Tools: Calipers

Well I have a shop in the garage so I figure I should start blogging about useful tools. This is the first post in a series of common (and uncommon) tools you’ll find in a machine shop.

The most useful tool, for a Mechanical Engineer anyway, seems to be a pair of calipers. I don’t leave home without them. I have 4 pair in my toolbox alone.

What are calipers?

8″ Mitutoyo Coolant Proof Calipers. This pair got sucked into a Monarch 10EE by some long steel chips whipping around but is still alright.

Calipers are a measuring instrument for distance, thickness, length, height, depth, diameter, etc. They can be used to accurately measure in one dimension (usually are marked in inches, mm or both) and will generally have a pair of inside and outside measuring jaws juxtaposed, as well as a depth gauge plunger. You may think “why not use a tape measure or ruler?” and the answer is: accuracy, precision and repeatability.

Accuracy, repeatability, and why if you’re cutting on a lathe you should be using a Micrometer, not calipers.

Accuracy – Closeness of a measured value to a standard reference

Precision (repeatability) – Closeness of two or more measured values to each other

Rulers are great for measuring how jacked the welds are on your hyperloop tube at a glance, but it’s hard to get an accurate reading without touching two surfaces. Calipers with a depth gauge or a weld gauge would be a better measuring tool for this application

Calipers are the weapon of choice for engineers primarily because they quickly can give you accurate, precise data about the length, thickness or diameter of an object, to a higher degree of precision than a ruler or tape measure, about .001″ or .025 mm.

Calipers are great for hand checking width of slots, hole diameters, parth thicknesses, overall dimensions, and distance between features.

This matters for making sure parts fit together, mating surfaces will be in proper contact, or two moving parts in an assembly will have adequate clearance. They are also superb for in process manufacturing checks, to make sure the proper amount of material has been removed, or a hole is large enough to clear a bolt.

In modern manufacturing, most bulk material removal processes tend to achieve around .001″ precision or less (at least for metallics, generally) , and more precise manufacturing environments generally use more precise tools.

Parts can be cut more accurately than you can measure with calipers on a lathe. Best stick to micrometers when checking turned parts, but in a jif, calipers can be used for layout or rough dimensional checks.

Lathes are capable of regularly turning out parts that have diameters precise to .0001″. For measuring tight tolerance parts that have a turning operation, you want to use a micrometer, as it can accurately measure down to .0001″ or less. These tolerances are normally necessary for press fit or slip fit parts, bearings bores, and optical assemblies.

From Machinery’s Handbook- This chart shows common processes and the tolerances they can achieve. Calipers can measure down to .001″ usually, or on the process list, from flame cutting down to about boring.

What types of calipers are there?

The oldest calipers had no markings to display the measured dimension, and were primarily used for transferring a measurement from one part to another, or to do a part layout

There are many kinds of calipers. The earliest calipers were merely two curved bows with a pivot in the middle. The common types of calipers you’d find in a shop for everyday measuring work are the following:

Vernier – old timey, hardest to break

These are seldom used in application in modern shops, due to a tendency for new engineers to misread the vernier scale.

Diagram of Vernier Calipers and how to read them from the Wikipedia article on calipers

Dial- The batteries never die

These are all over the place, a reliable instrument for quick checks, without batteries to die, they can sit in a toolbox for 10 years and never be dead when you pick them up. These are probably the most sensitive to damage, as a hard drop can break the fragile mechanisms inside.

My first pair of calipers in the professional world, a Fowler set I got at SpaceX. The dial is adjustable to set zero at any point on the slide, though you’ll have to remember what mark you started at.

Electronic- Easy to read, come in varieties (coolant proof, economy, etc)

For the lazy modern engineer, only accept the best. And buy some extra coin cell batteries because these calipers die when you need them the most. The first time, every time.

Even when your spherical bearing races are still covered in nuclear grade anti seize, electronic calipers make your life easy and your hands clean.

How do you use them? What can you measure with them, what can you not measure with them?

Direct Measurement of length, thickness

Direct Measurement of the thickness of a cube. Notice how I’m not using the sharp part of the jaws. This would scratch the part, and if I used too much force, may give me an undersized measurement. Watch out for burrs on metal parts, as they can throw off a reading. Hit a part straight off the mill with a deburring knife or stone before measuring.

The easiest ways to measure with calipers is to place an item between the jaws and close the jaws with the thumb wheel.

Depth measurement

Measuring the depth of the hole in the center of this tape roll. This isn’t going to be super accurate, but gives you an idea.
The business end of the depth measurement.

Most calipers have an integral depth gauge that extends when you open the jaws (be careful not to bend this guy when taking measurements). This allows you to measure hole depth, pocket depth, or features inside a hole like a shoulder or a counter bore.

Be careful to get the reference edge lined up well and set zero at the top of the hole before taking measurement. Keeping the calipers perpendicular to the measuring surface is more difficult with the depth gauge, so double check measurements.

There are special attachments for making it easier to hold the calipers perpendicular to a flat part surface when using the depth gauge feature.

Update: Thanks @cyclematch for this rad tip on measuring shallow steps.

Internal diameter / pocket width

I use the inside jaws to measure the distance between either side of the slot on this cube.
When measuring an inside diameter, move the jaws around while applying slight opening pressure to get to the largest diameter measurement. This may take a couple tries. Be careful with parts that scratch easily, caliper jaws are carbide or tool steel and will scratch parts readily.

