Router - Motor + Driver Combo Advice Needed

[harryn]

Hi,

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.

Plan
- 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.

Harry

[ger21]

Not sure why you seem to be trying to re-invent the wheel with every aspect of your machine.

I don't have any actual experience, but I don't think you'll be happy with direct drive. I have a feeling you won't be getting very smooth motion, especially at lower speeds. The mechmate guys like to use 3:1 reduction, and they say that a larger pinion than 1" gives noticably smoother cuts than a 1" pinion. So I'd think that a direct drive 1" pinion would be much worse than a 3:1 1" pinion. Also, I've heard that larger motors aren't nearly as smooth as smaller ones, especially when you get into the 4000oz range.

You would think that if direct drive was a viable option, someone would be doing it?

1) Don't know

2) I would think that 4000oz would be much too large for a Gecko. Gecko's have a switch for a Nema 42 mode, but I think they are best suited to smaller motors, say, 1200oz or smaller.

3) Don't know.

4) My guess is absolutely.

Why don't you just buy the CNC Router Parts drive components. They're a proven design that works. And when you start adding up the cost of 4000 oz motors and drives, I don't think there would be much of a cost difference.

[harryn]

Hi Gerry,

Thanks for the reply and suggestions.

Why re-invent the wheel - well, I guess that is just my nature.

For more specific reasons:

- If you look at the Atlanta Drives web site, there are a lot of things that DIY hobbiest are doing with rack and pinion that are "not ideal" compared to a professionally made machine. It isn't that the gear mashing we are doing can't be made to sort of work, but it seems it can be done a lot better as well.

ATLANTA Gear Racks

- Ahren at cncrouter parts is doing a great job for the 2 x 4 ft and smaller hobby market, but after testing some of the items, I have seen the limits of the designs for larger machines with higher spec and reliability goals. Will I achieve these higher goals in a reliable way? - not sure, but I need to try.

- Space - in an industrial environment, there is usually more space to work with than I have. Well done gearing takes up space and cost money. If I have to buy commercial quality gear drives, then the build might not happen at all.

- I am almost forced to make one axis of my router rack and pinion, so it is just easier if I make them all the same design.

- As far as 3:1 reduction vs direct drive, yes you are right, it seems like someone else would have tried it by now, and probably have. That is part of the reason for my posting these questions here - the conditions where direct drive stepper to gear rack will work seems to be a very narrow design and operating band. If I triple the motor rpms to deal with the 3:1 reduction gearing, I still can't see where the benefit is with the lost torque effect. It still seems like the same amount of motor power is needed either way?

- There are some trade-offs since I need to be able to build things with hand tools and a low end drill press, so that means precision requires paying someone else.

[BrianTee]

After about 4 rpm, microstepping a motor actually has less torque.

[ger21]

If I triple the motor rpms to deal with the 3:1 reduction gearing, I still can't see where the benefit is with the lost torque effect. It still seems like the same amount of motor power is needed either way?

Very large steppers have torque curves that drop of extremely fast. A motor 1/3 the size will probably have a flatter curve, and I'd expect at some higher rpm that the 3:1 geared smaller motor will provide more force than the larger direct drive motor.
You also gain 3 times the resolution, which is very poor with direct drive.

I also forgot to mention that if you plan on cutting at 600ipm, you'll probably want something in the area of a 5HP spindle, unless you'll be limiting your depth of cut to less than 1/4".