Hybrid Sphere

hybrid sphere
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Hybrid Sphere

I first became interested in woodturning after watching Tim Yoder on PBS. I watched how organic and creative the process was and it just looked fun. I saved up some money and bought a midi lathe which is what I still turn on today. At first, I didn't really want to make anything structured, like pens or bottle stoppers (even though I eventually learned to enjoy them). I wanted to take the biggest weirdest piece of wood and make it into whatever it was meant to be. I turned lots of bowls and platters until the market (our home) was saturated. Now instead of making something utilitarian like a bowl, I prefer to make more artistic pieces. I’ve used dyes and played with shapes and even made segmented turnings. I’ve recently discovered resin. Resin combined with wood opens up a ton of creative opportunities. I can dye the resin, dye the wood or both. I can also add pigments for added visual effect. All of these new ideas have brought about my latest creations, hybrid resin and burl spheres. I enjoy the challenge of learning the casting process and the creativity of adding color and texture to the spheres.

The first step in casting a blank is to choose a mold shape. I find that a disposable plastic cup makes a good mold as long as you use a resin that won’t overheat it. The inexpensive 1-quart measuring cups found at hardware stores work great, too. Depending on the type of resin you use, the casting will pop out and leave the cup perfectly clean and ready to use again. I prefer a round mold (like a disposable cup or a shop measuring cup). Since they’re already round, there’s less wasted resin and the turning goes smoother.

 

Marking the rough outline of the mold.

 

Sawing the blank on the band saw.

 

Disc sander table tilted to angle of plastic cup.

 

Burl sanded to a close fit in the mold.

 

Thin resin blend painted on burl to stop moisture. 

 

Don't guess at resin mix. Use a scale for accuracy.

 

Next you need an interesting piece of wood that fits the mold. Hybrid casts require a relatively small piece of wood, but it needs to be very dry and needs some character that draws your eye through the resin. For the piece in this article, I went with a small plastic cup and a gnarly piece of maple burl. The cup gets wider at the top which worked for this project because it would be wider right at the outermost part of the sphere. I used the cup to draw a rough circle onto the burl cap and cut just outside of the line on my bandsaw. I used the cup as a bevel gauge, to set up my disc sander and carefully nibbled away at the burl until it sat perfectly in the cup.

Once I got the wood to fit nice and snug, I set it on a silicone mat and removed as much dust and debris as I could. Dust and moisture are the enemies of a good resin cast. Moisture in the wood can react with the resin and cause bubbles. You can cook the piece in an oven and try to dry it out. You can stabilize the wood in a vacuum chamber with heat activated resin, too. Both of those processes can add a lot of time to an already lengthy process. I have had great success with painting the wood with a 2:1 penetrating epoxy. It penetrates deep into the wood and blocks any moisture from interacting with the casting resin. An added bonus is that when the penetrating epoxy becomes tacky and is almost set you can drop it into your mold and it will adhere, meaning it won’t float to the top when you pour the casting resin. I let the wood with the penetrating epoxy sit overnight before pouring the casting resin.

I used Alumilite Clear Casting Resin - Slow. It has a 12-minute open time and a demold time of 2-4 hours. To achieve a bubble-free casting I used a pressure chamber. Sounds fancy and complicated, but it really is just the pressure pot from an old used paint sprayer. The pressure reduces the bubbles to a microscopic size and will make the casting crystal clear.

I carefully measured the casting resin with a scale. This resin is a 1:1 ratio for the two components. Follow the manufacturer’s instructions if you use a different brand. I mixed the resin thoroughly for about a minute, scraping the sides of the container and keeping the stir stick in contact with the bottom of the cup to keep from creating bubbles. I then poured that mix into a clean cup and added my color. This ensures that your mix doesn’t have any unmixed resin on the sides. I mixed the dye until the color was nice and even throughout. With the resin mixed and the blank ready in the mold, it was time to pour. I poured the resin from a few inches above the wood nice and slow so that it didn’t fold over itself and cause bubbles (don’t worry too much because the pressure chamber will take care of the rest). I carefully took the mold with the freshly poured resin and set it gently into the pressure pot. I tightened the 4 handles and attached the air nozzle from the compressor to the inlet valve with the valve shut. Slowly open up the valve to let air into the chamber. Opening the valve slowly reduces the wind blowing the resin around in case you filled it to the top of your mold. I filled the chamber to 55 pounds of pressure and left it overnight.

The next morning, I pulled the casting out of the chamber and was thrilled to see a perfectly clear casting. That’s the payoff for being methodical in prepping and pouring the casting. I cut the side of the cup with a utility knife and most of the cup came off of the casting (the rest will turn off once the blank is on the lathe). With the disc sander table still at the angle that matched the cup I sanded the bottom flat and clean.

 

Cured casting being cut out of the mold.

 

Mounting blank to waste block. Aligning centers saves material.

