CNC Grinding
Also: precision grinding, surface grinding, cylindrical grinding, OD grinding, ID grinding, jig grinding, Blanchard grinding, abrasive machining, creep-feed grinding
CNC grinding finishes hardened parts with an abrasive wheel: surface, cylindrical and jig grinding hold ±0.0025 mm, flatness under 0.005 mm and finishes of 0.2 µm Ra.
646 precision grinding companies in the Noramark directory
Grinding removes metal with a bonded abrasive wheel (aluminum oxide, CBN or diamond) running at 30-60 m/s; each grain takes a chip a few micrometres thick. Surface grinding traverses a flat part on a magnetic chuck under the wheel for flatness and parallelism; Blanchard (rotary) grinding does the same faster on large plates with a coarser swirl finish. Cylindrical grinding spins a shaft between centers or in a chuck against the wheel for ODs, shoulders and tapers, and internal grinding does bores. Jig grinding uses a small high-speed spindle to finish hole positions and profiles to ±0.0025 mm (±0.0001") in hardened tooling. Creep-feed grinding takes deep, slow passes to form slots and fir-tree roots in superalloys.
It is the last operation on a hardened part. Milling at 60 HRC is slow and hard on tooling, and hardening distorts, so parts are machined with grind stock, heat treated, then ground to size. Grinding holds ±0.005 mm (±0.0002") as a matter of course and ±0.0025 mm (±0.0001") with care, at 0.2-0.4 µm Ra (8-16 µin), with flatness, parallelism and roundness in the micrometres. Bearing journals, gauge blocks, punches, die plates, spindle tapers, sealing faces and wear plates are ground.
Removal rates are low, so grinding is priced by the feature, not the pound, and a millimeter of stock is expensive. Aggressive grinding burns the surface (tempering, cracks, tensile residual stress), and soft, gummy materials (aluminum, copper, 304, titanium) load the wheel. Geometry is limited to what a wheel can touch: flats, cylinders and simple profiles. Non-magnetic parts need fixtures or vacuum instead of the chuck.
At a glance
| Typical tolerances | Standard ±0.005 mm (±0.0002") on ground dimensions; ±0.0025 mm (±0.0001") with care, and ±0.001 mm (±0.00004") on jig-ground holes and gauge work in a temperature-controlled room. Flatness and parallelism of 0.005 mm (0.0002") over 300 mm on surface-ground plates, 0.0025 mm with care; Blanchard grinding holds 0.025-0.05 mm (0.001-0.002"). Roundness of 0.001 mm (0.00004") on cylindrical grinding between centers. |
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| Size limits | Job-shop surface grinders take 300 x 600 mm to 600 x 1,500 mm (12 x 24" to 24 x 60") with 300-400 mm (12-16") under the wheel; Blanchard grinders handle plates to 1,000-2,000 mm (40-80") across. Cylindrical grinders swing 300-400 mm (12-16") over 1,000-2,000 mm (40-80") between centers; internal grinding reaches bores from about 5 mm (0.2") up. Jig grinders cover about 600 x 400 mm (24 x 16"). |
| Surface finish | Standard 0.2-0.4 µm Ra (8-16 µin); 0.1 µm Ra (4 µin) with fine wheels and spark-out passes; 0.025-0.05 µm Ra (1-2 µin) by lapping or superfinishing after grinding. Blanchard grinding leaves its 0.8-1.6 µm Ra (32-63 µin) swirl. The lay is directional on surface and cylindrical grinding, which matters for seals, so specify lay when it does. |
| Lead time | As a secondary operation, 3-7 business days per lot; a full routing of machining, heat treatment and grinding runs 3-6 weeks. Ground-flat plate and blanks ship in days. |
Typical of U.S. job shops; confirm with the shop for your part.
Materials
Hardened and case-hardened steels grind best: 52100, D2, A2, O1, M2, H13 and S7 at 55-65 HRC, 4140 and 4340 quenched and tempered, 8620 carburized and 1045 induction hardened. Cast iron grinds cleanly and is common for machine ways and plates. Hardened martensitic stainless (440C, 420) and 17-4 PH grind well; 304 and 316 load the wheel. Inconel and other superalloys grind with CBN and creep-feed methods. Tungsten carbide and ceramics need diamond wheels. Aluminum, copper and plastics are usually finished by turning, milling or lapping instead.
