Originally Posted by Cartierusm  Anyone Please? |
It's all trade-offs between rapid speed, machining force, and step rate. You use a reducer to get more force, but it also lowers max speed (or increases required motor RPM), and increases step rate. With servos, high RPM is not a problem, as that's what servos are really good at. With steppers, high RPM can be a problem, if you go too far. But, steppers have max torque at low RPM, which is where you need it, so you don't typically *need* to reduce as much as with servos.
My machine has 850 oz-in (peak)/170 oz-in (continuous) servos. With 2.5:1 reduction and 4-pitch screws, it has more than enough power for anything I've managed to throw at it. So, with 1200 oz-in steppers, and 2:1 reduction, you'll have oodles of power.
Servos are really good at high RPM - mine are rated for 4200 RPM. Steppers, on the other hand, are not good at high RPM, and large ones are often hard-pressed to get to even 1000 RPM while driving any significant load, so you have to be careful to avoid using so much reduction that your rapid speed is compromised. IIRC, you're planning on using 4-pitch screws. With 2:1 reduction, that's 8 turns on the motor per inch of travel. If you assume your top motor speed will be 800 RPM (I think that's quite conservative), then your rapid speed will be 800/8, or 100 IPM. Not record-breaking, but perfectly usable. In reality, I expect you'll do much better, as rapids don't require a lot of torque, so I expect you should be able to do at least 120IPM, which is reasonable. Despite the fact that mine *can* do 400IPM, I normally run only 150 IPM because it's just too easy to do something stupid and crash it at 400 IPM.
Now, another reason you might want to go to a higher reduction, is to reduce your step size. At 2:1, with 4-pitch screws, and 1.8 degree motors, you'll have 1600 steps/inch, or 0.000625"/step. Some would consider this a bit coarse. Of course, you'll undoubtedly be using micro-stepping drivers, which will reduce this by a corresponding amount (more or less - the microsteps are less accurate than the full-steps).
Another trade-off is the pulse rate your PC can handle. In the example in the preceding paragraph, if you're using Geckos with 10:1 microstepping, you're looking at 16000 steps/inch. To do 100 IPM, you need to be able to output 100 * 16000 / 60 = 26,667 steps/second. Almost any PC should be able to do that. If you went to 3:1 reduction, that would increase to ~40K steps/second, which would require a fast PC. Many/most PCs will not be able to go beyond 45K steps/second, so if your chosen reduction takes you above 45K steps/sec, then you either have to be very careful in your PC selection, or get a SmoothStepper, which can step up to 4MHz.
Now aren't you sorry you asked? :-)
Regards,
Ray L.
03-03-2009, 07:06 PM
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