I learned the hard way that QC in my little hobby shop is still important. In making a set of large hand screws for my woodworking interests, I cut some 1/2" rod for the 1 1/2" long swivel pins and threaded a mid-pin cross hole in each, half LH and the other half RH 3/8-16. I held the pins in a 1/2" 5c collet in a square block mounted in my vise. Drilled and tapped about 4 dozen and when assembling the clamps (after struggling with 9" of LH and 7 1/2" of RH threads on an 18" rod) found out that the threaded pin holes were not at 90° to the pin. They were off varying amounts up to 10°. How could that happen:? I puzzled over that for a long time until I realized that probably the unsupported end of the pin sagged under drilling pressure. Reading up about how collets hold work, I learned that only the ER collets (split on both ends) can adjust their grip to fit the rod. All the others like R8 and 5C force the end closed leaving the work "loose" inside the collet. So my pins sagged and I didn't notice. The problem could have been solved with a machinist jack had I checked for quality. So the replacements will be held against a stop in a grooved jaw of a freshly fettled machinist vice.
zu früh alt, zu spät klug
DanK