BALFORD.Metalworking engineering answers

Machining and secondary operations

How is surface finish specified and measured?

Reviewed by: pending engineering reviewStatus: draft6 cited sources

Answer

Ra is the arithmetic mean roughness of the surface profile, measured in micrometres (µm) or microinches (µin). As a rule of thumb: hot rolled steel arrives at 3.2–12.5 µm, cold rolled sheet at 0.8–3.2 µm, a laser-cut edge at 3.2–12.5 µm, a turned or milled face at 0.8–3.2 µm, a ground face at 0.2–1.6 µm and a polished face at 0.05–0.4 µm. A drawn part inherits the die surface, usually 0.4–1.6 µm, and an electropolished surface typically improves Ra by 30–50% compared with the same surface before treatment. Coatings add roughness of their own: a powder coat reads about 1–3 µm because of its orange-peel texture.

Sources

  1. Surface Finish Chart: Ra Values By Process
    Ra is the arithmetic mean roughness of the surface profile, measured in micrometres (µm) or microinches (µin). As a rule of thumb: hot rolled steel arrives at 3.2–12.5 µm, cold rolled sheet at 0.8–3.2 µm, a laser-cut edge at 3.2–12.5 µm, a turned or milled face at 0.8–3.2 µm, a ground face at 0.2–1....
  2. Metal Processing: 25 Fabrication Methods | Balford
    Planing processing typically offers a precision range of IT9 to IT7, indicating the level of dimensional accuracy achieved. The surface roughness, measured by the parameter Ra, falls within the range of 6.3 to 1.6 micrometers (um). The Ra value describes the average roughness of the machined surface...
  3. Surface Finish Chart: Ra Values By Process
    What is a good surface finish? It depends on function: 3.2 µm is normal for a painted or non-functional face, 0.8 µm suits most sealing and mating faces, and 0.4 µm or better is for sliding or optical surfaces. A tighter number than the function needs is added cost with no benefit. Does powder coati...
  4. Automotive Sensor Housing Design Guide: Stainless Steel
    Thread pitch and gauging The housing has to screw into the boss and seal Go / no-go thread gauges Sealing face finish Sealing depends on surface finish, not only on dimension Portable surface roughness tester (Ra) Length and step depths Sets how far the element sits into the medium Digital depth and...
  5. Ironing in Deep Drawing: Hold Both ID and OD | Balford
    ductility. On soft-magnetic parts the annealing step is also what sets the magnetic properties. Lubrication. Ironing is a high-pressure, high-surface-contact operation. The lubricant film is what separates a burnished surface from a scored one, and it is chosen for the grade and the reduction rather...
  6. Ironing in Deep Drawing: Hold Both ID and OD | Balford
    Does ironing remove the need for machining? Often, yes. On parts that need both diameters in tolerance - solenoid valve housings, cylinder sleeves, bushing shells - ironing can replace bar-stock turning entirely, and on cold extruded parts it can replace the turning pass that follows extrusion. It d...

What an engineer still needs from you

  • material and material condition (grade, temper)
  • material thickness, plus the key diameter, height and wall thickness
  • tolerances, burr direction and the visual acceptance standard
  • annual volume - it decides whether tooling is single-hit, progressive or transfer
  • surface finish, and any secondary machining, annealing or plating
Next step. Send the drawing for a manufacturability review, or send the part data for a process and cost route.
Submit a drawing / RFQDFM review

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This page is an engineering answer, not a quotation and not a dimensional acceptance. It is generated from Balford's published material and held to the verified company fact sheet.