On Wednesday 17 January 2018 08:06:22 Gene Heskett wrote: > On Wednesday 17 January 2018 02:42:51 Marcus Bowman wrote: > > On 16 Jan 2018, at 23:16, tom-...@bgp.nu wrote: > > > We are tying to cut some internal Acme threads on our lathe. We > > > have an internal Acme 8-pitch single point tool. From the edge of > > > the backside of tool to the tip of the cutting point is 0.490”. > > > The major diameter of our hole to thread is 0.506. So, you can > > > see there is very little clearance here. > > > > > > When we run G76 the backside of the tool slams into part when it > > > retracts after the cut. We are currently cutting (breaking is a > > > better word) wax until we are confident that something good will > > > result. > > > > > > This causes our tool to crash into the part when it tried to > > > retract after the cut: G0 Z0.100 > > > G0 X0.248 > > > G76 P0.125 Z-0.750 I0.005 J0.005 K0.0725 R2.0 Q14 L0 E0.0725 H2 > > > > > > In trying to measure things in the backplot window (not easy to do > > > accurately) it appears like the tool is moving back nearly the > > > full thread depth (0.075”). In this image the distance from where > > > the tip of the tool is on the first retract line up to the first > > > cut line which is the lower edge of that white band is about > > > .075”: https://www.bgp.nu/~tom/pub/IMG_5289.jpg. Considering we > > > only have ~0.010 this clearly wont work. > > > > I have no direct experience of using G76, but this problem reminded > > me a little of the retraction distance settings on peck drilling > > canned cycles in a milling machine, where the program response to a > > retract distance in, say, the G83 command depends on whether there > > has been a previous G99 (retract to the R value used in the G83) or > > a G98 (ignore the R and retract to the original Z value used before > > the G83 was begun. > > > > > It seems like the Drive Line is where the tool should come back to > > > on every pass. If it did it would always have clearance (assuming > > > it had clearance to get in the hole in the first place). But that > > > is not what is happening. From the GCode reference of G76: > > > http://linuxcnc.org/docs/html/gcode/g-code.html#gcode:g76 > > > > The documentation does say the tool should return to the drive line, > > but I don't immediately understand the diagram, which shows a > > different retract after each pass (the same your your own screen > > shot shows). This is either being done as a result of a previous > > command like a G99 or G98, or the G76 is not doing what it is > > supposed to do. I note the connection between G76 and G33, but Gene > > knows more about G33. > > > > > This is youtube video showing the tool hitting the back side on > > > retraction, twice in fact before the wax snaps off: > > > http://www.youtube.com/watch?v=yxHFyVMocpU > > > <http://www.youtube.com/watch?v=yxHFyVMocpU> > > > > Wax is a good idea. Must get some. I sometimes use plastic, but the > > wax looks even softer. > > > > > In typical threading the tool size, thread depth, and initial bore > > > diameter, are such that this problem may not be noticeable. > > > > Often the case, with this kind of problem. > > > > > But with an Acme thread where the size of the tool and the depth > > > of the thread (0.0725 in our case) leaves very little room for > > > error. So is the G76 code broken or is there something fundamental > > > we are misunderstanding? > > > > This is an important cycle, so it would be good to understand what's > > happening here. > > > > Marcus > > The more important point is to get the job done for Tom. But I > also for some reason had a fixation on the boring head. Put it back in > the box, its not needed. > > Does this enco have enough -x room to cut the thread with the tool > mounted as shown in the youtube video? > > If so, this can be done with the g33.1 if the enco can reverse its > spindle fast enough. > > Write a loop, executing the g33.1, but where one might adjust the z > for use with a tap, use an increasing - x. > > Something like this, after touching off so xz=zero with z at least a > turn out of the hole, and x centered in the hole. > > g7 g20 > S100m3 > g0z0 > g0x0 > #<_bottom_of_hole> = [-0.8750000 + enco's turnaround distance at 100 > rpms] #<_tmp_x> = 0.0000000 > > o100 WHILE [#<_tmp_x> gt -0.075000000] > > g33.1 z#<_bottom_of_hole> K0.12500000 > #<_tmp_x> = [#<_tmp_x> + -0.002500000] > g1 f20 x#<_tmp_x> > > o100 ENDWHILE > > (now do spring passes) > #<_tmp_h> = 3 > > o200 WHILE[#<_tmp_h> ge 0.0000000] > > g33.1 z#<_bottom_of_hole> K0.12500000 > #<_tmp_h> = [#<_tmp_h> -1.000000000] > > o200 ENDWHILE > m5 > m2 > > Obviously replace #<_bottom of hole> with a legal -z value if the hole > is not a thru hole. > And I just realized that it also needs a small z starting offset as the x is advanced, to duplicate the q function of a g76. The advance angle is 15 degrees so the advance is much less than the 29-30 factor used with a g76, the idea is the trailing edge does not cut, only the leading face and flat tip should cut. Another var and some trig can handle that. > Because you do have a wee bit of clearance, you will likely have to > adjust the value on the right of the gt in the first WHILE to get the > correct thread depth. Cutting wax sounds like a heck of a good idea. > > To determine the overshoot, put this in your hal file. > Obviously add the missing loadrt's and addf's to make it work. This is > straight copy paste from my sheldon, which may have to reverse a 40 lb > chuck! > > ========================= > # calculate spindle reversal overshoot distances. > setp sum2.ovrtrvl.gain0 -1.000000 # make sub2 out of sum2 > # feed accumulated counts to both sample-holds > net ovrtrvl1 <= hm2_[HOSTMOT2](BOARD).0.encoder.00.count > # Feed both sample-holds > net ovrtrvl1 => sample-dirchg.in sample-spndlchg.in > net spindle-is-reversed <= abs_encdir.is-negative => > sample-spndlchg.hold # that takes care of measuring reversal distance, > HOWEVER, it also measures the other end # of the travel, so the > halmeter jumps to a higher value at the end of the retract move as it > # reverses to fwd again. SO KEEP THAT IN MIND! Read it while the > retract move is running net ovrtrvl2 <= sample-dirchg.out => > s32_float_cmd.in > net ovrtrvl3 <= s32_float_cmd.out => sum2.ovrtrvl.in0 > net ovrtrvl4 <= sample-spndlchg.out => s32_float_spndl.in > net ovrtrvl5 <= s32_float_spndl.out => sum2.ovrtrvl.in1 > # sample-ovrtrvl.out can be watched with a halmeter, is in encoder > counts ======================== > Like the shoemakers kids, someday I'll display this in the gui... > Someday. :) > > Cheers, Gene Heskett
Cheers, Gene Heskett The above content, added by Maurice E. Heskett, is Copyright 2018 by Maurice E. 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
