Surface Finish

Surface finish is the texture a cut leaves on a machined part, and in engineering practice it is described by two numbers above all others: Ra, the arithmetic mean of the surface’s deviations from its centre line, and Rz, the mean height of its peaks and valleys. No machined surface is truly smooth — every cut leaves its own pattern of ridges and troughs — and the drawing tells the machinist how much of that texture is acceptable. Ra and Rz are how the shop and the inspector agree on the surface, and this entry sets out what each measures, how it is read, and how the machining decides the number.

Ra: the averaged surface

Ra — the arithmetic average roughness, also called the centre line average — is the single most used measure of a machined surface. The instrument traces the profile, draws a mean line through its deviations, and averages the absolute distance of the profile from that line over the sampling length. The result is a robust, readable number in micrometres — stable from one trace to the next, easy to state on a drawing and check on the bench — which is why Ra is the default specification on most machined parts. Its strength is also its limit: because Ra averages, it is almost blind to a single defect. A part with one deep scratch or a torn edge can still report a good Ra, since the one bad spot is drowned by the many good ones — and a functional surface can fail while its Ra number looks healthy.

Rz: the extremes of the surface

Rz — the mean peak-to-valley height — answers the question Ra cannot. The instrument divides the trace into sampling lengths, finds in each the distance between the highest peak and the deepest valley, and reports their mean. Rz is therefore a measure of the surface’s extremes rather than its average. Because a surface that seals, slides or carries fatigue load fails on its worst point rather than its average, functional drawings for sealing faces, press fits and heavily stressed parts often call up Rz as well as Ra — a part can hold a fine Ra and a poor Rz, and the poor Rz is the one that leaks or cracks. The two numbers are related only loosely: for machined surfaces Rz commonly falls between roughly four and seven times Ra, but the ratio shifts with the process and the material, so Rz is read from a drawing as its own requirement, never converted from Ra by a fixed factor.

Reading the surface with a profilometer

Ra and Rz are measured with a profilometer, most often a stylus instrument that draws a fine diamond point across the surface under a light, constant force, records the profile and computes the roughness parameters. The key of the computation is the cut-off length: a filter that divides the trace into the short-wavelength roughness the designer cares about and the longer-wavelength waviness and form error that belong to the machining operation rather than the finish. The cut-off must suit the surface — a cut-off of 0.8 millimetre is the common choice for ordinary machined finishes, with shorter and longer filters for finer and coarser surfaces — and the trace covers several sampling lengths, so the reading describes the surface rather than one lucky groove. For surfaces too delicate or too fine for a dragging stylus, non-contact optical instruments measure the same parameters without touching the part.

Where the finish comes from

On a machined part the surface is not an accident of the material but a direct result of the cut, which is why the finish is as much a machining decision as an inspection one. A milled or turned surface carries the marks of the tool’s advance: the feed per revolution and the nose radius of the insert set the height of the scallops the tool leaves, so a light finishing feed and a generous nose radius produce a finer finish, while a heavy feed leaves a coarser texture — the arithmetic this wiki treats in its entries on feeds and speeds and on tool geometry. Vibration and chatter print their own pattern across the surface and must be tuned out of the cut; a worn cutting edge drags instead of shearing and tears the finish long before it fails; and a flexible setup or spindle lets the tool deflect and smear the surface it should cleanly cut. The finishing pass is where the specified number is won: the operation that takes the final light cut, at the finish feed, with a sharp tool, leaving the texture the drawing names.

The values machined parts achieve

The range of surface finish runs from the broad marks of roughing to the mirror of fine abrasion, and a machinist reads where a requirement sits in that range to judge how hard it will be to hit. Rough milling and turning produce Ra values in the region of 3.2 to 6.3 micrometres. Standard machining with finish-grade practice reaches roughly 0.8 to 1.6 micrometres Ra, the everyday finish of CNC machining and the texture most drawings ask for. Reaming and fine boring bring holes to about 0.4 to 1.6 micrometres, and grinding reaches 0.1 to 0.8 and finer. Below the grinding range, in the few tenths and hundredths of a micrometre, lie honing, lapping and polishing — whole processes beyond the ordinary cut. That ladder is the economics of finish: the numbers near a machinist’s end of the range are earned by a careful finishing cut, while each step toward the bottom buys smoothness with a further process.

Specifying the finish on a drawing

The drawing states the requirement with the ISO surface-texture symbol and a value: a number in micrometres above it giving the maximum Ra the surface may show, and a finish allowance where a surface is to be ground or machined from a rougher state. Where the peak-to-valley character matters — sealing, sliding, press-fit and fatigue surfaces — the drawing calls Rz alongside Ra, and where direction matters the lay symbol tells whether the marks must run one way, as a grinding or turning finish naturally does. Reading the finish call-out belongs to reading the drawing as surely as reading its sizes, and the surface is measured with the same discipline this wiki treats in its entry on measuring a feature: clean the part, choose the right instrument and filter, take the trace where the surface is true. In the end Ra and Rz are the last two numbers of a job — the machine makes the part to size, and the profilometer confirms the texture that size is cut with, the finishing sentence of every machined surface.