Short thread-cutting

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

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

James

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

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.

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

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

Regards,

Andrew

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

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.

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

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

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

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

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.

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

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.

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

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

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

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

James