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Surface Plate Care and Checking
Guide · Updated October 27, 2026
Every gage block check assumes a flat reference under it. A worn or dirty surface plate quietly adds error to height gage settings, indicator checks and sine bar angles. Here's how to look after a plate and check it in the shop between calibrations.
Build reference heights
Get the fewest-block stack for any height in the free Gage Block Calculator. Free, no login.
Daily care
- Brush chips off and wipe the plate before and after use. Grit under a block or stand reads as error.
- Use a granite plate cleaner, or mild soap and water, and let it dry fully.
- Set things down gently, never slide heavy parts, and keep a cover on when you aren't using the plate.
- Use the whole surface, not just the middle, to spread wear.
- Keep the plate out of sun and drafts. Uneven heat bows granite. See temperature compensation.
Quick in-shop repeat check
- Clean and soak: Clean the plate and let it, the stand and the indicator reach room temperature.
- Set up a repeat reading: Zero an indicator on a stand (or a repeat-reading gage) at one spot on the plate.
- Sweep a grid: Move across the plate in a grid pattern and note the reading at each point.
- Look at the spread: The difference between the highest and lowest readings is the local repeat reading. Compare it with the plate's grade.
- Mark worn areas: Note low or high areas, work elsewhere, and send the plate out for calibration or lapping if it's out.
This finds local wear, but not overall flatness across the plate. For that, use a calibration service with an autocollimator or electronic levels, usually every one to three years depending on use.
Grades at a glance
| Grade | Typical use |
|---|---|
| AA (laboratory) | Calibration labs, gage block comparison |
| A (inspection) | Inspection rooms, height gage work |
| B (toolroom) | Layout and general shop work |
Exact flatness tolerances depend on plate size and the standard; read your plate's certificate.
Using gage blocks on the plate
Stacks on the plate are your height masters. Use them to check a height gage, check a dial indicator or set a sine bar. Clean the blocks first and wring them properly.
FAQ
How do I know if my surface plate is still flat?
Do a repeat reading check: sweep an indicator on a stand or a repeat-reading gage across the plate in a grid. Big changes point to wear. A full flatness certificate needs a calibration lab with an autocollimator or electronic levels.
What are surface plate grades?
Granite plates are usually sold as laboratory grade AA, inspection grade A and toolroom grade B. Each allows a flatness tolerance that grows with plate size. Check the certificate for your plate's exact figure.
Where does a surface plate wear?
Where people work most, usually the middle and the front edge. Use the whole plate, and rotate it if you can, to spread the wear.
How should I clean a granite surface plate?
Brush off chips, then use a plate cleaner or mild soap and water. Let it dry fully before you put gage blocks on it, and cover it when it isn't in use.
How do gage blocks help on a surface plate?
They are the height reference: you set height gages and indicators from a stack, and use stacks to check them. The free Gage Block Calculator at gageblockcalc.com builds the stack.
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General shop information only — follow your gage manufacturer's instructions and your quality system.