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Originally built to measure wall thickness behind an obstruction inside a tubular workpiece, this tool has proved useful in several other awkward measuring situations. It's not something one would use daily so it's offered here as an idea to recall when you run into a difficult measurement situation.
The tool consists of an aluminum frame with two stainless rods whose extension from the frame can be adjusted. A hole in the middle of the frame allows the rods to be positioned nearer each other when that is required.
At the ends of the rods are two brass measuring probes orthogonal to the rods. Held in place by setscrews, their position relative to the rods can be adjusted as required.
The longer probe is fitted with two adjustable collars that can be used to "remember" positions of the probe.
There are numerous ways to use this tool but I'll describe only my original use as an example. The clever people who frequent this forum can probably come up with many more, and perhaps better, applications.
The longer probe was raised such that the rod with the shorter probe could be fiddled through the hole in the obstruction and brought to bear on the inner surface of the workpiece. Then the longer probe was lowered until it touched the outer surface of the workpiece.
At this point the ends of the probes are, of course, separated by the distance I wish to measure. The problem is that I now have to retract the longer probe to get the tool past the obstruction and out of the workpiece.
So, the adjustable collar is slid down to the top of the stainless rod and locked in place with its setscrew, thus remembering the probe position. The longer probe is withdrawn and the tool is withdrawn from the workpiece.
Now, by sliding the longer probe down until the collar touches the stainless rod, I can reproduce the desired measurement between the probe tips and measure it with calipers.
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