Math check on keyway broach bushing design

[tominboise]

I am doing a project for a neighbor that requires an internal keyway cut. I need to broach a 3/16" keyway in a steel bushing, 2.4" long. I have a HSS broach and shim I bought from Shars but need to make a bushing to support the broach. The broach measures .410" deep at the wide end and the shim measures 0.051". I am aiming for my slot to be 0.106" deep.

The broach plus shim measures 0.461". The "stick out" when I am done cutting will be 0.106". So the slot depth in my bushing should be 0.461 - 0.106 = 0.355".

So I am planning on machining a bushing 2.4" long, with a collar on one end, 0.625" OD, with a slot milled into it measuring 3/16" Wide X 0.355" Deep. Does this sound correct?

Also, any thoughts on using aluminum vs steel for the bushing? I was planning on steel because I think it needs to be as hard as the broach, but maybe I don't need to.

 

[Ulma Doctor]

If you are doing it only once, the aluminum should suffice.
You may need to compensate in the event that the keyway is shallow, but that is easy with some shim stock or a sacrificial feeler gauge, cut to width for the purpose.

A 3/16”HSS tool could be shaped from a square blank,to perform cleanup duty of a shallow keyway, by chucking the bushing in a lathe and mounting the 3/16” tool on a boring bar.
After acquiring the correct height, you can use the crosslide to control depth and cut using the carriage to clear the slot with small adjustments to the crosslide on each pass of the carriage.
It is slow work, but it can be very accurate if you are looking for a close fit.

 

[Winegrower]

I think the major consideration is when the broach initially enters the hole. The bushing slot needs to be deep enough to allow the narrow end of the broach to just fit in the hole for the initial pass, then you'll adjust with shims for the desired keyway depth. I think from your numbers your bushing slot won't be deep enough. To be sure, measure the leading end of the broach and consider how it starts out.