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Long day machining my brake hanger hub yesterday. With some new tool bits and boring bars I had to spend quite a bit of time working out the right shims to fit the new tools in the tool post so the cutting height was correct.
Guess what? I buggered it up – obviously with the last cut of the day.
Boring the 37mm hole for the inner bearings I was measuring using vernier calipers and forgot that their measurement of an internal surface has to account for the thickness of the blades, the two outside edges of the blades touch the internal arc and the actual diameter is larger. The hole measured 36.95mm but the 37mm bearing fell in with about 0.05mm slop – too large a gap for bearing fit or even a grub screw. I should have tried the bearing itself to the bore holed rather than relying on the Vernier caliper Another school day!
I have two options here, 1) make another one or 2) sleeve the original with a bronze bush and fit the ball races into the bronze sleeve. There is still enough meat to do this, 6mm either side of the sleeve. I will however have to bore the bronze sleeve so that the wall of the sleeve will only be 1.5mm where the ball race is. If I make the sleeve too tight a fit then I could have trouble pressing it in whilst maintaining the integrity of the ball race location. Also, the outer diameter of the now made hub at 75mm does not allow much meat for the csk 8mm socket screws to bolt to the hanger plate, there is a danger the countersinking will encroach on the outer rim of the hub which will look bloody awful.
Whilst the original work piece is still in the 4 jaw chuck I will finish the existing one to allow the sleeve option. Then I will machine up another one with a larger overall outside diameter to prevent the countersinking encroaching on the outside diameter and hopefully with a bored hole that actually fits the bearings.
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