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Measuring: Telescoping Gauge Error

12 Jun 2023

I love tools - especially measuring tools - but don’t actually own a lot. I’ve started to change that, and one of my recent acquisitions include a set of telescoping gauges. If you haven’t used these before, they’re useful for grabbing inside measurements (ID of bores, or any inside measurement) in conjunction with a set of outside micrometers. They are a bit tricky to use, and if you’re going to be doing serious ID measurement you’ll probably get bore micrometers or proper inside micrometers. But, outside micrometers + telescoping bore gauges are a versatile combo if you want to be able to measure a lot of dimensions without totally breaking the bank.

I haven’t used telescoping gauges for about 12 years1, so my technique is rusty. As I was re-discovering it while measuring some cylinder bores, I started thinking about the error that corresponds to the gauge not finding the correct angle (circumferentially) when you swing it up and out. All the videos about using these show the process as one smooth angle that simultaneously finds the minimum in two directions by one smooth motion, and maybe that’s how it goes, but I still want to know how worried I should be about this circumferential angle.

If you are having trouble visualizing this, here’s a short Festo video showing the process. The angle I’m talking about is the angle in the plane of the 25 mm diameter being measured. If you stop and tighten the gauge when it’s in this plane, it’s often possible to feel that there’s a few degrees of play (which corresponds to more room for the gauge to expand!). This is, no doubt, down to my technique. But if this only leads to a 1/1000 th inch error in measurement maybe it’s fine.

Anyway, here is a visualization to explore this!2


  1. Whoa! Time flies. I used them while working as a staff member at the shop in the ME department while at school. 

  2. You can see the code here, although, same as with the false peaks post, I’m using this as an excuse for learning canvases so best practices may be lacking!