You could set the parameters of the G76 to cause it to do one pass. That would allow you to change the parameters for every pass until you reach your desired depth.
On Jan 17, 2018 1:43 AM, "Marcus Bowman" < marcus.bow...@visible.eclipse.co.uk> 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 . > -Tom > ------------------------------------------------------------ ------------------ > 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 ------------------------------------------------------------ ------------------ 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 ------------------------------------------------------------------------------ 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
