Flat enough to run without wobble under load, which is a property of the steel body and its tensioning, not of the teeth. A body that has been overheated loses that tension permanently and will never cut straight again.
A saw blade is a thin steel disc spinning fast and asked to stay in one plane while heating unevenly. It stays flat because it was made to.
Tensioning
During manufacture the plate is stressed — rolled or hammered in a ring around the center — so that it is under controlled internal tension. As the rim heats up in a cut it wants to expand and buckle; the built-in tension is what keeps it running true while that happens. This is invisible and it is a large part of what a good blade costs.
What destroys it
Heat. A blade pinched in a closing kerf, run far past blunt, or forced through a deep cut gets hot enough at the rim to relieve that tension. The tell is blue or straw coloring around the gullets. Once the tension has gone, the blade runs wide, wanders, and cannot be corrected by sharpening.
How to check a blade
Slide it onto a nail or a bolt through the bore, spin it slowly, and sight across the body against a straight background. Any visible weave is the end of it.
What flatness buys you
A narrower, cleaner kerf, less friction, less heat, and cuts that stay parallel in thick stock. A wandering cut in a long rip is often the blade, not the fence.
Last reviewed 2026-08-12