Just chipping in... So on your rotary axis, you have 60 000 steps per revolution. If you set up a dial gauge and do a simple G1 with 600 000 steps, or 10 rev's, is there much residual error?
Otherwise, if the accuracy is good enough for one rev, then, in fake code; - home rotary - cut one tooth with +3 deg, - 3deg (or whatever one tooth requires) - home rotary axis - move to 6 degrees - cut one tooth with +3 deg, - 3deg - start again, for next tooth So now each tooth should be cut within the maximum error of a single rotation. Regards Roland On 3 January 2016 at 08:02, Cecil Thomas <wctho...@chartertn.net> wrote: > My question is about what happens to the "leftovers" when the > precision of the g code commanded position cannot be met by the > hardware executing it. > Several years ago I wrote a program to "generate" involute gear teeth > by making multiple cuts of the same tooth from differing angles with > a rack shaped cutter. This eliminates the need for the different > cutters when making only one cut per tooth. I have used it many > times to cut relatively large gears with a relatively small number of > teeth with virtually no noticeable error. > > A few days ago a friend who repairs watches wanted to know if I could > figure out what gear (wheel to you watch guys) size, pitch or module > and number of teeth would be required to replace a missing one. (the > original was long gone). I had no problem working from the center > distance and the matching pinion coming up with the appropriate design. > > However, when I cut the gear I had the right number of teeth but the > last tooth was much too wide. > > It would appear that I had lost a bunch of steps on the rotary > axis. Further investigation reveals what I think is the root cause > but I would like someone with more knowledge than me to confirm or > disprove my analysis. > > The gear had 86 teeth (in the power train, not in the timing train) > and I made 9 cuts per tooth. That is 774 commands and about all but > 86 of them in the same direction. > > My rotary axis is a 200 step stepper into a 30 to 1 worm drive > microstepped by 10 so 1.8 degrees divided by 300 equals .006 degrees > per microstep or 166.6667 steps per degree. > > Unfortunately when the g code calls for a 1 degree move the motion > planner can only issue 166 steps since it can't issue .6667 > steps. That means that the actual movement of the A axis is only > 166/166.66667 or .996 degree. That is .004 degree lost as far as I > can tell. That might be close enough for one or even several > commands but after 688 comands in the same direction that constitutes > 688 x .004 or 2.7 degrees lost. > > That is a significant portion of a tooth on a high tooth number > wheel. Depending on the actual value of the command the actual lost > motion could be anything from nothing to essentially a whole step or > .0059999 degrees. > > I think that I can lessen the impact of the lost portion of the steps > by using the MOD operator to determine how much is left over after > dividing the commanded move by .006. Then use IF ELSE, IF the > remainder is Greater Than .5 steps then ADD a full step (command = > command PLUS .006 degrees) ELSE issue the commanded number (do nothing). > > This should statistically reduce the error by rounding up or down and > redistribute it randomly among all the cuts although it will not > eliminate it. The greater the number of cuts the better the > approximation will be. > > Sorry for the long post but I couldn't condense it much and get the > idea across. Can anyone confirm or disprove my observation or come > up with a better solution? Obviously I could add another reduction > stage to my rotary axis but I would like to avoid that if possible. > > Cecil > > > > ------------------------------------------------------------------------------ > _______________________________________________ > 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
