I'm not sure why it was added, but this restriction has some nasty side effects if you're doing rotary axis work. The pathological case is a G1/G2/G3 move with very short but non-zero motion in XYZ, and significant ABC axis motion. The minimum feed rate value applied is 0.1 units per min. Mixed XYZABC moves compute feed rates just based on the XYZ length. An example (starting from all axis zeros), assuming 2" diameter stock and a desired effective feed rate of ~10 IPM:
G93 G1 Z0.001 A10 F86.2 The XYZ distance is 0.001", and the computed feed rate would be 0.001 * 1/86.6 IPM or ~0.057 IPM. If this is forced to be 0.1 IPM, then the effective feed rate is about twice as high as it should be. You can see the effect get worse if the ABC motion is longer or the XYZ motion is shorter. This is a bit of a corner case, but it's nasty because an offending line is difficult to spot by eye in a complex program, and I doubt there's a way to prevent CAM from producing them (since the commanded motion is technically correct G-code). The good news is, I've got a fix that removes this restriction, and instead checks if the resulting feed rate is above the minimum that the rest of the system can handle (rebased onto 2.8): https://github.com/robEllenberg/linuxcnc-mirror/tree/hotfix/g93-minimum-feed-rate -Rob On Sun, Jan 10, 2021 at 7:02 PM andy pugh <[email protected]> wrote: > On Sun, 10 Jan 2021 at 23:59, andy pugh <[email protected]> wrote: > > > Does anyone have any background on the reasoning behind this? > > It's been that way since 2005.... > > -- > atp > "A motorcycle is a bicycle with a pandemonium attachment and is > designed for the especial use of mechanical geniuses, daredevils and > lunatics." > — George Fitch, Atlanta Constitution Newspaper, 1912 > > > _______________________________________________ > Emc-developers mailing list > [email protected] > https://lists.sourceforge.net/lists/listinfo/emc-developers > _______________________________________________ Emc-developers mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-developers
