On Monday 18 January 2021 18:47:25 John Dammeyer wrote: > > > So what does LinuxCNC do? Is the thread mucked up if spindle > > > speed is changed during a feed hold and then start? > > > > Feed hold has nothing to do with it John, you can't change the > > spindle speed in mid thread. Full stop. Period. I think it could do > > what is needed. But its like Yogi Berra once said about theory and > > practice. Which in this case means machine balistics are hard to do. > > Gene, > Are you saying that for threading on a lathe LinuxCNC _must_ have > control over the spindle speed? Or can I have a manual knob and > adjust the speed as it's threading. Or as it's returning back to the > start?
No, you do not need linuxcnc control over spindle speed, I have no software linkage to control the spindle speed on any of my machines, but the spindle speed is very tightly regulated by a PID on 2 of my machines, and by the slip angle between the vfd and the spindle motor on the other 2, but IF you change the speed mid thread, you will change the phasing between the spindle and the tool because it takes a finite time to bring the z axis into synch with the spindle speed as it starts the next stroke. The Z is at a complete stop when the index arrives, and it takes some 10's of degrees for the z speed to arrive at the correct speed, and actually become locked to the spindle. Change the spindle speed, and you change this delay to a greater effect than you would think because at a higher spindle speed, z has to reach a greater velocity, which takes more time. The position error once locked is more than 100% of the speed change, and can be almost square if doubleing the speed. But because once locked, its not a big deal if the cutting load slows the spindle 25%, its still locked at that phase angle and it will stay that way until the end of THAT cut stroke. But if YOU change the spindle speed, then YOU have changed the synch delay at the start of the next stroke and that changes the cutter position as it tracks the next stroke, cutting a wider groove with that next stroke. If you speed it up, the extra cut is the back edge of the tool because it synched later in the spindles rotation. Its the time it takes to accel to the locked state that eats your lunch. > Or are you saying that if LinuxCNC is cutting a thread keep your > fingers off the speed control? Yes, cut air at a reduced speed to check your pullout point at the left end of the thread, and when checking, remember that the cutter is advancing to the left per stroke. not only in depth of cut, its normally taking 99% of the cut off the front (left) edge of the tool, that is what you are controlling with the Q setting in a G76. Normally 29.5 degrees, the other .5 degree is to keep the top of the thread from smearing if your tool is dull. 30 degrees is perfect if the machine is, but you won't get quite as "clean" a cut because the back edge of the tool is skidding. At 29.5 for the advance angle, the right, back edge of the tools tooth is keeping things clean & smooth. 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 Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
