Introduction: Mini Bench Grinder/polisher
Basically all I've done is mount and motorise a 'toy' grinder to make it a viable tool. Whilst not a particularly taxing project, I think the motor mounting and drive belt construction may be of interest to others, the initial idea was to make a bench mount for a dremel arbour. Then I remembered I had a miniature bench polisher/grinder in a box from when I was little. whilst it was meant to be powered by a static steam engine and as such wouldn't normally be operated at speeds over a few 100 rpm there was no reason it shouldn't be driven with an electric motor. There is the possibility that a brass shaft in solid zinc alloy isn't going to be a good bearing combination for the much higher rpm the electric motor will deliver. The stands for the grinder and polish mops are identical it is just a case of swapping felt wheels for grinder wheels.
Step 1: Components
The basics are an old miniature bench grinder an old buggy motor (originally brought for an experiment in electric flight) a pulley, drive belt and a board to mount it all on. Whilst I happened to have all this and the other sundry items needed laying around doing nothing it doesn't count as free as it all had to be brought at some point.
A quick check on eBay revealed that even though it was 30 years old in its original box and unused the mini grinder could still be brought for under £5 so I'm not about to ruin something valuable.
The Motor is a graupner speed 400, according to the spec sheet 22000rpm 1 to 7 amps in normal opperation 40Amps when stalled in this installation that should never happen as the belt will slip.
The drive belt is 2.5mm Diameter nitrile rubber.
Step 2: Mounting the Grinder
I used a pice of 4"×3/4" wooden board some #6×1" wood screws and screw cups. Mark the position of the holes you are going to use. Make pilot holes I used a gimmlet but a very small drill would do. screw the grinder down to the board.
Step 3: Mount the Motor
A pulley smaller than the motor body would have been better in several respects but I didn't have one. I drilled the bore of the pulley out to suit the shaft adapter I had. then found a pice of strip wood that would give a clearance for the pulley over the mounting board. decided on a position for it and tacked it in position (in hind sight I should have established directions of rotation before I positioned the motor).I then drilled a hole on both sides to take a cable tie. I chain drilled a channel between the two holes and cleaned it out with a blade so the cable tie sat under the base of the board. next I wrapped the motor in clear plastic tape other plastic tape will do but it MUST be plastic as it stops the car body filler sticking to the motor and from entering the vents. secure the motor in position with the cable tie do not tighten it fully at this point. align the drive pullies using a straight edge . fully tighten the cable tie make sure the lug is on top of the motor this helps if you need to access it later to release it. To make the mount more robust I used 2 pack polyester car body filler, I find that credit card sized plastic loyalty/advertising cards make ideal spreader or moulding plates and I applied a heavy filet to each side of the motor. with a little care the cable tie can be released with a scribe point or a pin and because of the wrapping of plastic tape the motor can be cleanly removed if required.
Step 4: The Drive Belt
you may have noticed I haven't made any allowance for size or adjustment of the drive belt. This is simply because I don't need to as cyno acrylate glue (superglue) works brilliantly on rubber in tension. It started out as a much larger 2.5mm diameter nitrile rubber drive belt (I think it came from technobots.com) cut squarely with a razor blade using a small engineering square as a guide (you could just as easily use a lego brick as a guide as they have wonderfully accurate square edges) this length of drive belt was then looped around the pullies to establish the required length then cut slightly short (about 15mm)and square if it's too lose you can always cut a bit more out. (you could glue another section in if its too short but I wouldn't recommend it the fewer joints the better) put the belt around the pulley on the grinder apply a drop of superglue to one of the ends and hold the ends together until cured, the drive belt can now be streatched over the pulley on the motor. As a demonstration I made a second belt up with the left overs and put it over a small clamp and streatched it out as you can see from the photos it streatched a good 9cm without distressing the joint. You are using superglue you might want to use tweezers. I find if I do get any on my skin the easiest way to deal with it is absorb it with a tissue whilst this can leave you with a bit of tissue paper stuck on you it will rub off in under a day and it's better than being stuck to yourself or something else.
Step 5: Apply Power and Test.
The wheels need to turn downwards on the side you are using.I should have established the prefered direction of rotation for the motor and which way the wheels needed to rotate first, but I didn't fortunaly small DC motors seem quite happy to run in either direction, so I applied power temporarily from a battery pack to esablish the positive and negative needed to be. For a quick test I used the same battery pack as I did to check the direction of rotation, but at a 6Amp load a 1200mAh pack is going to last 12 minutes or less, so a proper power supply will be required. To test polishing I applied a little polishing compound to one of the felt wheels then applied power and tried to buff a flattened out pice of copper pipe after just a few seconds I had a nice bright patch.