Calipers

Part of Precision Measurement

Making and using outside calipers — the most versatile measuring tool for cylindrical and irregular shapes, essential for metalworking and woodturning.

Why This Matters

A caliper is the tool that bridges the gap between the workpiece and the measuring scale. You cannot lay a ruler against a turned cylinder and read the diameter directly — the curved surface doesn’t present a measurable flat face. The caliper wraps around the curve, captures the dimension, and lets you transfer it to a ruler for reading.

Outside calipers are among the oldest precision tools. They appear in ancient Greek, Roman, and Chinese metalworking. For any craft involving round or irregular shapes — turning, casting, forging, fitting shafts to holes — calipers are not a luxury but a fundamental requirement for achieving consistent results.

The principle is simple but the execution demands care: two curved legs that contact opposite sides of an object, joined at a pivot that allows the legs to be adjusted. When the legs contact the object snugly (not tightly), the span of the legs equals the diameter. Transfer that span to a ruler to read the measurement. The skill is in achieving consistent contact force — pressing too hard compresses the legs and reads too small; too light and the legs don’t fully close.

Types of Calipers

Spring-bow outside calipers: Two curved legs joined at the top by a tight spring (typically a bow of spring steel or springy bronze). The legs are forced apart by the spring; a screw between the legs draws them together. Adjusting the screw sets the opening. The spring provides automatic tension that keeps the setting firm against vibration and accidental bumping.

Firm-joint calipers: Two legs joined at a pivot with a nut-and-bolt that provides friction. The user opens and closes the legs directly and the friction holds the setting. Simpler to make but less precise — the friction can loosen with use, allowing the setting to drift.

Hermaphrodite calipers: One leg is a caliper leg (curved); the other is a scriber (straight, with a pointed tip). Used for scribing lines parallel to an edge or marking a center point from a turned surface. Not a measuring tool in the strict sense but invaluable for layout work.

Making a Firm-Joint Caliper

This design requires no spring steel and is buildable with basic metalworking tools.

Materials:

• Two pieces of mild steel bar, 6mm × 15mm cross-section, 200mm long (the legs)
• One bolt, 6mm diameter, with nut and washer (the pivot)
• File, drill, hacksaw

Steps:

1. Shape the legs: Mark the leg profile on each bar — a gradual curve from the pivot end to the measuring tip. The curve should bring the measuring tips to a point and have enough clearance to span objects up to 150mm in diameter. Hacksaw and file to the profile.

2. Drill the pivot holes: Clamp the two legs together and drill through both simultaneously using a 6.5mm bit (slightly larger than the 6mm bolt). Drilling together ensures the holes are aligned. Deburr both holes.

3. Assemble: Place the two legs together with the inner faces touching. Install the bolt with a washer on each side of the joint. Thread the nut on and tighten until the joint moves with moderate friction — the legs should move when pushed firmly but not move freely or flop.

4. File the measuring points: The leg tips must meet exactly when fully closed. Check by closing the calipers and looking at the tips against a light — any gap indicates the tips are not equal length or the legs are twisted. File the longer/higher tip until both contact simultaneously.

5. Test and adjust friction: The nut tightness determines friction. Too tight and the caliper is hard to adjust; too loose and the setting slips during use. The correct friction holds the caliper setting through a firm grip transfer from workpiece to ruler without slipping under its own weight if held at arm’s length.

Making a Spring-Bow Caliper

The spring-bow design requires a piece of spring-tempered steel for the bow. Sources include old clock springs, leaf springs from machinery, or hardened and tempered steel strip.

The bow: A U-shaped strip of spring steel, approximately 100mm long (from tip to tip of the U), 10mm wide, 1–1.5mm thick. It should spring apart naturally to about 40–50mm spread at the tips when unloaded.

Assembly: The bow connects to the tops of the two legs. Drill a small hole in each leg top and rivet or bolt the bow tips into these holes. The bow’s spring forces the legs apart; a small screw between the legs (passing through a threaded hole in one leg and butting against the other) draws them together and sets the opening.

The adjustment screw needs a knurled knob or cross-drilled hole for finger-turning. A wing nut works well.

Using Calipers

Measuring an outside dimension:

1. Open the calipers wider than the object
2. Close them against the object, applying just enough pressure to feel the legs seat against the surface — the calipers should just barely pass over the widest point with light resistance
3. Do not press hard — this is the most common error, causing false readings
4. Lift the calipers off the object, maintaining the setting
5. Transfer to a rule: hold one leg tip on the 0 mark, read the other tip’s position

Checking a turned cylinder: Set the calipers to a target diameter from the rule. Try to pass the caliper over the workpiece — if it passes, the workpiece is undersize; if it won’t pass, it’s oversize. When the calipers just pass over the workpiece with light drag, the dimension is correct. This is the most common use in turning and metalwork.

Testing caliper accuracy: Check that the tips meet precisely when fully closed (zero error). Check by measuring a known object of verified dimension. For a self-made caliper, measure a reference standard and record any consistent offset. Apply corrections during use or adjust the tips by filing.

Transfer Calipers

For comparing two dimensions — checking if a shaft fits a hole, for instance — the caliper is set to the shaft and then checked against the hole without reading the intermediate measurement. Set to the shaft diameter, then try to insert the caliper into the hole. If it passes, the hole is larger than the shaft; if it won’t pass, the shaft is larger than the hole. When the caliper just passes with light friction, the dimensions match.

This comparison method is faster than reading and re-setting and is more accurate than independent measurements of each piece, because it directly compares the two dimensions of interest.