 

When I’m bored, I turn waste blocks for future projects. I turn a dovetail tenon that just matches my 4-jaw chuck on various bits of scrap that’s around the shop. Then when I pull something out of a mold, I just grab one and go. I located center on one of my waste blocks and used a compass to draw a circle the size of the base of my blank. This will help center the piece when gluing them together. I used cyanoacrylate glue and an activator to join them. The activator is pretty instant but I still gave it a half hour before mounting it up to the lathe.

 

Waste block mounted in chuck and live center supporting outboard end.

 

This casting could turn into anything at this point – a bottle stopper, knife handle, drawer pull…you name it. But currently, I’m obsessed with spheres. I turned a full sphere, and I’ll explain the process, but you can make the project easier by turning “most” of a sphere and leaving a straight spigot on the end. Then it can become a gearshift knob, the top of a cane or any other connected sphere.

 

Present the tool at a downward angle for clean cuts on all plastics.

 

The downward angle prevents gouging the work and leaves a nice finish.

 

Rough-shape the sphere around the end with the tailstock removed.

 

Spheres can be intimidating. I learned how to make one using two shop-turned cup centers to hold the nearly-finished sphere. There are also sphere jigs on the market. They’re expensive but make the task very easy. Either way, the first step is to get the molded casting into something that starts to resemble a marble.

With my lathe speed set between 1200 and 1500 rpm’s, I started to rough the blank to a round shape. I used the Ultra·Shear Mid-Size Round tool, even though the Square tools are generally recommended for initial shaping of convex surfaces. I guess that shows their versatility. I also use a slightly different approach than the “factory recommended” flat and level. By bringing the handle of the tool up so that the tool is angled down (but still on the lathe centerline), I get a less aggressive cut. This really helps when working with resin castings or as in this case, resin-wood blends. It’s an approach similar to negative rake insert cutters, but I don’t have to change inserts and the superior edge on Ultra·Shear inserts gives me a better surface finish and no chip-out.

Once I had the sphere turned round all the way back to the spigot, I started sanding. I began at 220. Sometimes I start at 120 for all wood projects but I’ve found that 120 puts very deep scratches in the resin and they’re difficult to sand out. I sanded with the lathe at a low speed, then turned the lathe off and sanded by hand 90° to the turning direction and then at 45° Next, I wiped the sphere down with denatured alcohol and repeated the whole process with 320 grit sandpaper, then 400 grit mesh, then 600 grit mesh.  After sanding to 600 and wiping with alcohol I examined the piece to see if there are any noticeable scratches. This would be the time to address them and sand them out.

When I was sure there were no deep scratches from previous grits, I used Yorkshire Grit, a relatively new product which is a cream filled with micro-abrasives. I wiped it on the piece with a good even coat. Keep in mind that once you turn the lathe on, it can splatter so don’t use too much. I turned on the lathe and used a heavy-duty blue paper towel to polish the sphere. I use the blue paper towels since they are lint free and if it catches on anything, it will rip instead of grabbing it out of your hand. If you feel excessive heat and the Yorkshire Grit is drying out quickly, turn down the speed of the lathe. On something this size, 500rpm is about right. The abrasives in the product break down as you go through the process, so take your time. I usually go through the whole process a couple times and examine the piece to make sure there are no deep scratches. When you turn off the lathe and see a crystal clear sphere, you get the payoff for all the careful turning and sanding.

 

Sanding the sphere through a progression of grits.

 

Polishing the casting with Yorkshire Grit.

 

After Yorkshire grit the surface is flawless.

 

My next step was to part the sphere off the waste block. I used the Ultra·Shear Parting Tool-Ci pushing the parting tool into the waste block and not the burl. I parted it off with the parting tool almost all the way and finished with pull saw. This guarantees that the piece won't end up on the floor. I now had a sphere with a flat base. This is a great stopping point if you’re going to mount the sphere to something. To take it to the next level and make a complete sphere, you need to turn the base off with a pair of shop made cup centers.

I turned the cup centers from some walnut leftovers and checked the fit against the sphere as I went. I drilled the back side of the tailstock cup center to fit on the threads of my multi-tipped live center. I mounted the sphere between the cup centers, adjusting to get it as close to perfectly centered as possible. Using my Ultra·Shear Full-Size round tool, I slowly turned away the base until I just barely started turning with full contact all the way around. Then I re-sanded, using the same steps described earlier.

 

Cup centers hold the sphere for the final cuts.

 

Stem turned off. Ready for final sanding and buffing.

 

My final step was to use a 3-step buffing wheel system. It has three wheels and each has a different buffing compound. First is tripoli then white diamond and last is carnauba wax. I set the lathe at a medium speed, around 1000 rpm, and worked my way through the steps. I started with tripoli and slowly buff every side of the sphere, frequently changing directions. I then repeated with the white diamond wheel. The carnauba wax brings a soft glow and warm feel to the piece. I finished the buffing, stared into the sphere and enjoyed the view.

Between casting, turning, finishing, re-turning and re-finishing, hybrid resin spheres can be a bit tedious and certainly aren’t for everyone. But once you look at the crystal clear resin and perfect form, it’s all worth it. I certainly enjoy making these and even enjoyed the process of learning to make them, despite the “learning curve”. I hope I inspired you to try something new and to turn amazing things on the lathe.