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?

  • 1
    Digital or dialed machinist caliper turns up some good results
    – Kris
    Feb 15 '18 at 23:26
  • 2
    How about a block of plastic (of known dimension) and a regular caliper?
    – Hot Licks
    Feb 16 '18 at 2:23
  • Thanks @Kris and @Hot, great suggestions. I found it: External dial calipers are what I was looking for, but neck calipers definitely come close. Thanks for your input!
    – pcdev
    Feb 16 '18 at 2:49
  • I've also found vernier caliper extension returns lots of hits on Google images that are a good cheaper alternative to what appears to be an expensive and highly customised measuring device.
    – pcdev
    Feb 16 '18 at 2:57
  • 1
    i use a channel-lock, a piece of styrofoam, and a ruler. bite the surface with the block, then measure the handle distance. then remove, and re-tighen to match the measured handle distance. then measure the gap between the jaw and styrofoam.
    – dandavis
    Feb 16 '18 at 14:31

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

Neck Caliper Dimensions of caliper jaws

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.

Example of measurement geometry

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

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

Curre-Jaw Caliper in use

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

External Dial Calipers External Dial Calipers with wide opening

  • Yes, this is definitely close to what I was hoping for! I've done a bit more research and found some caliper extensions that do a similar job, I'll post soon.
    – pcdev
    Feb 16 '18 at 2:31
  • 1
    Found it! External dial caliper in Google returns exactly what I was looking for, but Neck caliper is a definite close second choice. If you add a small section on External dial calipers I'll mark your choice as the answer since you definitely got me on the right track with @Kris's comment on dial calipers - thanks for your help!
    – pcdev
    Feb 16 '18 at 2:53
  • @ocdev - Done! I'm glad you found something to suit your needs. Feb 16 '18 at 13:31

The instrument you are thinking of is the Machinist Caliper.

"spring" 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.

using caliper on framed object

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

  • Great, I can see how this would work and remember using these in shop classes at high school. I was kind of hoping that there was a combination machinist and vernier caliper, with the measurement device built in, but this certainly does the trick. I'll wait and see if anyone's got any other ideas.
    – pcdev
    Feb 15 '18 at 23:07

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.

Double-ended caliper

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:

Jewellers caliper

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.

enter image description hereenter image description here

  • Interesting, I wasn't aware of those. Thanks!
    – pcdev
    Feb 16 '18 at 5:20
  • 2
    All my life I have never seen any of these. I only wish I had something to measure so I could go out and buy one.
    – RedSonja
    Feb 16 '18 at 12:24

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

enter image description hereenter image description here

  • Yes, something like a micrometer would work, but more of a C shape than a G shape. The machinist calipers are definitely the sort of shape device I was looking for, but with measurement built in. Thanks!
    – pcdev
    Feb 15 '18 at 23:09

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.

  • see threadcheck.com/… and texas-mac.com/Measuring_Dovetail_Slot_Dimensions.html for cases where this is pretty well the only way of measuring something Feb 16 '18 at 11:57
  • I have several different sizes of digital micrometers 1", 2" kinda of spendy but very accurate.
    – Ed Beal
    Feb 16 '18 at 13:58
  • 1
    +1, with digital calipers this is especially easy as you can measure the blocks first, zero the caliper and then it shows the remaining thickness directly.
    – jpa
    Feb 16 '18 at 18:35
  • Absolutely, @jpa, thanks for pointing that out.
    – AnoE
    Feb 16 '18 at 21:43
  • 1
    For high precision when machinists do this they use parallel bars which are of a constant thickness their entire length, and also used for many other things so they are readily available.
    – hildred
    Feb 17 '18 at 16:31

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:

enter image description here

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

enter image description here

Thanks for all the other great answers!

  • I wonder if 3D printing some caliper extension tips could be workable. Another excuse to buy one :))
    – Criggie
    Sep 13 at 2:31

If you're look for quick and rough, an ordinary clamp for joinery will do the trick. enter image description here 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.

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