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How to Use Box Parallels

Guide · Updated October 28, 2026

Box parallels are hollow, ribbed precision blocks ground parallel and square on all faces. Being hollow they are light and stable, so they make tall setups on surface plates, grinders and mills without the weight of solid blocks.

Fill the height

Make up the odd height with a gage block stack. The free Gage Block Calculator builds the stack.

Step by step

Check the pair: Use a matched pair; check them against each other on a plate.
Clean: Wipe all faces and the table.
Choose the face: Pick the face that gives the height you need.
Set the pair: Place both under the part, parallel to each other.
Add blocks: Make up the odd height with gage blocks or parallels.
Clamp: Clamp over the parallels, not between.
Indicate: Indicate the top to check it is level.

Worked example: building a height

You need a part face 6.375 in above the table. A box parallel 6.000 in on its tallest face, plus a gage block stack of 0.375 in (0.125 + 0.250), gives 6.375 in. Fewer interfaces give less error, so use the parallel face nearest the target and as few blocks as possible.

Squaring

Box parallels are square on adjacent faces, so a part clamped to the side face is square to the base. Check with a cylindrical square before relying on it for tight work.

Why hollow?

Less mass means less heat soak and easier handling. The ribs keep them rigid.

Accuracy

Check yours: measure each face against gage blocks with an indicator on a surface plate. Treat them as setup tools unless measured.

Common problems

Part rocks: burr on a face. Height off: chip under the parallel. Clamping between supports bends the part; clamp directly over them.

Worked example: checking a pair

Put both parallels on a surface plate on the same face and sweep a test indicator across both tops. If one reads 0.0003 in higher, a part spanning both will tilt: across a 4.000 in span that is 0.0003 ÷ 4 = 0.000075 in per inch, about 15 arc-seconds. Mark the pair and keep them together, or use the faces that match.

Storage

Oil lightly and store in the case. Do not stack tools on the ground faces.

Tips

1-2-3 blocks: 1-2-3 blocks.
Angle plate: angle plate.
Square: cylindrical square.

Quick shop check

Check a box parallel for square by setting it on a surface plate and sweeping a face with a test indicator on a height gage. A typical precision box parallel is square within about 0.0002 in over its height.

FAQ

What are box parallels?

Hollow precision blocks ground square and parallel.

Why hollow?

Lighter and stable.

How do I make odd heights?

Add gage blocks.

Are they square?

Adjacent faces are ground square; check yours.

Where do I clamp?

Directly over them.

Related guides and calculators

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General shop information only — follow your machine and tooling manufacturer's instructions and your shop's safety rules.