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  1. #1
    Join Date
    Feb 2021
    Location
    Australia Brisbane Qld
    Posts
    51

    Default Help please. Timing of the half nut on my lathe

    Hi all. I’m attempting single point threading for the first time. I have successfully changed my gears to give me 18 tpi. I have been practicing on alum bar stock , scratch passes at this stage. I have read existing forum topics and plenty of YouTube. With the lathe turned on , and off I have been wiggling the half nut and practicing dropping it into position. I have identified lots of spots on my dial where I can drop the bar to easily engage the half nut so my tool can start doing scratch passes. But every time I back out and try to start again I never end up in the correct spot for my next scratch pass. It seems like the tip of my tool always lands in the middle of my previous cuts
    Approaching 2 days now with the same problem despite me watching lots of vids and lots of research. I fear my brain is about to explode soon

  2. #2
    Join Date
    Jun 2007
    Location
    sydney ( st marys )
    Age
    64
    Posts
    4,887

    Default

    Would it be correct to think your leadscrew is Imperial?
    Can you provide a chasing dial chart?
    Are you able to reverse the lathe?

  3. #3
    Join Date
    Feb 2021
    Location
    Australia Brisbane Qld
    Posts
    51

    Default

    I checked and my lead screw is metric. I don’t know what a chasing dial chart is. I have a switch to reverse the lathe direction. But I’m sure I must stop the lathe first rather than just slamming it into reverse

  4. #4
    Join Date
    Jan 2004
    Location
    Mackay North Qld
    Posts
    6,446

    Default

    Hi STEVEMORSE,

    Welcome to the MetalWork Forums,


    To help you explore the other areas of our forum here's a guide on to to navigate our Forum and find all the interesting places.

    Goto the FORUM box in the top left hand cnr of the page and click the down arrow. This will bring up a pull down menu that has Forum Home at the top

    Click Forum Home which will present a scroll down page.

    Our rules,the Terms of Service are right at the top ,we encourage you to read them.

    Below that are all the various areas and sub forums that make up out MetalWork forums.


    Welcome

    Grahame

  5. #5
    Join Date
    Jun 2007
    Location
    sydney ( st marys )
    Age
    64
    Posts
    4,887

    Default

    That's your problem.
    Cant cut an Imperial thread on a metric lathe by releasing the half nut unless you are very careful, you would be best to leave the nuts engaged and reverse the lathe.

  6. #6
    Join Date
    May 2013
    Location
    Rockhampton, QLD
    Age
    68
    Posts
    454

    Default

    Welcome to the forum.

  7. #7
    Join Date
    Oct 2007
    Location
    Alexandra Vic
    Age
    69
    Posts
    654

    Default

    Tread cutting can be somewhat fraught if your lead screw is metric and you want to cut an imperial thread. If you are lucky, the threading data plate will include information for the correct gear ratios/layout to cut both imperial and metric threads regardless of whether the lathe has imperial or metric screws, but the threading dial will only work consistently for threading in the same system as the lead screw, i.e. you can us the treading dial to cut imperial threads with an imperial leadscrew, or to cut metric threads with a metric leadscrew. Also for metric threads and leadscrew there may be multiple gears for the threading dial and you need to have the correct one for your pitch installed for the dial to work correctly.

    If you are cutting non native threads e.g. imperial threads with a metric leadscrew or vice versa, you need to keep the half nuts engaged and stop the machine, reverse the motor, wind back a few turns beyond the start of the thread and stop again, then start turning in the normal direction again. Any backlash in the gear train will be taken up as the carriage approaches the start of the thread you are creating and the tool will follow the scratch pass thread.

    Something that some people have done that has caused alignment errors in the past is to get to the end of the section to be threaded, stop and reverse the lead screw direction relative to the spindle then wind the carriage back with the motor running in normal direction, stop ad reset the lead screw back to the original direction relative to the spindle direction then make the next pass. This does not work as the gear train mesh is broken twice while changing lead screw direction and each break can allow the train to reengage after slipping one or more teeth, so the threading paths do not realign correctly.

    A couple of the better known YT guys with big industrial lathes have other ways to maintain alignment but they rely on special features built into the lathes that are not normally available in hobbyist lathes.

    Some info about the lathe in question and pics of the threading and feed charts and thread dial charts would help people to understand your particular lathe and provide direct answers to meet your situation. Similarly, it would help if your profile location listed a town or region rather than Australia, as someone experienced that is reasonably close may offer to visit and help solve your dilemma.

    Hope this helps.

    EDIT:

    More info has come to light since I started typing this, and it appears that you are cutting imperial thread via a metric leadscrew. As pipeclay suggests and I mentioned above, keep the halfnuts engaged and return the carriage a couple of turns beyond the start of the thread by reversing the motor and spindle. It is about the only way to do this successfully.
    I used to be an engineer, I'm not an engineer any more, but on the really good days I can remember when I was.

  8. #8
    Join Date
    Feb 2021
    Location
    Australia Brisbane Qld
    Posts
    51

    Default

    Thank I all very much for taking the time. I really appreciate it. I will jumo back on my machine. So much to learn

  9. #9
    Join Date
    Feb 2021
    Location
    Australia Brisbane Qld
    Posts
    51

    Default

    After learning that I can’t cut imperial threads on a metric leadscrew if I engage and disengage the half nut, I was still adamant about useing the half nut and dial so that I could easily stop and start where I wanted to .....so I changed my leadscrew gears to suit a metric 18mm 1.5 mm thread. This time , after my scratch pass I was always and very easily able to engage the half nut at exactly the right time , and I had no trouble cutting in precisely the right spot. And my new thing for attempting single point threading On a steel bar was a success , theN I made a bush and tapped a thread inside it and I tried the two parts together. There was only a tiny bit of play between my bush and steel bar ....so with more research and practice I’ll work towards getting excellent fitting results.... thank god for help on the Internet !

  10. #10
    Join Date
    Nov 2012
    Location
    Mid North Coast NSW
    Posts
    422

    Default

    Wow... don't know how you managed that.

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