I think I'm going crazy...(Long post)
I have been looking at the "Rack and Pinion" setup for the X-Axis (long axis). However, most servos and steppers have a pretty high rpm (1000 rpm+). Even those motors running 500 rpm are too fast for a rack-n-pinion.
If the pinion-gear diameter is 1" then that means it is going to travel 3.14" (Pi inches) per motor revolution. Assuming you have a slow 500 rpm motor and want a 100"/min travel then you still need a 15:1 reduction ratio. Most timing belt pully setups will only get you to a maximum of 5:1 or 7:1 reduction.
Ergo, I don't see how you can use a rack-n-pinion with even a slow 500 rpm motor and use a simple timing pully for reduction.
My servos are 2000 rpm. In order to gear it down I would need a minimum of 60:1 reduction (for 100"/minute). Now granted, a 60" diameter timing pully mated to a 1" pully on the motor would be quite impressive! But I think we can rule that out, LOL. I could do this with a worm-gear. They are very compact and it would only cost around $75. But a worm-gear is another source of backlash. Add to that any backlash from the rack-n-pinion system and your asking for trouble (Echoing Balsaman here).
Another thing to think about is this: My motors are 1500 oz-in. Whenever you do a reduction in speed it increases your torque. I calculate my ending torque after a 60:1 reduction to be 94247.77 oz-in. That is not only ridiculous, but a little scary!
So I was looking at plans for routers from MTC and CadCut and it looks like they are both using timing belt reduction on their rack-n-pinion at about 3:1 or 4:1. That is way too small for a 15:1 reduction needed for a 500 rpm motor. I couldn't find the rpm specs on either of the above so I have no idea how fast they run. But what do they know that I don't? Are their motors running a piddly 100 rpm??? I don't think so. So how do they get away with it?
Also, I have been looking at many Torque Graphs for servos and steppers (Just search Yahoo for "Torque Curve" and "Servo" or "Stepper".)
1. It seems that for steppers the lower the speed the more torque you have. It looks like a steep curve that is very high to start then drops quickly and flatens out with increased rpm.
2. For servos the torque is very high till you approach the maximum rpm and which point it falls quickly with increases in rpm. That is, The torque stays high for the first 2/3 of the graph as you increase rpm, then falls rapidly as you approach the maximum rpm.
3. In either case, low rpm means high torque, within reason. So what harm would it do to tell the software to run it at several hundred rpm instead of thousands of rpm?
Questions:
1. So, does anyone still think that I need to run my servos as close to their maximum speed to get the most torque from them? (opposite to what all the servo torque graphs say.) If so why? I'm still trying to understand this.
2. I don't see other rack-n-pinion systems doing big reductions. They are doing very small reductions. How are they getting away with that?
3. What is the best "inches per minute" to aim for? Is 100"/minute really best? Doesn't that limit you so you can never go over that? Why not set it for 150" or even 200", then just tell the software to make it go slower?
4. In light of the above, do you think a 5:1 reduction and then just telling the software to go slow would be enough for a rack-n-pinion?
5. Again, please answer any questions I'm too ignorant to ask!
P.S. I'm probably missing something very big here.
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