Hole spacing measurement

First measure the edge distance of the inside edges of the holes on the pattern. The inside edges of these holes on this 1-2-3 block are 1.849″ apart (probably ~1.850″ basic dimension)

To get a rough idea of where the center of two holes in a pattern are, measure the inside edge distance, then the diameter of the holes, and do some simple math.

The hole diameters are .350, so the holes are 1.849″+.35″ = 2.199″ center to center.

Other uses:

Layout / scribing

Dykem layout fluid drys very fast and provides a strong contrast for scribing lines. It also smells terrific.
Stock 6061-T6 Aluminium on a carbide tipped chop saw, with Dykem Layout fluid for scribing lines to cut 2.080″ cube blanks for the Swarf cube.

With some dykem and a pair of calipers you can scribe layout lines for cutting material to size with a saw, or making sure you’re leaving enough stock when taking a roughing pass on a manual machine.

Scribing a line .500″ inwards from the cut edge. This line will be parallel to the edge, so be careful to use good reference edges and not saw cut ones when doing layout.

This works best with aluminium, but will work on certain steels as well. In some cases with very hard material it’s best to use a carbide tipped scribe to make the actual line. Also be careful with scribing not to create a surface defect where corrosion or cracks can nucleate in parts that are sensitive to fatigue or surface imperfections, or have special coatings.

A rectangle offset .500″ from the edges of this piece of stock. This is great for setting up hole centers that are dimensioned from the part edges. It’s best to have machined reference surfaces when using calipers for layout. For quick layout that’s from a plane, consider using a height gauge on a surface block.

Go/No Go “gauge”

There is usually a thumbscrew that can be used to lock the moving jaw of the calipers. Don’t over tighten it.
No-go- the calipers won’t fit over the 1.000″ block when locked at .995″

using the thumbscrew to set a fixed jaw width is useful as a true / false test to see if a dimension on multiple parts is over or undersized. In this example, I set the jaw width to .995″ and the calipers will not fit over the 1.000″ 1-2-3 block, but when I set the width to 1.001″ the calipers slide over the part.

Go – The calipers fit over the 1.000″ block when locked at 1.001. This would make it very easy to check 5 or 10 blocks to make sure they were within .001″ of 1″ or further undersized

Non uses: hammer, screwdriver, pliers, butterfingers, knife, TSA pat down flag.

Hammers are generally better for hitting things.

Calipers are sensitive and can break from shock, misuse and abuse. It’s more of a pain than a world ending expense to need to replace calipers, but if a broken pair is in circulation, it can affect much more expensive parts. Be sure not to abrade or bend the jaws using the calipers as a screwdriver, don’t drop them, or use them to open packaging. Also they are a pointy metal object, if you bring them on a plane, consider checked baggage. ( I’ve made this mistake a few times, though I’ve never had them taken from me).

What brands are out there?

Brown and Sharpe

One of the older precision tooling shops. Started in Providence Rhode Island, got absorbed by Hexagon Metrology recently

Starrett

Starrett is basically the ground truth for machine shops and tooling houses in America, one of the oldest and best manufacturers of measuring and tooling equipment. Not cheap, but also high quality

Mitutoyo

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Mitutoyo is a great japanese maker of measuring tools. Middle of the road pricing.

Fowler

Founded in 1946 in Massachusetts, Fowler is known for high quality measuring equipment. My first pair of calipers were a Fowler dial 6″ set, and I still have them 10 years later, they still work well.

Tesa

Also part of the Hexagon Metrology conglomorate, Tesa is also known for high precision dial indicators.

Chinese / no name

Most calipers on Amazon or at Harbor Freight are a chineese / no name brand. These might be fine for fixing your lawn mower, but you don’t want to use them on more expensive parts where errors or inconsistent measurements might be costly.

How much should I spend?

You may be able to find a cheaper source than Mcmaster for some of these. What you need really comes down to your job or how big your parts are. Be sure to check the smallest increment the calipers will measure, and also how accurate they are.

Cheap

Good

Better

Best

Accessorize your Calipers

First you should probably put your name on them somewhere. Usually the back of the plastic housing for the display / battery.

The primary accessory for calipers is a data output cable, for taking many measurements and recording them on a computer. This is called SPC output for most calipers. Most home users won’t need these cables though, as they generally aren’t measuring hundreds of parts at a time.

Everyone needs a Holster for their calipers.

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Different tips are available for measuring special or hard to measure features. Mcmaster sells a kit.

Reference Gauges are used for checking calibration of calipers, and are precision ground to exact dimensions. You probably won’t need one for a home shop, but may if you’re trying to meet certain quality standards.

What are some specialty calipers?

Caliper for Pipe and Tubing – Mcmaster Carr $683.53
Caliper for Measuring Soft Materials, has a low jaw force. Mcmaster Carr – $858.07
Left Handed Calipers – Mcmaster Carr $169.85
Large Capacity Calipers, with Carbon Fiber Bow. Mcmaster Carr $1014
Calipers for measuring between hole centers- Mcmaster Carr $621.81
Caliper for measuring offset planes – Mcmaster Carr $504.17
Caliper for Small Holes and Grooves – Mcmaster Carr $419.00

More reading

Wikipedia Article on Calipers

Mitutoyo’s Origin History and Evolution of Calipers

The Starrett Story

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