Is gear reduction the answer?

[jharvey407]

Like most hobbyist CNC builders I started with more enthusiasm than education. My CNC router is in Stage III of construction, which means it is working well, but it could use some improvement.

At this point the main problem I have is the speed of my Z axis. My X and Y axis will both rapid at 750 ipm, although I have them set to 500 ipm simply because it seems more reasonable for the size of my machine. While the Z axis will only rapid at 80 ipm. I've set it to 120 ipm, but it stalls occasionally.

The first solution that popped into my head was a 2:1 gear reduction, but I realized I was just guessing it would work with no real knowledge or testing. So here I am once again on CNC Zone looking for help.

Here is what I have right now;

Keling KL23H286-20-8B 425 oz. stepper
Keling KL-4030 driver
24v power supply
5 tpi acme screw
1/2 step = 2000 spi

The picture below shows the Z axis. The tool it carries is a Porter Cable 690 router.

I don't have a torque curve for my stepper, and I'm not sure how to calculate the required torque.

Can someone explain, or link me to a source on torque calculations?

Would a 2:1 gear reduction allow me to run the Z axis faster?

I have the CNC router parts gear reduction R&P drive on the other axis, so I was looking to use the same belt and buy gears for a 2:1.

Any help is appreciated.

J

[mike_Kilroy]

....................
Can someone explain, or link me to a source on torque calculations?

Would a 2:1 gear reduction allow me to run the Z axis faster?

Free Motioneering will help u calculate the torques and speeds per ur request:

http://www.kollmorgen.com/en-us/supp.../motioneering/

do the Motioneering and it will let u know if 2:1 will help. no one can guess w/o doing the sizing....

[jharvey407]

Free Motioneering will help u calculate the torques and speeds per ur request:

http://www.kollmorgen.com/en-us/supp.../motioneering/

do the Motioneering and it will let u know if 2:1 will help. no one can guess w/o doing the sizing....

Thanks Mike, that looks like just what I need.

I'll let you know how it works out.

J

[ger21]

The reduction will probably be slower. A 1:2 "reverse reduction" might be a better option. The motor will have much more torque spinning at half the speed.
An even better option would be a much higher voltage. Those 425 oz motors have a very high inductance, and don't spin very fast on 24V.
Another option might be to try a damper on it, if resonance is an issue.

I use a Xylotex running a 250oz motor with a 4tpi acme screw and a 690, and I get 55ipm. I know I could get more with a damper, but I have a single shaft motor.

[jharvey407]

The reduction will probably be slower. A 1:2 "reverse reduction" might be a better option. The motor will have much more torque spinning at half the speed.
An even better option would be a much higher voltage. Those 425 oz motors have a very high inductance, and don't spin very fast on 24V.
Another option might be to try a damper on it, if resonance is an issue.

I use a Xylotex running a 250oz motor with a 4tpi acme screw and a 690, and I get 55ipm. I know I could get more with a damper, but I have a single shaft motor.

Actually this makes perfect sense. I can trade power for speed which makes sense because the two are typically interchangeable. So I have a lot of power, but low speed, so I'll convert the power into speed.

I know which direction to go now.

Thanks Ger,

[jharvey407]

I am investigating Ger21's idea of gearing up the motor to provide more speed for the lead screw. I figured I'd start by doing some calculations on motor RPM as it translates to linear motion.

Here is what I have now.

3/8" 5 tpi lead screw being directly driven via coupler by a 425 oz motor.

To achieve a linear speed of 80 ipm the motor must turn 400 rpm.

80 X 5 = 400

I would like to rapid at 250 ipm. To achieve this speed the motor would have to spin the screw at 1250 rpm. It will do this if there is no tool in the tool holder, but with the Porter Cable 690 in place the most I have ever been able run is 120 ipm, or 600 rpm.

By using a 1:3 gear ratio I could spin the screw at 1250 rpm with a motor rpm of 416.67. Using full steps that will result in 3000 spi.

Does anyone see any problems with this idea? I am running Mach3 with the 35k kernel speed so I'm sure it will handle it.

Thanks,
J

[ger21]

I'd use 1/4 or 1/8 microstepping if you can, as it should make it run much smoother.


Your 1:3 gear ratio would give you 333 full steps per inch, by my math. Not really good enough resolution.

If you're stalling at 400rpm right now, be aware that you'll need 3 times more torque to spin at 400rpm when geared 1:3. And you're not going to get it.

I seriously doubt you'll get 250ipm with that motor and 24V.
I'd try the 1:2 and be happy with whatever you can get.
If you really need to go that fast, I'd change to a 2 turn/inch screw, and double the voltage.

[jharvey407]

Your 1:3 gear ratio would give you 333 full steps per inch, by my math. Not really good enough resolution.

hhmmm.... let me see.

The motor is 200 steps per rev. with direct drive that translates to 200 steps per rev on the screw. The screw is 5 tpi, so 5 X 200 = 1000. 1000 steps will turn the screw 5 times, equallying 1 inch of movement.

Am I figuring this wrong? 1/2 step would increase it to 2000 spi.

A 1:3 gear ratio would spin the screw 3 times for each motor rev. so that gives us 200/3 = 66.666 steps per rev of the screw, the screw has to spin 5 times for 1 inch of travel 66.666 X 5 = 333.333 spi.

It seems I've been doing the math wrong. Ok, back to the spreadsheet.

But that's why I post here. It's always nice to get a second set of eyes on the problem.

Thanks Ger

[jharvey407]

If you're stalling at 400rpm right now, be aware that you'll need 3 times more torque to spin at 400rpm when geared 1:3. And you're not going to get it.

I seriously doubt you'll get 250ipm with that motor and 24V.
I'd try the 1:2 and be happy with whatever you can get.
If you really need to go that fast, I'd change to a 2 turn/inch screw, and double the voltage.

Ger,

I can actually rapid at 120 ipm, which is 600 rpm before the motor stalls, so there is a bit of extra torque at 400 rpm.

I've been hearing over and over that 24V isn't enough for these motors, which makes me wonder why Keling sells them in kit form with the KL-4030 drives and a 24VDC power supply.

I guess with the KL-4030 the highest voltage power supply I could use is 40VDC. I guess I'll look into getting a 40VDC power supply.

I'm also going to experiment with the gear drive as well. I'll keep you posted.

Thanks,
J