Motor Resonance
Stepper motors inherently can have an operational problem known as mid-band resonance. This can occur at different speeds (step frequencies) on different machines, and is motor, drive and voltage dependent. At the resonance point, the motor can loose torque. For example, the Sherline in the demonstration below, uses a 20 TPI screw (32000 steps per inch). This means high steps rates are needed even for small movements. This causes the motor to reach its resonance area sooner. If the system stays in the resonance area too long, the motors can run rough or even stall. Once past the resonance point in speed, the motor can regain torque and can continue on. Depending on the setup, it may be possible to accelerate on by the resonance point. For example, in one test performed, with settings for with 5 TPI screws (comparable to what a router might use with 8000 steps per inch) the motor could accelerate up to, and run at 98 IPM with no problem, simply by accelerating past the resonance point.
One way to reduce the effects of the this resonance if it does pose a problem to your system, is to install a resonance damper. Watch the videos below to see what a difference resonance dampers can make on a Sherline mill. Look at the bottom of this page to see how these dampers were made.
A Sherline Mill with Xylotex 269 oz.in 2.5A stepper motors driven by a Xylotex 2.5A 4 Axis Drive
The mill has dampers installed on the back of the motor.
The speed is about 44 IPM -the fastest Mach demo will allow-. I've used these dampers on a Sherline CNC Mill to get over 90 IPM using the DOS based program TurboCNC instead of Mach3. A machine using coarser thread like 10 TPI could go even faster. Ballscrew machiness at 5 TPI can achieve close to 200 IPM (no load).
The same mill as above with the dampers removed. Same software speed. Immediate stall because it looses power to resonance before it can get past the resonance point
The same mill again, but with no dampers, and the speed turned down to about 15 IPM. Runs fine at about 1/3 the speed since it never has to go through the resonance point.
The video below shows a Xylotex 3.5A 40V drive running the X axis of a Chinese router at 147 IPM (top speed Mach3 will allow in demo mode at the screw pitch) with a Xylotex 3.5A 425 oz.in. motor. By Switching to Mach3 with 45KHz I was able to get 185 IPM (but 147 IPM shown in the video).
Same setup but with a Resonance Damper shown below I was able to get a quick 255 IPM using the same setup.
Damper Parts
http://www.mcmaster.com
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McMaster # 89955K77
Easy-to-Weld 4130 Alloy Steel Round Tube .500" OD, .120" Wall Thickness, 6 foot Long
or use 1/2 aluminum rod and drill out hole
McMaster #6435K14
One-Piece Clamp-On Collar Steel, 1/2" Bore, 1-1/8" Outside Dia, 13/32" Width
McMaster #6435K16
One-Piece Clamp-On Collar Steel, 3/4" Bore, 1-1/2" Outside Dia, 1/2" Width
McMaster #2868T12
SAE 863 Bronze Sleeve Bearing for 1/2" Shaft Diameter, 3/4" OD, 1/2" Length
www.grainger.com
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Santoprene Washer, Fit 1/2 In, Pk 50
Grainger Item # 4PAL9
Flat Washer, Steel, Fits 1/2 In, Pk 100
Grainger Item # 6DZA4
The washers should be slightly loose between the shaft collars. This will allow them to absorb some vibrations. They should not be loose enough to make noise when spinning though. The weight of the shaft collars add mass (inertia) to the damper. The hole for the motor shaft should be about 1 inch deep and cut cross ways to allow easy fitting and compression over the stepper motor shaft. Never hit the damper or motor shaft to get the damper on or off the motor. The bronze bushing is used to convert the 1/2 inch shaft to the 3/4 ID shaft collar. It is slit to allow easy fitting and compression over the rod. The larger shaft collar provides a nice way to manually jog the motor. Once the large shaft collar has been fixed to the rod, the set screw can be ground to match the circumference of the collar. The number of the alternating types of washers starting with a steel washer is 1-1-3-1-3-1-3-1-1
