Picking up the thread.

[Adrian 2Participant @adrian2]

I have screw cut an internal 3/8" BSP thread on my Myford super 7. It is an American export machine with no 19 TPI gearbox setting.

Brian Wood's excellent book Gearing of Lathes for Screwcutting tells me to use a 29 tooth mandrel wheel and gearbox setting 23.

Cutting the thread was completed without disengaging the leadscrew . The job was removed from the lathe and the standard mandrel wheel replaced , half nuts released.

I now find I need to ease the thread a little more . My question, is it possible to reproduce my original set up so that the tool will continue to follow the same path and not produce a double thread? If so, how?

Over to you perhaps Brian.

Thank you .

[Adrian 2Participant @adrian2]

[Tony Pratt 1Participant @tonypratt1]

Yes, set the cross parallel to the lathe travel & reset the job as before. start the screw cutting procedure with the tool not cutting metal, stop the lathe once you are in the bore & adjust the tool forwards or backwards with the top slide to pick up the thread, slightly tricky but very doable.

Tony

[old martParticipant @oldmart]

As Tony suggests, and a magnifying glass might help.

[OldironParticipant @oldiron]

Lots of video's on Youtube to see the methods needed to realighn a thread.

Look at this one

IHTH

regards

[Speedy Builder5Participant @speedybuilder5]

If you are nervous (Like most of us) turn the chuck by hand until you are happy with the engagement.

[DC31kParticipant @dc31k]

Posted by Tony Pratt 1 on 10/02/2021 17:56:22:

Yes, set the cross parallel to the lathe travel & reset the job as before. Start the screw cutting procedure with the tool not cutting metal, stop the lathe once you are in the bore…

Leave the tool slightly slack in the holder and slide it forwards and backwards feathering the cross-slide until it drops into the groove of the existing thread. Tighten tool in holder.

[Nigel Graham 2Participant @nigelgraham2]

Yes, I've used the technique described, but if a tap is available would finish the thread with that, as it also trims the profile.

[Brian WoodParticipant @brianwood45127]

Hello Adrian,

I can do little other than endorse the advice you have already been given, with perhaps the added suggestion that you do the re-cutting by hand using a mandrel handle if you have one.

Best of luck

Brian

[Adrian 2Participant @adrian2]

Thank you to everyone. The method you all describe is the one I adopted before posting my question. Having carefully engaged the tool in the thread I then backed it out clear and cranking by hand I watched its path as it travelled into the bore. It was clear to see that it did not follow the thread but was very soon going its own sweet way.

I assumed this to be because of the use of the 29 tooth mandrel wheel and the 'timing' of my original set up has been lost. Certainly had it been a thread readily available straight from the gearbox there would have been no problem.

Adrian.

[Tony Pratt 1Participant @tonypratt1]

If the lathe is set to 19 TPI & the original is 19 TPI the tool will cut the same path i.e. 19 TPI & will follow the thread, something is amiss with your set up.

Tony

Edited By Tony Pratt 1 on 10/02/2021 19:49:49

[Howard LewisParticipant @howardlewis46836]

Try to find the point at which the first thread starts and mark the face of the job..

Bring the saddle forward by hand to set the tool against the mark and lock it temporarily.

Rotate the Leadscrew mandrel by hand, with a gear removed whilst gently trying to engage the half nuts

Hopefully, when the half nuts engage, it will be possible to refit the gear.

Once this is done, I would advance the tool into the work, but cleasr, before mbringing it out to engage the existing thread. The tool may ned to slacked in the toolpost to engage nthe thgread fully. Note nthe dial setting, move the tool inwards and back the Saddle out, without disengaging the half nuts. Return the Cross Slide to the dial setting and rotate the chuck by hand.

If you are lucky, the tool will follow mthe thread!

HTH

Howard

[duncan webster 1Participant @duncanwebster1]

3/8" bsp is only just over 9/16" root diameter so it must be very difficult to see what you're doing down the hole, especially as a lot of it will be full of boring bar.

Borrowing a tap is your best bet. Failing that DC31K's method sounds good, make sure you have the half nuts engaged and the saddle pushed back to take up the backlash

[Howard LewisParticipant @howardlewis46836]

Yes, Duncan is right, for 3/8 BSP, the easiest way is to use a Tap rather than screwcut, unless you are a masochist

Howard

[Adrian 2Participant @adrian2]

Thanks again to everyone, all is well now. As mentioned there is not much day light up inside with the threading tool entered. Interpreting what was going on looking at an angle was not easy .

The reason for screwcutting and not using a tap is that the thread on the valve I intend to screw in is undersize to the extent of being unacceptably sloppy .

Adrian.

[Brian WoodParticipant @brianwood45127]

Adrian,

I have been busy with other things today and would have posted earlier. From your last post you say everything is OK now, which is just as well really since I couldn't see why you had been unable to pick up the thread again. It is no different to cutting a metric thread when you have inadvertently opened the leadscrew clasp nuts before the job was finished.

Using the same set up a second time, 29 tooth mandrel gear and gearbox set for 23 tpi will still cut a thread of 19.04 tpi, just as it did the first time

Puzzled!

Brian.

[Adrian 2Participant @adrian2]

That's it Brian, just as you say. My being unable to see properly was the problem.

Thank you for your help.

Kind regards,

Adrian.

[Chris CrewParticipant @chriscrew66644]

'I assumed this to be because of the use of the 29 tooth mandrel wheel and the 'timing' of my original set up has been lost. Certainly had it been a thread readily available straight from the gearbox there would have been no problem'.

If I am reading you correctly, yes there would have been a problem and you would have had to use the methods already described to pick up the thread if the top-slide had been disturbed, the half-nuts dis-engaged or the work removed from the chuck. This would be necessary even if the gear-box settings had been available or just change-wheels used. Even when the use of the TDI is possible threads don't automatically pick-up in exactly the same position just because you drop the clasps nuts in at the indicated position. This is because you would have to be very sure the work was re-mounted in exactly the same position, both protruding from the chuck and the thread already cut in sync. with the lead-screw when it was cut. Both these conditions would be impossible to achieve.

In the home workshop I would suggest there is no 'wrong way' or 'right way' of doing any job. There is only 'your way' and if it worked, then it must be 'right', surely?

Edited By Chris Crew on 11/02/2021 21:15:38

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