On Sunday 26 July 2009, Chris Reynolds wrote: >I'm new to the group and I could use some help with a problem that I'm > having. I just built a new cnc machine and I keep running into a problem > that I can't figure out. I've been doing some test cuts, problem is they > always come up short on one end. I thought it was a backlash problem so I > put the dial indicator on the machine and ran it to an inch. It came up > like .040" short. So I tried adding a backlash command to the ini file and > that didn't fix it, so it wasn't a backlash problem. Then I tried messing > with the steps and finally got it to come out right. Run it to an inch, > dead on 0, run it back to 0 and end up dead on 0 again. So then I run the > program again and the whole thing is now to long. It always seems to come > up short on one end. Checking the math the steps are right, I have a 20 > tooth pulley on the motor and a 40 tooth pulley on the leadscrew, running > in full step mode, so that comes out to 1600 steps per inch, which is what > I have in the program. It's almost like it's a compounded problem, like > it's off a small amount but then when you cut through the whole part it > adds up. But the weird thing is that it's off on one end. The part I'm > cutting resembels a comb, where you have slots cut into the part every so > often, there's a wider section at each end, on the far end it's right, on > the 0,0 end of the part that wider section is to short. > >I'm using an older hobbycnc driver that I've had for a number of years, it's > not one of the newer pro boards. I've done a number of checks and backlash > doesn't seem to be the problem. Any help anyone can provide would be > greatly appreciated. > > Chris
>From the clues, Chris, I'd back clear off to pulling the motors loose, marking the pulley, or putting a pin in it to hit a stop, and see if the screws are good. Math shouldn't lie. If you have 10 tpi screws, then it should move .1000" per rev of the screw, or 1.000 for 10 turns. If the belt has no slack (pull it up snug enough to twang when plucked) then 1600 steps should = 1.000" If that's all good to the .00025" over a foot or so, then next I would scope the driving pulses with a real 100 mhz dual trace & see if a timing constraint of the driver is being violated, or possibly even noise is getting in. However, this sort of thing will usually preclude an accurate return to zero, an obvious error. Any chance the x/y table is sagging due to the weight of the motor on one end? Or if its a gantry, the rails are flexing enough as the weight of the gantry moves? But that also should be pretty obvious, park a good level on it & move for effect. Its a head scratcher, and I keep coming back to the doofy screws theory. >http://www.blowyourbrassoff.com >http://www.builderofstuff.com >http://www.myspace.com/builderofstuff >http://www.myspace.com/builderofbluesharps > > >Just a man, his shop, and a long list of unfinished projects... > Sounds like me & my mess. -- Cheers, Gene "There are four boxes to be used in defense of liberty: soap, ballot, jury, and ammo. Please use in that order." -Ed Howdershelt (Author) The NRA is offering FREE Associate memberships to anyone who wants them. <https://www.nrahq.org/nrabonus/accept-membership.asp> Q: Why did the germ cross the microscope? A: To get to the other slide. ------------------------------------------------------------------------------ _______________________________________________ Emc-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
