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Drawings & tolerances

Surface texture and drawing indications

Surface finish is a functional requirement, not a cosmetic number. Specify it where friction, sealing, wear, fatigue, lubrication or assembly performance depends on the manufactured surface.

PARAMETERS

Ra and Rz answer different questions

Ra · arithmetic mean roughness is the arithmetic mean of the profile deviations. It is stable for general comparison, but isolated peaks or valleys influence it only slightly.

Rz · mean roughness depth represents peak-to-valley surface height over the evaluation sections. It responds more strongly to local profile extremes.

DRAWING LOGIC

Read the symbol before the value

  • Basic symbol: manufacturing process is not prescribed.
  • Bar added: material removal is required, such as turning or grinding.
  • Circle added: material removal is not permitted; retain the produced surface.
  • Circle at the leader junction: the requirement applies all around the contour.
Design question Practical decision
Does the surface slide or carry a seal? Control roughness and groove direction; avoid a texture that promotes leakage or rapid wear.
Is the surface only non-functional? Do not demand unnecessary finishing. Preserve an economical manufacturing route.
Must inspection reproduce the result? State the parameter, limit and any necessary process or lay direction clearly.
SourcesSource basis: Ulrich Fischer et al., Mechanical and Metal Trades Handbook, form deviations, roughness and surface indications, pp. 93–96.
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