1610
« on: February 23, 2013, 04:26:21 PM »
OK, you have done some really good test. Do you see why I said it is your accel. now? Bottom line is this, backlash is not cumulative, you only see backlash when you change directions or have an intermitten tool load (think of a 2 flute cutter at a very low rpm, I mean low, 30 RPM low) imagine how that may push the gantry and then it would spring back between tool contact time. These are the only times you will see backlash. If you can hold .004 with your machine your positional accuracy will be .004 if you drill 1 hole or 1000 if this inaccuracy is due to backlash alone. However, inaccuracy due to missed steps can add up, they can be cumulative but most often are random. You have done the test now to know how severely under powered your machine is to be able to hold such a tight tolerance with speed. Now don't take this wrong, I'm not criticizing your machine or your ability, I'm pretty impressed with both. But, the simple fact is, we haven't figured out a way to beat the laws of physics. You see, the amount of backlash your machine has is irrelevant to distance you ask it to move. Backlash is just that, it is lost movement. So if your running a square for instance at 30 IPM. What ever accel you have tuned your motors to is what time will be allowed to slow the one axis to a stop and allow the other to get to speed (in exact stop, CV blends this transition). When you enable backlash, your requesting that motor to move your machine the distance planned by the tool path + the distance in backlash comp. in the same amount of time. Not only that, but the amount in backlash needs to be executed almost instantly else your tool path will suffer. Imagine you had a bus and a corvette that had to run the same road course in the same amount of time. That would be a big task in its self. How much harder would it be to accomplish if every corner was so tight the bus had to stop and back up to stay on the road? You see that lowering your accel to 1 your machine almost had the guts to cut the path you plotted for it. You also see that changing BLC speed had little effect. You could probably run the comp speed to 2 or 3 hundred % (I don't think mach respects anything over 100) because usually in BLC moves, you aren't actually moving very much mass, your just taking up space. But, just the inertia of a motor rotating and instantly having to reverse can be hard to hold. Couple that with the fact that when both axis get to the other corner, they need to be in the right place you can see why high production, high precision machines cost 100s of thousands of $ and weigh 10s of tons with no axis having over 15 inches of travel and motors big enough to match the price tag.
Brett