A Spin Casting Centrifuge by Gary Overman
This spin casting centrifuge designed and built by Gary Overman would be a great addition to any shop. This unit is capable of casting wax, resin and other mediums. We are grateful to Gary for submitting it here to be added to the tools section of the book. You can click the image to see a larger sized image.
Here is the Arm Assembly for the centrifuge. At the far left is the counterweight which is held in place with a large bolt and security nut. In the center is the slipring assembly which is bolted in place with 4 bolts through the flange.The slipring is constructed from common pipe fittings and bronze bushings are expoxied in place. The red hose provides a vacuum ling from the slip ring to the vacuum box at far right, which is just a plywood box that has been sealed with polyurethane. The box is bolted to the 5" alum channel arm and is constructed to sit tight in the channel. The box has thick rubber hinges on the left hand side, and Velcro latches on the right. A rubber seal allows the vacuum to pull the lid down very tightly and securely. Should the vacuum be broken and the lid loosened, centrifugal force is pulling against the rubber hinges and the lid remains closed with the simple aid of the Velcro. In other words, all forces are working to keep lid closed, rather than trying to open it. The drive pulley can just be seen on backside.
This photo shows the backside of the centrifuge without shroud or controls.In the foreground is the vacuum pump and filter.The pump,from salvage at about $60,is a gast dry rotary vane 4 CFM pump. It only achieves about 22"hg vac in this configuration at this altitude, but with about 350 RPM of centrifugal force behind the spin , it is MORE than sufficient to produce perfect castings.
The PVC "egg" at far right protects electrical connections. At far left sits the blue DC permanent magnet motor. It is a 90VDC 1/3 HP I bought from salvage for about $70.
The red hose connects the pump to PVC line which runs to the front of the machine and to a valve which allows operator to control vacuum in and out of box. Note that the Pulley is bolted to the 5" alum channel arm, and the whole of the arm assembly and pulley spins on a stationery shaft which is bolted to the steel angles which run midway of the horizontal plane of the frame(visible where the red hose meets the white PVC). This principle is easy to understand in use but difficult to describe on paper.
This is a view into the assembly from the top backside of centrifuge. It has a better view of the pulley. It also more clearly shows how the stationery shaft is bolted to the frame. I used 2 u-bolts for redundancy safety sake, though I imply none as you are totally on your own when it come to safety.
Close-up of slipring. Please note the use of a shaft stop collar that sits against bronze bushing. Not visible is a rubber o-ring that is covered in lithium grease and compressed between the stop collar and the bronze bushing.
Yellow box is a multi function box that a friend with electronics experience threw together for me. It is an AC to DC converter that has a relatively simple circuit inside that converts but does not regulate or filter current. A rheostat is used that purpose and since this is hooked up after the rheostat, the meter on the yellow box allows me to see the adjusted DC current that is leaving the combination (rheostat and yellowbox converter) of the two.
The box also has a cross-wired double-throw switch that allows me to reverse the direction of spin on the motor as well as turn off power to the motor should I elect to do so at this connection. It is fused for protection. Power flows from the yellow box ( now reduced for speed control ) to the pushbutton switch mounted in the white PVC fittings. Connection is simple jackscrew posts.
Here is the on-off switch to the motor mounted in a spring activated paddle. Spring pushes paddle off switch into an off position until weight of hand is placed on paddle. At that point, switch in PVC is turned on and motor begins spinning centrifuge. PVC switch can easily be removed for "remote" use since cord is 10 feet long.
Far left is simple homemade paddle ( as hand or foot ) switch that toggles vacuum pump on and off. Center is a 10 amp rheostat that limits ac voltage into ac-dc converter. This controls speed of DC motor. Far right is yellow-boxed AC-DC that is desribed in another photo.
This will explain why I use my centrifuge in the vertical position.I let gravity be my guide to control the start of the casting process, and then apply centrifugal force to overcome gravity and then use vacuum to avoid air bubbles. Here's where it all comes together.
This photo shows half of RTV mold prepared for use in vacuum centrifuge. The figure is of Jack Daniel whose famous still is 12 miles from my house. The mold has the typical lock and key features. Please note the air vent in the bottom left corner to allow vacuum to purge casting chamber ahead of the casting material as it enters the mold chamber when cast out of the copper pipe.
What is unique is how the wax ( or whatever you're casting) is held out of the mold until the centrifuge does its job. The copper pipe and cap holds the melted wax away from the casting chamber ( Jack ) until the centrifugal force spins the wax from the copper into the RTV where it quickly solidifies. The copper is hot the RTV is not and with vacuum and centrifugal force it is all done in just 2 minutes!
Here's a view of the completed slipring assembly. Please note the use of a shaft stop collar . Also note the rubber o-ring that is covered in lithium grease and compressed between the stop collar and the bronze bushing. The shaft and o'ring's are dissassemblied every 3 month or so and recoated with lithium grease. Because the unit is low RMP and only run for about 30 seconds at a time, wear and heat has not been an issue. I've used it hundreds of times and there has been so sign of wear or decrease in performance.
The red hose provides a vacuum line from the slip ring to the vacuum box at far left, which is just a plywood box that has been sealed with polyurethane. The box is bolted to the 5" alum channel arm and is constructed to sit tight in the channel. The box has thick rubber hinges on the left hand side, and velco latches on the right. A rubber seal allows the vacuum to pull the lid down very tightly and securely. Should the vacuum be broken and the lid loosened, centrifugal force is pulling against the rubber hinges and the lid remains closed with the simple aid of the velco. In other words, all forces are working to keep lid closed, rather than trying to open it. The drive pulley can just be seen on backside.
This photo shows how the frame can be laid on side to use the centrifuge horizontally like a merry-go-round rather than vertically like a ferris wheel if desired. But since my preferred molding method utilizes gravity to kept the wax out of the mold until centrifugal force is applied, the vertical orientation works best.
Here's the frame as I assembled and use it. The frame is 3" square 1/4" thick steel tube and weighs in at about 200 lbs. It's mounted on a wheeled platform with casters that are rated at about 250lbs ea. 2 casters swivel and 2 do not.
They are oriented so that the unit do not roll left to right as displayed in this photo. This is the ensure that if the unit is off-balance, it won't walk off on its own. In other words, the line of force is left-to-right and the wheels will only roll front to back.
I was going to cast a wax as a demo for the website , but wax doesn't show up well in photographs due to its translucency. Instead I mixed up some quick set epoxy with some colorant and cast it into the mold. I cut the sprue off while the epoxy was still soft and removed minor flash around hat with my fingernail. Other than that, this is straight from the mold. Wax because it is much more fluid casts even easier!
The plans and photos for this project were done by Gary Overman