How to Laser Engrave a Sphere

Laser engravers handle flat surfaces very well.  With an appropriate rotary accessory they can be made to engrave on a cylinder by making the cylindrical surface logistically flat.  To engrave a sphere there are two obstacles you have to overcome.  One caveat is that I only have experience with the Epilog Legend 24TT so all my designs, comments and results are targeted to that environment. 

The Problem

A client wanted to engrave company logos on a set of nested wood spheres.  These spheres, similar in concept to the Nested Russian Dolls, ranged from 1.25" in diameter up to 4" in diameter.  There were five nested spheres and each one would have something engraved on opposite 'faces' of each sphere along the joint where the top and bottom of the sphere split.  For reference sake the smallest sphere was solid.  All the others were hollow and were apparently made from a Russian wood called Linden wood.  This very blond, lightly grained wood is very soft, sands easily to a smooth finish and wicks sealer and finish rapidly to start.

Limit the Area to Engrave

If you limit the area to that needs to be engraved to a narrow ring  around the sphere you can effectively manage it as if it were a cylinder.  The width of the ring is determined by the radius of the sphere and the focal characteristics of the laser engraver.    The larger the radius the wider the ring.  Depending on your laser's focal characteristics you will most likely want to focus the beam on the highest point on the sphere.  You could focus on a spot to one side of the maximum diameter, but you have to be careful that the laser head does not impact your workpiece.

Holding the Sphere in a Laser Engraver - Plan A

Assuming you've defined a ring around the sphere in which to engrave the next hurdle is to hold the sphere so that the rotary accessory can reliably rotate the sphere.   Since most spheres will not have any sort of journal or tenon on the ends you will need to attach something so that it does.  A few solutions were suggested in the Saw Mill Creek forums on laser engraving (see this Saw Mill Creek forum thread ) including a cardboard tube such as a mailing tube.  This closely approached my idea which was to use PVC pipe adapters.  They are relatively inexpensive, can be found at most hardware stores, are made to fit / nest, come in a variety of sizes and can be machined if needed.  This seemed to be the ultimate solution for handling spheres of varying sizes.  

Thinking that this might not be the only time I need to engrave a sphere I thought a reliable, reuseable jig would be worthwhile.  So, I used a jig made of two halves that would become journals or tenons on the sphere.  Each half started with a 2" PVC coupler.  It is the right size to hold a 4" diameter sphere and the outside diameter came reasonably close to matching the diameter of the drive wheels of the rotary accessory (more later on why that wasn't so important).   Pictured here are all the parts for the entire jig.  One half is 'assembled' (not glued, just put together).  The black ribbon is a strip of rubber from an old bicycle inner tube.  The yellow ribbon is a packing strap material saved years ago for just this purpose (doesn't everyone?).   The clamp is a hose clamp often sold for assembling 2.5" diameter dust collection hoses.   All the PVC parts were chucked on the lathe, and slightly smoothed, especially the raised letters on some of the ends.  The 2" PVC coupling has a 1/4" hole drilled all the way through and a 1/2" hold drilled all the way through at a right angle to the 1/4" hole.  The 1/4" hole was made to hold a 1/4" brass rod that is used as a pivot point for the straps.  The 1/2" holes were used to pull the straps tight so they could be clamped on the outside of the coupling using the hose clamp.   Refer to the two assembed version pictures above to see what the whole jig looks like in operation.

The strap (one or the other, not both) is used to hold the two ends of the jig against the sphere.  

 Results and Critique

As you can see from this picture, I was able to engrave on the spheres.  However, the jig proved to be not at all useful on the smaller spheres and I opted to just engrave them while they sat still in the engraver the same as you would a flat surface.  The larger two I was able to use the sphere holding jig and you can see that the engraving goes about 40% of the way around the circumference of the sphere.

The good news is that this approach works.  The major limitation is that you must limit the engraving to a ring around the sphere as mentioned above.  The second limitation is that the strap going around the sphere means you cannot engrave all the way around the circumference of the sphere.  This could be solved if you created an external clamping mechanism that somehow pushed the two ends from the outside rather than trying to pull them together from the inside.

The bad news is that this is difficult to get set up.  I aligned the two ends of the jig on a lathe.  I chucked one in a chuck on the headstock and the other in a chuck on the tailstock (using a Oneway live center and appropriate adapter for the chuck).  This holds the two ends in perfect alignment while you adjust the sphere and everything else.  It allows you to tighten the straps and clamp them in place.  Then with everything aligned you dismount it from the lathe and take it straight to the laser engraver. 

Notice in the earlier pictures that there are small black and / or white pieces of material around the sphere where it touches the PVC jig.  The black pieces are more of the rubber inner tube.  The white pieces are double sided tape.  It turns out that double sided tape is not very effective on PVC.  It helped, but not nearly as much as I thought it would.

The bottom line is that I got the job done.  If I do more of these I'll consider a plan B jig that pushes the two halves to clamp them onto the sphere rather than trying to pull the two halves together.

Other Possible Solutions

Another plan offered was to turn two cylinders with some sort of 'V' groove around the waist or somehow contoured such that the sphere would sit in the groove or contour.  The two cylinders would be machined to fit inside the shoulders of the drive and idler wheels on the rotary accessory so that they would turn with the rotary motion of the drive wheels.  This sounds like a good plan and is worth of a test.  I didn't try it to start because there are two concerns.  If the two cylinders are not almost identical in profile and diameter then they may cause the sphere to run off course.  Likewise if the sphere is not perfect then it may not run true in the jig.  If you are trying to stay on a line  as I was on the joint between the top and bottom halves of these spheres even the smallest errors could cause big problems.  In this case I didn't have to time to set up a trial and create some test spheres.  I'll leave that as the perverbial exercise for the student.