The Anatomy of a Telescopic Gauge
Telescoping gauges, commonly referred to as snap gauges or telescoping bore 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 two styles of telescopic gauges. One type has two plunging telescopic rods. The other type only has one plunging rod, the second rod remains fixed. The telescopic rods are what measure the distance of the bore, with the help of a micrometer. The ends of the rod are the shape of a half ball, called the radius edge. It’s this shape that’s responsible for the telescopic gauge’s accuracy.
How to use telescopic gauges
Using the telescoping gauge is fairly easy. Although to obtain accurate measurements and to develop a good feel for the tool, a little bit of practice is required. Here’s a step-by-step process:
1. Collapse both the measuring heads by rotating the locking screw anticlockwise.
2. 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 too much because the rods are spring loaded.
3.Proceed to tilt the gauge so the rods are slightly inclined and tighten the locking screw.
4. Whichever end you lifted a bit above horizontal, force it slowly downward 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.
5. You are now ready to measure with a calibrated micrometer, since there is no scale on the tool itself.
Check out how machinist and YouTuber Adam Booth achieves accurate measurements using his non-traditional approach with a telescopic gauge.
Are telescope gauges reliable?
It’s typical for different machinists to have different techniques due to the “touchiness” of the telescopic gauges. A very real argument exists among the metalworking community that telescoping gauges are unreliable especially when it comes to repeatability. Once you develop your feel for the tool, telescopic gauges are a respectable option. Just like anything else, achieving successful and repeatable results takes a bit of practice. Since your success is very dependent on your growing feel for the tool, it’s recommended as you get started to double check your readings.
With two plunging telescopic rods, this set measures holes, slots and recesses up to 6 inches in diameter and comes with a protective case. These gauges work for measuring both to the tenth and to the thousandth.
Both legs of the gauge are plunging in this Mitutoyo set. If the type of machining you do calls for long reach, these would be the ones for you. Measures up to 6 inches.
The Fowler gauge features a rigid rod, which is helpful for grip and comes with vinyl casing for storage.
They get the job done, they’re no Starrett but you already knew that! As long as you check your work you will be good to go. Keep them safe in the included pouch!
Anytime Tools Bore Gauge 6 pc 5/16″-6″ Premium Telescopic High Precision T-Gage Set w/Hard Shell Case
Recommended if your project allows you to measure in fractions of an inch, not thousandths.
Beyond telescopic gauges, there are other tools that can help measure the diameter of a hole. If you’re interested in hearing what our members think about the topic and specifically telescoping gauges, join the conversation here.
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