On Thursday 10 December 2009, Erik Christiansen wrote: >On Thu, Dec 10, 2009 at 07:08:21AM -0500, Gene Heskett wrote: >> 3. face up & making a sprocket, or lathe style & carving gears, backlash >> will be a problem. So it may be wise to add a 2nd lock brake so that the >> table is locked pretty solidly when the motor is not moving it. I'm >> still figuring out how to do that with mine as it has a thumb screw with >> limited holding power. > >It was with some shock and disappointment that I discovered that my >brand new several-hundred-dollar rotary table has backlash, even with >the variable-engagement worm adjusted for maximum engagement. It has two >locking screw-clamps, each with a short captive tommy-bar, but if EMC is >to control them, then powerful pneumatic brakes and solenoid valves >would perhaps be easier to control.
Given the Tommy bar clamps on the one you showed in the link, a pair of small double acting air cylinders and an electric solenoid would be ideal. Unforch mine doesn't have those. Bear in mind then need for clearance when mounting it aligned with the X axis. >Perhaps I should disassemble the rotary table, to check how hard it is >to improve worm and wheel engagement so that both flanks make contact. >(I'm still aghast that a reputable manufacturer doesn't achieve that, >out of the box.) I doubt you will be impressed with what you see. I sure as heck wasn't. I was at Grizzly about thanksgiving picking up a dust collector for my planer and table saw and checked several of the more expensive tables on the display table, and they all turned much easier than mine cuz they had better bearings, and they all had some feel-able backlash. Most had provisions to adjust the worm engagement, and in my case I pushed it in till it drug over part of the turn, then ran it back and forth a few thousand times with emc to wear it in, but I don't think its possible to make it perfect. Much less than a degree, but its still there. Had the $200 one been tight, I would have dropped the card on the spot, but it was far worse than my 100 dollar one is now. The $200 version was also about 40 pounds heavier, a consideration on my micromill. >> <http://gene.homelinux.net:85/gene/emc/A-drv.jpg> >> shows the motor mount I made for a cheap 4" Grizzly table. >> >> The coupling inside it is a heavy steel rig, two cups facing each other, >> with 1/4" slots cut across, with a steel disk trapped in the middle which >> has a 1/4" wide fin, laying 0-180 on one face, and 90-270 on the other so >> it presents a sliding u-joint with very little backlash that can absorb >> the axles being out of line a couple thou. > >That sounds a lot like an Oldham coupling, Gene. For the OP, there are >some pictures here: http://en.wikipedia.org/wiki/Oldham_coupler Heck, I knew I wasn't inventing anything that wasn't just common sense, but my way leaves room in the middle for the nuts that attach it to the screws in the mill, and my center disk has the fins rather than slots. All carved from some very hard steel mine shafting from the scrap yard. I didn't count on its hardness though, they were hell on carbide to carve out. OTOH, I both didn't look at wikipedia, but wouldn't have known what to look for either. Such is life. :) >One idea I'm considering is driving via a toothed belt, from a motor >clamped nearby, in the T-slots. Just looping the belt over a pulley >added behind the rotary table handwheel allows it to still be used >manually, when it isn't worth mucking with gcode. Belt tightness I found is very important as I'm belt driving my Z and found I had to wedge a good sized bar in there when tightening the motor mounts slider to get the backlash down. With visible belt slop as it reverses, the backlash went from 3 thou to 50 thou! OTOH, my Z, formerly locked up tight with a 5 pound push on a drill bit, can now come down on the bathroom scales to 155 pounds before that 425 motor cogs back the first time. Drilling holes is no longer a problem. :) As you can see, there is a knob on the back of the tables motor that can be used when the motor is unplugged, (don't forget to shut off the psu when doing the plugging!) but seeing the vernier scales is a bit difficult. I didn't consider that because plugging in the motor and running it with emc is 10x more accurate anyway. >Erik -- 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> hardware stress fractures ------------------------------------------------------------------------------ Return on Information: Google Enterprise Search pays you back Get the facts. http://p.sf.net/sfu/google-dev2dev _______________________________________________ Emc-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
