Ra on Stainless Wire: What a Smooth Surface Actually Measures

Smooth is a visual judgement, and two coils that look equally bright can differ by a factor of three in roughness. Ra turns the impression into a number that can be ordered, measured and repeated. This page covers what Ra and Rz describe, how drawing sets the finish, and what changes downstream when the number moves.

Ra Is an Average, Rz Is a Peak

Ra is the arithmetic mean of the profile deviation from the mean line over the evaluation length. It is stable and easy to compare, which is why it is the default callout. Its weakness is that it averages: a surface with a few deep grooves and an otherwise polished field can report the same Ra as a uniformly fine surface.

Rz looks at the extremes, averaging the highest peaks and deepest valleys within each sampling length. For most wire applications the pair matters more than Ra alone, because deep valleys hold contamination and interrupt coating coverage while Ra stays acceptable. A specification that names only Ra leaves that door open.

How the Surface Is Measured on Wire

• Stylus instrument. The standard method. The cutoff wavelength and evaluation length must be stated, because the same surface reports different values under different settings.
• Measurement direction. Drawing leaves axial striations, so a reading taken along the axis differs from one taken around the circumference. State the direction, and take both if the process is anisotropic.
• Optical and confocal methods. Non-contact and useful on fine wire where a stylus may scratch, but reflective surfaces can bias the result; correlate against a stylus before relying on it.
• Comparator plates. Adequate for shop-floor screening, not for acceptance against a numeric limit.

What Sets the Finish in Drawing

1. Die geometry. A longer bearing length burnishes the surface and lowers Ra, at the cost of higher drawing force and more heat.
2. Lubrication film. Too thin a film produces pick-up and torn striations; too thick or uneven a film leaves a patchy finish. Film control is the single most common reason for a roughness shift between lots.
3. Reduction per pass. A heavy final draft shears the surface layer more severely and raises Ra; splitting the reduction over two passes usually gives a finer finish at the same total reduction.
4. Die material and condition. Diamond and carbide dies wear differently, and a worn die polishes less and marks more.
5. Final anneal. Bright annealing in hydrogen or vacuum can clean and slightly smoothen the surface, but it also replicates any scratch or die mark that was there before.

What Roughness Changes Downstream

• Plating and PVD. Rougher surfaces trap polishing compound and pretreatment residue, and coating thickness becomes uneven over peaks and valleys. Cleanliness and Ra are linked, not separate topics.
• Polishing time. Removing roughness is not linear: going from Ra 0.8 to 0.2 microns costs far more time than from 0.4 to 0.2, because each step has to erase the previous scratch pattern.
• Cleanability. In food contact and medical parts, Rz matters: a deep valley shelters residue that a brush and a rinse will not reach.
• Friction and feeding. A rougher wire grips harder in feeding, wears guide bushes faster and changes the friction condition assumed in a forming simulation.
• Appearance. Gloss is not the inverse of Ra. Waviness at a longer wavelength scatters light even when Ra is low, which is why a low-Ra wire can still look hazy.

Typical Targets by Application

Wire Specification

How to Write the Requirement

• Name Ra and Rz, not Ra alone, where cleaning or coating is involved.
• State the cutoff wavelength, evaluation length and measurement direction.
• State where the reading is taken: along the coil, at the ends, or on a sampled length.
• State the surface condition at the time of measurement, since a soap film hides the profile.
• Agree the instrument class, because stylus and optical readings on the same wire rarely agree to the last digit.

A finish specification is only useful if two people measuring the same coil get the same number, and that requires the parameters to be written next to the value.