snug fit - yeah, I know it's cheating :)

Rob

Cutting a 50mm M6 thread unsupported is going to be tricky what ever you do.
As Michael suggests, I think you will find the work is flexing and trying to climb up the tool which will cause the thread profile to be much larger than 60 degrees hence the wobbly fit.
You can get really fine centre drills but if you spot it with say a 2mm drill bit and use a centre as suggested by Rob this will help.
I would ditch the insert tool and use a really sharp HSS tool, grind a little bit of back rake on it too, I find this works better for fine threads and the effect on the profile is negligible.
Take a lot of fine cuts and finish off with a die if necessary.
Good luck.
Cheers,
Greg.

What I have done is purchased a new live center that has replaceable centers. I will turn a center to a small diameter @60 degrees with a decent length to fit into the live center body. A bit of an adaptation of Robs idea. I hope this will reduce deflection enough when cutting small diameter threads. I figure I will try the center option and if that does not work out then start forking out for the dies (expensive little blighters!).