Originally Posted by
peteeng
Hi MM - Your numbers check out BUT:
1) To answer about the rack tolerance - they are made to a tolerance so you have to ask the supplier what tolerance it is
2) To calculate the full operational tolerance you have to add the stepper motor tolerance (usually +/-5% of 1.8deg if you have std motors. Can be less if you have high grade motors eg some motors are 2%) then the gearbox tolerance it will have an arc radius tolerance or backlash tolerance then add the rack tolerance. Also there's the microsteps to consider and there is no tolerance on those, they are fuzzy steps that get corrected every mechanical step of 1.8deg to within the 5% tolerance. There is always the question does the controller cut the steps off or add one in when close to the target spot
3) Its unlikely you will get any torque at 1200rpm. The 9Nm is the stall torque or very close to 0rpm. I usually assume its linear from 0-1200rpm say so you have 4.5Nm at 600rpm. Do you have an actual torque curve for the motor driver combo?
4) So assuming you have 4.5Nm at 600rpm then you are moving at 8m/min rapid and have 140kgf available. Some of this torque is consumed by the motor inertia and the gears inertia plus various frictions. For instance seals on cars can be 2kgf each so 4 cars can be 8kgf drag. You can calculate that as well, the motor reflected torque can be considerable if accelerating. Since you only need 10-20kgf to cut the aluminium in a full slot I expect you will be OK depends on what feed speeds you actually want. Maybe 5m/min? at 5m/min the motor is at 312rpm so has good torque
5) if the machine is going to be used for aluminium exclusively I suggest you consider AC servos. They are more expensive but offer a flat torque curve 0-3000rpm and can be over driven. This means if the motor is loaded up the available torque increases overcoming the issue. Steppers if overloaded die immediately. Then the games over lost steps/lost position screwed job. To cut aluminium fast and effectively (commercially) you need high feed speeds to get good chip loads at high rpm spindle speeds. This means servos are the answer. Aluminium if not cut dry correctly or with mist or flood will gall on the tool and effectively weld the tool to the job stopping everything fast. Tools have to be uber sharp and changed regularly etc etc.
What tolerance do you need? and do you know the difference between tolerance, accuracy and repeatability? If you have high repeatability but a poor tolerance sometimes it does not matter. So depends on what work you are doing. If you are trying to compete with a commercial mill I don't think you will get there with a router... So what parts are you making? Peter