chiaphua gear cluster

[dlavery]

Originally posted by Dr.Bot
Be careful mounting the 80 tooth gear - it is very hard and difficult to machine. The best bet is to drill the face - I had this done at a machine shop. A carbide drill could probably do it. Cutting a broachway for key stock would be a real problem. I think a diamond file and careful hand work would be the only way.

If you don't have easy access to a machine shop, you can work on the 80-tooth gear yourself, but you have to do some prep first. The gear is hardened, and must be annealed before you can work it.

To anneal the gear, heat it in a torch flame until it is cherry red (around 1650 degrees F), then let it air cool. You should now be able to work with it easily. Once done, the gear will be too soft to use reliably, and will need to be re-hardened before you use it. To harden it, heat it again to cherry red. Rapidly quench it in water (OK) or oil (preferable). You want to plunge it straight in vertically (i.e. along the length of the piece), and not sideways, to make the stresses within the piece as even and consistent as possible. For the same reason, swirl the piece in the liquid in a figure-8 pattern, and not in a circular pattern, until it cools. This will ensure that fresh surfaces of the piece are exposed as it moves through the liquid, and makes the cooling as even as possible.

Once the piece has cooled down to about 100 degree F, the hardening step is done. The piece is nice and hard, but it is also brittle (with good tool steels, if you drop the piece on a concrete floor at this stage it will frequently shatter). You need to temper the piece to draw back the hardness of the surface material into a stronger, less brittle form, and relieve internal stresses. Take a file or some emery paper and scrape the surface of the piece to reveal fresh metal. You will need to evenly reheat the piece to the tempering stage, and then quench it again. While it is possible to do this with a torch, it is difficult. The easiest way is to heat it in a kitchen oven (I will leave it to you to determine if you want to ask before or after you do this), or even a small toaster oven. Depending on the alloy, you need to get the piece up to about 460 degrees F. At this temperature, this fresh exposed metal will turn a straw yellow color. Watch the color carefully - this is your best indication of when the steel has tempered. If it goes past yellow into brown or purple, it is too hot and you need to start over from the hardening step. Once it is straw yellow, take it out of the oven and requench it immediately. When it has cooled, you are all set.

This isn't a perfect process that you would use for large-scale production, but it is a reasonable small-lot method. Knife makers and blacksmiths have been using this technique successfully for ages. You will have a nice hardened piece that should stand up under the stresses your robot will see.

FYI, the first cluster from the jack (the one that was in the FIRST kit last year) can also be bored out and worked if you use the proper techniques. The material used in the first gear cluster is very hard. It also work hardens very easily. If you need to open up the bore to put the cluster on a larger shaft, use a TiN-coated drill bit, slow turning speed (~500rpm for a 3/8" bit), heavy feed rates, and LOTS of coolant. You want to get through the piece as quicly and smoothly as possible before it work hardens any more, otherwise you will be there all night.

-dave

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Y = AX^2 + B..... ehhh, whatever