Centerless Grinding
Also: centerless OD grinding, through-feed grinding, in-feed grinding, bar grinding, centerless ground bar, plunge centerless grinding, thrufeed grinding
Centerless grinding sizes round parts and bar between a grinding wheel and a regulating wheel, with no centers or chucks: fast, ±0.0025 mm, roundness under 0.001 mm.
54 shops tagged Centerless grinding among 646 precision grinding companies in the Noramark directory
The part rests on a work blade between two wheels: the grinding wheel removes metal and the slower regulating wheel controls rotation and, tilted a degree or two, feeds the part along. In through-feed grinding straight cylinders pass end to end continuously, thousands an hour, or bar stock 3-4 m long. In in-feed (plunge) grinding parts with shoulders, tapers or several diameters are held against an end stop while the wheels close. Nothing is chucked or centered, so loading is fast and there is no runout from the workholding.
It is the cheapest way to hold a diameter to ±0.0025 mm (±0.0001") in volume, and roundness is excellent because the part is supported on its own OD. Pins, dowels, shafts, rollers, needles, valve stems, hydraulic rod, drill blanks, carbide rods and medical guidewires are centerless ground, and so is the ground bar that Swiss shops buy.
Only cylindrical surfaces can be ground; ends, faces and non-round features cannot. Keyways, flats and cross holes on the ground surface cause chatter and lobing. Setup (blade height, wheel dressing, regulating angle) takes time, so small quantities are dear, and a wrong blade height produces odd-numbered lobing that a micrometer will not catch but a roundness tester will. Parts shorter than their diameter are hard to through-feed.
At a glance
| Typical tolerances | Standard ±0.005 mm (±0.0002") on diameter; ±0.0025 mm (±0.0001") with care, and ±0.001 mm (±0.00004") on gauge pins and bearing rollers. Roundness of 0.001 mm (0.00004") and straightness of 0.005 mm per 300 mm (0.0002" per foot) on through-feed bar; in-feed shoulders hold ±0.025 mm (±0.001") on length to the stop. |
|---|---|
| Size limits | Diameters from about 0.5 mm (0.020") to 150 mm (6") on common machines, and to 300 mm (12") on large ones. Through-feed length is unlimited, with bar to 6 m (20 ft) on supports; in-feed length is limited by the wheel width, typically up to 200-250 mm (8-10"). Parts shorter than their diameter are awkward to through-feed and are usually in-fed or ground another way. |
| Surface finish | Standard 0.2-0.4 µm Ra (8-16 µin); 0.1 µm Ra (4 µin) with a fine wheel, slow regulating speed and a finishing pass; 0.05 µm Ra (2 µin) by superfinishing after grinding. Ground bar is sold at 0.4 µm Ra or better with a bright, uniform surface. |
| Lead time | Job-lot grinding of customer parts in 3-7 business days; through-feed bar and pin production in 1-3 weeks. Centerless-ground bar in common sizes and alloys ships from stock. |
Typical of U.S. job shops; confirm with the shop for your part.
Materials
Hardened and soft steels: 52100 bearing steel, 4140, 1045, 8620, drill rod and 1018 pins. Stainless 303, 316, 416, 440C and 17-4 PH for medical and valve parts; titanium and Inconel for aerospace shafts and guidewires; M2 and carbide for drill blanks and tool rods with diamond or CBN wheels. Brass and aluminum can be ground with care and open wheels but are more often turned to finish. Plastics rod (PEEK, Delrin) is ground occasionally for precise bar.
What drives the cost
- Quantity: setup of blade, wheels and regulating angle is the same for 50 parts or 50,000
- Stock removal: more than 0.05-0.1 mm (0.002-0.004") on diameter means several through-feed passes
- Tolerance and roundness: ±0.001 mm (±0.00004") work needs a slow finishing pass, gauging and a stable shop temperature
- In-feed complexity: shoulders, tapers and multiple diameters need dressed wheel forms and an end stop
- Material: carbide and hardened superalloys need diamond or CBN wheels; soft materials load and smear
- Handling: sorting, packing to avoid nicks, and 100% gauging on critical pins
- Straightening long bar before or after grinding
When to use it
- Pins, dowels, shafts, rollers, needles and rods in quantities from hundreds to millions
- Diameters held to ±0.0025 mm (±0.0001") with roundness under 0.001 mm
- Sizing bar for Swiss machining, where the guide bushing wants h6 or h7 ground bar
- Long slender parts that cannot be chucked or held between centers without deflection
- Removing the turned finish and taper from hydraulic rod, valve stems and motor shafts
When not to
- A few parts with several diameters: cylindrical grinding between centers sets up faster
- Parts with keyways, flats, cross holes or threads on the surface to be ground, unless the shop can bridge them
- Ends, faces, shoulders and tapers that must be ground true to the OD: use cylindrical grinding
- Diameters that turning holds well enough; grinding adds an operation and a handling risk
Design tips
- Give the diameter tolerance and a roundness call-out, and say which is critical; ±0.0025 mm on diameter without roundness invites lobing.
- Leave 0.05-0.1 mm (0.002-0.004") on diameter for grinding after turning or heat treatment, more if the part distorts.
- Make the part longer than its diameter and chamfer both ends so it feeds cleanly and does not chip the wheel.
- Keep flats, keyways and cross holes off the surface that will be through-fed, or add them after grinding.
- For in-feed parts give a shoulder or end face to locate against, with a length tolerance to it.
- Specify straightness on long parts; through-feed grinding follows the bar and does not straighten it.
- Ask whether the shop can supply centerless-ground bar in your alloy instead of grinding finished parts; for Swiss work it is usually cheaper.
Frequently asked questions
- What is centerless grinding?
- OD grinding with the part supported on a blade between a grinding wheel and a slower regulating wheel instead of being chucked or held between centers. The regulating wheel controls speed and feed. It sizes round parts fast and holds roundness in the micrometres because the part rides on its own surface.
- What tolerance can centerless grinding hold?
- ±0.005 mm (±0.0002") on diameter is standard, ±0.0025 mm (±0.0001") is achievable with care, and gauge pins and bearing rollers are held to ±0.001 mm (±0.00004"). Roundness of 0.001 mm is routine when the blade height is set correctly.
- Through-feed or in-feed centerless grinding?
- Through-feed passes plain cylinders and bar straight through, fastest and cheapest, for parts with one diameter. In-feed plunges the wheel into a part held against a stop, for shoulders, tapers, heads and multiple diameters, at a slower rate.
- Centerless or cylindrical grinding?
- Centerless for volume, plain diameters, long slender parts and bar; it needs no centers and loads in seconds. Cylindrical grinding between centers for small lots, parts with several diameters, shoulders and faces that must run true to the axis, and anything that needs a center hole as a datum.