What drives the cost
- Stock to remove: 0.1-0.3 mm (0.004-0.012") of grind stock is right and 1 mm is expensive
- Tolerances below ±0.0025 mm (±0.0001") and finishes below 0.2 µm Ra, which need spark-out passes and gauging
- Geometry: interrupted surfaces, shoulders and internal grinding take longer than a plain OD or flat
- Material: carbide and hardened superalloys need diamond or CBN wheels; soft materials load the wheel and burn
- Fixturing: non-magnetic parts need custom holding, and thin parts need support so they do not spring on the chuck
- Quantity: setup per feature is amortized, but each pass still takes its time
- Inspection: work under 0.005 mm needs a CMM, air gauge or roundness tester at a controlled temperature
When to use it
- Finishing hardened parts: journals, bores, punches, dies, gauge surfaces, wear plates
- Tolerances tighter than ±0.013 mm (±0.0005"), or finishes below 0.4 µm Ra
- Flatness, parallelism and squareness within 0.005 mm (0.0002")
- Removing heat-treat distortion and scale from tool steel and case-hardened parts
- Bearing and seal surfaces, spindle tapers, hydraulic rods, mold plates
When not to
- Features a mill or lathe can hold to spec: grinding is a slower, extra operation
- Soft, gummy metals (aluminum, copper, 304) and plastics that load the wheel; turn or mill them to finish instead
- Large volumes of round bar and pins: centerless grinding is faster
- Complex 3D contours: 5-axis milling or EDM; grinding follows flats, cylinders and simple forms
Design tips
- Leave 0.1-0.3 mm (0.004-0.012") of grind stock after machining and heat treatment, and say so on the drawing.
- Add a relief or undercut where a ground surface meets a shoulder; the wheel corner cannot make a sharp inside corner.
- Give center holes on shafts to be ground between centers, and protect them through heat treatment.
- Put flatness, parallelism and squareness with a datum on the surfaces that need them, not in a blanket note.
- Specify surface finish and, for seals, the lay; a 0.2 µm Ra face with circumferential lay seals better than a cross-hatch.
- Pick a hardness that grinds well: 58-62 HRC tool steel grinds cleanly, and through-hardened parts distort less than thin case-hardened sections.
- Avoid thin unsupported sections; chuck pull and grinding pressure spring them, and they relax out of flat when released.
Frequently asked questions
- What tolerances can precision grinding hold?
- ±0.005 mm (±0.0002") is standard and ±0.0025 mm (±0.0001") is achievable with care. Jig grinding and gauge work reach ±0.001 mm (±0.00004") in a temperature-controlled room. Flatness and parallelism of 0.005 mm over 300 mm and roundness of 0.001 mm are routine.
- What is the difference between surface grinding and Blanchard grinding?
- Surface grinding traverses the part under the edge of a reciprocating wheel and holds flatness to 0.005 mm (0.0002") with a fine finish. Blanchard grinding uses a large rotating segmented wheel over a rotating table to flatten plate fast, holding about 0.025-0.05 mm (0.001-0.002") with a coarser swirl. Blanchard first, surface grind to finish.
- Can aluminum be ground?
- Yes, but it loads the wheel and smears, so most shops avoid it. Fine finishes on aluminum are usually reached by turning or milling with polished inserts, then lapping or polishing. When grinding is required, open-structure wheels and plenty of coolant make it workable.
- What is jig grinding?
- Grinding with a small, high-speed wheel on a machine that positions like a jig borer, to finish hole diameters, positions and contours in hardened tool steel to ±0.0025 mm (±0.0001") or better. It is used for die buttons, punch holders and precision fixtures after heat treatment.
- Why grind after heat treating?
- Hardening moves the part: a few hundredths of a millimeter of distortion and a layer of scale or decarburization. Machining at 60 HRC is slow and wears tooling. So parts are machined with grind stock, hardened, and ground to final size, which also gives the finish and flatness hardened parts usually need.
