SparkFun Forums

[dpolcari]

Hi everyone, I'm having a little problem with choosing the right type of motor for my project and would really appreciate any help. The goal of my project is to create a mini polisher/grinder. This means having a motor spin at a controlled speed while a metal disk is placed on top. This disk will have sandpaper on it such that I can grind some really small (micrometer size) glass tips. I need the motor to have 2 modes: slow (~40 rpm) and fast (~400 rpm). Can anyone suggest which motor would be best for this and how exactly i could go about controlling the speed of rotation. I really appreciate the help!

[Mee_n_Mac]

Can I assume that the grinding does not require a lot of motor torque ? If so then perhaps a small stepper would be the way to go. You'd need the motor, a stepper driver, a controller (micro-controller of some sort) a power supply and then the mechanical bits. The micro can precisely dictate the speeds, so long as the motor doesn't slow under load. A knob/potentiometer can tell the micro to "dial up" most any speed. Or a simple A/B switch to get the 2 you've mentioned.

A small DC motor could also work. You'd use a micro to control the speed via PWM and the driver could be a simple FET switch. You could even implement a feedback control mechanism to ensure the speed was maintained despite any (reasonable) load.
http://en.wikipedia.org/wiki/Pulse-width_modulation

How small a package are you looking to fit into and what level of $$s are you thinking of spending ? Is there a similar commercial product you could link to ?

[dpolcari]

Your assumption is right. You can essentially think of this as a grinder for a needle. I need to make that needle into a disk. Since it's glass I don't need that much torque. I'm a little confused about using the stepper driver and microcontroller. I thought it would be possible to use just a POT to vary resistance and thus change the voltage to decrease speed. Would this not work?

For budget, I wouldn't mind going up to about 200-300$. There isn't really a small commercial product... some people use a huge grinder:

http://www.struers.com/default.asp?doc_id=269&popup=1

I want to be able to do the same thing (much more crude) but much smaller scale. Realistically, I would like the total "machine" to be about 6-12 inches. I have no real experience in microcontrollers so any extra help you can give me in choosing one and using one would be great.

[fll-freak]

I would think at this scale, the accuracy of the bearings might be the biggest concern. Any wobble, side play, runout, or asymmetry might ruin your object.

[Mee_n_Mac]

^^ this above ^^

I was going to say that the electronics are the easy part of this project, getting the mechanicals correct might be difficult. To that end I might think that a very stable base would be on your shopping list, and that would be driven via belt by the motor. While you're musing over that, I'll make up a potential parts list.