Making a Variable Speed Bench Grinder

No. That sort of control needs a universal (brushed) motor, not an induction motor.

Buy a half-speed (1400 rpm) motor. Same thing, but the way the windings are connected is different. Good for HSS turning tools, still too fast for bench tools

Buy a geared-down slow, wet grinder. This is a _much_ more useful tool as it can sharpen nearly all woodworking edges. Prices for these grinders also range from the incredibly cheap upwards (quality to match, but a £30 machine is still useful)