On Wednesday 17 July 2019 16:51:30 John Dammeyer wrote: > There are a number of videos out there changing up the G0704 gear > driven spindle to belt drive. Some use toothed belts, others a double > set of pulleys. Most of the videos explain that they will soon but > the slotted RPM sensor disk back on but for most of them they've not > done that yet. > > For LinuxCNC what sort of encoder resolution is required on the > spindle to be able to do tapping. I know I can do it with one pulse > per rev with my Electronic Lead Screw to cut nice threads but I also > don't slow down the spindle at the end of a tap into a blind hole > while tracking spindle speed. > > My new 1.8kW AC Servo motor with a 2500 pulse per rev encoder can echo > the encoder pulses out of the drive so if I used a 1:1 toothed belt I > could use that. But if I want stepped pulleys to give me extra torque > at low speeds then I'm looking at V belts so I'll need an encoder on > the spindle. > > The spindle pulley on my G3616 type mill has a tapered cone. That > also suggests I want to stay with V belts and just mount a slotted > disk onto it rather than try to make a similar cone that holds toothed > GT3 belts. There's room. But how many slots? > > Thanks > John Dammeyer > I have a G0704 John, and have not managed to damage those plastic gears yet, but I found a 67 slot wheel replacing the original drilled tach wheel, with slot interruptors had way too much quantization noise to be used as spindle speed feedback. An amount to gain in a pid to hold the average speed close enough for rigid tapping, injected so much noise into the servo that it sounded like the spindle bearings had square balls slowly destroying their cages. The noise was actually the servo amp slamming the tooth clearances back and forth.
Some background: I took the OEM control box off, and built a power supply makeing a well filtered 126 volts dc, good for 20 amps surge. And I took the output of the spindle PID to a pwmgen module in a 5i25, and that PWM signal to one of Jons (Pico Systems)PWM-Servo amps which is heavy enough to control a PSU with that sort of muscle. The motor has a nameplate voltage of 0 at 9.7 amps for its one hp rating. So I've power enough to reverse the spindle from 2900 revs fwd to 2900 revs backwards in about 400 miliseconds, and back again. I have Jons amp set to current limit at about 17 amps and you can hear it chirp as it does this. But that homemade encoder was just too coarse, so without really figureing it out ahead of time, I bought on fleabay, an Omron encoder with a 1000 line quadrature A/B/Z output. Once it was in hand. I drilled and tapped the top of the motor shaft and planted a 5mm extension about 5/8" long and made a mount to put the encoder on top of the motor so it spins at motor rpms. Somewhere along the line I discovered the encoder was a low voltage differential output, so the A/B wires were interrupted by a pair of rs-485 interfaces, about $2 each on fleabay. They turned out to be bi-dir, so I had to jumper them for one way, and they gave me some purty 5 volt rail to rail signals to feed a 5i25's encoder inputs. The Z still comes from the old optical because it needs to mark zero degrees on the spindle. Then I put a notch in the speed knob that fitted the rollers of 2 small micro switches, and made a sandwich out of a pair of half circles cut from .030 alu sheet with the microswitches between them and arranged to see that notch, sending a pair of "I'm in gear so and so" back to the 5i25. Those face a mux4 which is used to both slow the motor to a crawl when neither switch is closed, and scale the SCALE from the ini file for feedback to the pid and video tach. I temporarily added some more hal to make me a scale reference, so the SCALE is scaled to the correct values regardless of which gear its in, and rigid tapping is correct in either gear. That gives me an effective scale of a bit over 7000/rev in high gear, and something over 14000 in low gear. Quantization noise has vanished! I won't claim its silent, but I have to either look, or listen real carefully to make sure the spindle is turning. The 2 unused inputs to the mux4 are fed about a 25 motor rpm offset voltage by a setp in the hal file, and the speed response is so instant that I can reach up and change gears while running wide open since by the time the knob has been turned 3 degrees, the motor has slowed to a crawl, the flat faced gears change silently and are completely engaged by the time the other switch closes. 200 millisecs later the motor is back to top speed and the spindle is turning at either 1500, or 3000 revs. All with zero possibility of damaging the gears. And I can run a Pgain of about 20, even as high as 40, but thats overkill, in the spindle PID. Worst effect is that until Jon's servo amp regulates, making the motors magnetics chirp, I haven't a clue how hard the motor is actually working. The spindle doesn't slow as the load of tapping a hole comes in. Makes me wish I could find a 15 horse motor as that would remove the need to peck tap the bigger holes :) If interested John, I can pm you a tarball of that whole config dir so you can borrow what you want. Theres also some alignment stuff that still needs fine tuning and which is waiting on me finding a camera that doesn't die about the time I get a good view. Teeny cameras with long lenses have seemed to disappear from the fleabay listings over the last 2 years. Darnit. Cheers John, 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
