On Wednesday 15 January 2020 22:59:20 Jon Elson wrote: > On 01/15/2020 09:10 PM, Brent Loschen wrote: > > 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. > > OH! I'm not sure you can do rigid tapping by plug-reversing > the motor on mains power. At our shop, where the mills run > off 480 V mains, the plug reversing is so fast you can't > even hear it. I doubt any Z axis could keep up. But, don't > worry about > the motor, it can handle this "abuse". I do rigid tapping > using a VFD and analog spindle speed control. Even with that, > I generally reverse the spindle in about 3/4 second, from > 1000 RPM forward to 1000 RPM reverse. That is slow enough > for the Z axis to follow. > > Jon > Whereas my G0704 is so fast I had to insert a limit3 to shape the turnaround so z could follow. The slowdown is not noticeable and the reversal is still around 1/2 second at the speeds I use. But I have to be carefull with blind holes, subtracting the overshoot else the tab bottoms and breaks. And I've not yet incorporated the gear shift effects into that shaping, needed because of the motor armatures increased effective mass due to the armatures 2x increase in speed in low gear.
A work in progress IOW. I have some hal code that measures this overshoot and displays it in the pyvcp area, both of the turns and the actual distance traveled, in the gui in the Sheldon's config, but haven't incorporated it into the G0704 yet. Need to get that done, and a tool holder made for the lathe to carry a top-hatted tap. Slowed progress because of the heart attack, but feeling perkier now. Currently working on the base shoe to install the BXA on the Sheldon, its a blond one bigger than the piston post I'll toss. The base shoe is supplied way oversized so it will fit any compound and I need to make it fit the spacer where the compound used to live. LinuxCNC is its own compound. Let me describe how the reversal works here in 3 machines. TLM can do it too. 1st, the direction change coming from motion, going to the motor controller is blocked by a sample-hold while the limit3 is shaping the motor slowdown, decaying to an effectively stopped spindle, which is detected by a retriggerable timer timing out which is fed from encoder A's pulse. That timer timing out then refreshes the sample-hold which allows the direction signal thru to the motor controller (vfd for the Sheldon), and releases the limit3 (which has a mux2 to select zero or set speed as input to the limit3) to charge back up to the set speed. With that all in front of the spindle PID, and Pgain at 20+ in the G0704, and no PID in the Sheldon, letting the vfd do it all, the G0704 is a touch more complex. >From that description, you should be able to achieve a reversal that Z can follow. It all happens so fast that you aren't aware its being dynamically controlled. On the Sheldon, belts squawled until I put new linkbelts in it, on the G0704 I can hear faintly, the current limiter in the pwm-servo amp I am driving its nominally 1 horse motor to about 2x its FLA draw, getting around 2 hp from it for those few milliseconds I can hear the limiter chirp. Been doing the chirp for about 4 years now. 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
