On Tuesday 07 August 2018 09:40:39 andy pugh wrote: > On 7 August 2018 at 11:39, Bengt Sjölund <beng...@tecno.se> wrote: > > Looking for a g code generator for scroll jaws. > > I have a Ø125mm 3-jaw chuck with 6mm scroll pitch and would like to > > be able to mill my own soft jaws. > > This sounds interesting, and harder than you might think. > > At first I thought it was just a set of curves at various radii, and > to an extent it is, but the centre moves. > In fact the curves are not constant-radius, but are probably close > enough for practical purposes. > > Also, each jaw is different. Whether you handle this in the G-code or > with an alternate workpiece placement is another question. > > The parts are easy to model in CAD. It might actually be easier to do > it that way and then CAM. > <Google> Hmm, the parts are not trivial to model in CAD. > https://www.youtube.com/watch?v=EYBiWhxgHTo > > (You could model a scroll, then intersect it with the three jaw > sections, then CAM each seperately.) > > In parametric G-code I would suggest that the answer is to start with > the nominal mid-point of each inner and outer thread flank > (separately) > > Then calculate how far the two edges of the jaw offset from that > nominal mid-point.... > > No, wait, all this is utterly wrong! > > Either of these approaches will lead to a jaw that is a perfect fit in > exactly one position and will bind in any other position. > > The outer flanks want to be machined to be at least as small a radius > as the smallest part of the scroll that they can touch. > The inner flanks want to be the same radius as the absolute max of the > scroll radius. > Correct, determining the radii of the inner arc and the outer arc, then step the center of the 2 fixed length arcs across the inner face of the jaw. It might take a pretty small mill to plow that, and if the jaws are wide, it might take a 3rd pass to clear the center of the cut at the edge of the jaw. My guess is that not, a 125mm scroll with a 6mm step. I'd cut a phenolic test piece to determine the near side/far side diameter of the tool, then add another thou for tool flex when cutting the metal. You should be able to use a 3mm tool as the narrowest part of the groove is at the centerline of the jaw.
Put an offset of 2mm for the second jaw, and 4mm for the third. That ought to boil down to less than 50 LOC unless I'm missing something. One might need, because the scroll has a small slant, want to offset the center point of the arc in order to more accurately place the actual scroll to jaw contact point at the center of the jaw. That visualizes to a 3mm offset from the centerline of the jaw for the arcs "anchor" point, I think. > I doubt that they are typically cut with any "slope" across the face. Thats what I'm referring to above, by compensating for that slope, the jaw to scroll contact would be centered on the bottom of the jaw, resulting it a stronger "tooth" on the jaw. And all that is, is a 3mm movement of the center point of the arc sideways, trivial to do in gcode. > They can only ever half line-contact with the scroll. -- Cheers, Gene Heskett -- "There are four boxes to be used in defense of liberty: soap, ballot, jury, and ammo. Please use in that order." -Ed Howdershelt (Author) Genes Web page <http://geneslinuxbox.net:6309/gene> ------------------------------------------------------------------------------ Check out the vibrant tech community on one of the world's most engaging tech sites, Slashdot.org! http://sdm.link/slashdot _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
