I have a motor from an older vacuum sweeper (I dont know any specifics about the motor, yet)that I want to use for a homemade arrow crester.  I was thinking about putting a rheostat switch on it to slow the rotation speed at a whim.  Would this harm the motor?

No, but it will heat up the rheostat if the motor is very large. What you really need is a variable transformer called a Variac, but probably too much cost for this project.

Would it be possible to use pulleys to reduce the speed instead ?

to use pulleys would require machining some sort of brackets with very tight tolerances.  The idea is to get the arrow to spin true and not wobble or the lines you are cresting will end up very ugly.  It is easiest to just machine a brass fitting that will fit the end of the "shaft" on the motor and screw into the arrow insert.  Does the motor size or supply voltage determine which rheostat I need?

One thing about those vacuum motors, the cheaper ones (like for your house) will burn out eventually if you use it a whole lot.  The commercial vacuum motors (for office buildings) would be the best bet because the motor is designed to run for longer periods.  Your crester will last much longer. Just an idea.

this was a residential model [green]freebie[/green]!!  and it doesnt have to last long as it will probably only be on for minutes at a time once year when I run out of arrows

Sounds great but...

Vacuum cleaner motors typically run at very, very high speed.  Like around 7000 rpm.  your arrow is going to have to be very tightly fixtured at even a quarter of that speed to prevent it wobbling out of control.  You will  need bearings of some kind on the free end and most likely a support in the middle.  You would be better served using a discarded gear motor that tops out at less than 200 rpm.  Think rotiserie(sp) and humidifier motors.

My two cents on the mechanical feasability,

Kent

The bigger the motor, the more current it will draw and hence the bigger rheostat you'll need.

If you pick one too small, it'll just heat up due to the current, no harm to the motor.

I'd suggest that you try one of the variable speed ceiling fan controlers from Home Depot or whatever.

[green]canoe[/green]-you are correct.  300rpms is ideal, that is why I wanted the rheostat to be able to control speed.

With commonly available fan controls you probably won't be able to control the vaccuum motor as it draws much more current than a ceiling fan and those little controls aren't made to handle it. Just varying the voltage will slow the motor somewhat but the speed is determined by the windings of the motor and the frequency of the current applied, thus to properly reduce the speed like you want to you need to reduce the frequency and hope that the motor will do it w/o heating & self destructing. Sounds like you'd be better to look around the thrift stores for an old rotisserie as mentioned before, they have a built in gear reduction and are made for similar duty and are small enough to use a fan speed control on. Then all you have to do is come up with a mounting fixture.

I think you can still use it. But in two different ways.
First off, a worm gear setup would give you the reduction you need. Also you already have the turning brackets if you modify the pieces that hold in the beater brush on the front of the vaccume. Cut it in half and extend it and you have what you need there. The worm gear and matching gear will be what cost you to reduce the speed of the motor.
Or since the beater brush was already belt driven, you could just find a bigger pulley to make the beater brush turn slower. Then you just need to fabricate a mount for the motor to keep the pully tight. It may be higher off the table since the pully will need to be very big.

If it's a vacuum motor it's probably a universal (It has brushes) motor. To control a universal motor, go to Your local (Or internet) woodworking store and purchase a Router speed control (PWM or "Chopper" for those who are into these things). This will have the "Power" to do what You are looking to do. Using a "Fan" control will be troublesome at best because they are for a different type of motor (induction).

Hope That Helps!
Tall Shadow

Just a second  thought.......

For smaller(5amps or less for a 600w dimmer) motors(Universal) a lamp dimmer can be used for a short time.

Be safe!
Tall Shadow

Lutron makes a high-current (5 amp) fan motor speed control (in addition to their more common one that's only good for a couple of amps). Lowes stocks them, for about $13, IIRC.

However, getting 300 RPM out of a motor designed to run at around 7000 RPM doesn't sound very promising. The motor will either stall or want to spin at a much higher speed. It sure won't have much torque at 300 RPM. Anyway, most dimmers and motor speed controls won't let you turn down the effective voltage below a certain point.