On Friday 28 February 2014 01:04:55 Jon Elson did opine: > 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. > Very very true, and is the reason we do 2 to 4 finish passes at .001" or less increments.
> 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 This latter is a common lament from all of us table toppers I imagine. I am amazed that they put in something that can only turn 2500 revs, then we put motors on it that are fully capable of moving fast enough to stall said motor even when its in low gear, and that of coarse leaves you with a pair of carbide pieces worth 10 to 20 bucks, to be dropped in the recycled carbide container. That was why I was asking a few weeks back about those 12k rpm R8 spindles on fleabay. You folks shot me down saying the run out was probably well over 3 or 4 thou. These little table toppers really ought to have spindles capable of marching that 1/8" 2 flute carbide bit, plowing a slot .06125" deep in an alu workpiece (with mist lube & prayers that its hard alu so it cuts clean & doesn't gum fill the bit=instant quiet little "tink" and some of the bit gets lost in the swarf on the floor) at 15 ipm because it has the huevos to do 10k rpms at 15 ipm. But it is what it is. None of these things have enough spindle speed or power to actually make the depth of cut per rev they are designed to do, perhaps 10%, so if we don't break them, we wear them out cutting slivers we usually re-cut several times before they are thrown out of the way. Dig cutting is almost impossible under those conditions. So we climb and hope the backlash doesn't eat our lunch. Sigh. Cheers, Gene -- "There are four boxes to be used in defense of liberty: soap, ballot, jury, and ammo. Please use in that order." -Ed Howdershelt (Author) Genes Web page <http://geneslinuxbox.net:6309/gene> NOTICE: Will pay 100 USD for an HP-4815A defective but complete probe assembly. ------------------------------------------------------------------------------ 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
