1. #1
    Chaz's Avatar
    Lives in Ickenham, West London, United Kingdom. Last Activity: 2 Hours Ago Has been a member for 6-7 years. Has a total post count of 1,362. Received thanks 111 times, giving thanks to others 65 times.
    Hi,

    I have 2 Leadshine EM806 drivers running two steppers for a lathe. On the X Axis I have no problem. On the Z Axis, I can rapid at 4000 mm/min easily enough. The ballscrew pitch is 5 mm from memory and the gearing is roughly 1 : 2 (motor:ballscrew).

    I did some threading testing on the lathe and noticed that I cannot thread whilst the RPM on the spindle is higher than around 200 rpm. If I run say 640 rpm, the Z stalls but only stalls feeding towards the chuck. I have no mechanical binding. I messed with acceleration rates (Centroid Acorn) but with threading it seems to ignore these.

    The motor is a 12.5 NM stepper - Nema34. It's being fed by a 70V Leadshine PSU and runs at max current (8.3A). Ive run the 'auto sense' flicking the dipswitch number 4 to sense the motor. I can hear the motor pulse.

    I dont have a cable or PC with serial port (old laptop somewhere) so checking the drive isnt easy. Is it possible that I can get better performance from the system by messing with the software or is that just fine-tuning?

    On my other lathe, I have no problems but the ballscrew pitch on that machine is very fine, hence torque is not an issue.

    Here's a quick video of it - you can see that it comes in, starts the thread, but then only goes in 1 direction but stalls in the other. Thoughts?


  2. #2
    Chaz's Avatar
    Lives in Ickenham, West London, United Kingdom. Last Activity: 2 Hours Ago Has been a member for 6-7 years. Has a total post count of 1,362. Received thanks 111 times, giving thanks to others 65 times.
    Wanted to add, the motor is 8 wired, wired in Bi Polar parallel.

    This is the motor.

    https://www.cnc4you.co.uk/Stepper-Mo...?search=nema34

    The specsheet here - https://cnc4you.co.uk/resources/Step...1%2012.5Nm.pdf

    Based on my current gearing the motor runs around 1600 RPM to achieve 4000 mm/min (I need to double check the gearing). Does anyone know what max RPM could be, nothing declared on the motor. I can email and ask the supplier but if I gear down some more, whilst 4000 mm/min is less important than stability, its nice to have high rapid rate on a large machine.

    Thanks

  3. #3
    Re: gearing.
    1:2 or 2:1.

    1600 rpm for a nema 34 stepper sounds extremely high.
    Typical is 800 rpm, or less.
    4m/min = 66 mm/sec.
    A 5 mm screw at 1:2 = 10 mm per rev of motor.
    So 12 Nm /2 == 6Nm stall and maybe 3 Nm 300 rpm.

    At 1:2 you would have half the torque vs direct gearing.

    For 10 mm /rev, 4000 mm/min = 400 revs/min.
    6.6 rps.

    Anyway, if as You said the threading ignores acceleration settings, then maybe / probably the problem is lack of torque on z past 200 rpm at spindle.
    The z axis tries to accelerate to a speed suitable for the spindle rpm, and cannot do it.
    The z axis maybe tries to accelerate "instantly" to the required sync speed, and simply does not have enough torque to get there before stalling.

    If gearing 1:2 motor/screw your screw would run at 3200 rpm at 1600 from stepper.
    That is probably wrong.
    2:1 is probably right.

    Anyway 4m/min on a medium lathe like yours, and gibs, is far too fast.
    It will wear out your mechanicals real fast.

    My lathe is about the same size, 12x24.
    On servos with 10x the acceleration and 5x the speed my rapids are much lower on purpose.

  4. #4
    Chaz's Avatar
    Lives in Ickenham, West London, United Kingdom. Last Activity: 2 Hours Ago Has been a member for 6-7 years. Has a total post count of 1,362. Received thanks 111 times, giving thanks to others 65 times.
    Quote Originally Posted by hanermo2 View Post
    Re: gearing.
    1:2 or 2:1.

    1600 rpm for a nema 34 stepper sounds extremely high.
    Typical is 800 rpm, or less.
    4m/min = 66 mm/sec.
    A 5 mm screw at 1:2 = 10 mm per rev of motor.
    So 12 Nm /2 == 6Nm stall and maybe 3 Nm 300 rpm.

