I’m intrigued by this, but for now will stick my cheap-and-cheerful solution. It would be interesting to compare an axle measured out of spec by your tool with the slip test in the slotting plus combi bushing and vice-versa.
I’ve been impressed with Slotting-Plus and NSR axles. I threw away several Slot.it axles right from the package after checking them via the slip method. I find it mildly interesting that it was all solid axles that failed. My inventory of hollow axles are all true by that test. Attributed to the manufacturing process, maybe?
I also tried hardened high-speed M42 tool steel rods ordered from McMaster-Carr. At approximately twice the price, they are expensive, but, oh man, they are straight and solid! Probably due them being made from cobalt steel. 🙂 (Hardness: Rockwell C65)
Build it strong.
Keep it simple.
Make it work.
(Leroy Grumman)