I quickly investigated the potential errors of using an undersized pin by building two concentricity gauges. Gauge A was the inverted one using 3/32" drill bit and a 0.001" precision dial indicator. Gauge B was a "new and improved gauge" employing a #41 drill bit and a 0.0005" precision dial indicator. A #41 drill bit is advertised to be 0.096" in diameter (the shank measured 0.0955" with our micrometer). If pin size matters, then Gauge B should give different (more accurate) readings than Gauge A; otherwise, they should provide nearly identical readings.sporty wrote:I do see different readings if I do not use a pin gage matching the size of the bore.
I took runout measurements from some handy Revell wheels having a bore diameter of 0.0960" (as measured using pin gauges), which matched the pin size of Gauge B. The runout of these wheels was rather large (a subject for another topic), but the readings between Gauge A and Gauge B always matched to 0.0005" (which was the precision of Gauge B). After doing this eight different times on different wheels, it seemed that a pin undersized by ~0.002" was not going to be a significant source of error with this device...
I then took some comparative readings with an old, pre-2009 BSA wheel (also having large runout). Its bore size was ~0.0975", this being the largest diameter bore we've ever encountered out of the box. I found with Gauge A (with the smaller 3/32" pin) that I could get differences of several thousandths in the run out measurements compared to Gauge B (with the larger #41 pin). These seemingly erroneous runout readings on Gauge A also tended to read smaller, or "better", as Sporty noted. So it seemed that a pin undersized by ~0.004" could be a significant source of error with this device.
Just how closely pins and bores need to be matched for a contraption like this may still be an open question, but based on these tentative results it seems reasonable to employ a #41 bit instead of the 3/32" bit to help maintain accuracy with "large"-bore wheels. (I'll also add that I mic'd the pin for the Pro-Wheel Shaver and it was ~0.095", which appears to provide external confirmation of that dimension being a useful choice.) Something like a pin gauge would seem even better, but a #41 drill bit is a regular hardware-store item and one would still need the drill bit to make a hole for a pin gauge anyway! Because of the tighter tolerances, it might be advisable to polish the shank of a #41 bit (with the caveat that polishing can remove the zinc-oxide coating or other protective plating).
I might also recommend drilling a second hole in the wooden chassis to hold a 3/32" bit for use with "small" bores, just in case you are lucky enough stumble across a wheel with a bore so small that it could get messed up if forced onto the larger #41 bit. One could swap out the different bits into their respective holes if / whenever this happens.
I might add that some vendors ream their wheel bores, sometimes out to 0.098", increasing the chance of run-out measurement error without a sufficiently matched drill bit or pin gauge; might a #40 (0.098") drill bit help in that case?sporty wrote:...make one or get one. In the long run, its going to make sure you get what you pay for...