Surface Finish & Roughness (Ra)

Tolerances

• As-machined finish is about Ra 3.2µm (125µin). Fine machining reaches Ra 1.6µm (63µin); grinding reaches Ra 0.8µm (32µin) and below; lapping reaches Ra 0.2µm (8µin); electro-polishing reaches Ra 0.05µm (2µin).
• Finishing changes dimensions. Powder coat adds 60 to 120µm; anodizing removes about 10 to 15µm per surface; electropolishing removes 5 to 25µm; hard chrome adds 10 to 50µm. Allow for the specific finish on any tight fit, and confirm the measured size after finishing, not just before.

Surface finish is the texture of a machined surface, most often measured as Ra (roughness average). Lower Ra means a smoother surface. Ra is set by the machining operation and any secondary finishing, such as grinding, lapping, or polishing, and each step finer usually adds a separate operation, so finish is a cost dial as well as a texture spec.

What Ra means

Ra is the arithmetic average of the microscopic peaks and valleys on a surface, reported in micrometres (µm) or microinches (µin). A lower Ra is smoother, and the scale spans more than two orders of magnitude, from a rough saw-cut near Ra 25µm down to a mirror polish at Ra 0.05µm.

Ra versus Rz

Ra is the most common finish metric because it is simple to measure with a stylus profilometer and it tracks well with how a surface looks and feels, but it averages out the extreme peaks, so sealing work may also need Rz, which catches the largest peak-to-valley heights Ra can hide. A surface reading is local, so a critical face is measured in more than one spot, and the finish a given operation reaches moves with tool sharpness, feed and speed, coolant, and the material itself.

The Ra scale and the operations

Each Ra band has an operation that normally produces it, which sets the finish a part gets before any secondary work.

Roughing and finishing passes

Sawing and rough milling sit around Ra 6.3 to 25µm. Face and end milling land at Ra 1.6 to 6.3µm, with a standard as-machined finish near Ra 3.2µm. A finishing end-mill pass or reaming reaches Ra 0.8 to 1.6µm.

Grinding, lapping, and polishing

Grinding spans Ra 0.2 to 1.6µm depending on whether it is a rough or finish grind. Lapping, honing, and electro-polishing take a surface from Ra 0.4µm down to 0.05µm. The implication is practical: if a drawing asks for finer Ra than the chosen operation can reach, the part needs a secondary process, and that secondary process is where the finishing cost lives.

Matching Ra to function

Match the finish callout to what the surface does, not to a generic fine target. A non-mating exterior is fine at Ra 6.3 or 3.2, the standard as-machined finish. A locating face or a tapped bore is comfortable at Ra 3.2 or 1.6. Bearing journals, seal seats, and hydraulic surfaces need Ra 0.8 or finer, which calls for grinding. Sliding or precision-guideway surfaces move to Ra 0.4, and gauge, optical, or implant surfaces need lapping, honing, or electro-polishing at Ra 0.2 down to 0.05. Specifying Ra 0.4 on a bracket exterior that only needs Ra 3.2 adds lapping cost for no functional gain, so the discipline is to call out Ra only on the surfaces that need it and let the general as-machined finish govern the rest.

Finishes and dimensions

Coatings and plating change the surface a drawing must hit, so plan the Ra callout together with any finish the part will receive.

Coating thickness by process

Powder coat adds 60 to 120µm and masks fine machining marks, so a powder-coated face should not carry a fine Ra callout. Anodizing removes about 10 to 15µm from an aluminum surface and largely preserves the underlying finish, so the machined Ra still matters underneath, and color anodize adds a little more. Hard chrome can build up 10 to 50µm and is often used to restore an oversize surface back to size, while zinc plating adds 8 to 20µm. Electropolishing removes 5 to 25µm and can be used to tighten a dimension as well as smooth it.

Planning for fit-critical features

On a fit-critical feature, either machine the pre-finish size to leave room for the coating, or finish the critical surface after coating, or the measured result after finishing will not match the drawing.

Measuring surface finish

Surface finish is measured by tracing the surface and reading its roughness, and the method sets what you can verify.

