Stuart Stevenson wrote: > This is and is not the answer I wanted. Of course, if using the > scale in the PID loop works then the idea of lead screw compensation > is moot.
I think that is the way to go to be perfectly honest. > I had no thought of changing the compensation point location on > the fly just the compensation amount. > The thermal growth of ball screws is nothing short of amazing. > They just don't seem to quit growing as the day progresses. Scales can > remove the ball screw thermal error from the machine positioning. > Thermal growth of the machine and scales is still a problem but it is > a very much easier and predictable measure. > For most machining projects the thermal compensation is not much > of an issue. On a part with +/-.010 inch tolerance the compensation > may may not even be necessary, depending on the size of the machine > and the part. But, the machine (G&L) I am working on is supposed to > have jig borer accuracy. It is large (X 100 inches, Y 100 inches, W 47 > inches, and Z (quill) 36 inches) and will do large parts and tools > (steel, graphite and aluminum). Thermal considerations are a must. Understood. >> However, it _would_ be rather straightforward to gather the comp data >> automatically. A HAL sum block can be used to subract encoder position >> from scale position to get the screw error, and the sampler component >> can capture that error, as well as the nominal position, to a file at >> preset intervals. So you could just start the sampler while the machine >> is at one end of travel, and do a slow G1 move to the other end, while >> sampler records the data. I'd go slow to minimize the chance of error >> due to time-skew between readings - maybe 5-10 minutes to go from one >> end to the other, capturing a reading every second or so. >> > For all but the very most insane accuracies needed the time skew > would be a minimal error issue. I'm curious, what is the resolution of the scale? and the encoder? Chris and I were discussing this last night a bit - I'm of the impression that its a lot harder to get high counts per inch in a scale, since you don't have the mechanical advantage of the screw working in your favor. > .500 inch increments is adequate for > almost all machines. In all honesty, if the machine needs increments > less than 1 inch then the machine has mechanical problems. When the > compensation amount is interpolated between the points, the accuracy > is very good. If a machine has error problems necessitating step > compensation between 1/2 inch increments it has problems the lead > screw comp cannot fix. > Even with the magic of EMC2 it is still not possible to make a > silk purse out of a sow's ear. Tell me about it. I have a sow's ear. (Chinese Shoptask 3-in-1) It has a periodic error on the Y leadscrew, probably from the crappy screw mount. I'm attempting to compensate it, and the backlash, but the real answer is a ballscrew with better end mountings. That is on my list. > I offer the use of the G&L as a test bed for your software. Might take you up on that. Regards, John Kasunich ------------------------------------------------------------------------- This SF.net email is sponsored by: Microsoft Defy all challenges. Microsoft(R) Visual Studio 2008. http://clk.atdmt.com/MRT/go/vse0120000070mrt/direct/01/ _______________________________________________ Emc-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
