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A Burnisher for the Mini-Lathe

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Roller Burnisher (Click to enlarge)

I saw a roller burnisher for a large lathe on Youtube and adapted the concept for the mini-lathe. The burnisher in the video works well on a large lathe but extends too far from the toolpost to be practical on a minilathe. The tool shown above duplicates the design concept but protrudes only about as much as a normal cutting tool so it works nicely on a minilathe.

Burnishing cold works the metal, flattening the peaks of tool marks and extruding them to fill nearby low spots left by the tooling. Cold working smooths and work hardens the metal, producing an almost polished looking finish. When starting with a fairly fine finish, burnishing will reduce the diameter a couple tenths, more if you start with a coarser finish.

As usual, my tool was made from material in my scrap box - the parts are shown at right. The 1/2" square HRS stock is 2.25" long. The 1" square piece is 1/4" thick with a 1/2" wide slot 0.050" deep across the center. The 1/2" square stock fits into this slot with a 6-32 flat head screw securing the two together; the edge of the 1/2" square stock is flush with the end of the slot. With these parts screwed together, scribe a line in the slot to mark the edge of the 1/2" square stock, remove the screw, and separate the pieces.

The ball bearing I used is a 100-kszz which is 26mm OD and 10mm ID but any (preferably shielded) bearing of this approximate size will do. The contact ball I used is about 0.342" diameter (from a large ball bearing) but any similar size hardened steel ball will do. Drill a pilot hole centered in the slot, half of the ball bearing width from the previously scribed line. Carefully enlarge this hole with a drill matching the diameter of the contact ball; drill until the tip of the drill causes a bump to protrude from the side opposite the slot. File or sand this bump off, insert the ball and check how much protrudes. Repeat until ball protrudes and a thin ring around the lower end of the hole retains the ball - click the picture above to see an example of what this looks like.

Turn a bushing to fit the ID of the bearing and part it off about 10 thou longer than the inner bearing race. Hold this in the 3 jaw chuck with a shim of 10-20 thou on one of the jaws to offset the work slightly. Drill from the tailstock with a 10-32 clearance drill to produce a slightly eccentric part, this to allow adjusting spacing between the ball and the ball bearing. Add a witness mark on the part with a punch adjacent to the shimmed jaw.

Assemble the parts using the 6-32 screw and insert the ball into the hole. Hold the bearing+bushing in place against the ball with the witness mark at 90 degrees from the ball. Use a transfer punch in the bushing hole to scribe an arc onto the 1/2" HRS by pushing the transfer punch toward the ball and sliding it up and down across the HRS. Mark the center point of this arc on the HRS, drill/tap 10-32, add a washer and assemble so it retains the ball. Adjust the eccentric (if necessary) so there is a few thou clearance between the bearing and the ball.

Add a few drops of oil on the ball bearing surface and on the work prior to burnishing. My preference is to use the vertical shear tool to produce a fairly smooth finish prior to using the roller burnisher. The burnisher is centered on the work and advanced until it touches, causing the ball bearing to rotate. The tool is then fed into the work a couple thou to produce the desired finish as it passes along the work. Flexible work may need support by a center in the tailstock.

At right is a piece of 3/8" mild steel where I did a skim cut followed by the vertical shear tool for 2/3 of the length followed by the burnisher for 1/3 of the length. I wiped the oil off with a paper towel (which left a couple dust motes) so oil would not add a false sheen. 5 thou infeed was used because the work deflected - support with a tailstock center might produce a better result. The first pass produces the biggest effect, subsequent passes produce only a little improvment. It's helpful to chamfer the work prior to burnishing else a sharp wire edge is extruded on the end, which needs a little filing to remove.

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