What are 1-2-3 blocks? How to use 1-2-3 blocks in your shop.

1-2-3 Blocks are one of the most common sights around a machine shop, and can be used for many other tasks beyond metalworking, such as woodworking, as well.

What exactly are 1-2-3 Blocks, and how can you use them in your shop?

What are 1-2-3 Blocks?

A 1-2-3 Block is a metal block (usually constructed of steel) which measures exactly 1” tall, 2” wide, and 3” long. Machinists use them for setting up and inspecting machines. Because 1-2-3 Blocks are made to very precise standards (commonly 0.0002”) with parallel edges, they can be used as a reference in shops for checking other work.

You’ll also find 2-4-6 blocks, with the same ratio, but in 2” x 4” x 6” dimensions for larger applications.

Metric 1-2-3 Blocks

In addition to the common 1” x 2” x 3” block in imperial measurements, metric blocks are also available and are used for similar purposes. For example, the 25mm x 50mm x75mm block closely approximates the form factor of a 1-2-3 imperial block.

Magnetic 1-2-3 Blocks

You’ll also encounter magnetic 1-2-3 blocks. These commonly use rare-earth magnets that pull the faces into flush alignment. The magnetic material also makes for handy storage, just clip them on to the side of a metal table to keep them close at hand, yet out of the way.

Why do 1-2-3 Blocks sometimes have holes?

You’ll encounter 1-2-3 blocks both with and without holes. Types with holes can then be threaded or unthreaded. Threaded holes can be used for clamping purposes, such as bolting a 1-2-3 block to a surface. Multiple 1-2-3 blocks can also be clamped together.

Holes are sometimes used to relieve material and lighten the blocks as well.

1-2-3 Blocks as a learning tool

Making your own set of 1-2-3 Blocks is a useful educational exercise. Blocks are machined slightly oversize, heat-treated, and then ground to final size.

Other uses for 1-2-3 blocks

1-2-3 blocks can be very useful even if you aren’t a machinist. For example, you can use 1-2-3 blocks:

• As a substitute for a clamp, by bolting two together
• To quickly measure 1”, 2”, or 3” lengths
• Attaching two pieces at a precise 90 degree angle (for example, gluing in woodworking)
• Verifying the accuracy of all manner of measuring tools
• As a paperweight!