I wish to update you my little experiment on the side.
1. Ideally we want all 8-Bolts to be milling thread (blind hole) to avoid aluminum shaving issue.
a. Total thread hole depth can be controlled around 4.85mm.
b. More than 3.5 thread is effective.
2. Milling Thread (Blind)
a. Milling Thread requires higher skill compensating the cutting parameter (for major and minor diameter of tooth) for production.
b. We can determine the tool life before tool wear and avoid compensating.
c. For the numbers of cranks, tool life can still be enough for production without too much added cost.
3. Breaking torque for Milling Thread is higher than Forming Tap. Given the same number of threads.
a. We control the total depth of milling thread for torque experiment with current production bolt. (Over 1.5mm in addition to the 4.9mm requirement)
b. Breaking torque of milling thread and forming tap are compared side by side.
Torque and dimensions between Milling Thread & Forming Tap Thread