On Tuesday 16 January 2018 18:16:04 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. > > 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 > > 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> > > In typical threading the tool size, thread depth, and initial bore > diameter, are such that this problem may not be noticeable. 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? > > -Tom
I am inclined to agree in that g76 is broken in this regard. My feeling is that it should retract to the drive_line, and no further. I have in most cases managed a work around with the tools I have by jumping in 2 directions, first being from USS threads to SAE, which are marginally less deep, and then to the next bigger bolt if there is room for it. But from previous discussions, it appears your bolt size is fixed. Given that, I only see one workable solution, which is to make, or buy if it can be found, a tap, and use the g33.1, rigid tapping. This requires a keyed to the spindle mounting for the tap, and adequate torque to turn the tap. I have neither but I have gotten away with it by writing a peck tap wrapper, advancing the end point only a small fraction of a turn per pass ever deeper into the hole. When the tap is 8mm and above, its very difficult to get a std er32 collet tight enough that the shank won't slip, and I am talking about wrenches 18" long. Ditto the drawbolt of an R-8 collet, with a 3/4" bore to hold a TTS toolholder. The TTS, installed in an R8 collet, doesn't offer enough traction unless its that proverbial 1/8 turn from broke. I'd call Nook and ask them where they buy the taps for making their nuts. I've made carefull note of how proud they are when the asking for a 1/2" 10 tpi bronze is right at $50/copy the last time I bought 2 of them. Its possible they make their own taps. Humm, useing the tool you have in a small boring head, driven by g33.1 might be workable as that would then back out at the same diameter it went in. Put a spindle shut down to adjust the boring head, and a click to run the next cycle, to the same depth, but a thou or 3 bigger each pass. A boring head like Andy's would be ideal as its self incrementing for diameter. Good luck Tom. 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
