Hello, I was reading the thread on the gent with the shizuka mill and leadscrew bearing backlash trouble. This sparks me to ask some general cnc questions, perhaps someone could answer. I assume backlash is a problem, because the mill has servomotors with rotary encoders, and any lash would lag the table movement with respect to the rotary encoders on the motors. Could this backlash condition be programed out by compensating lash at each direction reversal? I don't know much about cnc yet, but I believe some controllers have a place where you can input a axis' backlash. That way, a circular interpolation from an end mill would yield a truely circular hole. Also, if this machine had linear encoders, would drive backlash matter? The encoders would indicate true table position, so the computer knows where the table is at all times, so would not any lash be compensated for by the feedback loop of the encoders giving a "seek" signal to the motor? I would like to convert one of my mills to cnc, so I have been exploring what system is the best. Any thoughts appreciated, --Doozer
This is true with both servos with rotary encoders, and stepper motors, such as are found in low-end machines, and older ones, such as my antique Bridgeport BOSS-3 machine.
Normally, the backlash in a set of ballscrews is minimal, but as wear accumulates, it can become a problem.
That feature is certainly present in my old Compact-5/CNC, and I have read of it being present in other controllers as well.
Well ... sort of. With servos, the servos go to rapid motion when that reversal comes about, and result in some slight imperfections where it occurs.
Mostly -- yes. But if the mill is doing "climb" milling, the cutter can grab the workpiece and pull out the backlash -- much more quickly than the mechanics of the servos can react, even if the electronics are fast enough. And it is not just the servo motor -- it has to move the entire mass of the table (for the X-axis) or the table
*and* the saddle (for the Y-axis). That is hard to correct quickly and accurately at the same time.
Certainly the linear encoders are better in terms of absolute precision -- but at a significant cost. You can't use the cheap scales derived from digital calipers, because those take some time to encode the readout and send it (as an absolute position) in binary-coded-decimal form. The controller needs to be getting the quadrature pulses directly from the scale to have truly up-to-date information. Glass scales (the best for the purpose) are quite expensive compared to rotary encoders on the servo motor. Of course, the low-end machines and controllers simply feed pulses to the stepper motors and assume that they went where they were commanded to. The problems are:
1) Under heavy cutting loads, the steppers can miss a step, and eventually regain control typically four steps out of position. The controller thinks that it is four steps (or some multiple thereof) away from where it really is.
2) At high speeds, it is easier for a stepper to miss a step, and wind up again four steps out of position -- even with very little load. For this reason, high speed motion is best accomplished by a servo motor, and patience is the best bet with steppers. (There are tricks in the drivers to make them a bit better, but never as good as servos.)
The above are my opinions, and you will probably find others who will disagree.
Yes, compensated, assuming the backlash is predictible. Sometimes it varies, perhaps increasing along the length of the screw, or at certain areas due to wear, or even periodically depending on the angle of the screw turn.
But, backlash is also a kind of lack of rigidity, and metalworking is all about rigidity. On a manual machine it doesn't matter, since you don't straddle the backlash during a cut, but on CNC controls, where you reverse directions and straddle the backlash during curved cuts, backlash becomes slop that causes chatter or worse.
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