On Saturday 16 February 2019 14:28:33 Chris Albertson wrote: > Thinking about this as an electrical engineer, I'd say don't use DC. > DC current is as you say blocked by paint and oil films. Use AC. > I think a low radio frequency. Then the DC insulators would act like > capacitors and pass AC. even while blocking DC. To detect contact use > an AC voltage sensor, typically a diode and small capacitor.
The disadvantage there is the rc charge time. This means among other things a relatively leasure velocity to the first trip, and a much slower 2nd approach, where as in using the dc circuit, one usually uses a rotating with the spindle probe so even if it has some runnout, the contact surface is recorded an by the first contact discharging a .1 ufcapacitor, which takes long enough to recharge that the contacts logic zero, is captured and reported to LCNC instantly even if by the time the servo thread actually reads it, the contact has been lost again for .9 milliseconds. The rf circuit, cannot possibly respond in that time frame unless the closing velocities are also very slow. The DC method is inherently the faster method, but does require a very low ohmage connection in order to fully discharge the cap on a 10 microsecond contact. And we definitely DON'T have a low resistance circuit on this machine. I have one of this old tony's contacts about 75% made, but out of a brass tube instead of a steel probe and I'll take the machine out of it by grounding the workpiece, and wiring the tube straight to the probe input, using the same old cap for storage. The brass is long enough it can hit and be bent 1/4" without damage as it will just spring back. But its been a long day today. I'll figure out something thats hopefully repeatable. And I've got to figure what to do Monday as my lady adds another year to her journey on this ball of rock and water, making it to her 79nth. Something she'll appreciate without any physical effort. With copd, there's not much of that left. Sigh... That said, I think the DC method can be made to work with a separately wired probe, with both the ground on the workpiece and the hot on the probe wired independently from the machine. Run the probe cable as a shielded wire in the cable chain. At least this chain can be opened to add more wire, something none of the other cable chain I've bought can do. Nice! > I don't know if this is done commercially but the AC method should in > theory by MUCH more reliable. You can even use very long cables if > you use strong filters tuned to the frequency. Notice how well the > current passes from an AM radio station to your radio even over a > miles-long air gap. > > My guess is that a 100 KHz signal would go right through paint. > > On Sat, Feb 16, 2019 at 9:46 AM Gene Heskett <[email protected]> wrote: > > Greetings all; > > > > I've just found that because everything it painted before assembly, > > apparently including the inside of the spindle motor mount, that a > > ground to the bed frame can be anywhere up to 2 or more thousand > > ohms to almost anything else on the 6040, and apparently even > > includes the spindle bearings as part of the first 50 or so ohms. > > > > The net result is that using the workpiece as one contact, the the > > tool in the spindle as the other for the alignment function is > > fraught with enough variables I could break a tool against the edge > > of the workpiece, even damaging the workpiece, before a contact is > > detected. Since there isn't Z room enough for one of those $65 > > spindle mounted contact detectors, and it would take at least ten > > feet of ground braid strung thru the cable chains to arrive at a > > decent ground on the motor housing, which wouldn't solve the problem > > entirely because of the oil film in the spindle bearings, how the > > heck do I arrive at a reliable connection that only responds to a > > contact between the tool and the workpiece? > > > > A flying ground lead one could bring up and clip onto the tool would > > probably work, but sure resembles something Robe Goldburg would > > dream up as it would need to be long enough to reach the tool > > regardless of where it is on the table. > > > > That, or using a much higher voltage limited to a few microamps so > > as not to constitute a shock hazard. But basically use it to measure > > the air gap. I could make that work even before a physical contact > > was made but thats not a tasty idea in the long view either. > > > > Any other ideas out there? Hopefully something that doesn't involve > > changing tools to use. > > > > Thanks all. > > > > 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> > > > > > > > > _______________________________________________ > > Emc-users mailing list > > [email protected] > > https://lists.sourceforge.net/lists/listinfo/emc-users 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> _______________________________________________ Emc-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
