Sphere Turning

Contents

• The Problem
• The Tool
• The Result

The Problem

The spherical end on a bar

I had to make a replacement part. The original was made from 5/8" extruded aluminum bar, with a spherical surface cut on one end. The radius of the sphere was approximately one inch.

This spherical surface is almost entirely decorative. Square corners would not work well because it has to be easy to slip the end into a square hole. The rounding makes this easier, but just about any roundiung would suffice.

This part is needed for an (admittedly minor) museum piece. It would be nice if it exactly duplicated the original. I did not have a sphere cutting tool for my Taig lathe, so I set out to improvise one in order to make this part. The entire exercise of making the tool was fast enough that if I ever need to cut a different radius sphere, I might just make another dedicated tool.

The Tool

I decided to make a fixed-radius sphere cutter, a far less general tool than, for example, Forrest Atkinson's ball making tool, (see this photo). or Keith Shaw's hemispherical ball cutting tool, (see this photo). These tools would fit my Taig lathe, but they have an adjustable radius, making them complicated, and they are designed for cutting small balls, so they can sit quite high on the lathe cross-slide.

Sphere cutter parts

My sphere cutter has the following parts:

A base plate

About 1" by 2" cut from 1/8" aluminum. This rests directly on the cross slide and is drilled with a 3/8" pivot hole in one corner and supports the toolpost on the opposite corner. I could drill other pivot holes for different diameter spheres.

A toolpost

About 1.5" tall cut from 5/8" aluminum extrusion. After mounting it on the baseplate (with a minumum of 2 screws), mount the baseplate on the lathe cross slide and drill the toolpost on the diagonal to hold your tool bit. Also drill a perpendicular hole for a clamp screw to hold the tool bit. (Ignore the fins at the top of the toolpost shown here, and ignore the vertical hole down the center. I used a scrap that had these features.)

A pivot bushing

This is turned for a snug sliding fit in the pivot hole in the base plate, with a bottom side that fits a T-slot in the cross slide, drilled to fit a T-slot mounting bolt, and a thickness just slightly more than the base plate.

A T-slot mounting bolt

A T-slot nut

Washers

A top nut

The stack of parts making up the pivot clamps the base plate to the cross-slide while allowing the base plate to rotate. I opted to use a headless bolt so I can fix the bolt to the cross slide first, center it under the lathe axis, and then stack everything else on from the top. I used a stack of two washers because the larger washer I had in my junk pile seemed a bit on the thin side.

A cutting bit

I ground my bit from the 1/8" shank of a broken carbide micro drill. This choice of tool determined the size of hole drilled in the toolpost. Mounting the tool in a diagonal hole through the toolpost means that most of the tool is inside the toolpost, with only a tiny stub protruding at the rear.

A clamp screw

With a 1/8" bit, I used a 6-32 clamp screw. The screw shown probably began its life as a jack screw for a DB-25 connector before it landed in my junk collection.

Sphere cutter bit

This close-up photo of the cutting bit shows how I ground the end. The original carbide micro-drill had a 1/8" shank, a conical transition, and then a tiny twist drill. When such drills fail, they usually break right at the end of the conical section. If any of the twist drill survives, it can usually be sharpened, but if not, the remains make good lathe tools. In either case, you need a diamond cut-off disk grind tungsten carbide, using low speed, light pressure and patience.

To make a cutting tool, I begin by grinding the conical end of the broken bit down to the center-line to make a flat cutting surface. Both sides of the cone can cut, and with a flat ground across the tip, with a bit of relief angle, you end up with two good cutting corners.

With the tool mounted at a diagonal as shown, the cutting face of the cone approaches the work at a shallow angle. When I grind a new tool, I'll put a rounded end on it. The tool pictured has seen significant use.

The Result

The result in use

This photo shows the sphere cutting tool after using it to finish the end of a 5/8" aluminum bar. The part being replace is also included in the photo.

The bar protrudes far enough from the lathe chuck that it was supported on a dead center during most of the cutting operations. The bar was approximately 1/4" longer than the finished end, center drilled for a tailstock center. The excess length is determined by the depth of the center drill hole, since in the end, this hole will be cut off.

I began work with a regular parting tool, cutting off the excess length of the bar except for a spiggot to support the center. I found out the need for tailstock support the hard way. This workpiece is long enough that using a parting tool on it without a center ripped it out of the lathe jaws, producing the scrap that I later used to make the toolpost for my spehere cutter.

Once I roughed the shape, I backed off the tailstock, moved the carriage out from under the workpiece and attached the sphere cutter. Centering the cutter pivot under the lathe axis is essential, and once centered, lock the cross-slide.

For each cut with the sphere cutter, turn the workpiece so its largest radius point is facing the cutting tip of the tool, then pivot the tool and move the lathe carriage so that the tip just touches the extreme point. Then, move the tip just out of contact and turn on the lathe.

With a square workpiece, every cut begins as an interruptede cut. That means you should advance very slowly! Pivot the tool past the protruding corner of the workpiece and you should hear a slight buzz. If you did the initial setup correctly, you should not be cutting anything significant on this first pass.

After each pass, advance the carriage toward the lathe chuck by a hair, clamp the carriage and rotate the tool into the workpiece to make another pass. Thumb pressure on the toolpost is sufficient. I used both thumbs, one to steady the toolpost and one to advance it. Cut all the way in to the spiggot, but don't try to part off the spiggot until you have everything outside the spiggot turned to a spherical section.

Set the carriag stop to record your final sphere-cutting tool position before trying to part off or cut off the spiggot. If you use the sphere cutter to part off the spiggot, you will need to move the carriage back and forth a bit. Aluminum is soft enough that I simply sawed off the spiggot with a razor saw.

Oncer the spiggot is gone, back off the tailstock and use very very shallow cuts with the sphere cutting tool to finish the stub of the spiggot. When the spiggot is completely blended to the spherical end of the bar, use fine silicon carbide sandpaper to polish the end.