Holding torque vs rotational torque ?

[SScnc]

Does a higher holding torque rating on a stepper also mean that it has more actual rotational (real) torque ? I mean, say you have a nema 23 rated at 270 oz/in holding torque and a nema 34 rated at 500 oz/in HT, does the nema 34 also have more real torque or can it just "hold" more ?

I understand that the torque varies with speed but say both are running at the same RPM from the same power supply, the 34 is more powerful (right ?).

[ger21]

Originally Posted by SScnc I understand that the torque varies with speed but say both are running at the same RPM from the same power supply, the 34 is more powerful (right ?).

At 100 rpm, probably. At 1000 rpm, maybe not. It's far more complicated than that.

You can't just say "Both using the same power supply". You could be seriously restricting performance of one of the motors in your attempt at an "equal" comparison. Also, this would basically refer to voltage. But current plays as large if not larger role in torque. And it's very dependent on the drive. You can get 2 amp Nema 34's, and 7 amp Nema 34's, both with the same holding torque. The 7 amp motor will have far more torque at higher rpm, provided you were driving it with the optimal components. But this would likely require more expensive drives.

It's possible to get a 2000 oz-in Nema 34 that would have less torque than your 270 oz at 100 rpm, and it's also possible to get a 400 oz Nema 34 that would have much more torque at 1000 rpm.

It's all about properly matching the motor, drive, power supply, and drive mechanism. When comparing stepper motors, it's rarely apples to apples.

[SScnc]

Thanks for that information Ger21, i figured it was more complicated than I worded it, and it is. I have one nema 23 270 oz/in on my router table that is running both the Y axis screws via a timing belt. It's also got some reduction pulleys and another timing belt. I'll post some pics of the new build soon.

I have a Keling 36 volt PS and KL-4030 drivers running the nema 23's, what I'd like to do is to find a nema 34 for this axis that will have more torque and still work good with my setup. I'll contact Keling and see what they recommend and go from there. I've just read so many times here on the Zone that the torque rating on steppers refer to "holding" I had to ask.

Thanks again,

Steve

[ger21]

What you really should do, is figure out how fast you want to spin the motor and still have usable torque. That makes it much easier to decide what motor to use. If you want over 500 rpm, you'll probably want a 6 or 7 amp motor in the 400-600 oz range. Id also recommend a Gecko G200 series drive. Keling also sells a 570 oz Nema 23 that I plan on trying for my next machine, but again, you'll need a high current drive.

Have you thought about using a separate motor for each screw? That's what many people do. It may be a cheaper option. Acme screws waste 30-60% of the power due to inefficiency. Driving two screws can use up almost all the power and won't leave much left for movement.

Also, what screws are you using? Changing screws can give a tremendous change in performance.

[SScnc]

Hi Ger21,

Yes I did consider 2 steppers but decided to try one driving both via a timing belt and save that money to go towards good acme screws and nuts. Now I don't think that was a good decision as I think it's doing just as you said, using almost all the power of my 276 oz/in stepper. It does work reliably at 30 IPM max but that's just too slow, I'd like to achieve at least 50 or 60 IPM.

Here's the set up on the Y axis, one 276 stepper with a 22 tooth pulley driving a 44 tooth pulley that is connected to the left hand threaded rod. Then both threaded rods are connected via a timing belt running on 32 tooth pulleys on each rod. The threaded rods are 1/2-13 for now.

I've made a few test runs on this machine and the parts have turned out very, very good. This is my first attempt at building a CNC router, the main spindle is a Rotozip but it's speed control is shot so it runs at 30k rpm only, I put the old dremel style on it just for test because it's very quiet. You can see in the pics that the Rotozip is set on top of the carriage temporarily, it's normally mounted in an aluminum block I made on my X2.

The X2 was easy for me setup and is running perfect but I'm still trying to figure out the correct microstep setting on the KL-4030 drivers and steps per inch that work best on this machine. I think I have the Y driver set to 1/8 microstep and 42000 steps per.

Below are some pics, any advice is much appreciated, thanks,

Steve

For clarification, the pic of the stepper reduction belt drive is looking from the top down. And those are my attempt at harmonic dampers attached to the Y and X steppers.

[ger21]

Changing the 44 tooth pulley to a 22 tooth pulley should (or may) instantly double your speed.

You're doing a few things wrong.

One, the 1/2-13 is probably the most inefficient screw you could use.

Two, 13 turns /inch require the screw to turn 390rpm at 30ipm. My 1/2-8 2 start gives me 98ipm @390rpm.

Three, you spinning the motor twice as fast as the screw. The faster the motor spins, the less torque it has. So, at 30ipm, you're spinning at 780ipm. On my dual motor X axis with 253 oz motors, the max reliable rpm I can get is 640 rpm. But with my screws, I'm getting 160ipm. And spinning the motors slower than you are at 30ipm.

