Telescoping gauges

Telescoping gauges, sometimes called telescopic gages, are really useful for "measuring" holes in the 0,313 - 6" range. They don't actually measure anything on their own, but they transfer measurements to something that can measure - a regular micrometer for example.


How do you accurately measure a critical bore? An internal bearing seat? How do you get that last "thou" just right? Forget vernier or dial/digital calipers, unless you really know the relation between what the hole actually is and what they say it is. And you better find the true maximum diameter!

Spring calipers work well, they use the same principle as the t-gauges but with more "feel" and skill required. Go/no-go gauges also work, but have to be purchased or made. Internal (stick) micrometers work on bigger bores, but they are slower and you really cant use one in say a 25mm / 1" bore. Oh, the three-legged affairs do work, comes in a wide range of sizes but are really expensive. Really. As far as fast, fuzz-free measuring goes, telescoping gauges are the way to go in my opinion. They transformed precise boring work from frustrating to actually quite enjoyable, and the same set works for all measurements within the range, provided you also have the outside micrometers.

Mitutoyo telescoping gauge - Nikon D200, AF-S DX VR 18-200 @ 200, +1,7, built in flash

But the gauges must be used properly. You first set the telescoping ends a bit bigger than the bore, either by letting them snap open on an angle in the bore and lock them there (fastest way) or some other way outside the bore. The next step is to hold them, with the lock snugged tight enough, in the bore at an angle so the ends clear the bore, then swing or move the tips through the bore by levering the handle so the bore forces the ends to the true diameter. They will in effect get stuck, and as you force them through the bore they will find the centreline and biggest diameter "automagically", and as soon as you pass the centreline of the bore they will "pop" free.

It sounds complicated, but in practice its really simple. Snap open a little bigger than the bore, tighten lock, swing handle to force the ends through the bore, remove from bore when they pop free. Then comes the next part, measuring them to find out what the bore is:

Measuring over ends - Nikon D200, AF-S DX VR 18-200 @ 200, +1,7, built in flash

You simply rest one end on the fixed anvil of the micrometer, and rock or swing the other end while slowly closing the mic. As soon as it drags against the closing anvil thats your measurement. Its easier if you have something white or light behind the mic, then you can see the gap close and next comes the drag. DON'T look at the thimble while closing the mic. Keep looking at the anvil and swinging end, and when you feel a light drag - stop and see what your measurement is.

Be sure not to force the tips through the micrometer, because you could in effect mash the gage smaller. What you want is to recreate the drag felt in the bore when you set the gage in the first place.

Repeat the measurement at least once to make sure you didn´t bump or somehow affect the gages - you don´t want that bearing to rattle around when you (think) you are done! :-)

Advice: don´t buy cheap gages. I started cheap thinking I´d avoid spending a lot, but had to sell the cheap ones since they didn´t work properly. The plunger ends on those had a turned finish. Then I bought more expensive ones, still off-brand. They also went back, the plunger ends were smoother but the lock and action had a terrible feel. So I bit the bullet and ordered real Mitutoyo gages - and the light came on! Smooth action and feel, really well made, smooth, hardened plungers. Save yourself the frustration and get the good ones right away.

Ignore the quality of the next picture. Having said that, this job went smooth thanks to the gauges:

Repaired stud - iPhone 4s, shaky hands

This was a cast iron sheave with six equally spaced studs around the perimeter used as spring seats, all very worn. I turned the worn studs in the mill with a boring head until they cleaned up somewhat and then measured them. Bored the sleeves next, aimed for half a thou larger as measured with the t-gauge and the same micrometer, coated them with green loctite and tapped them home with a small hammer. Six different studs, six different measurements as the customer wanted minimum metal removal. They all fitted nicely. I aimed for a little clearance, toolmarks tend to smooth out as you tap things home and the loctite takes care of the rest. The harvester this sheave came from won't get that hot in service, so I felt confident using loctite. Had temperature been an issue, thermal shrink-fit would have been the solution.

So, telescopic gauges are really handy to have in the toolbox. You can easily measure work still in the lathe or mill, and using the correct technique and good quality gauges should enable you to work to really close tolerances. If you are like me you will measure bearing fits with ease. There are machinists who can hit less than a quarter thou, but that does take some skill, care and experience.

To be fair, you don't even have to use a calibrated micrometer to do bearing fits or fits in general. As long as you have the actual part that is to be fitted, what you care about is the difference in size between the parts, not their actual size. As long as you use the same mic for both internal and external measurements you'll be fine. 

Once set you can use them as a go/no-go gauge to check if you heated that shrink-fit part enough, or you can use them to...  well, thats for you to find out!