how I measure across a large diameter

[desbromilow]

I can't recall where I saw this, but hopefully it helps someone in the future...

using a framing square, and a sliding square, I can measure the diameter of things accurate enough to do belt length calculations.
refer to the 2 example photos showing the method.....
use a sliding square and the inside edge of the framing square as legs of a caliper - keep the sliding square in proper contact with the other leg of the framing square.
read off the value from the inner scale (or use another measuring tool to get the value)


and here the method is used for measuring the diameter of a lathe countershaft cone pulley...

you can see the use of a spring clamp to hold everything in place to get a better measurement

FWIW, I used a similar method to measure the headstock pulley diameters on the lathe, but instead of a framing square, I used another sliding square, set at 200mm, then simply subtracted the value on the ruler from 200mm to get the true pulley diameter. (no photos, sorry)