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Builder’s Tip – Making a Spinner
From January 2015 Bits & Pieces Newsletter
By Bill Evans, EAA Chapter 266, EAA 794228
Typically, commercially available spinners come with one or two bulkheads for attachment. Skullcap spinners may come with a simple metal strip bent into a U-shape. It has one anchor nut at the center for attachment. Many of the postwar Continental engines such as the 65, 75, or 85 series engines may be fitted with spinners. Even if I had one, I might buy a spare if they were affordable just to be sure of having a backup.
It would be hard to spin a prop spinner from aluminum. Metal spinning is not only a technology; it’s an art form. Most machinists cannot or do not spin. Metal blanks are often a lot smaller than the finished item. But they can be spun on a wood lathe. One (not 12) could be spun from a hardwood spin block. Great force is applied to the blank or plug that is forced against the spin block with sufficient force to thin and expand the aluminum blank.
This action stretches it from a 5- or 8-inch D-plug to a cone shape maybe 9 inches in diameter and 11 inches long. The spinning paddle has a metal end and is placed under the arm, and you place all your weight against the paddle. New spinners always have two or three very sore ribs, which may also explain why most machinists do not spin. You may have started with “O” condition (soft) aircraft aluminum. In this example, spinning work-hardens the aluminum to maybe T3 or T4 hardness. Thus you don’t need to have your spinner heat-treated. You will need a suitable grease or semi-grease to spin metal.
Humour me here. Surely you know someone with a wood lathe? I’d estimate you could make a fibreglass spinner for maybe $100 if you scrounged material for the spin block.
- fibreglass cloth – $10
- epoxy resin – pint – $15
- filling primer – $10
- colour topcoat – $20 (colour matched)
- attaching hardware – $5.
In this case, the spinner is bigger. I would glue pink foam blocks together, then glue them to a plywood block with fabric spray glue. The plywood piece is a template and should be turned to the maximum diameter of the spinner. The block would need to have a metal shaft attached which could be solidly installed in the headstock.
There are several ways to do this. Spin and carve the foam (glued to the plywood block) to the spinner shape you want. (Make a full-size drawing and get carpenter’s calipers.) Mark out the turning points with the calipers to the drawing dimensions and then make a cut every inch or so. Then use a wider tool to carve the foam to the shape those cuts indicate. Sand it smooth, with a block and paper. Cover it neatly with red plastic building tape. No folds. No wrinkles. Wax it well. You could paste wax it and polish the wax in the lathe.
Mount the newly turned spin block assembly tightly in a vise. It should be placed vertically. Place newsprint underneath. (Put on goggles and latex gloves.) Again avoiding wrinkles, lay up three layers of the fibreglass cloth sold at Canadian Tire, auto stores, or plastics supply companies. Don’t overlap the seams of the three layers. Place them 120 degrees apart. You could avoid overlaps by pinking the edges of the cloth, then using a zig-zag sewing machine or hand sewing for a binding stitch to join the edges into a sock, fitting the spin block closely. Hand sewing may be easier unless you sew well. The writer has five machines. I’d use a Bernina 530 or other zig-zag portable for this task.
I’d use epoxy resin at three times the cost of polyester, because the smell is much less and it is superior in strength and everything else. Use a spreader to carefully remove as much resin from your layup as you can without damaging the layup.
If you have never used fibreglass cloth and resin, this is a fairly neat and simple project to begin with.
Wrap the whole thing in several layers of plastic wrap like Saran Wrap. Squeeze the layup from top down to make it as tight as possible to the spin block. Get it tight! Wrap it again and squeeze it tighter and continue until it just can’t get tighter. This actually has a name: “the Saran Wrap squeeze”.
However, you could use the vacuum wrap if you have one. Let it cure a few days.
Trim off the overhang while still on the spin block. Maybe you could use a parting chisel and trim it off on the lathe. It gives you a nice, clean edge. Sand it smooth. (Do yourself a big favour; use a block and sandpaper to sand the spinner in the lathe, while still on the block.) Goggles and particle mask are needed.
Your layup needs to be tight and dry because it needs to be balanced and its mass even, as it will be turning at 3,000 rpm. The three fibreglass cloth seams should be 120 degrees apart. Unbalanced components cause vibration in the crankshaft.
The layups also need to be close to perfect; body putty adds weight to a rotating component and it would be hard to balance a filled spinner.
Auto parts stores such as Carline sell expensive body putty and catalyst. It might be $40 per can.
Use hardboard strips between the layup and red-taped plug to separate the spinner from the block.
Sometimes layups distort after fabrication. Who knows why? It might be smart to keep the spinner on the spin block until you have made bulkheads. Once fibreglass is fully cured, it usually does not distort.
Put on paint mask, goggles, and gloves for sanding, etc. The spinner can be primed and block sanded, then painted using the filler primer. Prime and sand until it is perfect under a strong light. As always, apply the topcoat with a fine, thin spray to start, then 10 minutes later apply the thinnest coat you can.
Karl Heindl showed me how to spray-paint in about 5 minutes. No runs and no sags. You need those halogen lights or the equivalent on a stand. Bright is the operative word here. You operate your spray gun/can a foot or so from the spinner. It is tilted at 45 degrees sideways. You spray the spinner vertically and position yourself to be able to see in the paint reflection whether the paint line is continuously wet.
Apply paint to that line, paying constant attention to getting a surface that is just barely but continuously wet. No speckled spots. Keep your eyes where they can see that the paint being applied is wet (shiny) on the surface or not. Judicious application is the operative term here. Once the sprayed surface is “wet”, turn the spinner enough to allow the next row of paint to be sprayed. The whole area and the paint line must be just wet. And so on, around the spinner you go. Once a line of paint is all wet, resist all temptation to spray on more. You should have achieved a spray paint application free of sags and runs. Don’t ruin it now by adding more.
When done, rotate the spinner in your work lights so you can see the light reflection in the paint and see that the painted surface is wet and blemish free. Very carefully touch up if you must.
In the same way, it might be good to apply clear coat at least to the leading edge, since wind erosion occurs there.