Guide To the Best Micrometers

What Is a Micrometer?

A micrometer is a precision measuring instrument used by mechanical engineers, machinists and other workers in technical or laboratory settings. It measures length in microns, or thousandths of a millimeter.

Woodworkers seldom need as much accuracy as machinists. But they still use a micrometer for measuring dowel diameters, shim thicknesses and the depths of slots, holes and countersinks.

In its basic form, a micrometer is C-shaped with a fixed anvil on one end of the open C and a movable spindle on the other. You advance the spindle by rotating a thimble with threads precisely tooled to move it a fixed amount with every rotation. The measurement — the distance between the spindle and the anvil — may appear on a graduated scale on the spindle or display on a digital screen.

DIYers seldom need to measure with micrometer accuracy. A caliper — a ruler with sliding jaws — can perform many of the same measurements. A good set of calipers can measure with a tolerance of ± 0.001 inch. But plenty of measurements call for even more accuracy than that, such as:

• The diameters of thin wires;
• The diameter of an O-ring and the width of the groove it fits into;
• The depth of a slot in a workpiece;
• The thickness of a piece of sheet metal, paper or plastic sheeting;
• The thickness of the wall of a pipe or tube.

Types of micrometers

Different types of micrometers measure different things. The main types are:

• An outside micrometer, which measures thickness;
• An inside micrometer, which measures the inside diameter of a hollow object;
• A tube micrometer, which measures wall thickness of pipe or tubing;
• A depth micrometer, which measures the depth of a slot or hole.

Features to consider when buying a micrometer

• Resolution: Top-quality micrometers typically measure up to one inch, but those with lower resolution can measure up to six inches. If you don’t need the most accurate micrometer, choose one with a wider range and lower resolution. You’ll save money that way. For a woodworking project, you seldom need a tolerance better than 1/64-inch, which is roughly 0.4 millimeters or 400 microns.
• Digital vs. analog: There’s also a wide price difference between digital micrometers and less accurate, less expensive analog models. Although a digital display is easier to read, it comes at a premium you probably don’t need unless you’re doing precision machine work.
• Tip options: The tips of the anvil and spindle vary in shape.
  • Flat tips are the most common. If you’re looking for a general-purpose measuring tool, these are the ones you need.
  • Screw thread tips fit between screw threads, measuring their depth.
  • Ball tips contact an object at a single point and are best for measuring round objects.
  • Disk tips fit inside small spaces and are best for measuring inside diameters.