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How to Check an Angle Plate
Guide · Updated October 27, 2026
An angle plate is only as good as its faces: they need to be flat and square to each other. A worn or burred plate puts that error straight into every part set up on it. All of these checks happen on a surface plate.
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Step by step
- Clean and deburr: Stone off burrs lightly and wipe the plate and the surface plate.
- Check face flatness: Use a straightedge with a feeler or light gap, or sweep an indicator over the face.
- Check squareness: Run an indicator on a height gage up the vertical face, or use a master square and a light gap.
- Measure error over height: Gage blocks under the indicator point or a known stack give you a reference. Record the error per inch of height.
- Check parallelism: Indicate the top and side faces against the base.
Indicator squareness method
- Sweep a test indicator from bottom to top on the vertical face.
- Total reading divided by the travel gives the out-of-square per inch.
- Turn the plate 180° and repeat to separate plate error from setup error.
Fixing errors
- Small burrs: stone them.
- Real out-of-square: shim the setup, or have the plate reground or scraped.
- Note the error on a tag on the plate.
Related: checking squareness, flatness and parallelism, checking a test indicator.
Quick shop check
Put the angle plate on a surface plate, mount a test indicator on a height gage, and sweep the vertical face from bottom to top. A Grade B plate may be within about 0.0005 in over 6 in; if the indicator changes 0.0010 in over 6 in, the face is out about 0.00017 in per inch. Rotate the plate 180° and repeat to separate plate error from setup error.
Worked example: checking the vertical face for square
Set the angle plate on a clean surface plate with a precision square or cylinder square against the vertical face. Run a test indicator on a height gage up the face from bottom to top, about 6 in, and note the change. A total change of 0.0003 in over 6 in means the face is that far out of square over that height; compare that with the tolerance your job needs. Flip the plate to check the other face the same way, and mark the results on the plate so the next setup uses the better face.
FAQ
How square should an angle plate be?
Precision ground plates are often within about 0.0002 in per 6 in. General plates are looser, so check the maker's spec.
Can I check an angle plate without a master square?
Yes. Use a test indicator on a height gage swept up the face, or the cylinder-square method.
How often should I check one?
Check it after a drop and on the shop's calibration schedule, or yearly for working plates.
What reference do I use for height?
Gage blocks give exact heights. Build stacks with the free calculator at gageblockcalc.com.
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General shop information only — follow your gage manufacturer's instructions and your quality system.