[Paul Copioli]

When dealing with the wheels that many teams make from solid 2 inch thick by 4, 6, 8 inch diameters and machine down we may want to think about castings. Quantity is your friend when dealing with castings, but you need the quantity to get the price right. Below is the example for the kit transmission development:

The kit gearbox housing is made from die cast aluminum. The prototypes were machined from solid. The gearbox design mimics team 217's design strategy for the past 5 years: enclosed, machined housing. The main problem was we would only build 4 to 6 per year so castings were out of the question. We had everything machined from solid by one of our sponsors. This is actually quite expensive (around $300 to $400 a pop) due to machining time. The kit gearbox was going to be ordered 3600 at one time so castings were the way to go. There are several types of aluminum casting processes to choose from but two are the most popular: sand casting and die casting. Which one you choose depends mostly on quantity, but wall thickness and surface quality also come into play. Sand castings are good for quantities of 25 to about 500 and die castings are usually more cost competitive for the higher quantities (>500).

I received quotes for both machined from solid and die casting for the housing. The machined from solid was $52 per piece for quantities of >2,000. The price we paid for the die casting was $11 per piece (a factor of 5 difference). Now, this doesn't come for free because we had to pay an up front tooling charge of $11,000 for the die. Amortize the cost of the die over the initial 3,600 units and you get an adder of about $3.06 per unit so the price this year for the boxes was $14.05.

Now, sand castings have about 1/4 to 1/2 the cost of die castings for the tooling, but the piece price is slightly higher and the high quantities can't be achieved by a single tool (so many tools would have to be purchased in order to make the higher quantity). Sand castings seem to be the better fit for a company willing to sell aluminum wheels due to the lower initial cost.

I think wheels of 4", 5", 6", 7", and 8" could be made from sand cast aluminum machined with a stock bore for about $25 to $35. If key ways or hex broaching is required, the price could jump another $10 or so.