Here is a lapping spindle made for the Taig Micro Lathe. No doubt, it could be re-dimentioned to fit a Sherline or other small lathe.
The spindle can hold a lap made of copper (or a grinding wheel) to grind or lap very hard materials such as hard steel, carbide or jewel material. The spindle is driven by an accessory drive which has its own motor and speed control, using a fine rubber belt.
The spindle can be swiveled to allow grinding at an angle. The drive can be repositioned to make it possible.
The spindle frame is attached to a disk with JB Weld. The disk clamps with two brass clamps at the top and the bottom of the disk.
The drive is based on a magnetic clamp, normally used as a base for measuring instruments. The clamp can be clamped to steel screwed to the lathe board, or (as shown here) to a heavy bench block used for ballast.
The upper pulleys of the drive can be moved up and down to adjust belt tension. They can also "walk" from side to side on a long arbor to track with the belt as the spindle is moved laterally during operation.
The motor is an ordinary sewing machine motor. I power it with a variable speed pulse width modulation control.
The spindle runs in ball bearings in the frame, which is made of one piece of aluminum. While it runs in ball bearings, it is also made to be a smooth sliding fit inside the bearings to permit the reciprocating motion used when operating this kind of spindle.
The bearings are 1/4" ID by 1/2" OD, with a flange. The spindle ends with a knob which is free-turning on the spindle. This permits grasping the spindle with the fingers to produce spindle movement when it's running.
Here I have some grinding wheels mounted. The one on the left is a beautiful little resinous bond 180-grit diamond wheel from Harbour Freight. It mounts on the 1/4" spindle against a brass retaining ring now installed on the spindle nose. The hole in the ring was made undersize the diameter of the spindle by a couple of thou, then the ring was heated to expand it, tapped onto the spindle, cooled and then trued.
On the right is a resinous bond cutoff wheel made by Dremel. It's installed using a 2-56 set screw in the end of the spindle. Thus abrasives can be mounted in two ways.
It's hard to find vitreous bond grinding wheels in such small sizes, but there are many available in rubber bond from Dremel, Cratex and others.
The spindle frame is made from 1"x2" aluminum rectangle, milled in my Enco mill-drill. (The Taig mill would likely handle the job). The 1/2" bearing holes are drilled in the milling machine undersize, then reamed with a 1/2" reamer. Both holes are drilled and reamed simpultaneously to be sure they are aligned.
The spindle is made from 1/4" drill rod, polished in the lathe with emery cloth to a smooth sliding fit in the bearings. The disk is made of 1/8" aluminum.
The base plate is also 1/8" aluminum. The pin in the base plate is made from 1/4" drill rod. The 1/4" hole in the base plate which holds the pin is drilled undersize, then reamed to 1/4" minus 0.001" to produce an interference fit.
The knob on the end is retained in place by a collar which is drilled undersized, then tapered with a clockmaker's broach to make an interference fit. This is a tight grip, but the knob can be removed to make machining the nose of the spindle.
The brass clamps which clamp down the disk have a shoulder which overrides the disk, while the lower part is slightly under the thickness of the aluminum disk. The mounting bolts are 3/8" x 8-32, spaced at 1", which is the spacing of the Taig T-slots.
