Ra 1.6 µm (about 63 microinch), a good turned or milled finish.

What do three triangles on a drawing mean?

An old way of marking a fine finish, roughly Ra 0.4 to 1.6 µm, usually ground or finely turned. Check the drawing's notes, as the convention varies.

Can turning reach Ra 0.4?

Fine turning with a sharp insert, a large nose radius and a small feed can reach about Ra 0.4 to 0.8. Lower values usually need grinding.

Surface finish converter: Ra, N grade, Rz and microinch

I know theRa (µm)

Nearest N grade: N7 (Ra 1.6 µm)
Ra: 1.6 µm
Ra in microinch: 63 µin
Rz (about): 6.4 – 11.2 µm
Old drawing symbol: ▽▽▽
Usually made by: turning, milling, boring, drilling with care

Rz depends on the process; 4 to 7 times Ra is a common range. The triangle symbols are an old convention; read the drawing's own note if it has one.

Conversions used

N grades follow ISO 1302: N1 is Ra 0.025 µm and each grade doubles, up to N12 at Ra 50 µm.
1 µm = 39.37 microinch.
Rz is not a fixed multiple of Ra; 4 to 7 times Ra is a common range.

Questions

What is N7 in Ra?: Ra 1.6 µm (about 63 microinch), a good turned or milled finish.
What do three triangles on a drawing mean?: An old way of marking a fine finish, roughly Ra 0.4 to 1.6 µm, usually ground or finely turned. Check the drawing's notes, as the convention varies.
Can turning reach Ra 0.4?: Fine turning with a sharp insert, a large nose radius and a small feed can reach about Ra 0.4 to 0.8. Lower values usually need grinding.

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Results use standard formulas and typical values. Use them for estimating and planning; check critical figures against the drawing, the material certificate or the maker's data.