I am working on a DIY wood router with the following goals:
- approx. 5 x 10 ft sheet capability.
- 600 in / minute cutting rate
- 3 axis
- The router is a moving table design concept
- So far, I am using an assumption of needing 100 lbs of force at any given time to hold / move the router position and avoid chatter. The actual number I measured (hand holding) was between 50 and 80, but 100 seems about right.
- Rack and pinion using a nominal 1 inch dia pinion on all axis
- Direct drive the pinion mounted right on the stepper motor - no gearing
- 1 inch dia x 3.14 / 200 steps = 0.0157 inches / full step
- If I can actually use the 10 microsteps for resolution, then 0.00157 in / microstep, which is more than enough for wood, at least that is my perception. So far, no one seems to recommend going to finer microstepping than this, for many reasons, but if it worked, I would consider it.
- With this setup, I can keep the stepper motors running in the 0 - 200 rpm range, which is where they have their best torque, and microstepping works well.
I am aware that many people recommend against using microstepping for resolution, and that it should only be used for smoothing, but it seems like this is primarily because the stepper combo they chose lacks sufficient torque in microstep mode to actually move the head - at least that is what it seems to be.
In order for this to work though, it takes a powerful stepper motor if my calculations are right ?
(100 lbs force) x (16 oz / Lb force) x (5 X up rating needed to deal with force during microstepping vs motor rating) x (0.5 inch pinion radius) = 4 000 oz inches.
In theory at least, these loads can be divided into two motors per axis, but this is not always the case, because the load might be on one side of the table vs another, so distribution of forces is not equal. Nonetheless, at least 2 of the three axis will end up with 2 motors (1 on each side)
Question 1 - Is my torque derating for microstepping too aggressive vs the stepper motors "rating" ?
Question 2 - What combination of a Gecko product drive and stepper motor can do this ?
Question 3 - Will this setup actually provide 100 lbs of force in microstep mode to hold / move the router head ?
Question 4 - Since this means that the microsteps will be quite strong, will that cause artifacts when carving letters or fine shapes ? I sometimes see artifacts in cnc router carvings, even in units using screws. Perhaps these are from a curve or line being constructed from individual X, then Y moves to make a diagonal line ?
Here is my thinking, and please correct me if I am way off:
- If I run the stepper motors at higher rpms, and gear down, there really isn't any clear benefit, because the motors will just loose torque (rapidly) above 200 rpm anyway.
- If I add in a gearing setup, and especially if I build it, it will likely have worse accuracy performance than any non linearities in the stepper motor and driver.
Obviously servos run at very fast rpms in order to make up for their low torque ratings, which requires massive gearing down. For that reason, I am really not considering them. In addition, I am not really ready to take on servos, as I am just starting to figure out steppers.
I would rather give up on the linear speed goal than gear down of possible.
This is a hobby, so it will take a while to build this thing. If a motor + driver combination is going to be available during the next 6 months, that might still work. I noticed that there aren't any nema 34 or 42 stepper motors on the Gecko site right now, so maybe some new ones are coming ?
Thanks for any suggestions and comments.