Auto & Shop InformationLesson 9 of 17
Measuring and Marking Tools
Rules, tapes, calipers, micrometers, squares, levels and gauges.
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Half the shop section is vocabulary and the other half is knowing which tool suits which job. Measuring is where that choice counts most, because the wrong tool does not give you a wrong number - it gives you a number that is not precise enough to be worth having.
The measuring tools
| Tool | What it measures | Typical precision |
|---|---|---|
| Steel rule or scale | length directly | to about 1/64 inch |
| Tape measure | longer lengths, flexibly | to 1/16 inch |
| Folding rule | length, rigidly, in a confined space | to 1/16 inch |
| Vernier caliper | outside, inside and depth | 0.001 inch |
| Dial caliper | the same, read from a dial | 0.001 inch |
| Micrometer | outside diameter and thickness | 0.0001 inch |
| Feeler gauge | the width of a gap | by the leaf selected |
| Depth gauge | how deep a hole or recess is | varies |
| Dial indicator | how far something moves or runs out | 0.001 inch |
A caliper measures three ways - jaws closed on the outside of a part, jaws opened inside a bore, and a rod extending from the tail for depth. That versatility is why it is the shop's general-purpose precision tool.
A micrometer is more precise but far less versatile. It measures across an outside surface and little else.
A feeler gauge is not really a measuring instrument, it is a comparison. You select the leaf that just slides through the gap with slight drag, and the number stamped on that leaf is the answer. Spark plug gaps and valve clearances are read this way.
A dial indicator measures movement, not size. If a question asks how much a rotating part wobbles, or how far a surface is out of true, the answer is a dial indicator.
Reading a vernier caliper
Two steps.
Read the main scale at the vernier's zero, taking the last mark the zero has passed. In the figure the zero sits just past 1.225, so the main scale contributes 1.225 inch.
Find the one pair of lines that coincide. Only one pair can, and the number on that vernier division is the number of thousandths to add. Here it is division 15, so add 0.015 inch.
1.225 plus 0.015 is 1.240 inch.
Why only one pair can line up: the vernier's 25 divisions are spread across 24 of the main scale's, so every vernier division sits a fixed sliver ahead of the one before. That sliver is the instrument's precision, and it is what lets a scale you could not read by eye be read exactly.
Reading a micrometer
Three steps, and the first two are both on the sleeve.
Read the numbered mark the thimble has uncovered. Each is a tenth of an inch, so the 2 in the figure is 0.200 inch.
Count the small marks past it. Each is 0.025 inch. Three of them is 0.075 inch.
Read the thimble against the datum line. Each thimble division is 0.001 inch, and 13 is on the line, so 0.013 inch.
0.200 plus 0.075 plus 0.013 is 0.288 inch.
The reason a micrometer works is a screw, which makes this the simple machines lesson in a measuring instrument. The spindle advances a fixed distance per turn, so a large rotation of the thimble becomes a tiny movement of the spindle, and angle becomes distance. The ratchet or friction thimble on the end matters for the same reason: it slips at a set torque, so every user closes the instrument with the same force and gets the same reading.
Marking and checking tools
| Tool | Job |
|---|---|
| Scriber | scratches a fine line on metal |
| Awl | marks or starts a hole in wood |
| Center punch | dimples metal so a drill bit will not wander |
| Chalk line | snaps a long straight line |
| Combination square | checks 90 and 45 degrees, and measures depth |
| Try square | checks 90 degrees only |
| Framing square | large square for laying out framing |
| Level | checks horizontal with a bubble |
| Plumb bob | checks vertical with a weighted string |
| Protractor or bevel gauge | sets and checks other angles |
Square, level and plumb bob are the three most confused tools in the subtest, and each answers a different question. Square: are these two edges at right angles to each other? Level: is this horizontal? Plumb: is this vertical?
A center punch exists because of a real problem. A drill bit set on smooth metal slides before it bites, so the hole ends up somewhere other than the mark. The dimple gives the bit a place to sit.
A scriber is for metal and an awl is for wood. A pencil line on metal is too wide to work to when you are after thousandths.
Know it by sight
| Item | How to recognize it |
|---|---|
| Steel rule | a flat, rigid steel ruler |
| Folding rule | hinged wooden or plastic sections that fold up |
| Tape measure | a retracting steel tape in a case, with a hook on the end |
| Framing square | a large steel L, about 24 by 16 inches |
| Protractor | a half circle marked in degrees |
| Spirit level | a long bar with bubble vials |
| Torpedo level | a short, tapered level with vials |
| Bullseye level | a round vial that shows level in every direction at once |
| Outside calipers | legs that bow inward to measure across the outside of a part |
| Inside calipers | legs with tips that turn outward to measure inside a bore |
| Dial calipers | sliding jaws with a round dial readout |
| Digital calipers | sliding jaws with an LCD readout |
| Outside micrometer | a C-shaped frame with a spindle and a thimble |
| Inside micrometer | a straight rod with a thimble and no frame, set across a bore |
| Depth gauge | a crossbar base with a rod or blade that slides down into a hole |
| Feeler gauge | a fan of thin steel blades, each marked with its thickness |
| Thread gauge | a fan of blades with toothed edges, matched against a thread |
| Wire gauge | a flat disc with numbered slots around its edge |
| Awl | a pointed steel shaft in a handle, for scribing and starting holes |
What you can skip
Across the 59 questions on this topic in our bank:
- Laser measures and laser levels never appear.
- Machinist layout tools - the scriber, the try square - never appear; the dial indicator comes up once.
- Metric conversion. Metric measuring comes up once. The micrometer and vernier reading in this lesson are in inches, which is what the questions use.
Where people lose points
Saying a caliper is more precise than a micrometer. It is the other way around.
Using a level to check vertical. A plumb bob does that.
Thinking a square measures length. It checks angle.
Reading the micrometer's small sleeve marks as thousandths. They are 0.025 inch each.
Forgetting to add the two scales. A vernier or micrometer reading is always a sum.
Calling a dial indicator a size measurement. It measures movement.
Work one in under a minute
A technician needs to know whether a brake rotor is warped, and by how much, to within a thousandth of an inch. Which tool is correct, and why not a micrometer?
A dial indicator.
The question is about movement, not size. A warped rotor is one whose surface moves toward and away from a fixed point as it turns, and that variation is what has to be measured.
A dial indicator is mounted to something fixed with its tip on the rotor face, and the rotor is turned by hand. The needle sweeps through the amount of warp directly.
A micrometer would measure thickness at chosen spots, which is a different fault. A rotor can be perfectly uniform in thickness and still be warped, because warp is about the surface's position, not the metal between the two faces.
The trap is that both tools read to a thousandth. Precision was never the question; what was being measured was.
Where this leads
Every tool lesson that follows assumes you can lay out the work first, and the micrometer's screw is the simple machine you already know.
Related lessonsReference
- Striking Tools - the next tool family
- Drilling and Boring Tools - where the center punch is used
- Inclined Planes, Wedges and Screws - the screw a micrometer runs on
- Shop Safety and Practice - measuring twice as a habit
Practice this topic
Check that this lesson stuck. Answer questions on measuring and marking tools only, and see the right answer and why after each one.
Practice Measuring and Marking Tools questions