Originally Posted by
Bad Lag
My guess - it failed because there is no radius at the bottom of the milled channel. A radius down there would have reduced the stress concentrations and could have prolonged the life/prevented failure. You would not/should not use a straight cutter for this operation.
For a beam subjected to these loads, it is not a good idea to place the web on one edge. It belongs in the middle of the section. He should have milled inwards towards the middle from both the left and right sides. On one side only, the beam torques (twists) with every power stroke.
Add in some notch sensitivity of aluminum and corrosion and aluminum's general susceptibility to fatigue failures and well,...
That's what I was thinking.
I have a pair of Stronglight 99s that I've been thinking about milling a little more. My plan is to deepen the groove on the face by about a millimeter or less, and then match that with a groove on the backside, leaving a web in the middle that I will NOT drill or otherwise weaken. I'm going to use a carbide cove or box bit chucked into a router mounted in a table, and take my time with very light cuts.
I don't want to lose the cranks, or my leg, or my life. Just a few grams.