How do I measure the thickness of something with a lip or frame?

Question

I'm looking for a device like vernier calipers (ie 0.1mm resolution or thereabouts is fine), but in more of a C-clamp configuration which allows me to measure the thickness of some object's face where there is a thicker frame, lip or ridge around the outside which makes it impossible to use verniers.

A few examples:

• Thickness of glass plate in a framed mirror
• Thickness of the side walls of a container with a narrowed top
• Thickness of a lipped lid, say for a barrel

I know there must be a range of measurement devices to do these sorts of things, but my Google-fu is failing me today. Micrometers are the closest I can get, but (1) I don't need that sort of accuracy, and (2) they still don't work if the lip/frame is too thick.

What should I be looking for?

Answer 1

There are special forms of Vernier caliper such as these known as neck calipers.

These neck calipers enable measurements in hard to reach places.
They offer you the following benefits:

• You can measure wall thickness inside bores and recesses.

Obviously, the machinists calipers in A.I.Breveleri's answer will be cheaper and will probably work for a much wider range of situations.

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Another type is the Curre-jaw caliper - again with a vernier scale - though both these jaw types are available on calipers with electronic digital readout

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There are also a very wide range of external dial calipers, as kris and pcdev identified.

Answer 2

The instrument you are thinking of is the Machinist Caliper.

To use it on a lipped or framed plate where you cannot remove the caliper without opening it, you must insert a plate of known thickness into the measurement.

The caliper has no readable scale -- you use it to transfer measurements to a ruler.

Answer 3

There are double-ended calipers which have the same gap at both ends, allowing you to measure the accessible end while set on the thing you are measuring.

There are also calipers with a direct readout scale attached to the accessible end - in this case, using the different location from the pivot point to expand the scale for easier/more precise reading (this one being a jewelers version and optimized for small/precise measurements) - there are others with the same idea but in other sizes:

Reproduction of Studley-chest caliper (which goes two ways, as can the double-ended one) which has the opposite scaling compared to the jeweler's caliper (less movement on the scale as the scale is closer to the pivot point than the tips of the legs are.

Answer 4

Micrometer would be what I can think of with a C shape, other than that, Spring joint calipers maybe?

Answer 5

If you do not need it that often, then you can just use any old vernier or other measuring tool you have around. Then grab two metal objects (possibly square) that are wide enough to go past the lip on both sides. Hold them to your mirror glass or whatever you are measuring, and measure the total width.

Then, measure just the width of the two objects, and subtract.

And, of course, with a digital measuring device, measure the blocks first and zero your instrument on that.

Answer 6

My question has definitely been answered (neck calipers and machinist calipers being good solutions, but external dial calipers was really what I was looking for). I thought I'd add a couple of interesting solutions I came across in my research:

My favourite was these homemade calipers found here. This is essentally exactly what I had in mind when I asked the question. A bit rustic but beautifully simple:

I also found caliper extensions which are an excellent and low cost solution to the problem:

Thanks for all the other great answers!

Answer 7

If you're look for quick and rough, an ordinary clamp for joinery will do the trick. Set the clamp to the thing you're measuring, put ruler against the neck to measure the distance between the base of the two arms, then remove the clamp and reset the clamp to the distance you measured, then measure the space between the two gripping pads.

Answer 8

I made a thickness gauge from wood and right angle braces, long bolt and one nut soldered to the underside of the upper arm. A wooden anvil is attached to the other metal right angle brace. The bottom anglebrace is fixed and the top one is hinged, so you can go around any molding that would prevent direct measurement, as a fancy tabletop. You open the upper jaw, slip it into place and close. Then adjust the bolt till both the underside touches the anvil and the bolt touches the top surface. Then lock in the bolt with two nuts. Then open the jaws, extricate the tool from the item (tabletop). Then just measure the gap.