Stylus and optical methods

The classic tool is a stylus profilometer, which drags a diamond tip across the surface and records the height variation over a sampling length to compute Ra or Rz. Optical methods, such as confocal or interferometry, measure without contact, which suits soft or freshly coated surfaces a stylus could scratch. Either way, the reading is local to the traced line, so a critical surface is measured in more than one place and more than one direction, because finish varies with the lay of the machining marks.

Environment and verification

For a fine Ra, the measurement also needs a stable environment, because thermal drift and vibration move the reading at the micron scale. A finish you cannot measure is a finish you cannot enforce, so plan the inspection with the callout, and confirm the shop has the tool (profilometer or optical) and the environment (clean, temperature-stable) to verify the value you ask for.

Finishing operations and their effects

Secondary finishing operations change both the finish and the dimensions, and each one behaves differently, so the choice follows the function and the material.

Grinding, lapping, and honing

Grinding removes a small amount of material with an abrasive wheel and reaches Ra 0.8µm and below with good form control, which is why it is the standard route for bearing and seal surfaces. Lapping uses a loose abrasive on a flat plate and reaches Ra 0.2µm and below on flat or cylindrical surfaces, suited to gauges, optics, and sealing lands. Honing uses expanding abrasive stones to finish a bore to a tight size and finish, common on cylinder bores.

Electro-polishing, anodizing, and powder coat

Electro-polishing removes a controlled layer from a stainless surface, smoothing it to Ra 0.05 to 0.2µm while also improving corrosion resistance, which is why it suits medical and food-contact parts. Anodizing builds an oxide layer on aluminum that hardens the surface and can hold a dye, removing only a little base metal. Powder coat lays a polymer skin 60 to 120µm thick that protects and colors but masks fine finish.

Plating

Plating adds metal: hard chrome builds a wear-resistant layer, zinc plates for corrosion, nickel for appearance and wear. Each operation has a dimensional signature, so the finishing plan is part of the tolerance plan, not an afterthought applied to a finished size.

Choosing a finishing route

A finishing route is the sequence from the machined surface to the final surface, and choosing it well means matching the operation to the function, the material, and the tolerance. A bearing journal points to grinding, because it needs Ra 0.8µm or better with good form. A sealing land on a stainless part points to lapping or electro-polishing, depending on whether the priority is flatness or corrosion. A decorative exterior points to bead blast plus anodize (on aluminum) or powder coat (on steel), which hides machine marks and adds protection without a tight Ra. A medical implant points to electro-polishing or tumbling, which smooth and clean the surface. The route also has to fit the tolerance: if a dimension is tight, finish the critical surface after coating, or size the pre-finish part to leave room for the coating, so the final measured size lands in band. Planning the route at the design stage avoids the common trap of machining to size, applying a finish, and then finding the part out of tolerance.

Checklist

• Ra called out only on surfaces that need it (bearing, seal, sliding, optical, medical).
• General as-machined finish (about Ra 3.2µm) governing the rest.
• Operation that produces the target Ra identified, with secondary work planned.
• Coating and plating thickness allowed for on fit-critical dimensions.
• Rz considered for sealing surfaces where Ra alone may hide peaks.

Common finish mistakes

• Calling a fine Ra on every surface, which adds grinding or lapping cost where the part does not need it.
• Ignoring coating thickness on a fit-critical dimension, so the part measures wrong after powder coat or plating.
• Asking for an Ra the chosen operation cannot reach, forcing an unplanned secondary process.
• Relying on Ra alone for a sealing surface, when Rz would catch the extreme peaks that cause leaks.
• Leaving the finish off the drawing entirely, so the shop defaults to as-machined regardless of what the function needs.

Design rules

• Specify Ra only where function needs it, such as bearing, seal, optical, or medical surfaces. Over-specifying finish adds grinding, lapping, or polishing cost across features that do not need it.
• Account for finishing thickness on fit-critical dimensions. Powder coat, anodize, and plating each add or remove material, so size the pre-finish feature to land in band after the finish is applied.
• Match the Ra to an operation the shop can run. If the target Ra needs a secondary process, plan it and its cost up front rather than discovering it at inspection.