Outside Calipers

Part of Precision Measurement

Using spring and firm-joint calipers to transfer and compare external dimensions without a graduated scale.

Why This Matters

Outside calipers are the oldest and most fundamental measuring tools in the machinist’s kit. They predate the micrometer by centuries. A caliper transfers a dimension from a workpiece to a rule, or compares two workpieces directly. In skilled hands, a well-made pair of spring calipers is accurate to 0.1 mm — entirely adequate for most workshop fitting work.

More importantly, calipers do not require a graduated scale on the instrument itself. They can be set from a standard, carried to the workpiece, and the fit checked by feel. This tactile skill — feeling when the caliper leg touches lightly, neither gripping nor sliding freely — is something that takes practice but never requires electricity, complex instruments, or anything that can break.

For a rebuilding workshop, a set of well-made calipers is an essential first tool, long before micrometers or vernier scales become available.

Types of Outside Calipers

Spring joint calipers:

• Curved legs with bowed outer shape
• Spring tension holds them open; adjusting nut closes them
• Most common workshop type
• Good for repeated checking of one dimension

Firm joint calipers:

• Simple pivot with friction — no spring
• Set by feel, hold position by joint friction
• Simpler to make; less convenient to adjust

Odd-leg (hermaphrodite) calipers:

• One leg straight, one curved
• Used for scribing parallel lines, finding center of round stock
• Not strictly for outside measurement but in the same tool family

Transfer calipers:

• Lock the setting with a screw after measurement
• Allow transfer to rule or other instrument without disturbing the setting
• Valuable for measuring inside dimensions and transferring them to outside

Type	Best Use	Accuracy
Spring outside	Repetitive checking, turning work	±0.1 mm
Firm joint	General comparison	±0.2 mm
Hermaphrodite	Layout, centering	±0.3 mm

Making Outside Calipers

Basic calipers can be forged or cut from steel plate:

Materials:

• 3–4 mm thick mild steel flat bar for legs
• Rivet or bolt for pivot
• Steel spring strip (from old saw blade) for spring joint type

Process:

1. Cut two identical leg blanks — curve the ends to the desired shape
2. Drill pivot hole in both, ensuring holes align perfectly when overlapped
3. Round and smooth the measuring tips — they must be hard, smooth points
4. Rivet together at pivot with enough friction to hold position but allow adjustment
5. For spring type: cut a U-spring from spring steel, rivet between legs above pivot, add adjusting screw and nut through the spring arms

Leg tip treatment:

• File to a smooth, rounded point — not needle-sharp (will mark soft work) nor blunt
• Harden the tips by heating to cherry red and quenching in water or oil
• Test by trying to scratch with a needle file — should resist

Sizing Calipers

Make multiple sizes. A 100 mm caliper for small work, 200 mm for medium, 300 mm for larger stock. Longer legs become unwieldy and flex too much. Never force a caliper to measure well outside its design range.

Using Outside Calipers

Setting to a dimension from a rule:

1. Open the caliper slightly wider than the target dimension
2. Hold the caliper and rule in good light at eye level
3. Close the caliper until one leg aligns with zero on the rule
4. Adjust until the second leg aligns with the target graduation
5. Lock or note the setting; verify by measuring again

Checking a turned diameter:

1. Open caliper to slightly more than the workpiece diameter
2. Hold caliper loosely at the top joint between thumb and forefinger
3. Lower onto the workpiece — the caliper hangs by gravity on the part
4. The correct setting feels like: caliper falls under its own weight with slight drag
5. Too tight: caliper won’t pass or requires force to pass
6. Too loose: caliper falls freely without any resistance

The Feel Test

“Just passes under its own weight” is the traditional test. Practice on stock of known size — measure with a micrometer, then learn what that caliper feel corresponds to at each diameter. An experienced machinist reads 0.1 mm differences by feel alone.

Comparing two workpieces:

1. Set caliper on the reference part (master or first part)
2. Move directly to the comparison part without adjusting
3. A part that is the same size will give the same feel
4. A part that is larger will not allow the caliper to pass
5. A part that is smaller allows free passage

This comparison method is faster than measuring both with a micrometer and is used constantly in production environments for go/no-go checking.

Caliper Arithmetic

Calipers alone cannot add or subtract dimensions — you need a rule or reference block for that. But they excel at:

Checking taper: Set calipers to diameter at one end; compare feel at other end. Any difference is taper.

Checking roundness: Measure in two perpendicular directions without moving the workpiece. If the caliper feel differs, the part is oval or eccentric.

Checking parallelism: Set calipers to width at one end; slide to opposite end. If feel changes, the part is tapered in width.

Checking concentricity: With part between centers, set calipers on the diameter; slowly rotate the part. A part with runout will vary in feel as it rotates.

Caliper Care

• Keep pivot joints clean and lightly oiled
• Protect tips from damage — a nicked or mushroomed tip gives inaccurate feel
• Check that legs spring back fully when adjusting nut is loosened
• Store hanging or flat, not piled where legs can be bent
• Periodically check that both legs are the same length — bent legs give false readings

A good set of outside calipers, well maintained, will last generations. The skill of using them — the feel, the light touch, the calibrated sense of drag — is equally valuable and equally durable.