Why is imported plywood 5.2 mm?

[Jamie Buxton]

That thin "quarter-inch" plywood we grumble about is imported, and manufactured to a thickness of 5.2 mm. That's the spec. I can understand global industry using a spec in millimeters -- like 5 mm or 6 mm. But does anybody know why they'd choose an oddball spec like 5.2 mm?

[Rod Sheridan]

0.2mm sanding allowance?

regards, Rod.

[Bob Ross]

Thicker costs too much, thinner would tear :D

[Chris Padilla]

Sounds like a tolerance to me. 5 mm +/- some %.

[David Keller NC]

Because that's the minimum allowable thickness for the size standard. There are standards for each size and grade of plywood (can't remember the name of the organization and standard - perhaps someone else will).

When the standards for english measurement plywood were agreed upon, they were centered around a nominal size +/- a percentage for the unpredictable spring-back when the sheets left the glue press.

Since that time, the engineering knowledge and machine tolerances have gotten a lot better, and manufacturers figured out that they could save money by putting their process setpoint near the minimum specification. That's partly why plywood is always undersized.