On Wednesday 31 January 2018 22:17:59 Jon Elson wrote: > On 01/31/2018 10:18 AM, Kenneth Lerman wrote: > > Jon, > > > > I have one question I have that I haven't seen a good answer to. > > > > Why is there a need to do a second touch? Can't you just back off at > > low speed and see when the probe is no longer touching? > > As far as I know, the G38.2 function in LinuxCNC does not > permit this. Also, backing off would reverse the backlash. > I guess if all probe moves only registered on the > backing-off move, then that would be consistent. > > > I didn't see a significant difference when I did some tests. > > Everyone seems to do it the way you're doing it, but I'd like to > > understand if there is any evidence that way is better. > > I KNOW, by measurement, that MY PROBE has a significant > delay in reporting the trip. Renishaw-style probes may have > much more immediate response. But, my Blum TC50 probe has a > laser sensor and a microprocessor in it, and sends serial > data via an IR link. I was able to measure holes at > different speeds, and had to slow to 1 IPM before the size > measurement didn't change any more. The added time delay of > the second measurement is SO SHORT I can't even tell it is > making the second touch. So, it really doesn't slow things > down much. > > Jon
For somewhat different reasons, based on the rpm of the probe wire and how far it moves in one revolution, its best if my 2nd touch is even slower as the can impinge on the readings, so F.1 or even .05, and yesterday, spinning at 1250 revs, that error is in the 4th digit to the right of the decimal point. Then I am working on mechanical etching of pcb's, I switch the target to the end of a piece of 1/8" brass pipe stuck it one corner of the micarta pallet, and the probe becomes a reformed needle threader wire. This inverts the circuit since the needle wire is grounded thru the spindle bearings, and the brass pipe is the insulated target. Again, the results are close enough that I can drill 40 thou into the pcb where a hole goes, turn the pcb over in the pallet, and run the drill file for the 2nd side, with there being no visible "ledge" where the two meet. Parts leads that just fit, still fit the hole. This I've observed, is a case where having a backlash comp setting thats about half a thou too little lets you do the backaway bounce a lot faster, and lets the mass of the tables inertia finish the backlash move. Too much backlash comp seems to have an effect on the repeatability. Someday I'll spend some time with a scope and see if that can be correlated somehow. In any event, if a thou is close enough, shrug. I am getting better than that. I never did get to the garage to work tonight, too many other housekeeping chores like this is put out the trash night and my legs were screaming for insulin by the time I'd done that and a few other things. I'm a DM-II so insulin is not used unless I'm in the shop like the pulmonary embolism that damn near put a ~30~ on my story 3 or 4 years ago. So I went to bed, but Nova was reshowing the solar airplanes round the world trip. And I didn't have the sense to shut it off. Sigh. I don't recommend getting old, avoid it if you can. Sitting around, thinking about the man I once was, and who now isn't even good once, is not an enjoyable way to spend your retirement years. -- 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 [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
