Telescoping gauges are indirect measuring devices used to measure the internal diameter of a bore, hole, groove, slot, etc. This T-shaped tool consists of a handle, two telescopic rods and a locking screw.
There are telescopic gauges with two plunging telescopic rods and others with only one. With the help of a micrometer, the telescopic rods measure the distance of the bore. The ends of the rod are the shape of a half circle (called the radius edge), which is responsible for getting accurate measurements. Gently twisting the locking screw clockwise secures the telescopic rod(s) into position. When twisted counterclockwise, the screw is released, and the telescopic rods can move freely again.
How to Use Telescopic Gauges
Developing a good feel for the tool requires a little bit of practice. Collapse both measuring heads by rotating the locking screw counterclockwise. Place the gauge into the bore with one head held in place against the wall of the bore. Begin to loosen the screw, but not by too much because the rods are spring-loaded. Proceed to tilt the gauge so the rods are inclined just a touch above horizontal, then tighten the locking screw (clockwise). Whichever end you lifted first, force it downward (the opposite way), slowly through the bore. As you do this, wiggle the gauge back and forth. This will allow the gauge to find its smallest size on the spring. Finally, measure with a calibrated micrometer.
Are Telescope Gauges Reliable?
Due to the touchiness of telescopic gauges, different machinists have different techniques. Some metalworking community members argue telescoping gauges are unreliable in terms of repeatability. However, once you develop your feel for the tool, they are a good option. Repeatable results just take a bit of practice.
Here’s machinist and YouTuber Adam Booth with his non-traditional approach.
With two plunging telescopic rods, this set is best for measuring both to tenths and thousandths.
Best for projects requiring long reach. The set has two plunging legs and is known to provide good feel and lock up.
Features a rigid rod which is helpful for grip.
They’re no Starrett, but you already knew that! Doublecheck your work and you will be good to go.
Best for projects that need to be measured in fractions of an inch, not thousandths.
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This article was prepared by Practical Machinist, which is solely responsible for its content.