Have you tried lowering the acceleration rates in LinuxCNC. Sounds like you are losing pulses probably in both directions only sometimes and it take a long time for the errors to show up as an offset. Did they have issues like this with Mach3 ? Was the machine a lot slower with Mach3?
Dave On 12/11/2015 3:19 PM, Viesturs Lācis wrote: > Hello! > > This is more like a hope that somebody (especially any fellow > jewelers) might share their experience, where to look for source of > problem. Please feel free to suggest whatever possible ideas that I > already have not tried out. I will try to provide as detailed > description as I can (sorry, if it ends up too long and boring to read > all the way through). > > The machine is relatively small bench-top mill, used by jewelry maker > to mill wax models to be used in lost-wax casting process. That is a > 4-axis machine with rotary head (I do not have particular data of what > motors and stepper drivers are used there). Original controls come > with mach3. I only tested with my replacement control pc with Mesa > 5i25 and LinuxCNC 2.6.8 > > The problem is this: when milling out a ring, the tool passes back and > forth on X axis (parallel to center-line of rotary axis) and changes Z > height during those passes to create the profile of the ring. Between > the passes there are small indexing moves of A axis to make for 0,02 > mm stepover from previous pass. The ring is about 23 mm diameter, so > it takes something around 6000+ passes to complete whole circle. The > problem is that the Z position in the last pass is approximately 0,5 > mm lower than in the first. > Here is a picture that shows the issue (as they say something about > one picture being better than 10^3 words): > http://picpaste.com/IMG_8289-lyaSJDQI.JPG > > Things that have been tried: > 1) looked at the code and Z heights match (at least to +/- 0,01 mm, > which could not be observed to human eye), so it is not direct issue > with the code; > 2) checked for any hardware issue: loose clutch on motor shaft, any > slop in linear bearings; > 3) swapped the stepper drivers and motors > 4) I reorganized the wires on the machine to move any motor power > wires as far from incoming step/dir signal lines as possible > > The end result from these actions - no improvement at all. Client says > there are total at least 4 attempts of milling the ring, no > improvement has been observed. > > There are 2 things that have affected the result that do not make any > sense to me and I would also appreciate any hints for these: > 1) we made "test code" - the same 3d model was used, but code was > generated with 0,2 mm stepover (instead of 0,02) to make for shorter > run time - the result: _no problem_ at all; IMHO this excludes any > effect from actual cutting forces as there was 10x more material to > remove; I would have expected 10x smaller error (still easily > observable, compared to overall surface finish roughness), but it was > all equally smooth; > > 2) the same original code was executed on a smaller workpiece (tool > was touched off closer to centerline of rotary axis, actual diameter > reduced from ~23 mm to ~14 mm); the position drift was smaller (client > says 0,3 mm instead of 0,5 mm). IMHO reducing the actual diameter > decreases actual stepover distance and distorts overall proportions of > the part produced, but it should have _zero_ effect on Z axis > positioning precision. > > Thanks in advance for any [almost] meaningful ideas! > > Viesturs > > ------------------------------------------------------------------------------ > _______________________________________________ > 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
