On 02/27/2014 09:45 PM, Gene Heskett wrote: > On Thursday 27 February 2014 22:35:15 Jon Elson did opine: > >> On 02/27/2014 07:16 AM, Gene Heskett wrote: >>> I should also mention that servos do their magic by >>> exploiting the error budget, limits set before throwing an >>> error in the .INI file. The way we gear steppers, in the >>> distance they move per full step, normally 1/200 of a >>> revolution, will generally translate to an error less than >>> may exist dynamically in the servo system. >> On both of my mills (a Bridgeport manual mill converted to >> analog >> velocity servos of my own design and a minimill with PWM >> servos that I sell through Pico Systems) I can get under 200 >> uInch >> error even at the maximum velocity. I don't know how that >> works out in steps in some particular stepper installation, >> but it is a pretty SMALL error. >> > Correction there Jon, damned small error. ;-) My point was that the > steppers can pretty well duplicate that unless we were making another > hubble mirror. > > And can it do it when the servo motor is exerting half its design torque > while counteracting cutting forces? I am sure that represents a pretty > heavy cut, on either of your machines, heavy enough to deflect the tool > more than that. > > Yes, of course, deflection of the machine frame, leadscrew, various attachments, etc. greatly dwarfs the servo error.
I did a little more thinking, and with a standard stepper motor, that's 200 full steps/rev (forget microstepping, it has nothing to do with positioning accuracy) and if you have a 5 TPI ballscrew, then a full step of positioning error is only .001" (200 x 5). And, if you have a finer pitch leadscrew, it is likely of small diameter, and therefore compressible, so although the finer pitch gains something, the compressibility loses more. But, of course, if the tool or workpiece deflects more than the rest of the machine, then you are doing the best you can, and no further improvement of the positioning system will give any benefit. I do notice that solid carbide cutters in the 1/8" size are a LOTTTT stiffer than HSS cutters, and thus hold dimensions better. Since I found you could get these for ~ $3 on eBay I have started using them a lot. My machine doesn't have a fast enough spindle speed to get the best from them, but they still work quite nicely. Jon ------------------------------------------------------------------------------ Flow-based real-time traffic analytics software. Cisco certified tool. Monitor traffic, SLAs, QoS, Medianet, WAAS etc. with NetFlow Analyzer Customize your own dashboards, set traffic alerts and generate reports. Network behavioral analysis & security monitoring. All-in-one tool. http://pubads.g.doubleclick.net/gampad/clk?id=126839071&iu=/4140/ostg.clktrk _______________________________________________ Emc-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
