On Tuesday 17 December 2019 12:44:47 Dave Matthews wrote: > On Tue, Dec 17, 2019 at 9:22 AM Peter C. Wallace <[email protected]> wrote: > > You can get really bad base jitter if you try to run the base thread > > faster than the machine is capable of responding (the latency-tests > > default 40 KHz base thread is too fast for some systems so you might > > get considerably better reported latency at 20 KHz for example) > > > > > > Peter Wallace > > Mesa Electronics > > For those of us in the cheap seats how does the base thread frequency > relate to the maximum number of words per second sent to the parallel > port? I figure 20k words / second would work out to 375 inches / > minute of travel on my homebuilt CNC router. Far faster than I would > need and thus in the range of not needing to worry about it all that > much. > > Dave > There is also the effect of variable latency to concider, and its very very important with software stepping. Imagine when moving that near your calculated speed limit, and something disturbs the timeing of the steps.
The motor effectively stops, then the high speed signals resume. But they resume at the current speed without the acceleration rampup in speed that accompany's the speed change. Unless you have motors are wound on air cores, and the copper in the windings is also zero weight, a massless rotor in effect, the motor is not capable of resumeing that speed so it locks in place until the step rate received is low enough to allow it to restart. But because it will restart at any of it 200 positions, lcnc has lost the reference point to the axis only and your part is wrecked. This is your REAL speed limit, and because the table has varying drag, increaseing as your approach ends of travel, your need additional head room. The end result can often be 10% or less of what your calculations show. Generally speaking that is still faster than the spindle horsepower provides for, and as those limits apply only to the rapid moves, and will probably be cutting air. And its a lesson that cost me some broken tools and wrecked parts to learn. And since every system involving springs has a resonant frequency that is often well below this speed limit, the motors magnetic bounce will cause the stalling to manifest itself at even lower speeds, so torsional "dampers" will often result in much hugher speeds as they dampen the resonance, in one case here a 435 oz motor as the vertical (Z) motor on a baby hf mill, went from an 8 ipm speed limit, to 34 ipm. That axis was not a ball screw, but a double nutted NOOK acme that I could adjust for about .0005" of backlash in a rotating nut design. You can see pix of these dampers on my web page. Those are alternating big steel fender washers with powdered (talcum) rubber disks cut out of 1/8 thick rubber but the center holes in the rubber are smaller than the spools hub, so the center of the washer when installed is few thou fatter. So the steel to rubber slips and the slippage is the resonance absorber. There are of coarse other designs. Choose your poison. Cheers, Gene Heskett -- "There are four boxes to be used in defense of liberty: soap, ballot, jury, and ammo. Please use in that order." -Ed Howdershelt (Author) If we desire respect for the law, we must first make the law respectable. - Louis D. Brandeis Genes Web page <http://geneslinuxbox.net:6309/gene> _______________________________________________ Emc-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
