On Thursday, June 07, 2012 05:30:43 PM Steve Blackmore did opine: > On Thu, 7 Jun 2012 09:39:12 -0400, you wrote: > >I did notice yesterday while cutting that 4mmx0.7 thread, which btw > >came out real nice, that the spindle speed you start at should not be > >changed during the run as it will widen the cut thread, leaving a > >distorted profile at the root of the thread. > > That should not happen? I thought LinuxCNC compensated for spindle speed > change by using an encoder and modifying the feed to suit? One of the > selling points over Mach I'm told ;) Oh, that part works well indeed Steve.
The problem is that it has to do the Z axis backlash comp move AFTER the index pulse goes true, when it should already have that done by the time its waiting on the nxt index pulse. What I want is for it to purposely overshoot the retrace motion by 2 or 3 thou (its out in the air anyway, or in a clearance cut as the case may be), and then, perhaps at the same time as running the x back in, run the Z back by that same amount of overshoot, thereby triggering the backlash comp move BEFORE the index pulse arrives. That would, in my little bitty mind, remove an unknown distance move and prepare the z to be locked in step with the spindle as the index pulse arrives. As is, there appears to be perhaps 30 degrees of spindle turn at 300 rpms before the Z can become locked after it has completed the backlash move, and that would be 60 degrees at 600 revs etc. Whatever, it is more than sufficient to be seen with a good glass when you run a couple repeats to about 1/2 the needed depth at 200 revs, get tired and set the spindle up to 300. The carriage moves sideways with that 100 rev difference more than enough to see it in the bottom profile of the thread being cut. In this case since I wasn't at full depth, I slowed the spindle back down as I was doing a touch off to go .1mm deeper, and the distorted thread was then repaired by the next .1mm or .2mm deeper cuts. > >I had started it at an S200 and had run it up > >to 300 with the spindle override for a couple of loops as I was > >touching off to the final size. Slowing the spindle back down to 200 > >put it back on track and it cleaned it up nicely. As for cutting, > >with that fine a thread, I could have cut it at 600+ rpms, z was not > >speed challenged. > > > >My guess is that there is a get up to speed lag in the z accel after > >seeing the index before z is actually phase locked to the spindle. I > >have always started a thread well off the end of it in air, and I'm > >thinking this phenom could be backlash related. To that end, would it > >be possible to incorporate in the retrace motion of G76, and extra 2 > >thousandths of an inch to the right of its park & wait for index, > >which is then brought back to the z starting position with a 2 thou > >move to do nothing but take up the backlash while it is waiting for > >the index. This 'turn around' move would then take up the backlash, > >hopefully preventing this spindle speed related slippage between the > >spindle and Z. If the index arrives while the backlash move is in > >progress, skip it and wait for the next index of course. > > Starting off the actual end off the stock is normal for threading and > the carriage should have all the backlash gone by the time the tool hits > the work. My point is that it cannot lock until the backlash move has been completed. That is pretty much a fixed delay, and the faster the spindle is turning, the more degrees it turns before the lock can kick in. > I always start my threads 5mm off the real start point and > never have a problem. I would think the minimum distance would have to > be considerably larger than the actual backlash amount to be certain. So do I, by .1" or thereabouts. My backlash is not more than 1/5th of that. IIRC z is about .00147" right now, phenomenally tight for a mini- lathe being moved by its original screw, its support bearings and half nut. > That said I never use G76 and only have a couple of tenths of a thou > backlash (0.005mm) on both axis on my lathe. That is tight, as quality machinery should be. This is a decade plus old mini-lathe with lots of wear in the screws and bearings. And I likely have it currently setup tight enough it has 1/4th the backlash it had new out of the carton. OOTB, this thing was a huge POS. Cheers Steve, Gene -- "There are four boxes to be used in defense of liberty: soap, ballot, jury, and ammo. Please use in that order." -Ed Howdershelt (Author) My web page: <http://coyoteden.dyndns-free.com:85/gene> Gravity brings me down. ------------------------------------------------------------------------------ Live Security Virtual Conference Exclusive live event will cover all the ways today's security and threat landscape has changed and how IT managers can respond. Discussions will include endpoint security, mobile security and the latest in malware threats. http://www.accelacomm.com/jaw/sfrnl04242012/114/50122263/ _______________________________________________ Emc-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
