Session 6: Caliper Use & Precision
'F' → Fullscreen
Build
Four stations. Your caliper travels with you; the parts stay where they are. Three readings at every station, and at every station you record what feature you measured — not just a number.
Technique: The Three Rules
- Zero it first. Close the jaws fully and confirm the reading is 0.00 — then do it again at every station, not once for the whole hour.
- Close the jaws snug, not forced. Station 2 is going to show you exactly why.
- Read it straight-on, not at an angle.
Powering on, zeroing, and a full measurement — start to finish.
Station 1: Thin Materials — Filament
- Measure the filament's diameter at three different points along its length — not the same spot three times.
- Then measure point A again with your own 0.1 mm caliper.
| Reading | Caliper | Diameter |
|---|---|---|
| Point A | 0.02 mm caliper | |
| Point B | 0.02 mm caliper | |
| Point C | 0.02 mm caliper | |
| Point A again | 0.1 mm caliper |
Did your three precision readings agree? And could the 0.1 mm caliper see the difference between them?
Both Units
Point A again, on the 0.02 mm caliper — once on each setting of the switch.
| Measurement | Millimeters | Inches |
|---|---|---|
| Filament diameter, point A |
Station 2: Compressible — Foam Wheel
Measure the outer diameter two times, changing only how hard you close the jaws. Then do the same for the inner diameter of the bore, using the inside jaws and changing only how hard you open them.
| Jaw pressure | Outer diameter | Inner diameter |
|---|---|---|
| Barely touching | ||
| Snug |
Put your “snug” reading on the board. That is one of the two class numbers.
Station 3: Features — Two Bolts
Two bolts from the parts bin, not a matched pair. Measure both — and notice that “how long is it” turns out to have two answers.
| Feature | Hex bolt | Carriage bolt |
|---|---|---|
| Thread diameter, across the crests | ||
| Length, under the head to the tip | ||
| Length, top of the head to the tip | ||
| Across the flats (hex head / square neck) |
Which of your two lengths do you think a supplier means when it sells you a bolt?
Both Units
The hex bolt's under-head length, on each setting of the switch. Session 8 uses this exact number, so double-check the feature before you write it.
| Measurement | Millimeters | Inches |
|---|---|---|
| Hex bolt, under head to tip |
Station 4: Hollow — Metal Tube
This one needs both ends of the caliper: the outside jaws and the inside jaws.
| Measurement | Value |
|---|---|
| Outside, across the marked face | |
| Outside, across the other face | |
| Inside, across the marked face (inside jaws) | |
| Wall thickness, measured at the cut end | |
| Wall thickness, calculated: (outside − inside) ÷ 2 |
Put your marked-face outside reading on the board. That is the second class number.
Your two outside readings are not the same. That is not a mistake — say what it tells you about the part. Your measured wall and your calculated wall probably disagree too. Which one would you trust to order a part?
Both Units
The marked face, outside, on each setting of the switch.
| Measurement | Millimeters | Inches |
|---|---|---|
| Tube outside, marked face |
Keep this sheet. Next session you will convert one of these millimeter readings to inches by hand — and then check your answer against the number the caliper already gave you.
Station 5: The Blue Wheel
Exactly what you did at station 2, on a rubber wheel instead of a foam one. Same two pressures, same two diameters.
| Jaw pressure | Outer diameter | Inner diameter |
|---|---|---|
| Barely touching | ||
| Snug |
Put three parts in order by how much the reading moved between your two pressures: the metal tube from station 4, this rubber wheel, and the foam wheel from station 2. You never measured the tube at two pressures — say where it belongs anyway, and why.
Station 6: The Black Extrusion
This one needs the third way of measuring — the depth rod, the thin blade that slides out of the tail of the caliper as you open it. Nothing else today has used it.
| Measurement | Value |
|---|---|
| Overall width, across one face | |
| Width of the slot opening | |
| Center bore diameter | |
| Depth of the slot — depth rod |
This extrusion is sold under a size. Ask what that size is, write it down, and put your own overall-width reading next to it.
| Sold as | What you measured |
|---|---|
Station 7: The Flange (Bonus)
Everything here is measurable with the caliper except the last row — and that is the point.
| Measurement | Value |
|---|---|
| Flange outer diameter | |
| Raised boss outer diameter | |
| Center bore diameter | |
| One bolt hole diameter | |
| Two opposite holes, near edge to near edge | |
| Bolt circle diameter — work it out |
You cannot put the jaws on the bolt circle. Its two ends are hole centers, and a caliper cannot touch a center. Use the two readings above it to get there, and write down how you did it.
Compare as a Class
Two columns go on the board: everyone's tube reading and everyone's foam reading. Find the range of each — smallest to largest.
| Class low | Class high | Range | |
|---|---|---|---|
| Metal tube | |||
| Foam wheel |
Same room, same calipers, same technique — and one of those ranges is far bigger than the other. Name what made the difference. It was not the tool.
Identify a Source of Variation
Name at least one specific, physical reason that measurements of the same object came out different, and say which station showed it to you most clearly.
Checkpoint
Before you move on, check yourself against these:
- Did you re-zero the caliper at every station, not just the first?
- Does every reading you wrote down have a feature name beside it?
- Station 1: are your three precision readings on the same strand, at three different points along it?
- Stations 2 and 4: did you use the inside jaws for both of the inside measurements — the wheel's bore and the tube's?
- Both class numbers — tube and foam — are on the board.
- Where your own readings disagree, can you say whether it was the object, your technique, or the caliper's limit?
Reflection
Answer these on your own. Reflections are individual, even when the rest of the session was team work.
- Rate yourself 1–5: how confident are you that if you measured the same object again tomorrow, you'd get the same number? Then name the one thing most likely to make it come out different.
- You wrote three of today's measurements down in both millimeters and inches. Pick one of those pairs. The two numbers are different — did the object change size? Say exactly what changed and what did not.
Submit
Submit four things:
- Your readings from all four stations, each one with its feature named.
- The class range for both the tube and the foam.
- Your named source of variation, and which station showed it.
- Your two reflection answers.
Stations 5, 6, and 7 earn no points, and you only go to them if your teacher sends you. Hand it in anyway if you got to station 7 — especially your working for the bolt circle.