I'm concerned that you are about to make a mistake, and waste a lot of money.
450 Oz-in. steppers are pretty hefty devices. The mill we use at work has servo motors rated for approximately 500 oz-in. maximum, and they are expensive to replace. These have all the grunt we need for a lot of heavy milling. The steppers you have should be able to keep up. Steppers lose torque at high speed, but most heavy cutting takes place at feed rates that have a stepper motor putting out full torque. I followed the link to the Keling website, and the heftiest motor they listed was a maximum of 6.3N.m, which I assume (correct me if I'm wrong) means Newton-Meters. The conversion calculator I used gave me 56 inch pounds, a little more than a tenth of the torque your steppers should be able to put out. If you get outright stalls with 450 inch pounds, you will more often get outright stalls with 56 inch pounds. If you are missing steps, it's probably not the motors' faults. Something else is probably at work here. The only valid reason I could see for upgrading would be to get faster rapids. If you are trying to drive the steppers faster than they can go, you may lose steps, but otherwise you should look at another cause. First find the drive voltage, and the coil inductance of the motors. With a switching stepper motor controller, 70 volts can get a 1mH coil up to 7 amps in about 100uS.if you give it another 100uS to stay at 7 amps, you can have almost full torque for 200uS per step. For a full stepped standard 200 steps per rev motor that would be 1500 RPM. Your motors probably don't have that low inductance. Divide by 2 for 2mH = 750RPM. Divide by 3 for 3mH = 500RPM. That should be plenty to keep up with milling at even 50 IPM. I wouldn't want to go far above 3mH for something that has to be fast. These guys have a NEMA 34 motor with 7A (parallel) coil current at 1.2mH per coil with nearly 600 oz-in. http://www.homeshopcnc.com/index.php?page=shop.product_details&flypage=gk_flypage.tpl&product_id=29&category_id=5&option=com_virtuemart&Itemid=1 <http://www.homeshopcnc.com/index.php?page=shop.product_details&flypage=gk_flypage.tpl&product_id=29&category_id=5&option=com_virtuemart&Itemid=1> That would be a pretty fast stepper, with good torque. I suspect your problem is either electrical/wiring getting old, or if you have a rubber resonance damper in the system, and it has become petrified. Rubber gets old. you may just need to replace a resonance damper, as resonance can drop the torque to 0.0 well before the top RPM should occur. If the resonance damper is not too far gone, it could be causing seemingly random errors. Also 7A is a lot of current. Dirty connectors can limit that current, or at least the speed that current ramps up at. You could connect some wires to a coil on the stepper that's giving you the most trouble, and connect it to a DMM, See if the voltage is what it's supposed to be when the motor just comes to a stop. If the wires are hooked up to screw terminals, and that has gotten corroded, a little cleaning with Scotch Brite or fine sand paper, and re-tightening may solve your problem. Plug in connectors could be tougher to fix. I hope you don't have old wire that needs replacing. Of course, if your problem is A: bad wiring or B: a hardened resonance damper, or C: too much resistance in the high current wiring to the motor; AND... if you replaced the whole system with an expensive servo system with new wiring and no resonance dampers; your problem should go away. I suspect that if it's a simple problem with the stepper system, a simple solution would be just as satisfying. On 06/13/2012 11:48 AM, Todd Zuercher wrote: > I am thinking about trying to upgrade some old routers from steppers to > servos. I am seeking advice on a good and inexpensive BLDC motor to > use. > > The steppers on them now are NEMA 34 with a 3/8" shaft, rated at 7amp > and 450 oz-in. and are half stepping from an old Anaheim Automation > unipolar bi-level drive. > > We are tired of lost and missed steps not to mention outright stalls > ruining pieces. ------------------------------------------------------------------------------ Live Security Virtual Conference Exclusive live event will cover all the ways today's security and threat landscape has changed and how IT managers can respond. Discussions will include endpoint security, mobile security and the latest in malware threats. http://www.accelacomm.com/jaw/sfrnl04242012/114/50122263/ _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
