The shaft comes back from the CNC lathe, the print says ±0.01 mm, and it still won't seat right in the bearing block. Sound familiar? It's the moment most engineers first search for cnc cylindrical grinding - and the moment it pays to know what the process actually fixes. Cylindrical grinding isn't a more expensive version of turning. It's a finishing step with its own rules: tighter tolerances, better roundness, smoother surfaces, and a wheel that treats the part differently than an insert does. This guide walks through the checks that separate a good cnc cylindrical grinding quote from a guess, in the order the decisions actually come up.
Turning or Grinding? Know When the Job Needs the Wheel
You don't send every shaft to the grinder, and you shouldn't. Turning holds ±0.02 to ±0.05 mm on a good machine and leaves Ra 1.6 to 3.2 µm - plenty for a lot of parts. Grinding earns its cost when one of these shows up: hardness above HRC 45, where carbide inserts start struggling; a diameter tolerance inside ±0.005 mm; a surface finish below Ra 0.4 µm; or a roundness and concentricity spec that turning can't hold across the length. Ask yourself the questions in that order. If none of the four applies, save the money and ship the turned part. If even one does, that's the signal to spec the job for cnc cylindrical grinding and stop pretending the lathe will get lucky. Turning is the cheaper process right up to the point where cnc cylindrical grinding becomes the only process that holds the print.
OD, ID, Centerless, or Between Centers - the Setup Decides the Result
Cylindrical grinding splits into a few setups, and the one you quote changes the numbers. OD grinding works the outside diameter of a shaft held between centers or in a chuck. ID grinding goes after the bore with a smaller wheel on a long spindle. Centerless grinding supports the part between a grinding wheel and a regulating wheel - no centers, no chuck - and it runs high volumes of pins and rollers at consistent diameters. Between-centers work is the go-to for stepped shafts and anything where concentricity between diameters matters, because the centers define one true axis for every pass. Most cnc cylindrical grinding shops run all four setups, but each one changes the wheel, the holding, and the price. When you ask for a cnc cylindrical grinding quote, name the setup you need; a supplier that quotes "grinding" without the setup is quoting a guess.
Tolerance, Roundness, and the Finish the Quote Must Name
A good quote names three numbers, not one. Diameter tolerance: production cnc cylindrical grinding routinely holds ±0.002 mm, and precision work gets to ±0.001 mm - but that's the wheel, the coolant, and the shop temperature all behaving at once. Roundness and cylindricity are separate specs. Roundness is how far the cross-section drifts from a true circle, and it matters for bearing seats and sealing surfaces where runout shows up as noise and wear; it's the spec where cnc cylindrical grinding beats turning outright. Surface finish lands at Ra 0.2 to 0.4 µm as standard, with mirror finishes below that on request. And the drawing has to show grinding allowance - typically 0.1 to 0.3 mm on the diameter - because a part sent without allowance gets charged for a grind that only kisses the surface, or worse, gets re-treated.
Material, Hardness, and the Wheel Spec
The wheel is where cylindrical grinding gets interesting, and it's also where buyers get burned. Aluminum oxide wheels handle most steels up to about HRC 55. Above that, CBN - cubic boron nitride - earns its price, because it holds geometry on hardened steel far longer than oxide does; hard chrome and die steels come off CBN wheels for that reason. Rim speed matters too: 30 to 60 m/s at the wheel rim takes the cut clean, and a wheel that's too soft or too hard shows up as chatter or burn. Coolant isn't a detail. A grinding burn that turns the surface blue-gray and softens the case is a scrapped part, and the fix is flood coolant plus the right feed, not a stronger wheel. When you spec cnc cylindrical grinding on hard steel, name the hardness and the required finish, and let the supplier pick the wheel. If they can't name it back, that's your answer.
What the Quote Should Say
Before you commit, the cnc cylindrical grinding quote should answer six lines: the setup - OD, ID, centerless, or between centers; the grinding allowance; the tolerance and roundness targets; the surface finish; the wheel and coolant plan; and the inspection report, whether that's a roundness trace, diameter readings, or a CMM report. Add the material and hardness on your side, and the supplier has everything needed to quote a real job instead of a safe one. Volume buyers should also ask whether the shop runs the same wheel day after day; a shop that changes wheels mid-batch quotes a different part than it ships.
FAQ
What tolerance can cnc cylindrical grinding hold? Production work holds ±0.002 mm on diameter; precision setups reach ±0.001 mm, depending on material, length, and geometry.
How much grinding allowance should I leave? Typically 0.1 to 0.3 mm on the diameter - enough for the wheel to take a clean pass without pushing you into a second heat treatment.
Can you grind a long shaft? Between-centers machines on most shops' floors handle lengths around 1000 mm; longer parts need special setups, so ask before you send the drawing.
Does grinding hard steel burn? It can - that's why the coolant and wheel call matter. A blue-gray surface means the part was burned and the case may be softened.
Do I get a report with the parts? Ask for a roundness trace and diameter readings on the RFQ. A shop that can't inspect the tolerance it quotes can't hold it.
Spec it in the right order - setup, allowance, tolerance, finish, wheel - and the cnc cylindrical grinding quote tells you who knows their work. Our surface grinding guide covers the flat-work side of the same process, our tolerance guide explains what ±0.01, ±0.005, and ±0.002 mm actually mean on a drawing, and our inquiry page is where you send the shaft drawing, the hardness, and the finish spec for a quote. A part that needs the wheel gets it cheaper when you spec it right the first time.