Why are flywheel keys square?

I am currently building the Redwing engine and have got to the flywheels. The keyway has to be broached but I don't have a proper broach and therefore have to use a square ended tool and 'plane' the metal away. The problem is the casting is so hard thet I struggle to take any metal off at all. I have made a cast iron plug to fit snugly into the centre and drilled through at the intersection, this removes most of the metal and I have now managed to get a decent square groove,

It occured to me that if one used a round key then all this fuss would be avoided - wouldn't it? I know one would have to use a round nosed cutter to mill the corresponding groove in the crankshaft but I reckon that's easier than broaching.

I bet there is something I have not realised so would someone please enlighten me?

Thanks, John

Because there is no sudden change in section like you get when you come across the wall of a key, the round type is gradual and in time will roll over and extend the 1/2 hole.

John S.

Edited By David Clark 1 on 24/04/2012 21:01:30

They must be very hard castings if you can't plane them after drilling. I've used the planing method for keyways 3/16" wide and 1" long in steel without drilling first, just 1 or 2 thou at a time and its soon done see this

Though I have now bought a set of miniature broaches from the states at a very reasonable price its a minutes work to press them through, don't think I will be going back to planing.

One of the reasons for a square hole is that the gib headed key can have a slight taper in its height so it tightens up as pushed in, you won't do that easily with a round hole.

J

John S is right of course, however I have used round keys with a bit of locktite to overcome the tendency to roll and never had a problem. If you can drill both the wheel and the axle with it assembled you can get a good fit easily.

Russell.

For small bores (say 1/4" to 5/32" I use a 'top hat' shaped filing guide with a slot in it and a square file.

Neil

That must be a tiny file. What size key do you use on a 5/32 shaft!

Russell.

very neat flywheel fitting (and engine). But some future engine restorer is going to have a b.....r of a time removing them!

russ

well perhaps I meant 5/16"

Neil

Many thanks for the posts which have helped me get my head around the subject. Also thanks for the other ideas, all useful info.

Cheers John