Introduction to CNC Grinding

• 1
  How an Abrasive Wheel Removes Metal
• 2
  Surface, Cylindrical and Creep Feed Grinding Compared
• 3
  Why Grinding Burns Parts and How to Stop It
• 4
  Where Grinding Beats Milling and Where It Does Not
• 5
  Grinding Process Selection Guide

A grinding wheel is a milling cutter with millions of tiny, irregular teeth. Each abrasive grain is a hard mineral held in a bond. When the wheel touches steel, individual grains shear off chips that are far smaller than a milling chip, often a few micrometres thick. That is why grinding reaches tolerances and finishes that turning and milling cannot hold on hardened material.

The grains do not stay sharp. They dull, fracture, or pull out of the bond, and new edges appear underneath. This self-sharpening behavior is the whole trick. A wheel that glazes over keeps rubbing instead of cutting, and the part heats up fast. A wheel that sheds grains too quickly loses its form and the size drifts. Dressing is how the operator resets both conditions.

Heat is the real limit. Almost all the energy pushed into the contact zone turns into heat, and it leaves through the chip, the wheel, the coolant, and the workpiece. When the workpiece takes too large a share, it expands, burns, or cracks. So the practical rules are modest depth of cut, adequate coolant, and a wheel that stays open.

Hardness matters less than people expect. A 60 HRC tool steel and a soft aluminium part use completely different wheels, but the same machine can run both. What changes is the abrasive, the bond, the wheel speed, and the feed. Match those four and the process is stable.

• 1
  GrainAluminium oxide for steels, silicon carbide for cast iron and non-ferrous, diamond and CBN for carbide and hardened alloys.
• 2
  BondVitrified covers most precision work; resin bonds tolerate higher speed and shock.
• 3
  Grit sizeCoarse for stock removal, fine for finish, but a fine wheel on soft metal loads up.
• 4
  GradeSoft grade for hard material, hard grade for soft material. The counterintuitive part.

Surface grinding puts a flat or formed face against a reciprocating or rotary table. It is the standard route for dies, plates, and any part where flatness and parallelism carry the tolerance. A magnetic chuck holds ferrous parts; non-ferrous and thin parts need fixtures, and thin parts will still spring, so light passes and support blocks matter.

Cylindrical grinding spins the workpiece against a wheel, with the part held between centers or in a chuck. Outside diameter work holds roundness and taper on shafts, spindles, and pins. Inside diameter grinding uses a much smaller wheel, so the wheel speed drops unless you run a high-frequency spindle, and the arc of contact is long, which traps heat.

Creep feed grinding takes one deep pass, often 1–6 mm, at a slow feed rate instead of many shallow passes. The wheel is open and soft, the machine needs high pressure coolant, and the process suits slots and profiles in hard alloys. It replaces milling on materials that destroy carbide cutters.

Centerless grinding is the odd one out and worth knowing. The part is supported by a rest blade between a grinding wheel and a regulating wheel. There is no center to locate, so throughput is high on small pins and bushings, but the setup is fussy and the process only works on rotationally symmetric parts.

• 1
  SurfaceFlats, steps, die plates. Reciprocating or rotary table.
• 2
  CylindricalShafts and bores. Between centers or chucked.
• 3
  Creep feedDeep slots and profiles in hard alloys, one pass.
• 4
  CenterlessSmall pins and bushings in volume, no centers.

A burn is a tempering mark. The surface got hot enough to change the microstructure, and the visible discoloration is the mild version. The serious version is a rehardened layer with tensile stress under it, which cracks later in service. On a hardened shaft, that crack is a fatigue origin.

Coolant delivery matters more than coolant type. Flooding the general area is not the same as aiming a coherent jet into the contact zone. High-pressure coolant through the wheel or through a shoe gets fluid where the heat is generated. On creep feed work, this is not optional.

Wheel speed has a ceiling set by the bond. Vitrified wheels commonly run 30–35 m/s, resin-bonded wheels higher. Running a wheel above its rated speed is a safety issue, not a productivity one. Running it well below the optimum makes the grain rub and the part burn.

Feed and depth are a pair. A light, fast pass often runs cooler than a heavy, slow one, because the grain cuts instead of rubbing. If a part burns, the first thing to change is usually the wheel grade or the coolant aim, not the depth of cut.

• 1
  BurnTempering discoloration, usually blue or straw on steel.
• 2
  CrackRehardened layer under the burn, found by inspection, not by eye.
• 3
  ChatterWheel out of balance or too hard a grade, shows as regular marks.
• 4
  LoadingSoft metal fills the wheel pores. Use a coarser, softer wheel.

Grinding wins when the material is hard. Carbide, hardened tool steel above 50 HRC, and many superalloys will not take a good surface from a carbide cutter for long. Grinding also wins when the tolerance is tight on a hardened part, because you cannot harden a finished milled surface without distorting it.

Grinding loses on soft material in simple shapes. Aluminium, brass, and mild steel machine faster by milling or turning, and grinding them only loads the wheel. If the drawing calls for Ra 1.6 μm on an aluminium bracket, a good mill and a finishing pass will do it.

Grinding also loses when the geometry is complex in three dimensions. A five-axis mill reaches pockets, ribs, and angled faces that no grinding wheel can enter. Grinding is a finishing process, not a shape-making one, for most work.

Cost follows setup, not cycle time, on small batches. A grinding fixture, a dressed form, and a proof part take time before the first good piece. On one-off parts, that setup can exceed the machining cost. On a run of 500 hardened pins, it disappears into the piece price.

• 1
  Choose grindingHardened steel, tight roundness or flatness, fine finish, thin hard parts.
• 2
  Choose millingComplex 3D geometry, soft material, loose tolerance, fast turnaround.
• 3
  Choose bothMill soft, heat treat, then grind the critical faces only.