Superfinishing and polishing

Fine stones or buffs remove the last microscopic peaks

Superfinishing and polishing are the gentlest abrasive steps: they take an already ground surface and shave off its last microscopic peaks. Superfinishing makes bearing rollers and crankshaft journals run smoother and last longer; polishing gives taps, watch cases and medical implants their mirror shine. Both remove only a few micrometers or less.

Photo: Mgeurts, CC BY 3.0, via Wikimedia Commons

superfinishing and polishing · tools and magnified surface profilesCircled numbers match the steps below; the bottom row magnifies the surface so you can see what each step removes.

How it works

Start from a ground surface. The part arrives already ground to size. Under a microscope its surface is still a field of tiny peaks and scratches.
Press a fine stone lightly. In superfinishing, a stone or abrasive film with very fine grit (tiny abrasive particles) is pressed gently against the turning part.
Oscillate fast. The stone shakes back and forth in short, rapid strokes, so its grains cross the old grinding marks and slice off the peaks.
Let it stop by itself. As the peaks disappear, the stone starts to ride on a film of oil and cutting almost stops. The valleys stay, and they later hold lubricant.
Polish for shine. For looks, a spinning cloth wheel (a buff) loaded with compound (fine abrasive in a wax bar) is pressed on the part, through finer and finer grades, until it reflects like a mirror.

Key facts

Materials: Hardened steels, stainless steel, brass, aluminum, chrome-plated parts, precious metals
Typical parts: Superfinishing: bearing races and rollers, crank and cam journals, piston rods. Polishing: almost any shape, flat or curved
Part size: From watch parts to rolls and rods several meters long
Precision: Superfinishing removes about a micrometer and barely changes size; polishing targets looks, not dimensions
Surface finish: Superfinishing can reach roughness around Ra 0.01–0.1 µm; polishing gives a mirror finish
Batch size it suits: Superfinishing: mass-production lines. Polishing: one-offs by hand to automated or robotic lines
Tooling and machine cost: Stones, films, buffs and compounds are cheap consumables; automated machines are a significant investment
Speed: Superfinishing: seconds to under a minute per surface. Polishing: seconds by machine to hours by hand

Strengths and limits

Strengths

Very smooth surfaces that cut friction, wear and noise in bearings and shafts
Superfinishing keeps the valleys, which hold oil
Little heat, so the hardened surface is not damaged
Polishing makes parts attractive and easier to clean

Limits

Cannot fix size or shape errors; the part must already be ground accurately
Superfinishing needs dedicated heads or machines for each type of part
Hand polishing is slow, skilled work and can round off sharp edges
Polishing dust and used compound must be extracted and handled

Where it's used

BearingsBall and roller races, rollers
AutomotiveCrankshaft and camshaft journals, shock absorber rods
MedicalHip and knee implants, surgical instruments
Consumer goodsWatch cases, jewelry, faucets, cookware
HydraulicsPiston rods and seal surfaces

How to recognize it

A superfinished journal looks bright but slightly satin, with no visible grinding lines.
A polished part reflects your face like a mirror, often with faint swirl marks in raking light.
Rounded, softened edges on decorative metal parts hint at buffing.
On bearings, the running track is noticeably brighter than the ground faces around it.

Compare with

Grinding5.7sets the size and shape first; superfinishing and polishing only refine its surface.Honing5.8a similar slow scrubbing with stones, but inside bores and removing more material.Electroplating11.5also gives a bright finish, but by adding a metal layer instead of removing peaks.

Watch it

Supfina · A clear manufacturer explainer of the principle

Cutting Tool Engineering · Superfinishing bearing parts in production

Balu Forge Industries Ltd · Abrasive finishing of crankshaft journals in a factory

Avid Welder · Hand polishing through finer and finer grits to a mirror

See real photos

Wikipedia: Superfinishing — the principle and its through-feed and plunge variants, overview with photos
Wikipedia: Polishing (metalworking) — grits, buffing wheels and compounds, with photos
Wikimedia Commons photo search

Remember

These steps remove only the last microscopic peaks of an already ground surface.
Superfinishing improves function: smoother running, longer life, oil kept in the valleys.
Polishing improves looks: a mirror finish built up through finer and finer abrasives.