Brian wrote: > Jon, > > If I had encoders on the screws, the method described on the wiki > would work. This is basically what Dave suggested a few replies back, > and what my config already does to some degree. This method will also > work with steppers, until the motors are disabled, and you move the > machine by hand. Then the wiki method no longer holds water. Trust > me, I have tried lots of different things to come up with a usable > solution without modifying anything. I have thought this problem up > and down, to no avail. > Well, I simply don't understand why this doesn't work. If your only position feedback is from the linear encoder, and you have significant backlash, and you set the system up as a stepper-servo, then you would likely have servo hunting behavior, with the motor running to the end of the backlash until it bumps the table, the PID senses the move and then commands it to run to the other end and bump it back. The only solution to this is to set the deadband equal to the backlash, which is probably not desirable. > With respect to backlash being a significant hindrance to my CNC > system, it is a problem, but with a full screw map, it really isn't a > huge deal. Well, I (and many other CNC users and machine tool builders) believe it IS a big problem, and central to any accurate machining. If the motors are not able to control axis position, then all else is hopeless. The problem is the cutting forces can move the table over the length of the backlash, suddenly and without warning. It takes time for the motor to zip to the other end of the backlash and try to compensate. > It is plenty accurate surprisingly so as a matter of fact. > Because I still use the machine manually, I am not planning on > changing over to ball screws. Moreover, the fact that I use a machine > that has acme screws with lash is not a good reason not to allow EMC2 > to correctly utilize the data available to it. > > Why is there so much resistance to this? I must not be effectively > explaining the situation. Yes, I think this must be so. I think I understand how this is supposed to work in EMC2. Although I don't use the screw error compensation, I think I know how this is designed to work, and that it does work on other machines where it is used. I most certainly switch my machine to E-stop and move manually, and can pick up where I left off without any problems, just by hitting F1-F2. Now, the screw mapping with backlash may not work at all for manual moves. I believe the code uses the COMMANDED change in direction to decide which value to use from the table, so it clearly will not compensate correctly for manual moves.
The "resistance" you sense is historical. This battle has been fought on rec.crafts.metalworking for some 15 years now, and on various EMC-related lists for a decade. Those who have fought the battle know the answer, and are tired of the fight. I'm one of those who believe that backlash is a terrible problem, and makes everything else SO much harder. I do practically ALL machining in the climb-milling direction, and this alone would be a disaster if I had a lot of backlash. If you can't change to ballscrews (some problems with back-driving the handles as well as the COST!) then it is often possible to adjust the nuts to reduce backlash. .001" is quite achievable, and will pretty much solve all these problems. Jon ------------------------------------------------------------------------------ This SF.net email is sponsored by Make an app they can't live without Enter the BlackBerry Developer Challenge http://p.sf.net/sfu/RIM-dev2dev _______________________________________________ Emc-developers mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-developers
