Metrology Monday: Calipers

What Are Calipers For?

The title seems a little silly for this one since most everybody has at least seen calipers if not used them. They are used for a reasonably precise method (I have already covered at least one unreasonably precise method) of measuring the distance between two surfaces in a quick and convenient way. Presuming you can find them, of course. Note that no vernier scale caliper is pictured above even though I own one…

The most used measurement is the outside one. I will often use calipers for marking out a part as well as measuring an existing object to find out what size it is. Handy if you have unlabeled metal stock lying about. In the picture, we can see that the shank of my 3/8″ end mill is about .001″ under sized according to my inexpensive caliper. If I needed more precision, I would get out a micrometer.

Using the other set of blades, one can measure inside distances between flat surfaces. It turns out my 21mm wrench is actually a 21.11mm wrench.

Calipers can also be used to measure depth in two different ways. The first is what I have used for years, the other end of the caliper.

The second is one I only learned about recently and while it is somewhat specialized, it allows a more reliable reading of a step using the end faces of blade end. I held it above the piece for what I fondly hope is clarity.

Alternatives

The alternatives to calipers are fairly well known and are all a tradeoff between precision and convenience. There are probably people out there doing fine work with just a rule or just micrometers but they are giving something up.

Options

Calipers come in Vernier, dial and digital models. Vernier versions are the most robust and proof against fluids (cutting coolant) but slower to read. Many have both Imperial and metric scales on them. Dials are quick and easy to read but only come in one set of units. Digitals can convert between Imperial and metric, can be zeroed at any point and can give implausibly precise readings. And they need batteries which is why my pictured one is not getting used for any of the action shots.

Calipers come in a vast range of quality from very cheap plastic models (1.99!) to very uncheap ones you would be careful not to breath on. They also come in lengths from 6″ to 24″, at least at my local tool supplier which lists 83 different models ranging in price from $50CDN to over $500. Mine are much more in the $50 range (probably less) but do a good enough job anyway. I use them to get close and then check final dimensions with a micrometer if it matters.

One thought on “Metrology Monday: Calipers”

  1. There are also calipers without any graduations, which are used to make things to fit. Typically one would use inside calipers to obtain the diameter of a borehole, and then adjust some outside calipers to just touch them, so that when turning a rod/axle/stopper for the bore, it can be turned down to the requisite fit, testing with the outside calipers.
    Of course, one can use the inside and outside legs of (say) the vernier calipers in the same way – measuring one sets the other automatically – but these should not be used on rotating workpieces (it can bend the legs) whereas comparator calipers, adjusted with a screw and thumb wheel, can be so used.
    (In really skilled hands – not mine, that’s for sure – they don’t even need the screw adjustment, just a nice tight but not too tight pivot.)

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