What does the bar on a surface finish symbol mean?

Material removal is required: the surface must be machined. A circle in the V means material removal is not allowed. The plain check mark leaves the process open.

Is Ra in micrometres or microinches?

ISO drawings use micrometres (Ra 1.6). US drawings usually use microinches (63). Ra 1.6 µm is about 63 µin.

The direction of the tool marks on the surface. A symbol to the right of the V gives it: = parallel, ⊥ perpendicular, X crossed, M multidirectional, C circular, R radial, P particulate.

Mechanical prints: lesson 5 of 7

Surface finish symbols

Read the symbol and answer. Get 10 right to finish. ASME practice shows microinches, ISO shows Ra in micrometres.

This US drawing is in inches. Roughly what is the 32 on this symbol in micrometres?

The three basic symbols

Any process

Material removal required

Material removal not allowed

ASME Y14.36 and ISO use the same three symbols. The difference is mostly units and where the value is written: US drawings usually give the roughness in microinches next to the V, ISO drawings write the parameter and value in micrometres under the long leg, such as Ra 1.6.

Ra (µm)	0.4	0.8	1.6	3.2	6.3
Ra (µin)	16	32	63	125	250

Lay symbols

Lay is the direction of the tool marks: = parallel to the surface line in the view, ⊥ perpendicular to it, X crossed, M multidirectional, C circular, R radial and P particulate or non-directional.

Common mistakes

Reading a plain check mark as "do not machine". It means any process. The circle means do not machine.
Mixing units. 63 on a US print is microinches, not micrometres.

Questions

What does the bar on a surface finish symbol mean?: Material removal is required: the surface must be machined. A circle in the V means material removal is not allowed. The plain check mark leaves the process open.
Is Ra in micrometres or microinches?: ISO drawings use micrometres (Ra 1.6). US drawings usually use microinches (63). Ra 1.6 µm is about 63 µin.
What is lay?: The direction of the tool marks on the surface. A symbol to the right of the V gives it: = parallel, ⊥ perpendicular, X crossed, M multidirectional, C circular, R radial, P particulate.

Explained in our own words and checked against several textbooks and published tables. For the full rules, see ASME Y14.5, ASME Y14.36, ISO 286, ISO 965, ISO 1302 and ISO 21920-1.