On 2/17/2014 3:47 PM, Steve Blackmore wrote: > On Sun, 16 Feb 2014 16:51:06 -0600, you wrote:
>> Sorry, but I don't consider changing the machine to fit the tool to be >> an acceptable solution. > > Then your stuck with a known problematic design. I wish I had a pound > for each complaint/query/moan I've seen and heard about twin steppers on > a gantry machine. > > Other fixes include servos or closed loop steppers but keeping both > sides in synch with software without any feedback is nigh on impossible. It's such a simple design concept to run a shaft across the gantry to connect to a chain or belt or rack on both sides, or to use a screw down each side and connect them at one or both ends with a chain or belt. Never racks, ever, and doesn't use up two motor control outputs from the control electronics. A friend of mine has a big Torchmate table, uses a rod across the gantry with a rack on each side, rod is turned by a cogged belt. Don't ask me why they used an ACME threaded rod when nothing runs on the threads! Plain shaft would work just as well. A while back on this list, someone was attempting to replace the control on a Thermwood gantry router with a PC and LCNC. Thermwood used a motor on each end of the gantry, and to make it especially challenging the gantry stretched across the long dimension of the table. Never had any racking problems with the OEM control but the person doing the refit couldn't get it to work with LCNC. I would have just got out the tools etc and fixed the @#%$@% thing to use a cross shaft rather than screw around trying to use software to fix a hardware design problem. What might provide some insight into an anti-racking system is to study how pivot sprinkler lines are made to handle fields which are not perfectly flat. They have sensors at each wheel pylon that detect if that point is advancing or lagging then the speed is adjusted to get it back in line so the pipe doesn't end up kinking and wound up around the pivot like a dog that's too dumb to change directions when it gets its chain wound up. Design the gantry with the end bases long and stable then have the cross bar mounted so it can pivot just a little. Add sensors to detect the tiniest amount of racking then have a system to automatically adjust the speed of each side to straighten it out. That could be completely independent of the CNC control. But still far more complex than mechanically coupling the ends of the gantry. ------------------------------------------------------------------------------ Managing the Performance of Cloud-Based Applications Take advantage of what the Cloud has to offer - Avoid Common Pitfalls. Read the Whitepaper. http://pubads.g.doubleclick.net/gampad/clk?id=121054471&iu=/4140/ostg.clktrk _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
