V-Block Diameter Adapter for Calipers (x10 Reading)

A small 3D-printed adapter that lets you measure the diameter of a cylinder using the depth rod of your caliper instead of the jaws.

⚠ Read this first

If you can measure a part with the caliper's jaws, do that. It will always be more accurate than this adapter. This tool is meant for the cases where the jaws can't be used, for example:

• Pipes, posts or bars where you only have access to one side

• Diameters larger than the jaw opening of your caliper

• Parts with flanges, caps or shoulders that block the jaws

• Quick checks on tubes or rods that are already installed

How it works

The adapter clips onto the end of the caliper. Rest the cylinder against the V, push the depth rod through the slot until it touches the surface, and read the value on the display. Multiply the reading by 10 to get the diameter.

Example: the display shows 3.95 → the diameter is 39.5 mm.

The V angle is 112.89°, which makes the depth exactly one tenth of the diameter. The geometry does the work, so the angle needs to stay true (see the printing tips).

Accuracy

• Typical precision: ±0.1 mm on the diameter

• The x10 factor multiplies any reading error by 10, so the main source of error is how hard you press the rod against the part. Use a light, consistent touch and average 3-4 readings.

• Tested against a calibrated caliper on a 39.49 mm and a 51.99 mm cylinder, with errors within about ±0.1 mm.

• Round parts only. A slightly lobed (non-round) cylinder can give a wrong reading in a V.

Range

• Recommended: 30 to 170 mm

• Theoretical limit: about 175 mm. Above that, the cylinder stops touching the flat face of the arms and the reading becomes invalid.

Printing tips

• Material: PETG or a rigid PLA. The angle is what gives the accuracy, so avoid materials that warp.

• Print it flat with the V faces as clean as possible, and lightly sand them if needed.

• Don't leave it in the sun or in a hot car, since heat can change the angle.

• Arm height: 8 mm.

• Check it once against a cylinder of known diameter before trusting it.

Compatibility

Designed for a 150 mm (6") caliper with a flat depth rod. Check the width of your caliper beam before printing, since the slot is a tight fit and other brands may differ.