Inside Calipers

Part of Precision Measurement

Inside calipers measure internal dimensions — the diameter of a bore, the width of a slot, the span of a groove — and transfer that measurement to a rule or comparison standard.

Why This Matters

External dimensions can be measured with outside calipers or a rule laid against the workpiece. Internal dimensions cannot be directly measured this way — a ruler cannot fit inside a bore and show its diameter. Inside calipers reach into the cavity, span the widest point, and bring that measurement out where it can be read or compared.

For metalwork and woodworking, the ability to measure internal dimensions is essential wherever parts must fit together: shaft to bore, peg to socket, beam to mortise, pipe to coupling. The bore of a wheel hub, the inside diameter of a bearing housing, the width of a dovetail groove — all require inside measurement. Without accurate inside measurement, fitting is entirely by trial and error.

Inside calipers are also used in pairs with outside calipers for comparative measurement: set the inside caliper to the bore diameter, then transfer that span to an outside caliper, then measure the outside caliper span with a ruler. This chain of transfer is less accurate than direct measurement but can achieve ±0.3mm in skilled hands, which is adequate for most practical fitting work.

Design Differences from Outside Calipers

Outside calipers have legs that curve inward — they wrap around the outside of an object. Inside calipers have legs that curve outward — they spread inside a bore or slot. The legs must be thin enough to enter the bore, and the tips must be rounded so they contact the bore wall smoothly without catching.

The pivot and adjustment mechanism is identical to outside calipers. The key difference is leg geometry:

• Outside caliper legs: sweep inward, meeting at the tips when closed
• Inside caliper legs: sweep outward, reaching their maximum spread when fully extended

For a bore 20–50mm in diameter: inside calipers with legs approximately 100mm long and tips spread to the bore diameter. The curved section of the leg extends about 50% of leg length from the tip, creating the outward-reaching geometry.

Making Inside Calipers

Materials: Same as for outside calipers — mild steel bar stock, a pivot bolt and nut.

Leg geometry: Start with two pieces of 6mm × 15mm flat bar, 150mm long. Mark the inside caliper profile:

• From the pivot (one end): straight section 40mm, then a gradual outward curve
• The curve sweeps the tip outward 25–30mm from the centerline at the tip
• The tip itself: shaped to a rounded point (not sharp — a sharp tip can catch on surface irregularities)

File or grind to the profile. The curve must be smooth — any kink or flat spot in the leg causes the tip to contact the bore at an angle, giving false readings.

Tip rounding: After shaping the basic profile, carefully round the measuring tips with a file and fine abrasive. The tip should feel smooth to the touch from any direction and leave no scratch when drawn across a flat metal surface.

Assembly and adjustment: Same as outside calipers. Pivot bolt with friction nut, or spring-bow design.

Critical test: Open the calipers to a known diameter cylinder (turned accurately, or a calibrated bar). The inside caliper tips should just contact both sides simultaneously with a light, equal feel on both sides. If one side contacts first, the legs are unequal in length — file the longer leg tip.

Using Inside Calipers

Measuring a bore:

1. Partially close the calipers to a smaller span than the bore
2. Insert the tips into the bore, keeping the caliper axis aligned with the bore axis
3. Gently spread the legs by adjusting the screw until both tips contact the bore wall simultaneously — you feel a slight drag that is equal on both sides
4. Lock the adjustment if using a screw-lock design (spring-bow calipers hold automatically)
5. Carefully extract the calipers without disturbing the setting
6. Measure the span by closing over a rule, or transfer to an outside caliper and measure that

The “feel” step is the critical skill. Both tips must contact simultaneously with equal force. Rotating the caliper slightly within the bore helps confirm even contact — if the caliper rocks, one tip is not contacting. The correct feel is a gentle, consistent drag as the caliper is rocked slightly.

Measuring a slot or groove: Insert the tips and spread until they contact both walls. For deep slots, the caliper must reach to the floor — ensure the adjustment doesn’t loosen as the caliper travels deeper.

Comparative measurement (checking if a shaft fits a bore): Set inside calipers to the bore. Transfer the setting to outside calipers by holding the inside caliper tips inside the outside caliper tips and expanding the outside calipers until they just contact. The outside caliper is now set to the bore diameter and can be compared directly to the shaft.

Transfer Errors

Every time a measurement is transferred (inside to outside caliper, caliper to ruler, ruler to workpiece), there is a potential for small errors. To minimize:

1. Make all transfers immediately — don’t walk across the workshop between setting and reading
2. Minimize the number of transfers
3. Use the comparative method (check shaft directly with bore-set caliper) rather than double-transfer
4. Check the feel of each caliper contact carefully — consistent contact force is more important than trying to “feel” fractions of a millimeter

For critical fits, use an inside micrometer or a telescoping gauge (see below) instead of caliper transfer — these measure the bore directly with a digital or vernier scale.

Telescoping Gauge

A telescoping gauge is an alternative to inside calipers for smaller bores. It consists of two spring-loaded plungers in a T-handle, the plungers reaching out to contact opposite sides of a bore. The plungers compress as the gauge is inserted, then spring out to contact the bore walls. A locking screw holds the expanded dimension. The gauge is extracted and measured across the plunger tips with outside calipers.

Making a simple telescoping gauge:

• Two steel pins in a drilled hole through a handle (wooden or metal)
• A light compression spring between the pin ends (inside the handle)
• A locking screw through the handle that can press against one pin to prevent extension

The spring force ensures consistent contact pressure in the bore. This eliminates the feel-judgment required with inside calipers, making the gauge more repeatable for inexperienced users.

Measure the telescoping gauge tips with outside calipers after extraction to read the bore diameter. With careful design and use, accuracy to ±0.2mm is achievable.