@Malcolm Russell said:
John, I see that three-point gauges are still listed in the store. I believe these should be used to get gauge widening on tight curves. I have two of these bought from one of your predecessors around 1980, and I was puzzled as to how they could possibly work. The outer pair of studs have their slots perfectly in line – probably milled in situ. The outer rail fits snugly in the slots as a perfectly straight line and cannot adopt a curve to match the inner rail. It has recently dawned on me that the gauges would work as intended if the outer pair of studs could rotate. On my samples they are rivetted up tight, but I managed to get them to rotate freely with penetrating oil and tightly gripping pliers. Do your stock items have rotating studs?
All the gauges i have come across, including a P4 gauge that I have in stock, are of similar designs. That is 3 machined slots on a triangular base (no rotation). The gauges we currently have are not the ones that you purchased many years ago as we no longer have the drawings. They do, however, work on the same principle. The riveted slots I don’t think were ever meant to rotate, more a manufacturing set up.
It is possible that the slots in the gauges you have are slightly undersized and so when used on a tight curve are difficult to fit. I have come across some of the roller gauges we have in stock that were manufactured some years ago have to be forced onto the rail.
Theoretically you are correct in that both the inner and outer slots should in fact be points and not slots. However if you think of the relative distances involved (Radius of curve verses length of slot and distance between slots) the amount of “free play” needed in the slots is tiny and probably well within machining tolerances. That is probably the reason why nothing more sophisticated has ever been produced (to my knowledge anyway)