If you haven't seen this, it explains how different motor-screw combination's can give vastly different results.
http://pminmo.com/PMinMOwiki/index.p...chanical_Power

[SScnc]

No I hadn't seen that site, I'll take a closer look at it. As for changing the 44 to a 22 tooth, I originally had the stepper mounted directly behind the threaded rod connected via a lovejoy and I still could only get 30 IPM reliably. Any faster it would stall, With the stepper removed everything turns freely with no binding so I thought gearing it down would work better. The damper is there because I thought that was the problem but it really made no difference because the stepper is just stalling at higher rpm's.

I know I need to upgrade the screws and thanks for the details on what you're using, I'll go with those 1/2-8 2 start also. Do you think I should get a higher torque nema 23 stepper for this machine as it's currently setup ?

[arizonavideo]

Originally Posted by SScnc I have a Keling 36 volt PS and KL-4030 drivers running the nema 23's, Steve

This is the problem.

Your screws are slow but your voltage is also way too low for your steppers.

The 4030 driver is a 36V max driver so you have slow screws with slow steppers and a slow driver.

The steppers would work fine with 60v and direct drive, a Gecko 201 would work here.

Faster screws would help lots too and the cost would be less than new drives. You want at least 5 TPI

Going to a larger stepper wont really help with your 4030 driver. You can only make so much power with 36v at 3A.

Have you looked at the Gecko G251 drives? 50V at 3.5A is a lot better than 36V at 3A so you would get a fairly good speed increase but someday you will still have to go to a faster screw.

Don't tell anyone but a Gecko G201 will run two steppers together if they are smaller ones and that would let you run about 70V on your two Y steppers at 3.5A for only the cost of one drive. Even with the fairly high inductance of your steppers you will be able to go 700 RPM or more. They will be some hot buggers at 70V, do you live somewhere cold?

I like the look of your machine nice and clean.

With a high inductance stepper and a low voltage driver the torque will start dropping off at a vary low RPM like about 200 RPM. You might try a simple test and step up your gearing to 1.8 to 1 or 2 to 1 over driven with your stock screws and drivers. You should make about the same amount of force at the tool tip at 50 IPM as you do now at 25 IPM. It sounds strange but your motors right now are making peak power at 200 or so RPM if your cutting at 25 IPM now with whats looks like about 2 to 1 step down gearing then the motor RPM is about 500 RPM and at that speed it will be making about 1/2 full torque.

[johnmac]

You have a fine looking router! Changing out the screws should be the first thing to do. Steppers will produce best power at slower speeds. They will be happy at speeds slow enough to counts the turns!

John

[SScnc]

Thanks for all the helpful info guys ! I'm going to change the screws just as soon as I can. I now see that the steppers should turn slow for torque, I have another question while we're at it...

I have timing pulleys on each screw and a long timing belt connecting them to turn in unison, I don't remember the tooth count on these but they're about 1-1/8 in diameter, do the size of those pulleys matter ? I'm wondering if they are causing some power loss and should be as small as possible ?

Just for the heck of it, I reversed the 22 and the 44 tooth drive pulleys to make a 1:2 drive, it didn't work except for extremely low stepper rpm, like 20 rpm, but now I know.

Thanks for the compliments on the router, I had planned to build from Rockcliff's plans (model D) and then just before I started to round up all the raw materials I changed my mind and came up with this design while walking around the scrap yard. It's kind of a mix of a couple designs I've seen but the main detail is the tube rails patterned after the Sosylva (?) design. The tubes are the pipes used above garage doors that the horizontal springs are wound on. They are much thicker than electrical conduit, the scrap yard had a bunch of them piled up, brand new and straight.

Thanks again,

Steve

[arizonavideo]

What current are you driving the steppers at and what is the OZ rating?

With a 2/1 belt drive I would think you could do some good speeds unloaded but taking deeper cuts might be a problem. What is your acceleration set at?

I'm a bit surprised at the 20 RPM thing so you may have more drag than expected, at least the cost was low to do the test. What happens as you go faster than 20 RPM?

You do get a good amount of friction from the belt too but as they break in the drag will be a lot less.

[SScnc]

I had the current set at 2.7A and then changed it to 3A and I think my accel. was set at 4. When I tried to go faster than 20 IPM the stepper would stall and make that awful noise we've probably all heard at some time, very loud too.

I think the belt was fairly well broken in as I had made a warmup program for the router like I did for the X2 and ran it a lot while checking it out and also have made several parts with it. The parts turned out very good. I was about to order new screws and then found THIS post about belt drive. I'm expecting the new Kevlar belts and a couple more pulleys to arrive today from McMaster-Carr and have already converted the machine for belt drive. I didn't want to keep possibly dealing with the same problems. And, the belt setup is 1/2 the cost of new acme screws.

Before deciding to go belt drive, I turned the feed rate way down one this axis and watched the writing on the belt and it moved 48" in about 6 seconds. I grabbed the belt to see how much force it could handle and it tried to pull me into the machine ! Not very scientific I know but made me see that it had a lot of torque while moving the belt fast. EDIT: I should point out that I had changed the drive pulleys back to 2:1, 22 tooth on the stepper and the 44 tooth on the shaft driving the long belt.

I'll see how it does and post back with some results.