Newbie- Stepper motor size needed??

[Crevice Reamer]

Originally Posted by btrager1978 One question, that is the force required to turn the shaft at the actual shaft dia and not at the outside handwheel dia right. Thanks for all the info it is greatly appreciated. Joe

Yes, but it is easier to MEASURE the force at the outside diameter. Then you can, multiplying by the radius, translate the force to the shaft diameter.

The force will change depending on whether you use ball screws or the stock Acme screws. Ball screws are super easy to move and retain little backlash. Acme screws need to have gibs and anti-backlash nuts fairly tight to reduce backlash--and consequently need more turning force.

CR.

[btrager1978]

I checked out the last set of images, some usefull ideas for sure.
I agree the tweak to eliminate the head, side to side movement is interesting, I will look into that some more, I think the part with the bearings wouldnt hurt to be built with some more beef, definitly a good idea. It's also got a few other brainstorms started that I'll have to look into.

Thanks, this is getting more fun the deeper I get into it and I still have a ways to go :-)
Joe

[jalessi]

Joe,

You will have to know what type of screw you will be using before you determine the stepper force requirements.
If you are not looking for maximum performance it is not that big of a deal as long as you don't under size the steppers to much.
There have been so many RF31 type CNC conversions done already using 425 motors with pulley reduction, so that wont be a issue.

If you did decide on direct drive you might need a bigger motor maybe if the ways are super super tight.

Pulley reduction will slow down the fpm however you wont ever machine that fast anyway!

There are guys using 250 oz servos with pulleys that have snapped off 3/8 tooling.

If you have a 5 tpi screw and 2 to 1 pulley reduction you have 156.25 lbs of force.

That is a lot more that you will ever need.

[btrager1978]

Ok, yeah That makes sence, I'll do that.
I tryed a tweak to help eliminate backlash on the stock screws, it has done wonders useing it in manual mode, It involved pressure fitting delron rods from both sides in the nut,so the leadscrew rides mostly on the delron for minimum friction and backlash.

Thought it will be worth giving it a shot, I can always upgrade to the ballscrews if needed.
Thanks
Joe

[jalessi]

Joe,

I may have jumped the gun a bit suggesting stepper torque values because when I hear cnc conversion I assume "ball screws"

Due to the inefficiency of the stock lead screws the torque requirements are larger.



Jeff...

[btrager1978]

No problem I'll look into that tommorow and see what I come up with. I know this doesnt give the info needed yet, but with the delron tweaks i did on the nuts it turns much easier then it was turning with the anti backlash screws adjusted to were the backlash was somewhat manageable.
I'll have to measure the backlash, to the eye i cant see any so it has improved considerably.
Thanks
joe

[Crevice Reamer]

I'm sure YOU know this all Joe, but some may not:

BACKLASH: When reversing direction, any handle movement that does not also move the axis (or table or head/quill) is backlash. It is measurable directly by the dial on the handwheel.

For CNC, backlash must be checked and adjusted often. Backlash will turn a circle into a vague blob..

ACME SCREWS are the standard for most manual mills. They are just a relatively close tolerance screw thread and give fairly high precision and backlash while the adjustment lasts.

Acme screws and nuts wear quickly. Usually the screw wears most in the middle and less on the ends. After a while, you can't use the ends because it's too tight.

Even relatively cheap ballscrews, which HAVE some backlash, are better because the backlash does not vary so often. Mach3 (The hobbiest defacto best computer software for machine control) can compensate for backlash that doesn't keep getting worse

BALLSCREWS have large threads that allow a ball bearing to roll IN them. The ballscrew nut contains many small steel balls that recirculate inside to reduce friction. The ball nuts can be extremely tight to eliminate backlash--yet still have little friction.

Once ballscrews are installed, manual control is lost. Because ballscrews turn so easily, the table or head will not hold a position, but is free to move on its own. So while you COULD install hand cranks on double shaft motors, you would have to constantly lock the gibs on the other axes and it's just not practical.

Ballscrews come in two types: Rolled and ground. Ground ballscrews are best, but can cost thousands of dollars for just one screw. We small-time automators usually can't afford them.

Rolled ballscrews come in several grades. The better they are for accuracy and low backlash per length, the more they cost. We usually use a medium grade.

If you buy say a six foot length of ballscrew, it needs to first be cut to axis lengths. It is hardened material, so this is usually best done with an abrasive cutting disk.

After they are cut, each end is turned down on a lathe. Because they are hardened, this is difficult to do. One end is usually turned to one diameter to fit a bearing. The other end may be turned to several decreasing diameters to accomodate thrust bearings, threaded for clamp nuts, and turned at the end to fit stepper coupling or pulley.

Once you have determined the LENGTH of the screws you need, there are companies who will make your ballscrews to order.

PULLEYS are used to increase torque by gearing down the motor RPM. However, stepper motors get weaker as speed increases, (To a limit of 800-1500 RPM depending on PS voltage--up to 20-25 times motor rated voltage if the drivers can handle it.) so most of the gain in torque results in lost speed. That's why most stepper motors are connected direct drive.

STEPPER MOTORS usually have a hardwired step count of 200 per revolution. At this FULL STEP resolution, 200 computer pulses will turn the motor once. Micro stepping theoretically increases accuracy at the expense of speed, and promise extremely high accuracy by increasing steps per revolution, but practically 8 or 10 microsteps are the limit. There is a limit to how many pulses the computer can put out, and the higher the step count, the slower the motor will run.

SERVO MOTORS, which are more expensive, do not have the starting torque that steppers have, but they maintain what torque they have into high rpms. They are usually geared down 2 or 3 to 1 to gain starting torque. Even geared down, they can still attain thousands of RPM, so speed is not a problem with pulleys.

Servo motors are also equipped to tell the computer (through encoder feedback) exactly where the motor is at any given time so there are no missed steps. Stepper motors can stall and miss steps unbenownst to the operator until the finished part is measured.

Each system has its pros and cons. Steppers used with proper power supplies are reliable, consistent and cost effective--That's why most hobby applications use steppers.

NEMA SIZES: Both steppers and servos may come in different Nema flange sizes. We usually use either the smaller Nema 23 or the somewhat larger Nema 34. The torque may overlap between the sizes, but generally the larger motor has an easier time.

For example, a 500 oz Nema 23 stepper motor will be working hard (and getting hotter) to attain the torque at which a 500 oz Nema 34 will be easily cruising. Generally, power is added by extending the length (stack) of the motor.

CONTROL DRIVES: Stepper drives are the electronics that translate the pulses from the computer into useable current for the motors. They are fairly expensive and many are subject to being destroyed easily.

Generally, the more expensive drives offer the best features like overheat protection, micro stepping and speed morphing. Steppers tend to get hottest standing still. Morphing drives micro step at low speeds but vary to full steps at high speed.

Actual expensive commercial servo drives use a different system than stepper drives, but OUR affordable servo drives use basically the same pulse system as stepper drives.

GECKO DRIVES are generally acknowledged as the best. Gecko "Vampire" drives are virtually unkillable.

BREAK OUT BOARDS: Mach3 uses the many wires in a parallel port (printer) cable to send control from the computer to the drives. Rather than fastening each tiny wire in the cable to its destination, the breakout board accepts the cable plug and then puts each wire on an accessable screw terminal.

The new low-cost Gecko G540 board will combine tiny cheap drives with a "Vampire" breakout board so that all you need to connect is the parallel cable, power wires, and motor cables. In a couple of months, CNC conversion is going to be a LOT easier.

CR.