Grinding
Grinding uses an abrasive wheel to remove very small amounts of material. It is the precision finishing process — used after heat treatment, when tolerances are tighter than machining can hold, or when surface finish must be below Ra 0.4 µm. A grinding wheel has thousands of abrasive grains, each acting like a tiny cutting edge. Material removal is slow (micron-sized chips), but accuracy and finish are unmatched.
Why Grind Instead of Machine?
| Reason | Example |
|---|---|
| Material is too hard for carbide | After heat treatment to HRC 58–62 (bearings, dies) |
| Tighter tolerance than turning/milling | ±0.002 mm on shaft diameter, roundness < 0.001 mm |
| Better surface finish | Ra 0.1–0.2 µm for seal surfaces, bearing races |
| No burr or thermal damage | Precision components that can't tolerate burrs |
Common Grinding Types
Surface Grinding
The most common grinder. A rotating wheel reciprocates across a magnetic chuck holding a flat workpiece. Used for die plates, fixture bases, parallel blocks, and any flat surface that needs precision.
- Typical accuracy: ±0.005 mm flatness, ±0.002 mm parallelism
- Depth of cut: 0.005–0.02 mm per pass (very light)
- Wheel: aluminum oxide (A) for steel, silicon carbide (C) for carbide/ceramics
Cylindrical Grinding (OD Grind)
The part rotates between centers while a large grinding wheel traverses along it. Used to finish hardened shafts, pins, and rollers after heat treatment. External cylindrical grinding is the most common — internal grinding bores precision holes.
- Typical accuracy: ±0.005 mm diameter, roundness < 0.002 mm
- Stock removal: 0.05–0.2 mm total (rough + finish passes)
Centerless Grinding
The part rests on a work blade between a grinding wheel and a regulating wheel — no centers needed. Used for high-volume production of pins, dowels, and cylindrical parts. Excellent diameter consistency but limited to simple cylinders.
Internal Grinding (ID Grind)
Small grinding wheels reach inside a pre-bored hole to finish it. Used for bearing races, die bushings, and precision hydraulic cylinders. Very delicate — the small wheel has limited rigidity.
Grinding Wheel Basics
| Abrasive type | Use for | Color |
|---|---|---|
| Aluminum oxide (A) | Steel, carbon steel, alloy steel | Brown / white |
| Silicon carbide (C) | Cast iron, aluminum, carbide, ceramics | Black / green |
| CBN (cubic boron nitride) | Hardened steel (HRC 50+), high-volume | Black (superabrasive) |
| Diamond (D) | Carbide, ceramics, glass, composites | Silver (superabrasive) |
Grinding burn: if you grind too aggressively, the surface overheats and gets a "burned" temper — a blue/green discoloration that ruins surface integrity and can cause cracking. Use light cuts, dress the wheel regularly, and always flood with coolant.
When to Use Which Process
| Requirement | Process |
|---|---|
| Rough shape, general tolerance | Milling / turning |
| ±0.02 mm, Ra 1.6 µm | Finish milling / finish turning |
| ±0.005 mm, Ra 0.4 µm | Finish boring / fine turning |
| ±0.002 mm, Ra 0.2 µm, hardened steel | Grinding |
| Mirror finish, extremely flat | Grinding + lapping / polishing |