Stepper Motors

[Karl H]

I'm thinking of making a small(15"x19")CNC gantry milling table but am not sure of the stepper motors. How are they rated and what is a good brand to buy? Thanks -K-

[MTNGUN]

It's been years since I did my CNC conversions so I'm rusty on the subject, but I bought Vexta stepper motors and they have worked flawlessly.

I believe they are rated based on torque. Torque requirements for a given machine are hard to pin down, I suggest you look at similar (store-bought) machines and see what size motors they use.

Drivers have to be compatible with the motors, driver amps must be enough to satisfy the stepper. I decided to use Gecko stepper drivers, which are rated for 7 amps, and then selected the biggest Vexta stepper that would run on 7 amps. Likewise the DC power supply had to be sized big enough to supply both motors. You won't need that big of a motor for your little gantry, I'm just pointing out some of the issues in selecting components.

Even if you don't use Gecko drivers, the Gecko website has some excellent "Stepper 101" type technical papers that are a good read.

http://www.geckodrive.com/support.aspx?q=10003

Gecko also offers suggestions on sizing your system: http://www.geckodrive.com/support.aspx?n=667486

[kf2qd]

You might look into some of the Brushless Servos out there. They give much higher performance and are much more configuarable. And they give you option to operate in analog mode as well as stepper.

http://www.automationdirect.com/adc/Shopping/Catalog/Motion_Control/Servo_Systems/Low_Inertia_(100W_-_1KW)_Servo_Systems/100W_Servo_System_(Low_Inertia)

Automation Direct also has some stepper motors, but if you could afford it brushless servos are a real nice option.

[Paul Alciatore]

Be careful when you look at torque ratings. Like everyone who sells things, the motor manufacturers like to use the largers numbers they can get away with (example: 2x4s don't measure 2" X 4") and stepper motor manufacturers and sellers are no different. A common spec used it the "holding torque". This refers to the torque needed to force the motor to rotate at least one step, while being held at a full step position. This is the highest torque number that can be assiciated with the motor in normal operation.

But, the useful torque the motor will generate to actually move things is a lot less. A LOT LESS! It could be as little as 10 to 25 percent of the holding torque rating. And to make things worse, if you use microstepping, the torque drops a lot in this mode of operation. You may need to make enough microsteps to comprise a very large percentage of a full step to even make the motor move at all from a stopped position. And even then, the torque will be only a small fraction of the full step torque (which is only a fraction of the holding torque). Microstepping is not micro positioning unless there is no load.

This is why you see statements like MTNGUN's "...and then selected the biggest Vexta stepper that would run on 7 amps..." A lot of guys buy the biggest motors they can fit on the machine and and hope for the best.

The torque needed for a machine can be determined with a torque wrench on the handwheel while making the worst case cut. Of course, this only works if the machine is already built and in operation. I am working on a conversion on my Unimat lathe and took this more conservative path. I tightened the feeds down to a point that felt like more resistance than I had ever experienced while cutting and used a torque wrench (torque screwdriver in this case) to turn them, observing the reading. I then multiplied the torque figure I got by a somewhat conservative factor of five to convert from holding to running torque. I purchased some steppers with an available higher number for a holding torque rating, that will fit, and that weren't too expensive (about $20 each). Once I get them up and running, I will see how well this worked. If I need to, I will then purchase larger motors. My drive boards have a good excess drive capability and the power supply is separate so it can also be easily upgraded as needed. In fact, I am using a supply that is only enough for one axis while doing the development work, so I will need another supply in any case.

One thing not often spoken of in these considerations is that motors and systems have different characteristics and requirements for high and slow speed operation. For heavy cuts in metal (milling machine or metal lathe) the actual torque developed may be the most important consideration to prevent missing a step. For lighter cuts or just positioning the spindle for drilling or for wood work, the speed of motion and the ability to accelerate the weight of the table or gantry up to a speed may be more important. All of what I said above is based on torque ratings and slower speed motions as needed on a machine tool. If higher speeds are needed, as in wood working or for rapid slewing, then the Gecko sizing method seems more appropriate.

[outback]

Stepper motors are rated by oz/in. Or ounce per inch of torque. I have never needed to size a stepper motor because friends and former co-workers give them to me so I often use what I have on hand.

I would think about how heavy your machine will be, how fast it will move, and the horse power on the machine spindle. I always direct drive my stepper motors (with a flex coupling) to the feed screw. However, some people use timing belts to connect a stepper to a feedscrew to increase the torque and increase accuracy.

Probably the most important thing is the power supply to power the steppers.
Steppers seem to thrive on current more so than voltage. Most of the time the power supply voltage is increased to increase the current. Steppers need high electrical power when running at high speeds and under heavy loads. Don't worry about a power supply large enough to supply MAX power to all three steppers at the same time. That need just never happens. Even when X and Y are both rapid traversing at 45 degrees they are only using 50% of max power. Lots of people miss this.

I just built my third, 3 axis milling machine. Each axis is made of castiron dovetails. The table is 4" X "18. X and Y travel is around 6" X 12". I'm using 450 oz/in steppers made my Superior Electric. Again they are direct drive. Plenty of power. I also have plenty of preload on the feedscrew thrust bearings to minimize backlash.

Someone told me my Bridgeport has 1200 oz/in steppers. Nowdays new 1200 oz/in steppers are half the physical size of the original Bridgeport steppers and can me bought for around $120 ea.

Try to stay with 4-wire stepper motors. Steppers with 6 or 8 wires can be used but be sure you get a wiring diagram with those motors.

Outback