BIG Potentiometers

Joined
May 15, 2005
Messages
260
where can I get one? Something like a DAMN BIG light dimmer, or fan speed and direction controller... The ones I have told of, are gray about 3 in tall , about 2 in mostly round.. red nob handle. looking for speed/dir control for my belt grinders. any ideas?

tnx/ al
 

polish avenger

Gold Member
Joined
May 28, 2002
Messages
1,531
If you're thinking of using a variable transformer or Powerstat for your grinders, I think you'll be disappointed. They work alright for applications where no real torque is necessary, but won't give you the power you need for grinding.
-Mark
 

Stacy E. Apelt - Bladesmith

ilmarinen - MODERATOR
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There are several ways to control the speed of a knife grinder, but one of them isn't by reducing the power input. You will either need to go with a VFD or change to a DC drive and motor.
Stacy
 
Joined
Mar 24, 2008
Messages
608
I'm going to jump in here with a question of my own.

Let's say I do have the 24VDC motor (1/3hp, permanent magnet). What speed control system should I use.

It's not for a knife grinder, but it's a problem that's been bugging me for a while.

Phillip
 
Joined
Jul 4, 2007
Messages
915
Rpaladin & Firebert: what kind of RPM are we talking about?, and Bladsmith is spot on on telling you you should never reduce power input, instead use step pulleys even a small manual transmission would work
 
Joined
Mar 24, 2008
Messages
608
Thanks guys for helping.

For my purposes, a transmission would not work well. The system is a direct drive (C-face mount). The mount has been calibrated so that the spinning disc runs true, any slop would lead to unacceptable errors.

At full voltage the motor runs at a few thousand rpms. I'd like to be able to bring this down to below 500. A loss of power would not be the end of the world as the friction generated during sanding and polishing is very low in this case.

Isn't there a way to preserve torque while reducing rpm.

This is for a gemstone faceting machine. It has been my pet project for a while. Most of the mechanics are figured out; I just suck at electronics.

Phillip
 

Stacy E. Apelt - Bladesmith

ilmarinen - MODERATOR
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firebert,
That type motor can be run with a simple DC drive. Ebay is full of them. K&B electronics makes commercial drives for DC motors. A lot depends on the motor, but for your use an simple drive will do fine.

A few comments on faceting machines - The table (disc) should be rock solid. Use a belt drive to transfer the motor power to the lap (VS is still a good thing on the motor).If you are using the motor shaft and the bearings of the motor to mount the dics on, it will have far too much side play. It may feel solid, but when doing a facet, a few ten thousandths of an inch will mess everything up. The mast or table shaft for the quill is the same. You can't overbuild these two things. For an 8" lap,I would go with a 1.5" shaft and the best bearings you can find on the disc table, and the same size and rigidity on the mast (or quill table shaft). I have been faceting for a long time, and can tell you that the rigidity is what you are paying for when you purchase a faceting machine. If you build one, you have to go as strong or stronger. Compare it to a 2X36 belt sander from HF and a Bader grinder. Both do the same thing, but for knife making the Bader will hold up better and longer. The Bader costs the same as 20 HF units, but is built 20 times stronger. Look at this photo of an Imahashi faceter and you will get some good ideas.
 

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Joined
Mar 24, 2008
Messages
608
Thank you for the help Stacy. I'll look into getting a silicon controlled rectifier.

I know what you mean about building it solid, the base of the machine I'm working on weighs over 35 pounds without the motor of faceting head. Direct drive was recommended to me as a way of eliminating possible deflection from the belt tension. I couldn't get any measurable slop form the mounted laps, but perhaps that isn't enough. If it doesn't work with direct drive, I suppose I can modify the system without screwing with everything too much.
 

pso

Joined
Oct 29, 1998
Messages
494
firebert

You can use an SCR to control the speed of a DC motor but it will take a fair amount of know-how and work. In the end, you will probably spend more money than if you bought a ready made DC drive as Stacy recommended.

Phil
 
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