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?
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
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.
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.
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.
What types of calipers are there?
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.
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.
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.
How do you use them? What can you measure with them, what can you not measure with them?
Direct Measurement of length, thickness
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
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.
Thanks for the good article on calipers. One more method to add for measuring step height, I learned it a couple months ago from my machining professor. Thanks Prof LaFont! pic.twitter.com/0BqJ3a299f
— Kevin Matocha (@CycleMatch) May 17, 2020
Internal diameter / pocket width
Hole spacing measurement
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.
Other uses:
Layout / scribing
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.
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.
Go/No Go “gauge”
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.
Non uses: hammer, screwdriver, pliers, butterfingers, knife, TSA pat down flag.
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?
One of the older precision tooling shops. Started in Providence Rhode Island, got absorbed by Hexagon Metrology recently
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 is a great japanese maker of measuring tools. Middle of the road pricing.
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.
Also part of the Hexagon Metrology conglomorate, Tesa is also known for high precision dial indicators.
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.
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?
Cool article! Great info on what tools to buy. Learned something.
Thanks! Come back in a week or so and I’ll have another article.