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Thread: Thread cutting

  1. #1
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    Default 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

  2. #2
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    Default

    Use method A.
    Chuck should not unscrew if you have put it on correctly.
    When reversing make sure too go into reverse after a couple of seconds after taking it out of foreword, if you change to quickly the lathe will stay in the first selected direction.

  3. #3
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    Default

    as said use A. You will most likely lose syncronisation if you use the tumbler gears.

  4. #4
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    Default

    Hi Tony, Guys,

    This is where I often cheat and thread away from the chuck. Running the lathe in reverse with the tool at the back. Saves running into the chuck, particularly in a blind hole. I use the same technique and keep the half nut closed and either wind back using the motor or often with a lathe handle. Don't forget to wind back further than you need in order to account for the back lash.
    Best Regards:
    Baron J.

  5. #5
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    Default Thanks for the info

    Pipeclay, thanks for the reply. I had thought as much but as mentioned, was unsure as I could not find any reference in the various manuals etc.

    Following on from the reversing question, the reverse tumbler - is it used for normal direction turning but having the apron move Left to Right vs Right to left.

    On my lathe (Sheraton 9AR) in the forward position the apron moves right to left and the cross slide moves out from the centre.
    In reverse the apron moves left to right and the cross slide moves into the centre - which would seem ideal for facing the end or for parting.
    I'd have thought that forward would be left to right apron travel and inward movement of the cross slide given they would be possibly the more common used movements.

    I've also been contemplating the fitting of a Hercus tumbler gear set with the self locking detent pin in the handle Vs the need to undo the square head bolt every time you want to change the tumblers. Do you know if this is as simple as swapping the tumbler unit over or will I need to make some modifications to the lathe head?

    thanks again
    Tony


    Quote Originally Posted by pipeclay View Post
    Use method A.
    Chuck should not unscrew if you have put it on correctly.
    When reversing make sure too go into reverse after a couple of seconds after taking it out of foreword, if you change to quickly the lathe will stay in the first selected direction.

  6. #6
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    Default Thanks for the info

    Baron
    thanks for the reply.
    I saw a youtube clip on threading with the lathe in reverse and the tool holder upside down. It was quoted as being safer than normal threading due to having removed the risk of crashing into the chuck. I wasnt sure that it could be safely done with a threaded chuck as it might unscrew in reverse unless the chuck was given quite a good locking twist when fitted.

    Im curious about the "lathe handle" you mention - what is it?

    thanks
    Tony

    Quote Originally Posted by BaronJ View Post
    Hi Tony, Guys,

    This is where I often cheat and thread away from the chuck. Running the lathe in reverse with the tool at the back. Saves running into the chuck, particularly in a blink hole. I use the same technique and keep the half nut closed and either wind back using the motor or often with a lathe handle. Don't forget to wind back further than you need in order to account for the back lash.

  7. #7
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    Default Thanks for the info

    Zsteve
    Thanks for the reply confirming the need to reverse the lathe using A.

    There is so much to learn with these machines. It would be awesome to spend time with a master and learn the trade and no doubt its many tricks.

    Tony


    Quote Originally Posted by Zsteve View Post
    as said use A. You will most likely lose syncronisation if you use the tumbler gears.

  8. #8
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    Default

    Is leaving the half nut engaged for metric threading a requirement on all lathes or an option to constantly engaging/disengaging? I thought the latter but the more time that goes by the more I realise i know nothing!

  9. #9
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    Default

    Quote Originally Posted by Mk1_Oz View Post
    Is leaving the half nut engaged for metric threading a requirement on all lathes or an option to constantly engaging/disengaging? I thought the latter but the more time that goes by the more I realise i know nothing!
    Generally only required when cutting threads that are opposite to the leadscrew thread ( imperial leadscrew metric thread and vice a versa ), or if you may not have a chasing dial.

  10. #10
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    Default

    Tony,
    in regards to the direction with the tumblers, on a Hercus the up position is foreword and down reverse.

    The Hercus uses 14.5 PA ( pressure angle ) gears and Sheraton 20 PA gears.

    Can not tell you if you can change the tumbler set up without any hole drilling in the headstock, someone else may know or you will need to remove yours to compare to the Hercus, you would also need to confirm that the tooth count of your existing tumbler gears are the same as Hercus ( 32 teeth ) and that the bore is the same ( 5/8" , .625" ), as you may need to change out the Hercus tumbler gears with your existing gears.

    Just curious , how often are you using the reverse tumblers to warrant the change over.

  11. #11
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    Quote Originally Posted by ajbvicau View Post
    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.
    Just so you are aware there are many ways to skin a cat. You can open the halfnuts at the end of the thread, then turn the lathe off and then reverse, but then you MUST "pick up the thread" on the way back.(so its best if you stop the lathe before the thread dial has made a full turn or you will have to count the turns)

  12. #12
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    Hi Tony,

    To cut on the reverse, the tumbler will still be the same as you go toward the chuck, you just have to put the cutting tool upside down. If you reserve the tumbler you will get lefthand thread. I have done a few times on the Hercus without problem.

    If you are cutting metric thread on imperial leadscrew and you have to disconnect the 1/2 nut (if in case of threading on blind hole). Just take note on the number on the dial indicator when it is engaged, disconnect the 1/2nut at the end so the carriage is not moving forward then crashes into the work, on reverse once the mark goes back to the same mark/location on the dial you will have to engage the 1/2 nut again to bring the carriage back out.

    Trong

  13. #13
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    I once heard that there was a method that can be used when thread cutting say a metric thread on an imperial lathe that does require the lathe to be run in reverse
    Once the all the prep/setup work for thread cutting has been done and you are ready to start cutting then with the motor off you engage the half-nuts - with a felt tip marker an alignment mark is put on the chuck and headstock where the chuck attaches to the the headstock - a similar alignment mark is put on the leadscrew and lathe bed at the point where it emerges from the gearbox - with this done turn on the motor and start the first cut. At the end of the cut disengage the half-nuts retract the tool and wind the saddle back leaving the motor going, adjust the cross slide for the next cut - observe both of the alignment marks(motor is still going) - at some point they will all line up at that moment engage the half-nuts for the next cut, then as they say rinse repeat.
    I was always wanting to try this, it sounds plausible but I don't do a lot of thread cutting, has anyone heard of this and actually tried it ?

  14. #14
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    Quote Originally Posted by familyguy View Post
    has anyone heard of this and actually tried it ?
    Though it was awhile ago, I've seen reference to it and tried for giggles, not on a job. But are you sure it was for cutting non-native threads and not just picking up native threads without a threading dial?(or possibly course threads?).
    Also I think you've left one step out(or at least its not clear), pretty sure you need to wind the carriage back to the same starting position each time.
    Last edited by Stustoys; 10th Jun 2021 at 11:46 AM. Reason: y

  15. #15
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    It would probably make sense to wind the saddle back to the same position - possibly use the tailstock as a stop to make sure the saddle stopped it the same spot each time it may have been mentioned at the time but it has been quite a few years. It was told to me by a tool maker many years ago - my first lathe did not have threading dial - he said it works for both native and non native threads, with a native thread I recall he said if the thread you are cutting is a whole number multiple of the lead screw thread then the two alignment marks will line up quite frequently as the lathe runs, if cutting non native threads then the alignment marks will lineup only once every so many turns and you may have to wait up to 10 or 15 seconds (depending on the lathe speed) for the marks to line up.

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