How to: Calipers? Call who?

Calipers seems to be simple enough, but it might be more to them than one would think. I’ve gotten a few eureka moments over the years, so let’s look into a few that measured very small.

Although the impact of said eureka’s has not been small at all!

Calipers, or as machinists would call them: guesstimeters, are actually invaluable tools to have in your shop. But how do they work? For us woodworkers, they might not be familiar. They are, however, invaluable in the shop for a number of applications. They seem simple enough to use, but you might be surprised. And no: don’t use them as an adjustable wrench/spanner!

Choosing a good caliper

Machinists use micrometers for their work, as they need that level of accuracy. For us woodworkers though, anything below .5mm (slightly less than 1/32”) will be more than adequate for almost all applications. The reason is that wood will compress, it swells and it moves. You do not want extremely tight joints before glueup; when you apply glue the surface starts to swell immediately (and you need room for the glue anyway). However, your accuracy won’t be better than the weakest link, so we need good quality calipers. Starrett, Mitutoyo, Limit and Diesella are renowned and trusted brands in the industry.

There are three main types to choose from: Vernier (shown here), dial and digital. The dial caliper has a dial that looks like a clock. The hand spins and show the fractional measurement while the ruler shows the larger measurement. The digital gives the readout on a display, and needs batteries to work.

Accuracy: my Mitutoyo vernier calipers is rated at ±0.05 mm or ±1/128”. Accounting for instrument and/or operator error and any zero-point error (meaning that the calipers does not read zero when fully closed), I regard the accuracy as ± 0.1mm or 1/256”. In physics, we refer to “the level of uncertainty”. Meaning that the result will vary between some known variable. In this case measuring 10mm tells us that the actual measurement of the part is between 9.9mm and 10.1mm.

In relation to woodworking, that level of accuracy is “dead on”! However, since calipers can be useful for other applications as well, having a tool with a high degree of accuracy in the shop is beneficial. I would avoid plastic and lesser-quality (that means cheap) measuring tools like the plague when ever a high level of accuracy is required. A folding ruler is great for construction and rough measurements, but if you put a couple of them next to each other you may find that they measurements differ. As long as one measuring device is used for everything, it does not matter much if the long sides of a box is measured to 80cm but in reality is 79.8cm – both sides will be equally accurate.

I recommend that you get calipers of decent to high quality. One nice thing about my premium quality Mitutoyo is the satin finish – it makes it easy to read. I have some polished stainless steel rulers in the shop, and they are much harder to read. Sometimes it is worth spending a few bucks more, especially for lifetime tools.

Reading Vernier calipers

This model has both inches and millimeter scales. The accuracy is printed on the scales, letting us know how accurate our reading will be at best. Add half of that as an unknown factor (operator, instrument or environmental – meaning clumsiness, using calipers as a wrench at times and dirt). I’m focusing on millimeters from here on.

In this case, we can see that zero on the moveable jaw is between 4 and 5 millimeters, ergo we have a measurement of four point something. If we then look along the same scale, we find where a line on the moveable scale lines up with a line in the millimeter scale. In this case, the 5.5 line. This gives us a measurement of 4.55 millimeters with an accuracy of ±0.05 millimeters. This tells us that the thing we measured is between 4.5 and 4.6 millimeters.

As we can see, the 5 an 6 lines are slightly offset while 5.5 is dead on. Sometimes it is hard to really tell, but for woodworking it really does not matter.


Now, how do we use the calipers?

Measuring thickness:

This board is 14.2 ± 0.05mm in thickness. On the main scale we read 14mm, and on the fraction scale (the one on the bottom / the moveable jaw) reads 2; the line at 2 lines up with a line on the mm scale. This gives us a reading of 14.2mm – which means that the actual measurement is between 14.1mm and 14.3mm, accounting for accuracy.

Measuring width:

Insert the “birds beak” into what ever you want to measure (dado, inside of a washer, etc.) and open the calipers until the “beak” jaws fit snug. Remove carefully and read the measurement – best practice is to read the measurement while the jaws are engaged in the gap you are measuring.

Depth measurement

Place the “foot” of the calipers on the edge and open the jaws. A thin metal rod slides out at the end. Plunge it to full depth and read the measurement. This is a very neat way to check that your mortise is deep enough. Measure the tenon either by this method or the step method (see below), then – without altering the setting – check that the mortise depth is slightly deeper. This method is great for very narrow places.

Step measurement

To measure a step, open the calipers slightly and insert the calipers into the step as shown above and open until the calipers bottom out. Read the measurement.

Other uses

The sharp parts of the jaws fits in between threads on screws, giving us the diameter of the “shaft”. If you need to predrill for your screws, this measurement tells you which drill bit you need.

The jaws are parallell, which means you can check for square and parallell faces on smaller items. Opening up the jaws gives you a small square for checking two sides. My calipers opens up past 15cm, meaning I can check that the faces are square and parallell for planks up to 15cm in width.

You can also utilize the sharpness as a marking gauge, but I do not recommend dragging the jaws like you would do with a marking gauge. You’ll introduce wear, and your measurements becomes more inaccurate. I find that the “bird beak” is great for placing pin marks, which then can be used to locate the tip of the marking knife. If you do not have dividers, you can use the beak for laying out dovetails

Calipers are great to have in your shop. I have a couple, and I use them often. I have one placed near the band saw and jointer/thicknesser to check dimensions before thicknessing, check for square and for parallell faces.

There are lots of variants of calipers. Some are highly specialized, so I recommend that you get the type I’m showing. It is a very versatile tool with more uses that one might think.

As for dial, vernier or digital – that one is up to you. I personally do not like the digital ones because of the button cell batteries. Once you get accustomed to reading the Vernier scale, the digital readout really is not any faster or more convenient. At least not if we consider the fact that at some point, the battery dies. And you don’t have a spare at the ready now, do you? It might be me lacking confidence and knowledge of how this works, but I do not trust the accuracy to be maintained over decades of use when it comes to digital calipers. The Vernier calipers, and to some extent the dial type, seems more reliable to me. The markings can’t shift, although they might wear. Besides, I advocate environmental awareness. Button cell batteries may contain lithium, silver (and old ones had mercury in them, but those are banned in the EU/EEC from 2020 onwards) or other rare substances. Vernier calipers are more eco friendly. It might seem like nothing, but even Mount Everest is made up of atoms..

If you don’t own calipers, go fetch! They are great to have in the shop. One advice though: do not get too hung-up in accuracy. A board that is 5.38mm today will be a different thickness in a week. Remember that the most important measuring tool in a woodworking shop is the word “-ish”…

For woodworkers and metal workers alike, calipers certainly measure up!

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