Modern CNC mills are remarkably accurate, and for most flat surfaces a good face-milling pass is all a part needs. But some parts call for surface grinding: very tight flatness and parallelism, a fine finish, or material that’s already been hardened.
Knowing which process a feature really needs keeps your parts in spec without paying for an operation you don’t need.
What milling does well
Face milling removes material quickly and produces flat surfaces that are good enough for the large majority of machined parts: brackets, plates, housings and fixtures. It’s fast, economical, and done in the same setup as the rest of the part’s features.
Milling gets harder when:
- Flatness or parallelism requirements get down to a few ten-thousandths of an inch over a large area
- The part is thin and wants to move as stress is released or as it’s clamped
- The material is hardened tool steel that’s difficult or slow to cut
- The print calls for a finer surface finish than a cutter leaves
What surface grinding adds
A surface grinder removes very small amounts of material with an abrasive wheel while the part is held on a magnetic chuck. Because each pass takes off only a few tenths of a thousandth, the process can generate surfaces that are extremely flat and parallel, with a fine, consistent finish.
- Flatness and parallelism to tenths of a thousandth on suitable parts
- Fine surface finish, typically much smoother than a milled face
- Hardened materials such as heat-treated tool steels that would quickly wear out cutting tools
- Square and parallel blocks with tightly controlled thickness
- Form grinding: profiles dressed into the wheel to grind precise shapes and angles
The classic case: after heat treat
Heat treating changes a part’s dimensions slightly and can cause distortion. For die components, punches, wear plates and precision tooling, the common approach is:
- Machine the part, leaving a small grind allowance on critical surfaces
- Heat treat to the required hardness
- Surface grind those surfaces to final size, flatness and finish
Having milling and grinding under one roof makes this sequence simpler: one shop plans the stock allowance, coordinates heat treat and grinds to the final print.
How to call it out on your drawing
- Use flatness and parallelism callouts on the surfaces that matter, rather than tightening every dimension
- Specify the surface finish (Ra) where it’s functional
- Note the hardness and whether dimensions apply after heat treat
- If you’re unsure whether a feature needs grinding, ask. We’ll recommend the most economical route.
A quick rule of thumb
If your part is soft, reasonably rigid and your flatness requirement is in the thousandths, milling is usually the right call. If it’s hardened, thin, or needs flatness, parallelism or finish beyond what a cutter can reliably hold, plan on grinding.
Our grinding department is led by a specialist with more than 30 years in the trade, running an Okamoto ACC-1224DX with a 12″ × 24″ chuck. See our precision grinding capabilities or send us a print.