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Author Topic: Need advice for retrofit of millrite  (Read 4419 times)

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Need advice for retrofit of millrite
« on: September 15, 2008, 05:32:14 PM »
I have done most of my homework; got all software except MACH3, have a dedicated PC with dual flat panel monitors and decent videocard, found the 3 axis packages I want, designed the motormounts in solidworks, checked backlash (X = .029", Y = .022, Z = .015), found the timing gears and belts I need, now I just can't decide between the 1200 oz in steppers or the 1125 oz in servo motors. It is $250 more for the servos and from the info I have come across there is no clear answer to this. This machine would be for home use only making all different kinds of things from woods, plastics, to stainless and aluminum. I am ready to buy the kt I just can't figure which would be better. The millrite has an 8" x 32" table, weighs around 1200 lbs, has 7/8" 5TPI acmes, and is rock solid. HELP

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Re: Need advice for retrofit of millrite
« Reply #1 on: September 15, 2008, 05:48:21 PM »
Stepper systems set up correctly are great but if you want speed and acceleration then servos are the way to go. From what you describe I think Steppers would suffice.
 I love AC servos but have steppers on my Bridgeport and it has done me proud for the last few years, I am doing another slightly bigger mill at the moment, it has AC servos but I want the accel and speed as I make my living from my machines.
Hood
Re: Need advice for retrofit of millrite
« Reply #2 on: September 15, 2008, 05:51:00 PM »
Speed is not a huge concern for me and I intend to gear the motors down 1:2, but are the steppers less accurate than servos? If so, by how much?

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Re: Need advice for retrofit of millrite
« Reply #3 on: September 15, 2008, 05:58:02 PM »
No steppers are every bit as accurate as servos, your resolution may be better with servos depending on the encoder but unless you get something like a SmoothStepper so you can use the potential of the big encoder count you are not going to see that and even if you did you probably wouldnt need it as there would be more flex in your mill than the resolution.
 With 2:1 gearing and Geckos you will have 4000 steps per revolution of the motor so if you have 0.2" ballscrews that gives you a pretty good resolution. Granted the microsteps are not going to be spot on but it will be somewhere between a few microsteps which is minimal.
Hood

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Re: Need advice for retrofit of millrite
« Reply #4 on: September 15, 2008, 06:08:31 PM »
Sorry meant to say 4000 steps per Rev of the ballscrew, not the motor .
Hood
Re: Need advice for retrofit of millrite
« Reply #5 on: September 15, 2008, 06:27:42 PM »
Speed is not a huge concern for me and I intend to gear the motors down 1:2, but are the steppers less accurate than servos? If so, by how much?

Worrying about accuracy differences between servos and steppers is a little silly in a machine with such high backlash.  That backlash will be by far your biggest problem if you expect better than wood-working accuracy (if that) out of that machine.

Regards,
Ray L.
Regards,
Ray L.

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Re: Need advice for retrofit of millrite
« Reply #6 on: September 15, 2008, 06:31:41 PM »
Oh man never even noticed that backlash was in inches, I work metric so when I saw these numbers I just presumed it was metric, as Ray says with that amount backlash accuracy of steppers/servos  is not going to a concern.
Hood
Re: Need advice for retrofit of millrite
« Reply #7 on: September 15, 2008, 07:40:53 PM »
I was under the impression that with Mach 3 the backlash does not really matter so long as it is consistent because you can compensate for the backlash with Mach3. Your input is greatly appreciated and here are the kits I am considering and 95% sure on the steppers; they have a 1 year warranty on the motors, breakoutboard, and powersupply with the standard gecko warranty for the drivers. What do you think?

Stepper http://www.kelinginc.net/CNCNEMA34Package.html

Servo http://www.kelinginc.net/CNC34ServoMotorPackage.html
Re: Need advice for retrofit of millrite
« Reply #8 on: September 15, 2008, 07:56:46 PM »
I was under the impression that with Mach 3 the backlash does not really matter so long as it is consistent because you can compensate for the backlash with Mach3. Your input is greatly appreciated and here are the kits I am considering and 95% sure on the steppers; they have a 1 year warranty on the motors, breakoutboard, and powersupply with the standard gecko warranty for the drivers. What do you think?

Stepper http://www.kelinginc.net/CNCNEMA34Package.html

Servo http://www.kelinginc.net/CNC34ServoMotorPackage.html

Mach3 can mitigate backlash, and reduce it's impact on the quality of your work, but backlash *always* matters, and you'll never get truly good results with backlash in the system.  The cutting forces will sometimes work to push the cutter away from the desire path, regardless of Mach3's best efforts.  With such large amounts of backlash, the machine will never work particularly well, and your work will nearly always have visible artifacts of the backlash.  With as mch backlash as you have, those artifacts will be *very* visible.

Regards,
Ray L.
Regards,
Ray L.
Re: Need advice for retrofit of millrite
« Reply #9 on: September 15, 2008, 08:22:40 PM »
I think what they're trying to tell you is to start your project with a set of ball screws.
Thats where I would start. acme screws are not designed for any cnc.

Ed
Ed VanEss