Okuma Grinding Machines: How Precision Grinding Actually Works

• 1
  What makes Okuma grinding machines different
• 2
  Cylindrical, surface and double-disc grinding compared
• 3
  Abrasive choice, dressing and coolant control
• 4
  Burning, chatter and taper: what the part tells you
• 5
  When milling beats grinding, and why
• 6
  Holding ±0.005 mm across a production run
• 7
  Matching the process to the feature

Grinding removes material with a rotating abrasive wheel instead of a cutting edge. Each grain acts as a tiny negative-rake cutter, so the chip is far smaller than in milling. That is the whole reason grinding reaches Ra 0.2–0.8 μm and holds ±0.005 mm on a hardened shaft where a carbide end mill would chatter and wear.

Okuma builds grinding machines around a double-column structure and a rigid wheel head. The column carries the wheel head on preloaded linear guides, and the table moves on hydrostatic or roller ways depending on the model. Rubber dampeners in the table assembly absorb vibration before it reaches the workpiece, which shows up directly as better surface quality and fewer feed marks.

The control side matters as much as the iron. Okuma's CNC reads spindle load, wheel position and thermal drift, then compensates in real time. On a cylindrical grinder that means the machine adjusts infeed as the wheel wears, so the tenth part of a run sits in the same tolerance band as the first.

Thermal stability is the quiet factor. High-performance alloys and composite materials in the bed and column resist heat growth during long cycles. A grinding machine that grows 5 μm over four hours will drift out of tolerance on a Ø50 mm journal even if the wheel is perfect.

Cylindrical grinding turns an OD or ID true. The workpiece rotates against the wheel, and the machine controls roundness, taper and cylindricity in one setup. This is the standard route for shaft journals, hydraulic spools and bearing seats that must run concentric to a datum within a few microns.

Surface grinding flattens faces. A magnetic chuck or fixture holds the part while the wheel reciprocates across it. It suits die plates, wear pads, guide rails and any face that must be flat and parallel to a reference. Flatness of 5 μm across 300 mm is routine on a well-set machine.

Double-disc grinding works both faces at once. Two opposed wheels grind the part from each side, which holds thickness tolerance and parallelism without a second setup. It is fast on bearing races, valve shims and pump plates, and it avoids the re-chucking error that comes from flipping a part.

The choice follows the feature, not the material. If the critical callout is roundness or a diameter, use cylindrical. If it is flatness or parallelism, use surface. If it is thickness on a thin disc, use double-disc. Running the wrong family costs cycle time and still misses the tolerance.

• 1
  CylindricalOD and ID of shafts, spools and bearing seats
• 2
  SurfaceFlat faces on die plates, rails and wear pads
• 3
  Double-discParallel faces on thin discs and shims

The wheel is the cutting tool, so its specification decides the result. Aluminum oxide suits carbon and alloy steels. Silicon carbide handles cast iron and carbide. CBN (cubic boron nitride) holds form on hardened steel above 45 HRC and lasts many times longer, which matters when a wheel change stops the spindle.

Grit and bond set the finish and the removal rate. A 46–60 grit wheel with a vitrified bond removes stock fast and leaves Ra 0.8–1.6 μm. An 80–120 grit wheel with a softer grade pushes toward Ra 0.2–0.8 μm but cuts slower and loads up sooner on soft material.

Dressing restores the wheel's cutting edges and keeps it concentric. A single-point diamond dressed 0.02–0.05 mm per pass, at a crossfeed of 0.1–0.2 mm per revolution, opens the face without wasting abrasive. Skip dressing and the wheel glazes, the load climbs, and the part burns.

Coolant does two jobs: it cools the contact zone and flushes chips out of the wheel. Straight oil gives the best finish and lubricity, while water-soluble fluid runs cooler and is easier to handle. Flow should be direct and heavy, 20–60 L/min at the nozzle, aimed at the contact point rather than the wheel guard.

Grinding burn shows as a blue or straw tint on a steel surface, with a softened layer under it. It comes from too much infeed, a dull wheel or starved coolant. The fix is to reduce depth per pass to 0.005–0.02 mm, dress the wheel, and check the nozzle alignment before the next part runs.

Chatter leaves evenly spaced marks across the surface. The cause is usually wheel imbalance or a workpiece that is not rigidly supported. Balance the wheel, shorten the overhang, or add a steady rest. On a long shaft, a center rest placed at mid-span often removes the pattern in one adjustment.

Taper on a cylindrical part means the table is out of parallel with the wheel axis, or the tailstock is misaligned. Indicate the table travel and the centers, then realign. A taper of 0.01 mm over 200 mm is visible on a micrometer but easy to miss on a caliper, so measure at both ends.

Size drift over a run points at thermal growth, not at the wheel. Let the machine warm up, keep the coolant at a steady temperature, and check the in-process gauge. Once the machine is stable, size repeats within ±0.005 mm without touching the offsets.

Grinding is slow per cubic millimeter of stock removed. If a feature starts as solid bar and needs 3 mm off the diameter, mill it to within 0.1 mm first and grind the rest. Removing all the stock on a grinder can multiply cycle time by five or more.

Soft materials rarely justify grinding. Aluminum and mild steel cut cleanly at Ra 1.6–3.2 μm with a sharp end mill, and grinding soft aluminum loads the wheel and tears the surface. Reach for grinding when the material is above roughly 45 HRC, or when the tolerance and finish callouts are tighter than milling can hold.

Geometry matters too. Deep pockets, sharp internal corners and complex 3D contours belong on a 5-axis mill. A grinding wheel is a rotating disc, so it cannot enter a slot narrower than its own width or cut a sharp internal radius. Design for the wheel or accept a two-process route.

Cost follows setup count, not just cycle time. A part that needs a mill setup, a heat-treat step and a grind setup pays for three fixturings. On low-volume work it is often cheaper to mill the feature slightly oversize in soft state and grind one critical surface, rather than grind everywhere.

Warm up before the first part. Run the spindle and coolant for 30–45 minutes so the structure reaches steady state. A machine that starts cold will grow a few microns in the first hour, and every part in that window carries the drift.

Measure on the machine whenever the geometry allows. An in-process gauge or a probe reading closes the loop between the wheel and the control, so size corrections happen between parts instead of after a rejected batch.

Dress on a schedule, not on feel. For a vitrified wheel in hardened steel, dressing every 15–30 parts keeps the cutting edges sharp and the size predictable. Log it, because a wheel that was dressed early is cheap and one that was dressed late has already burned parts.

Keep the environment steady. Coolant temperature, shop temperature and fixturing all feed into the final size. A grinding cell held within ±1 °C repeats far better than one that swings with the weather, and the operator can see the difference on the gauge.