a quick easy way to get a keyway on inside diameters
a broach set
an arbor press
(both available at TechShop)
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using the broach set, fit the correct insert into your ID ( this example uses a .375 diameter insert)
grab the broach that goes with the size key you want and get the shim ready. (this example uses a 1/8 broach)
using the arbor press "push" the broach through the insert into your part. this will leave only a slight key way in your part.
place the shim into the slot of your insert and run the broach through again. this will give you the depth you need for your key way.
and there you go - a key way on the id of your part