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Thread: Epoxy Granite Portal Machine

  1. #13
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    Default Re: Epoxy Granite Portal Machine

    Steppers and servos both are capable of 0.001“ and way finer but first off, you're trading speed for resolution in some cases (the pitch of your ballscrews). Second, that won't be the problem in my opinion. It'll be backlash and slop and flex in the entire system which probably can't total more than like 0.0005 for you to hit your goal. So not only do the bearings and ball nuts and mounts all have to have less than that amount of backlash, but your system, across the largest spanned member, has to also flex less than that under load.
    I'm taking some liberties in the math here but just trying to make the point that the motors are probably the easiest part to get "accurate".
    That said, you're making this because you want to and many people have made great machines on their own! Just be realistic about what it'll do.

    Sent from my SM-G960U using Tapatalk



  2. #14
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    Default Re: Epoxy Granite Portal Machine

    I'm taking some liberties in the math here but just trying to make the point that the motors are probably the easiest part to get "accurate".
    Yup. In theory, steppers - when under no load to speak of - can be just a hair more accurate than servos of equal resolution, because servos have following error. However, in practice; a) you wouldn't have a servo with equal resolution; you'd have an encoder on it with finer resolution than the stepper; b) following error can be as little as +/- one encoder count with a well tuned servo; and c) in any case we are talking about errors on the same order as the quantization error of the control system - so it's not something to really worry about.

    Where servos and closed loop stepper systems have an advantage is dealing with momentary mechanical overload - open loop steppers can get knocked from one step detent to the next; thus "loosing a step"; and throwing off any moves that come afterwords. Closed loop systems will detect this and make a correction - you might loose a little bit of surface finish (although, that might be a non-issue, if as is likely, the overload was during a roughing operation) but you won't ruin the part.

    That said, you're making this because you want to and many people have made great machines on their own! Just be realistic about what it'll do.
    Yes; if your expectations are reasonable - or straight up low - you can be pleasantly surprised when your creation outperforms them. However, if you have unrealistically high expectations, you are guaranteed to be disappointed when it fails to live to up the "pipe dreams"...

    Do steppers and servos have the same mounting in case I wanted to swap later on? (I honestly cant seem to find conclusive data.)
    Most steppers of the sort that you'd use for a CNC machine are made in NEMA standard frame sizes - 17, 23, 34, or 42, typically. Some servo motors are also available in those frame sizes; however there is much more variety in servo motor frame sizes than there is in steppers.

    However, it's typically not a major problem to make some adapter plates; should one need them.

    I have no personal experience with this company's products, but they have been recommended by other people on this forum... and they do have servos in NEMA frame sizes.
    DMM | AC SERVO DRIVE | AC SERVO MOTOR | ROTARY ENCODER






  3. #15
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    Default Re: Epoxy Granite Portal Machine

    Thank you for your responses.

    Does anybody know of a good spindle servo? (1.8 Kw, 8000 RPM) Is direct drive necessary if I want to do rigid tapping?



  4. #16
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    Default Re: Epoxy Granite Portal Machine

    Thank you for your responses.

    Does anybody know of a good spindle servo? (1.8 Kw, 8000 RPM) Is direct drive necessary for rigid tapping?

    - - - Updated - - -

    Thank you for your responses.

    Does anybody know of a good spindle servo? (1.8 Kw, 8000 RPM) Is direct drive necessary for rigid tapping?

    - - - Updated - - -

    Oops. DIdnt mean to double post. Sorry.



  5. #17
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    Default Re: Epoxy Granite Portal Machine

    Does anybody know of a good spindle servo? (1.8 Kw, 8000 RPM) Is direct drive necessary if I want to do rigid tapping?
    No, it's not required; but it can't be a friction-belt drive (like a V-belt) - they will slip a little; putting a large amount of force on the tap. Gear drive works if the backlash is low enough and you can setup the control to handle it; but timing belt drive works best if it's not directly coupled. It helps if the pulley/gear ratio is an integer multiple; but depending on the control may not be a strict requirement.



  6. #18
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    Default Re: Epoxy Granite Portal Machine

    Quote Originally Posted by Richmaple View Post
    Steppers and servos both are capable of 0.001“ and way finer but first off, you're trading speed for resolution in some cases (the pitch of your ballscrews). Second, that won't be the problem in my opinion. It'll be backlash and slop and flex in the entire system which probably can't total more than like 0.0005 for you to hit your goal. So not only do the bearings and ball nuts and mounts all have to have less than that amount of backlash, but your system, across the largest spanned member, has to also flex less than that under load.
    I'm taking some liberties in the math here but just trying to make the point that the motors are probably the easiest part to get "accurate".
    That said, you're making this because you want to and many people have made great machines on their own! Just be realistic about what it'll do.

    Sent from my SM-G960U using Tapatalk
    Hmmm.
    On a mill I've used some decent AC bearings. Double nuts shimmed as tight as they can go, jaw couplings, a fair bit of work.
    I can't get lower than 0.03mm (0.0012").
    Some expensive linear rails on it then maybe I'll half that but still.



  7. #19
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    Default Re: Epoxy Granite Portal Machine

    Quote Originally Posted by ando600 View Post
    Thank you for your responses.

    Does anybody know of a good spindle servo? (1.8 Kw, 8000 RPM) Is direct drive necessary if I want to do rigid tapping?
    Most I've seen is about 6000rpm (think was Mitsubishi or something). That's like about £1000 plus. Then you need a drive to match.
    Generally ones less than £1000 go to 3000rpm.
    Likely have to overdrive one with a timing belt.



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