View Full Version : Gib-Head Keayway Cutting....

02-01-2010, 02:10 AM
Hi all, i need to fit a gib-head taper key into a flywheel. Is the hub slot tapered or not? If so how is it cut, i am also fitting parallel keys in other bores with broaches but i presume these only cut parallel ways?

Any info appreciated



02-01-2010, 02:19 AM

Machinery's Handbook 27 page 2366 - under indexed heading "Keys and Keyseats".


02-01-2010, 06:34 AM
yes i have a machinerys handbook, and have studied the pages on keyseats but it does not tell me how to create the taper?

Ian B
02-01-2010, 07:37 AM
Does this help for the dimensions:


Looks like the shaft has a parallel keyway, and the taper is in the hub / flywheel.

Are you cutting the hub keyways using a broach? These usually come with a plug and shims. Can you make an additional shim, with a 1:100 taper? This should result in a tapered keyway.


02-01-2010, 07:45 AM
My experience of tapered shaft keys is with Lister CS engines.

The slot in the shaft and flywheel are both parallel and only the key is tapered.

02-01-2010, 09:25 AM
IanB; No that wont work, whatever the shim shape, the broach will always cut a paralell way as the whole broach has to pass through the bore.

EvGuru; That was what i thought too, the taper is 1:100 i think so its pretty fine.

As i cant see any detail listed in any book for a taper keayway i will assume they are parallel and cut to suit.



Looking at the page that IanB forwarded, it does mention "Section at deep end of keyway" so it must be tapered. The only way i can see of doing this is to make a tapered broaching bush to suit, but this will mean the broach is not travelling square to the job which sounds like a recipe for breakage???

02-01-2010, 09:41 AM
Dave, it does appear the keyway broached in the hub is tapered. As you said the only way is with a tapered slot in the bushing but you would also have to angle the hub in the press while broaching. Since it is a standard angle you could make an angled plate to put the hub on.

02-01-2010, 10:21 AM
Yep, that does make sense.

Thanks all


Rich Carlstedt
02-01-2010, 10:25 AM

You do not have to cut a keyway in the flywheel.
Folks here are looking at professional methods.
For model work, a gib head key works perfectly even
without a hub cut keyway.

Make the flywheel slide on the shaft as normal--no drive on fit !
Cut the keyway in the shaft and slide in the gib blank and file a taper so that the lead end is flush with the shaft, and about .002" higher at the head end for the right amount.

If you are nervous about filing near the shaft, clamp the key in a vise and file the bottom to get the same effect

I did this on a Stuart Turner # 7 over 35 years ago and it has never come loose.
The taper locks so tight that you can even take a skim cut (.001) on the finished flywheel to resolve runout


Ian B
02-01-2010, 10:47 AM

When I mentioned a tapered shim, that was indeed to convert a normal (parallel) broaching bush into a tapered one (I thought you might have the bush already, as they often come in sets with the broaches). This will put the broach off the centreline axis and give a sloping top face to the keyway, as shown in the link I posted.

Pressing a broach through off-axis; well, a 1:100 taper isn't a lot - a bit of trig will tell you how much - about 0.6 degrees I think. A tapered washer, thicker on one side than the other under the hub would compensate for this in the press - same 1:100 taper.

Is the idea to make a keyway that also tapers along the thickness of the key, not just its height? Ooh, that looks a lot more difficult... I can see why a side tapered key would have been advantageous - it'll give the tightest fit to prevent the flywheel rotating on the crankshaft. A full size engine probably had a key 1 1/2" wide or so, possible to get in there with files & scrapers to ease a taper out. To be effective, I suppose the keyway in the crank would also have to be tapered widthwise.

To get the sides to lock up with the flywheel in the right position on the crank, and to get the top slope to mate at the same time looks like an interesting bit of fitting!


02-01-2010, 10:52 AM
Thanks Rich, thats very interesting.

In this case, the flywheel is on a half-size traction engine and will likely be used for belt drive of equipment at some stage. In view of this torque loading on the rim, i would feel happier with a positive seat & slot fitting.

There is enough info here now for me to either fit the key to a parallel slot or to modify my bush to create a tapered slot.

I would guess that most of the torque is taken on the sides of the key so a parallel slot with the taper only being used for locking should be adequate. I would imagine a full-taper contact with the key boshed in would be nearly impossible to remove?



Rich Carlstedt
02-01-2010, 07:53 PM

Thanks I didn't realize it was a working flywheel.
You are correct in that respect.

Keep in mind that any taper less than 7 degrees is a locking taper