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Thread: Thread cutting
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9th June 2021, 10:10 PM #1Most Valued Member
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Thread cutting
Looking for some help to clarify when / how to reverse the lathe for thread cutting.
My lathe is by default an imperial thread cutting lathe with an 8TPI lead screw.
In simple terms, as I understand it I set the lathe gearbox up for the desired thread TPI and when ready to cut the thread I engage the half nut on an appropriate thread dial index.
When I get to the end of the thread I disengage the half nuts, wind out the cross slide and wind the apron back to the start and when ready I engage the half nut on the required thread dial index.
To cut metric threads I change the stud, compound and screw gears and using the conversion chart select the required imperial TPI to match the metric thread I want to cut.
I read that I dont use the thread dial, I engage the half nut when ready to cut the thread but when it gets to the end I must withdraw the cutting tip, stop the lathe, leave the half nut engaged and reverse the apron back to the start. I then set the lathe to cut forward, adjust the cutting tip and start the lathe going forward again.
My question I need clarification on is about the reversing of the lathe.
Having stopped the lathe, do I -
a/ reverse the motor - thus leaving every gear engaged as originally selected and keeping the synchronisation of it all, or do I
b/ select reverse via the tumbler gear reverse feed control?
If a/ is there not a risk of spinning the chuck off the headstock when the motor is put into reverse (my chuck is threaded onto the headstock spindle)
I am using the back gears and lowest speed 60 RPM for threading so it is not as risky as the faster speeds but a risk none the less of seeing the chuck fly off the lathe.
If b/ is there not a risk that the synchronisation of the head/chuck and piece being threaded could be lost when selecting reverse via the tumbler gear and again when selecting forward after the apron gets back to the start thus resulting in a mismatch or the thread cutting or am I over analysing the task?
My apologies if this is a dumb question. I just cant find a clear answer in any of the manuals, they just say to reverse the carriage back to the start - not how.
thanks for your help
Tony
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