Hi Gene, But it's also been reported that with LinuxCNC it's possible to stop the spindle while still in Threading Mode and then turn the spindle by hand and the carriage will follow. This is of course on a lathe. Not something you'd do on a mill I think.
The question then becomes what is the minimum number of encoder edges required to do this without mucking up a thread. A 60 Tooth encoder gives 240 edges which is still 1.5 degrees. On a 1" shaft being turned to say 8 TPI with an 8 TPI leadscrew the circumference is 3.14" or 0.0087222..." per degree or 0.01308333..." per 1.5 degrees. So if the spindle stops on an edge and then is further rotated by hand manually for 1.5 degrees the carriage will lag by 0.013" or so before the system realizes it needs to move. My understanding of carbide tooling is that's enough to break the tip. But if the encoder is 10x that with 2400 lines then at 600 RPM (for example) we get an encoder edge every 42 uS or so and we've move 0.15 degrees or and 0.0013083333...". At this point probably not an issue for carbide and as long as the carriage can track that it's not a problem. And as long as the system can respond to that encoder change fast enough. See the issue? John > -----Original Message----- > From: Gene Heskett [mailto:ghesk...@shentel.net] > Sent: February-05-21 1:16 AM > To: emc-users@lists.sourceforge.net > Subject: Re: [Emc-users] Spindle speed changes with threading. > > On Friday 05 February 2021 02:57:38 John Dammeyer wrote: > > > Granted this subject is a bit old I've now had some time to dive back > > into the TI F2837xD which has dual processors and other features that > > will make it a good test bed for trying out stuff. > > > > It has a hardware Quadrature Encoder Interface (QEI) so theoretically > > I should be able to grab encoder counts in any resolution and > > calculate spindle position relative to Z axis position and create > > moves to track. > > > > I'll report back when I have some more information. > > John > > > As has been told to me, its the time delay between starting the z on the > passing index, until z has reached the synchronous speed and is then > locked at whatever phase exists when sync speed has been achieved, This > is done anew for every stroke of a G76 or a G33.1. Changing the spindle > speed after such a cut has started, changes this delay, therefore the z > position during the instant stroke as this synching is done fresh for > each stroke of a G76 or a pecked G33.1. > > The effect is that of changing the lateral position of the thread, with > of course a bad thread being the result. The raised speed slides the > thread to the left since it increases the delay because it has to reach > a higher speed to get synced which takes longer. So the only way to > reduce this effect is to lower the MAX_VEL and increase the MAX_ACCEL, > and in some cases slow the spindle. This was the case with the original > 1600 oz/in z motor on my G0704, which could only do 29 ipm. A 940 oz/in > motor can do 90 some, which helps some. As long as z can keep up, this > delay won't change by more than an edge spacing from the encoder. Change > the spindle speed and you change the locked z to spindle phase. > > I think some of my sloppy rigid threading on the G0704, is not at the top > of the stroke, but at the bottom turnaround and back out, due to the > spindle reversal being too quick for z to follow. Since that turnaround > is a programmed turnaround in my hal file, I should try slowing the > decel/accel as the z can't keep up. But I keep forgetting to run that > experiment.:( > > As its currently set, it can do a 3000 rpm turnaround in about 350 > milliseconds. Lots quicker at the 300-500 revs I normally tap at. But if > I slow that turnaround down, that will increase the overshoot at the > bottom, Dangerous to the tap in a blind hole. > > And that could cause a change in the phase lock while backing out, making > the tap cut sloppy on the up, backout stroke. > > Stay well and safe John. > > 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 _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
