On Wednesday 15 January 2020 22:10:37 Brent Loschen wrote: > On 1/15/2020 3:22 PM, Gene Heskett wrote: > > I should probably interject here that with a good high-res encoder, > > it won't care about the wobbly speed mechanics as long as it keeps > > on turning under load. The z axis rigid tapping motion is 100% > > controlled by the spindle position as it turns. Since the Z is > > typically the slowest axis, you will have maximum spindle speeds to > > deal with because Z can't keep up. My G0704 can reverse directions > > much faster than Z, so I have some hal trickery in the form of a > > limit3 to shape the reversal decel/accel's into something resembling > > what the tv folks call a "sine squared" signal, where the peak of > > the delta in speed matches the zero speed and the z motor _can_ keep > > up because it has maximum torque at zero speed. This "shaping" of > > the reversal of the spindle has almost no effect on the actual > > reversal times. > > > > Hi rez in the encoder allows much higher Pgain in the spindle motors > > PID, and with the speed being helped by the air drive if its wired > > up to the high.on.low signals and fast enough in reaction times, > > rigid tapping should not be a problem. Haveing some experience with > > split sheeves needing several revs to adjust, I can see that being a > > problem, but it would be an educational experiment to see how big a > > problem. > > In addition to the split pulleys, my "mechanical" concerns are more > along the lines of the heavy duty electrical contactors clunking every > time the fixed speed, 3 phase AC spindle motor is called to switch > back and forth; increased heat in the motor windings due to those > reversals; and the air powered, mechanical spindle brake snapping on > and off hundreds of times. Yes it was very tempting, and yes, I'm > sure some of my concerns could be mitigated, however I feel that this > old mill (cira 1985) just isn't the right tool for the job. > This is where all those clunking contacts get replaced with a vfd of suitable size. I have a cnc'd 11x56 Sheldon lathe that was in pretty bad shape when I paid way more than it was worth, but it was a Sheldon. It came with a 3/4 horse 110 volt single phase motor. I went touring the local recycle places looking for a stronger 3 phase, finding an air compressor recently removed from the local horsepistol during a remodel.
Probably 50 years old. One big tank, two compressors with 1hp 3 phase 220 volt motors, 50 bucks got all 8 bolts blown off. Ebay supplied a 1.5 hp vfd. One motor ran quietly the other had pretty rusty bearings, so I ordered up $28 worth of replacement bearings and rebuilt the noisy one, the 2nd set went in a drawer. Its in that lathe now, & I have done some rigid tapping with it. With the original v-belts spinning an 8" 4 jaw, reversal is silent except for the screaming v belts, like Michelin tires on hot blacktop. The vfd gives full 4 quadrant control useing dynamic dc braking to stop, and current controls so as not to overstress the motor by setting the motors max amperage at low speeds to the FLA on the motors nameplate. I was curious a year or so ago so I wrote a program that reversed the motor, with the spindle at 400 rpm, every 5 seconds, and let it run for an hour. At the end of that hour it was still comfortable to lay a hand on it. That motor will run like that for another 100 years if the bearings last. The motors windings are not being stressed anything like they would be with clanking relays doing all that. The vfd won't last as most of those come with a note that they will need the power capacitors replaced at 5 year or so intervals. As that sort of thing is my cup of tea, I am a CET, thats less of a problem than living long enough to need to do it since I am now 85 with a parts list thats beginning to look like the 6 million dollar man. Turn around overshoot I have measured and I do measure it live, its displayed in the linuxcnc gui, both in revolutions and in overtravel distance the tap travels because of the overtravel of that big chuck at 400 revs is about 5 turns so if tapping a blind hole, I have to subtract that travel from the depth I code to tap. So generally I tap at slower speeds to reduce that overshoot. At 200 revs, the complete turnaround time is 1 fat second for a nearly 40 lb chuck. Tapping is absolutely not a problem. My spindle encoder is watching the 60 tooth bull gear, so the tap drive that determines where the carriage should be gets a fresh reading of where the spindle is in its rotation every 1.5 degrees of that rotation. And while I could use the encoder to help govern the speeds, I don't, the vfd does a more than adequate job without it. And the computer running the show? Is a raspberry pi 4b. 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 Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
