I had a job in the shop once which required measuring the diameter of some large glass tubing while it was being worked on the lathe. My regular vernier calipers would open up wide enough to measure the glass (around 100 mm) but they did not have a deep enough throat to reach the centre of the glass tube. I could have gone out and bought an expensive pair of machinist's calipers but as this was an infrequent job, I decided to build my own. I bought two twelve-inch shelf brackets (flat steel with a small bump to keep them from falling into the shelf support). I hammered the bump flat, cleaned it up with a hand file, then joined them together.
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