    At 1:2 you would have half the torque vs direct gearing.

    For 10 mm /rev, 4000 mm/min = 400 revs/min.
    6.6 rps.

    Anyway, if as You said the threading ignores acceleration settings, then maybe / probably the problem is lack of torque on z past 200 rpm at spindle.
    The z axis tries to accelerate to a speed suitable for the spindle rpm, and cannot do it.
    The z axis maybe tries to accelerate "instantly" to the required sync speed, and simply does not have enough torque to get there before stalling.

    If gearing 1:2 motor/screw your screw would run at 3200 rpm at 1600 from stepper.
    That is probably wrong.
    2:1 is probably right.

    Anyway 4m/min on a medium lathe like yours, and gibs, is far too fast.
    It will wear out your mechanicals real fast.

    My lathe is about the same size, 12x24.
    On servos with 10x the acceleration and 5x the speed my rapids are much lower on purpose.
    Thanks. Ill check the maths later.

    You are right, its the initial acceleration to sync to the spindle which is a problem. What I struggle to understand is why does this only happen towards the chuck and not from it and why does the first pass work? I did this a few times yesterday, same result.

    My other lathe has a much much finer ballscrew, it has no problems and whilst I dont know the exact motor spec, its not nearly as powerful.

    This is why I love servos. Oh well. Time to gear the Z down somewhat I think.

  5. #5
    Chaz's Avatar
    Lives in Ickenham, West London, United Kingdom. Last Activity: 2 Hours Ago Has been a member for 6-7 years. Has a total post count of 1,362. Received thanks 111 times, giving thanks to others 65 times.
    The motor has the small pulley, the ballscrew the larger pulley.

  6. #6
    That would be 2:1 with 2 turns in gives 1 turn out

    As written it sort of looks like a gearing up but that is the convention for gearing down to get less speed but more torque.
    Building a CNC machine to make a better one since 2010 . . .
    MK1 (1st photo), MK2, MK3, MK4

  7. #7
    Chaz's Avatar
    Lives in Ickenham, West London, United Kingdom. Last Activity: 2 Hours Ago Has been a member for 6-7 years. Has a total post count of 1,362. Received thanks 111 times, giving thanks to others 65 times.
    Quote Originally Posted by routercnc View Post
    That would be 2:1 with 2 turns in gives 1 turn out

    As written it sort of looks like a gearing up but that is the convention for gearing down to get less speed but more torque.
    Yep, counted the pulleys, 18 tooth on one, 36 on the other. So making the larger one bigger will give me more torque. Just need to look at space and belt options (assuming there is no problem elsewhere and the drive is working as well as possible).

  8. #8
    Chaz's Avatar
    Lives in Ickenham, West London, United Kingdom. Last Activity: 2 Hours Ago Has been a member for 6-7 years. Has a total post count of 1,362. Received thanks 111 times, giving thanks to others 65 times.
    So I can go to a 72 tooth pulley but no standard belt size works if I want to keep the Centres similar distance. Going to a 60 tooth pulley will give me more torque and with a 400mm belt versus 330mm, is virtually the exact same distance between centres.

  9. #9
    Ger21's Avatar
    Lives in Detroit, United States. Last Activity: 8 Hours Ago Has been a member for 7-8 years. Has a total post count of 635. Received thanks 88 times, giving thanks to others 0 times. Referred 1 members to the community.
    If your spinning the motor faster to try and get more torque, you may actually end up with even less torque.

    Those huge motors usually have very little torque over 500-600rpm.
    Gerry
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  10. #10
    Chaz's Avatar
    Lives in Ickenham, West London, United Kingdom. Last Activity: 2 Hours Ago Has been a member for 6-7 years. Has a total post count of 1,362. Received thanks 111 times, giving thanks to others 65 times.
    Quote Originally Posted by Ger21 View Post
    If your spinning the motor faster to try and get more torque, you may actually end up with even less torque.

    Those huge motors usually have very little torque over 500-600rpm.
    Thanks Ger. This is what has been recommended to me too. With the Acorn tuning where there are 2 parameters, it will thread OK.

    My 2nd lathe has an issue with the encoder which I hope to fix today, will then test back to back and see.

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