This suggestion is obviously a hack, but might be worth trying. Try pre-loading your carriage in a direction opposite your cutting direction in an attempt to remove the backlash. If you have a small lathe with a light carriage and your ways are well lubed; your carriage might be "bouncing or wandering" in your half nut. Try something like a bunge cord or extension spring to pull the carriage back towards the tailstock and see if that makes a difference.
A cable running over a pulley to a weight running up and down vertically would do the same thing. That may allow you to run without any backlash compensation. Dave On 6/8/2012 8:30 AM, gene heskett wrote: > On Friday, June 08, 2012 07:29:59 AM John Thornton did opine: > > >> I've cut the same threads at rpm's from 100 to 2000 (by accident) and >> the pitch is perfect. I've seen a video where a guy was turning his >> spindle by hand back and forth and EMC was keeping perfect pitch, iirc >> the guy was positioning the stock for a recut of the threads. >> >> John >> > > I can well imagine that, in a setup with 2 start ball screws and next to > zero backlash. But I am running on the OEM screw& half nut. Backlash is > un-avoidable unless I take it all out and cut air while its being taken up, > its out in the air at that point anyway. Maybe that is what I should be > doing? With no comp move, probably no problem. Hummmm. > > Backlash comp moves in the 15 thou range are required, and that takes time. > If the backlash comp move could be done while waiting for the index, it > would seem to be advantageous in this case. But it is not, the backlash > move is done after the index arrives. The synchronized move cannot be > established until the backlash has been taken up. And the spindle doesn't > stop& wait, not with this control setup which has no braking dump > facilities. > > And that is what I'm asking the G76 cycle to do, to issue a .001" move left > while advancing X to the next cut depth, so that the backlash move is > complete by the time it pauses for the index pulse arrival. Heck, rather > than moving Z's movement range according to the angle given G76 as an > argument as X deepens the cut, retrace to a fixed point for both X and Z, > then move X to the surface, then move both X and Z to trace that advancing > into the thread angle (nominally 30 degrees) would take care of the > backlash move while waiting for the index. > > Because my Z, a 16 tpi acme screw, is driven by a 2/1 geardown, I doubt I > could get to 2000 revs as that would result in some pretty wild z motor > speeds. My reliable Z move max is about 15 ipm. Its an 8 wire motor, in > series at about 2.5 amps. More speed would need to wire it parallel and > set that driver at max, 4.2 amps. And find a bigger transformer. With the > X axis being run in parallel and wide open, that tranny warms up nicely as > it is. > > I did order an all steel QC toolpost kit yesterday, with an extra cutoff > tool holder so I can preset the bit heights properly without having to > spend a lot of time putzing with that when I turn the cutoff blade around > to use the threading tooth on the other end. This one has proper wedges& > should be several times more rigid than the all alu rig I have been using. > > That thing, even with its bottom hollow ground, wasn't very stiff, leaning > over into the work and killing the spindle and fuse, not to mention a huge > gouge in the work, entirely too frequently. Hopefully that will be nearly > $200 well spent. > > >> On 6/7/2012 8:39 AM, gene heskett wrote: >> >>> Hi Guys; >>> >>> 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. 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. >>> >>> Am I making any sense here? >>> >>> Cheers, Gene >>> > Cheers, Gene > ------------------------------------------------------------------------------ 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
