Variable speed is a BIG plus for knife sharpening. Unfortunately it is an expensive choice too. I have a
Burr King 1.5 HP variable speed motor running on standard wall current (2x72" Knifemaker model). I'm also in the process of upgrading my 2x72" Coote belt grinder to a 3hp variable speed motor running off of a 240 volt 30 amp single phase line. The motor itself requires three phase current and the VFD (variable frequency drive) converts single phase to 3 phase current. For variable speed you need a 3 phase AC motor. I can go into a good bit more detail if anyone is interested. Another alternative is DC motors - yet another topic. I'm upgrading from a treadmill motor that is on the Coote now. I have a little 1x30 Delta belt grinder. These days it acts primarily as a belt hook for the larger belts
It is similar to the Harbor Freight unit. Tough to beat for the price, but there are things it just can't do. There's a reason for more expensive units. The Kalamazoo is attractively priced as a step up from the HF or Delta level of units.
When I was using my HF, I redid the platen by attaching some float glass to it. This gives a much nicer platen for flat grinds and adds a bit more tension to the belt too. A simple upgrade.
Another alternative for belt grinders that do not directly link the motor shaft to the drive pulley is to get a pulley with multiple diameters so you can select the pulley size and therefore the speed of the belt.
I go from 24 grit belts to 4000 grit belts with steps inbetween. Beyond that I use leather belts with and without compounds. Not a big fan of felt or cork belts. For compounds, I use CRO and Boron carbide, but have recently introduced CBN pastes for leather belts, extending the range of options from fairly coarse grits on up to tenth micron CBN. These CBN paste preparations stick well to leather and condition the leather itself too. In the future I will be introducing nanocloth belts, for 1x30, 1 x 42 and 2x72" belt grinders. This should give exceptional final finish results. It is a work in progress. I find leather belts have rough 'joins' and the thump from the joins annoying, but used well they give excellent results. I may produce some kangaroo leather belts in the future, but especially for the 2x72" size, I'm hoping to find a genetics engineer who can grow some 8 foot tall kangaroos for me
I can review various belts if anyone is interested.... in a followup posting.