A connecting rod is a highly stressed part, with conflicting material requirements. Aluminium is an ideal material but some high strength alloys are not suitable when hot and others wear rapidly. Most are difficult to machine. I make the con-rod from a 7075 T6 aluminium alloy. This is a particularly hard and strong alloy that has good bearing qualitities when cool and well lubricated, as at the lower crankpin end. At the hotter upper end, the gudgeon pin is pressed firmly into the con-rod and it reciprocates within the hardened and diamond honed piston bores.
Using my Feeler capstan lathe, I firstly machine a dumbbell shaped blank from bar-stock. The grain structure is aligned with the length of the bar and so that of the con-rod. I mount four blanks at a time in a fixture on the milling machine where I sequentially machine one side flat, centre drill, drill and ream the holes, without removal. I use quick change tooling to mount each tool and electronic digital readout for positioning. This ensures that all are accurately spaced and aligned. I invert the con-rods to mill the opposing side and then chamfer the holes. I swing-mill each end using a special rotary fixture and then vibratory deburr the parts for three hours before micro-honing the crankpin bore to final clearance size.
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