Hello Viesturs, This is a long shot - but some stepper drivers are very sensitive to active high vs active low on the step signal. Noticed this with two makes of stepper driver so far.
One way round the drives would slowly and randomly lose steps (first noticed on the Z axis), when the step output signal was inverted the problem vanished. Probably not that, but it's a fairly simple thing to test if it is. Good Luck Ben -----Original Message----- From: Viesturs Lācis [mailto:viesturs.la...@gmail.com] Sent: 11 December 2015 20:19 To: Enhanced Machine Controller (EMC) Subject: [Emc-users] Z axis stepper gradually losing position 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
