Keying it Up
 

Hey,
Ok, so we ordered our steel shafts for the drive, and we have sprockets that have keyways as well as hubs that have keyways. We do not however have the ability to key our own stuff using a CNC or lathe, any other ways to key shatfs? I was thinking about putting a drill bit in a press, and moving the shaft by hand, but I'm almost sure the bit will just snap. Any ideas? Thanks.

if you have a drill press, why dont you just pin the sprockets onto the shaft? That's what we're doing. Our sprockets have a keyway and set screw as well, but we're just going to pin them.

-anton

ps. if you pin it, disassembling it isnt as easy

If neither your sponsors nor your School system have the equipment available, you'll have to do what we did, and find a shop in the yellow pages or ask at a local metal store.

We got lucky, and the guy with the shop has become a good friend, and a great help. And in the meantime, we have acquired some simple machine facilities of our own.

After all that, have you thought of using a Woodruff Key Seat Cutter. It's like a thick circular saw blade with a perpendicular shaft through its centre, and it cuts a round bottomed hole the width of the key into the side of your shaft. If you're extremely careful, you could use a really good drill press and cut a key hole in your shaft.


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So you don't have a CNC or a mill. No worry!

Pinning is certainly a way to go, but a key and keyway will provide a much more reliable connection. A pin will easily break under a lot of load, say like... on a drive train.

So, the next thing you can do is drill the hole off center, in fact, drill it so that it's tangent to the shaft, so when you slide in a pin, it will fit straight right in between the edge of sprocket and shaft. The compression will keep the sprocket spinning with the shaft. You can probably do two of them on the sprocket if you don't think one is good enough.

On the other hand, if your sprockets don't have a hub, and don't have too much room to work with, then the key and keyway is still the way to go.

You can buy keywayed shaft at places like McMaster Carr or other places. Please don't not try that method of yours... I am sure drill bits were never designed with that in mind. Besides, your shaft is probably steel, so it will take forever to try to work them with normal press.

Last year we pinned our drive shafts. We broke two pins. As luck would have it we didn't break the pins during and qualification or elimnation rounds :)
This year, with our earth moving drive, we will be keying the shafts.

Wayne Doenges

Last year, we broke 2 shafts on our arm at West Michigan. Those of you that went there may remember our spectacular fall from the bridge Friday during our second practice round (they played it on the highlights video at the party 20,000 times!). When we tried to use the arm to stand our robot back up, we broke the keyed 3/8 shaft on our "elbow." We did it again in our last qualifying match. We never solved the problem, but with some driving adjustments, we never broke another shaft.

Don't even consider doing it on a drill press! I have broken two woodruff cutters using a proper mill and vice. Now, I don't really know what I am doing, apparently, but it seems like these small woodruff cutters are kind of delicate and any vibration or sloppiness and they snap. Plus for the key to work it needs to fit tight.

Best: purchase shaft with the keyway already cut. This is a common, cheap, item available just about everywhere. Or, you could order it from McMaster. Just look in the yellow pages under machine tools supplies.

If you pin, consider using rolled pins: they have a bit of "give" and don't snap like the hardened steel pins.