Surface roughness (Ra)

Where it comes up

• Production machining, turning, and sawingCNC milling, turning, and production sawing from bar, plate, castings, and forgings, from prototype to production.
• Screw machine productsTurned pins, fittings, shafts, and bushings made from bar stock on multi-spindle and Swiss screw machines at volume.

Related terms

Chatter

Chatter is a self-sustaining vibration between a cutting tool and the workpiece that leaves a wavy, patterned surface and shortens tool life. It comes from long tool stick-out, thin walls, weak workholding, or the wrong speed and feed. Chatter marks usually fail surface finish callouts.

SPI finish

SPI finish is a mold surface finish grade defined by the Society of the Plastics Industry, now the Plastics Industry Association. Grades run from A-1, a diamond-buffed mirror, through B paper and C stone finishes to D bead-blasted textures. Glossy grades cost more to polish and maintain.

Tolerance

Tolerance is the total amount a dimension is allowed to vary from its nominal value, such as ±0.005 in (±0.13 mm). Tighter tolerances raise cost through slower processes, extra operations, and added inspection. Hold them only on features that affect fit or function.