On Sunday, February 26, 2012 08:25:35 PM Cecil Thomas did opine: > Gene, > I CNC'd one of the red Harbor Freight 7x10's by puttting a gearmotor > with encoder (servo) on the lead screw and another on the x axis by > mounting it on the BACK of the saddle with a sliding coupling made by > drilling a cross hole in the end of the cross slide lead screw and > another in the motor shaft into which I inserted a 3/32 pin which was > exposed on one end only.
I think I stumbled over that rear cross slide motor setup someplace on the net & was wondering how that coupling was done. That would need a long slot unless the motor was mounted to the carriage. I don't recall that detail either since its been a couple of yonks since I saw it, and it may not have even been your machine. > The two shafts are connected by a hollow > tube with a (tight) 3/32 slot running its length. I was warned that > the slop in a gearmotor would be detrimental to a CNC setup but those > of us who have been anywhere near these little toy lathes know that > the backlash in the gearmotor is the least of our worries. Absolutely. Backlash in the crossfeed is 10 thou, and it is NOT in the nut, the end play is in the front hub so I need to machine its length a bit. If I can, that may get some teeny ball bearing thrust washers installed, like I did to the hub/bearings of the X & Y on the mill, which allowed me to set those up with a skosh of preload for zero lash there. At the same time, I will probably install similar ball thrust washers on the left end of the lead screw which should allow me to run with clearance on the right end & reduce the overall friction there too. > The gearmotors are spur gear reduction and are about 150 rpm at full > throttle. The spur gearing and low ratio allows the crossfeed to be > turned by hand in manual operation. > > By the way, I run my spindle as the A axis (fully encoded with a 256 > line hp encoder) the motor is a 36 volt servo that Surplus Center > used to sell for $30 with no encoder. Fortunately it had a tiny 1/8 > inch or so rear spindle that was there for mounting an encoder. > I can run up to about 1000 rpm using step generation from EMC2 with > no problem. The servo is mounted behind the machine and uses the > original belt to the countershaft. I have two configs depending on > whether the spindle is in Hi or Lo but I rarely use the Hi > configuration for threading. > I haven't figured high was good for very much, except for throwing alu swarf all over the building. The ponies to cut steel more than 1/4" in diameter simply aren't available in this pasture regardless of how hard we whistle. For that, I'd have to whistle up the horses we had in 1944, King & Colonel, a pair of Perches that totaled up to 4300 lbs. But that was another time & place, long ago & back in the middle of Iowa when I was 10yo. I saw those 2 horses digging trenches with their hooves, bellies on the ground with 75 foot of 5/8" log chain's center up in the air, but the tractor I had gotten stuck that outweighed those two horses by 2x, did come loose too. Needless to say they got an extra can of oats that night, and split a pound of sugar cubes that cost us dearly in war ration coupons. > I think you could drill a hole through the rear of the existing motor > into the end of the armature shaft and press in a pin which would > then give you a place to mount a HP style encoder on the motor. I > have done that on other DC motors to turn them into servos. Yeah I > know all about skewed rotors and anti cogging and all the reasons you > can't just take any old 24 volt DC motor and make a servo but I > mostly go for what works.....I just don't talk as much about the > stuff I tried that didn't work. Chuckle. You, me, & Thomas Edison. One could say that I've BTDT myself. And like most, when it works better, I brag a bit. > I am using Gecko servo drives. > I only use the spindle in servo mode when threading. I have a switch > on my Power supply/driver box that I can switch to "free run" on the > spindle by just disconnecting the drive and connecting DC to the > motor and using it as a free running DC motor when doing simple > turning not requiring spindle coordination. > > The up side to my spindle setup is that the spindle is not just > "synchronized" to the x and z by index but is a fully functional axis > with better than .5 degree resolution. I can cut any thread you can > imagine in straight or taper form inside or out. > > The down side is that I have to write all my own threading routines > because the "canned routines" don't work with an A axis spindle. I > personally like writing my own routines because I am involved in this > CNC world for my own self improvement and not for production. I expect that is my fate here also. Come on over some night & we can cry in each others beer. It seems to me there ought to be a way to do this without resorting to the ABC settings with its inherent limits. But I suspect this will be an entirely different, probably checkered horse, with interesting games in the .hal files. Thanks Cecil. 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) My web page: <http://coyoteden.dyndns-free.com:85/gene> Satire is what closes Saturday night. -- George Kaufman ------------------------------------------------------------------------------ Try before you buy = See our experts in action! The most comprehensive online learning library for Microsoft developers is just $99.99! Visual Studio, SharePoint, SQL - plus HTML5, CSS3, MVC3, Metro Style Apps, more. 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