Milling & Grinding

Part of Machine Tools

Combining milling and grinding operations to produce precision flat surfaces and accurate forms in hardened steel.

Why This Matters

Milling leaves tool marks — ridges and valleys on a machined surface that, at best, represent several microns of variation. For many purposes this is fine. But hardened cutting tools, precision gauges, flat reference surfaces, and tight-fitting parts require smoother, more accurate surfaces than milling can produce. Grinding solves this: an abrasive wheel removes material in tiny increments, producing surfaces accurate to micrometers when done well.

The combination of milling and grinding defines the capabilities of the precision machining era. You mill to near-size quickly, then grind to final dimension and finish. This workflow is how measuring tools, precision gauges, thread gauges, and accurate machine components are made. Without it, precision is limited to whatever lapping and hand-scraping can achieve.

This article covers surface grinding (grinding flat surfaces) and tool grinding (sharpening and shaping cutting tools) — the two forms of grinding most relevant to early industrial capability.

Surface Grinding Principles

Surface grinding holds the work on a flat table and passes it back and forth under a grinding wheel, taking thin cuts of 0.01-0.05mm per pass. The result is a flat, smooth surface accurate to 0.01mm or better.

Wheel selection: Surface grinding uses a cup wheel (grinding on the end face) or a straight wheel (grinding on the periphery). For ferrous metals, aluminum oxide abrasive; for non-ferrous, silicon carbide. Grain size: 46-60 grit for roughing, 80-120 for finishing. Bond: vitrified (clay-fired) for longest life; shellac-bonded for finer finish.

Spark stream reading: The spark stream tells you what is happening. Long, bright, branching sparks indicate heavy cutting. Short, faint sparks mean you are barely touching. No sparks with obvious wheel pressure means glazing — the wheel needs dressing. Dull orange sparks with smoke mean the work is overheating.

Coolant: Surface grinding generates significant heat in a small contact zone. Flood coolant prevents thermal damage to hardened work — heat above 200 degrees C can draw the temper of hardened tool steel. If coolant is not available, take very light passes (0.005-0.01mm) and allow the work to cool between passes.

Improvised Surface Grinding

Without a commercial surface grinder, a reasonable approximation is achievable:

Lathe with grinding wheel: Mount a small grinding wheel in the lathe chuck (or better, on a separate spindle mounted on the cross slide). The work is held in a fixture on the carriage and traversed past the wheel. Requires careful attention to alignment but produces usable flat surfaces.

Straight-line grinder: Two flat rails of known accuracy support a carriage. The grinding wheel is mounted on the carriage and moves over the stationary work. Medieval knife and sword grinders used exactly this principle.

Lapping: For flat surfaces on hardened steel (gauge blocks, lathe centers, flat parallels), hand lapping on a cast-iron surface plate with fine abrasive paste (aluminum oxide or silicon carbide in oil) is time-consuming but effective. The surface plate and workpiece are moved in a figure-eight pattern to distribute wear evenly.

Tool Grinding

Every lathe tool, milling cutter, drill bit, and chisel requires precise grinding of its cutting geometry. The angles described in cutting tool theory must be ground accurately to get good performance.

Grinding lathe tools: Forged high-speed steel or carbon steel blanks are ground to shape on the bench grinder. The tool is held against the wheel face at the required clearance angle, traversed smoothly across the wheel to form a flat face. Check angles with a small protractor. The crucial check is that the cutting edge is straight and the clearance and rake angles are correct.

Grinding drill bits: A properly ground drill has both cutting lips at equal length and equal angles (typically 59 degrees per side from the drill axis for a 118 degree included angle). An asymmetric grind makes the drill cut oversize. Practice the freehand drill grinding technique on worn drill bits before attempting to regrind good ones.

Grinding milling cutters: End mills and form cutters are more complex to grind than lathe tools because the clearance angle must be maintained all around the curved cutting edge. A simple grinding attachment that rotates the cutter while advancing into the wheel (a cutter grinder) is needed for serious milling cutter sharpening.

Tolerances and Surface Quality

Surface roughness is measured in Ra (arithmetic mean roughness). Typical results:

• File: Ra 3-5 micrometers
• Lathe turning fine feed: Ra 1-2 micrometers
• Milling fine: Ra 1-3 micrometers
• Cylindrical grinding: Ra 0.2-0.5 micrometers
• Surface grinding: Ra 0.2-0.8 micrometers
• Lapping: Ra 0.01-0.1 micrometers

For most machine-tool work, ground surfaces (Ra 0.5 micrometers) are adequate. For precision gauges and slip gauges, lapping to 0.05 micrometers is required.

Flatness is distinct from roughness — a rough surface can be flat, and a smooth surface can be curved. Check flatness with a precision straight-edge and feeler gauges. For workshop use, a straight-edge check to 0.01mm is sufficient for most work.