Short thread-cutting

[JamesFParticipant @jamesf]

I have a need to make an adapter with an M50x1 female thread on one end and 2" 24tpi male thread on the other. I have a certain amount of leeway, but the length of each threaded section will probably be 5mm at the most.

It's been suggested that the easiest way to do this might be to turn the lathe by hand.

Does that sound like the best approach? Would it be worth making up a mandrel to fit the spindle and fitting a crank on the end to be able to turn with a bit more control and torque?

James

[JamesFParticipant @jamesf]

[JamesFParticipant @jamesf]

Should have said. This will be in aluminium or mild steel. Probably the former.

James

[AndyfParticipant @andyf]

Unless you have a very slow back gear, James, doing it under power would need quick reactions. And with only 5mm length to go at, there isn't really room for a wide run-out groove.

Either a hand-crank, or if the work is in the chuck, just use the handle of the chuck key as a tommy bar. After all, you only need about 5 turns for each thread.

If the threads end up against internal and external shoulders, a very narrow run-out groove might help get each of them neat and clean at that end.

Andy

[BazyleParticipant @bazyle]

For the terminally lazy you can run backwards with an upside down tool or one in the rear toolpost, starting at the shoulder end. That allows you to use a drilled hole instead of a runout groove.

[Speedy Builder5Participant @speedybuilder5]

Can you drill a hole thro the tube to provide an end of thread run-out ? – if you thread in reverse as Bazyle says, make sure the chuck doesn't come off.

[JamesFParticipant @jamesf]

Posted by Speedy Builder5 on 21/05/2013 15:58:55:

Can you drill a hole thro the tube to provide an end of thread run-out ? – if you thread in reverse as Bazyle says, make sure the chuck doesn't come off.

The problem with going straight through is (I think) the similar size of the threads. I'm not at all sure there'll be anything left between the inner M50 thread and the outer 2".

They're actually so close I bet with a bit of PTFE tape I could probably do away with this adapter and screw the two parts together directly, but that quite deeply offends my sensibilities

James

[Anonymous]

Ah, do the inner and outer threads overlap? If so, you'll probably end up with a pile of swarf.

Regards,

Andrew

[JamesFParticipant @jamesf]

My plan is for them not to overlap, but I think I need the piece to be as short as I can feasibly make it.

James

[BazyleParticipant @bazyle]

The hole I was talking about is not a massive crater. Just a small hole depth = thread depth and dia say 1.5x pitch. It is just so the thread ends in a small space not a sudden stop. I hadn't spotted one thread was female but then drilling from outside would still work.

[Ian PParticipant @ianp]

Posted by Bazyle on 21/05/2013 15:54:12:

For the terminally lazy you can run backwards with an upside down tool or one in the rear toolpost, starting at the shoulder end. That allows you to use a drilled hole instead of a runout groove.

I am unclear why cutting threads (in the direction) away from a shoulder is any easier than having the tool approaching normally.

Whilst I can see that you can take your time aligning the tip in the drilled hole, how do you ensure the backlash has been taken up?

Normal screwcutting with these fine pitch threads, providing the spindle speed is low enough or manually rotated, is not too difficult.

Ian P

Ian P

[Kevin FParticipant @kevinf]

Why don't you make the threads longer and then face off or part the threads to the required length

[JamesFParticipant @jamesf]

Oooh, I do like a bit of lateral thinking

I have to admit that as something of a newcomer to metal turning, working out what order to perform operations in to make the job most straightforward is one of the places where experience seems to count bucketloads and not just in relation to this particular piece of work. The number of times I've been making pieces for practice and planned out how to do it, then got halfway through and thought to myself "You know, I really should have started this differently…"

James

[JamesFParticipant @jamesf]

Thanks to all who contributed to this thread. It was all a bit "sweaty palms" trying to turn a 3.5" diameter aluminium workpiece in the Conquest Super, but I managed to get the part made and then screwed my courage to the sticking place and turned my first ever thread (M50x1 female thread with a restricted length — not exactly making life easy for myself partway through the middle. In the end I opted to do it all by hand turning the chuck with the chuck key.

It all went well and fits the existing telescope part I wanted it for very nicely, so I'm feeling rather smug for the time being. Aesthetically it's not going to win any prizes because the finish perhaps isn't as nice as it ought to be. I could do it better a second time having learnt from the experience. But I'm not exactly doing this every day and six months ago I'd never even used a lathe.

James

[Chris HeapyParticipant @chrisheapy71135]

Buy/make yourself a mandrel handle for the lathe so you can turn the spindle by hand easily, you'l find it one of the most useful accessories for threading operations.

[JamesFParticipant @jamesf]

Yes, I have that on my list of things to do. I was just keen to get this sorted so opted to do it by turning the chuck. I can see that the handle/mandrel makes a lot of sense in the longer term particularly because there's always the danger of undoing the chuck when using the chuck key as a lever. As a fair bit of astronomy kit has threads that are realistically too large to cut with a tap or die (typically in the range of 1.25" to 2" diameter) it will probably see a lot of use.

James

[Russell EberhardtParticipant @russelleberhardt48058]

A trick I've used for large diameter fine threads is to use a smaller bottoming tap in the toolpost as a thread cutting tool. That results in the correct thread form. You can grind a bit off the end to get the thread to the bottom of the recess if necessary.

I always screwcut under power but then I can go down to about 7 rpm.

Russell.

[Russell EberhardtParticipant @russelleberhardt48058]

deleted

Edited By Russell Eberhardt on 04/06/2013 16:33:27

[JamesFParticipant @jamesf]

Not sure about imperial, but metric thread profiles are defined in terms of the pitch, aren't they? So if a 1mm pitch is required then any tap for a metric 1mm pitch thread will do the job?

James

[NobbyParticipant @nobby]

Hi James
I use one of these handles sprung loaded for safety for fine threads . And leave the half nut closed and after retracting the tool wind back etc

Nobby

[JamesFParticipant @jamesf]

Where does the spring loading come in? I can't work that one out.

James

[AndyfParticipant @andyf]

Posted by JamesF on 04/06/2013 16:49:51:

Where does the spring loading come in? I can't work that one out.

James

My guess is that Nobby is avoiding the problem I have with my simple crank. If I forget it's there and turn the motor on, the imbalance makes the lathe try to do somersaults while the crank takes vicious swipes at my elbow. I guess Nobby's crank is loose on the shaft until pushed bodily inwards against a spring, to engage a dog clutch.

Andy

[Russell EberhardtParticipant @russelleberhardt48058]

Posted by JamesF on 04/06/2013 16:43:17:

Not sure about imperial, but metric thread profiles are defined in terms of the pitch, aren't they? So if a 1mm pitch is required then any tap for a metric 1mm pitch thread will do the job?

That's right. 6 mm x 1mm is a common size. Perhaps a bit fragile though. You should be able to find 10 mm x 1 mm which has a bit more "meat".

Russell.

[JamesFParticipant @jamesf]

Ah, yes, eminently sensible to protect oneself from firing up the lathe with the handle still engaged

James

[JasonBModerator @jasonb]

Posted by JamesF on 04/06/2013 16:43:17:

So if a 1mm pitch is required then any tap for a metric 1mm pitch thread will do the job?

Its best to use a fairly close size otherwise the steeper helix angle of a small dia tap can start to rub if used for a very large diameter. I've used the method several times but never done more than about 1.5 times the tap dia. If I recal this was a 1/2" x 40tpi thread cut with a 3/8" tap

 

J

Edited By JasonB on 04/06/2013 17:15:58

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