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How to Check a Square with a Cylindrical Square
Guide · Updated October 28, 2026
A cylindrical square is a precision ground cylinder with ends square to its axis. Because a cylinder line contact is easy to see and hard to tilt, it is the reference used to check machinist squares and parts on a surface plate.
Read error with blocks
Use gage blocks or feeler stock to size any light gap. The free Gage Block Calculator builds the stack.
Step by step
- Clean plate and tools: Wipe the surface plate, square and cylinder.
- Stand the cylinder: Place it on the plate on its ground end.
- Light gap method: Bring the square blade against the cylinder and look for light at top or bottom.
- Indicator method: Mount an indicator on a height gage or stand; sweep the cylinder at two heights and zero.
- Sweep the work: Sweep the part or square at the same two heights.
- Compute error: Difference between readings over the distance between heights.
- Check both sides: Rotate the square 180° to separate square error from setup error.
Worked example
Indicator heights 1.000 in and 5.000 in apart (4.000 in span). Zeroed on the cylinder at both heights. On the square blade: 0.0000 in at the bottom and +0.0006 in at the top. Error = 0.0006 in over 4 in = 0.00015 in per inch. Over a 6 in blade, that is 0.0009 in out of square. If a gap of light appears and a 0.0005 in feeler stock just slips, the gap is about 0.0005 in.
Why a cylinder
Line contact on a round surface does not depend on the cylinder edge, and any rotation shows the same profile, so it is easy to verify the master itself.
Common problems
Dirt under the base tilts everything. Heat from your hand bends a thin blade. Reading only one side confuses setup error with square error.
Tips
- Squares: check a machinist square.
- Squareness: check squareness.
- Plate: use a surface plate.
FAQ
What is a cylindrical square?
A precision cylinder with ends square to its axis used as a squareness master.
Light gap or indicator?
Indicator for numbers; light gap for quick checks.
How do I express error?
As a gap over a length, such as 0.0006 in over 4 in.
Why rotate the square?
To separate its error from setup error.
How accurate are cylindrical squares?
Good ones are within about 0.0001 in over their length.
Related guides and calculators
- How to Check a Machinist Square
- How to Check Squareness
- How to Use a Surface Plate
- How to Use a Height Gage
- Feeler Gage Basics
- How to Set a Bore Gage
- How to Use Adjustable Parallels
- How to Use Box Parallels
- All free machinist calculators
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