Surface Finish Chart: Ra, RMS and N Grades, With Symbols Explained

What are the common surface finish values?

Charts list roughness in a fixed series of steps, each about half the one before. Metric drawings give Ra in micrometres (µm), inch drawings give Ra in microinches (µin), and ISO drawings may use the N grade. One row of the chart below is the same finish in all three.

To convert, multiply micrometres by 39.37 for microinches, or divide microinches by 39.37 for micrometres. Charts round the result to the familiar numbers, so 3.2 µm prints as 125 µin rather than 126. The N grades come from ISO 1302; its successor, ISO 21920-1, drops them, but they are still on a great many drawings.

What is the difference between Ra, RMS and Rz?

Ra is the average height of the peaks and valleys measured along a line. RMS, written Rq today, is the root-mean-square of the same heights and runs about 10% higher than Ra. Rz is the average of the five biggest peak-to-valley heights, so it is several times larger than Ra.

• Ra (arithmetic average). The default on almost every modern drawing in both inch and metric. When a finish has no letters beside it on an inch drawing, it is Ra in microinches.
• RMS or Rq. About 1.1 times Ra on a turned or milled surface. US drawings from before the 1960s often say RMS; for ordering a part, treat 63 RMS and 63 Ra as the same finish, because the gap is smaller than the spread between two machines.
• Rz. Common on German and other European drawings. Roughly 4 to 7 times Ra, depending on the process, so Rz 16 is in the region of Ra 3.2. There is no exact conversion; ask the shop if a fit depends on it.
• Rmax or Rt. The single biggest peak-to-valley height. Used where one scratch would cause a leak, such as a seal face.

A profilometer measures all of these from the same trace, so the drawing’s letters only decide which number the inspector reports.

What surface finish does each machining process give?

Each process has a typical roughness range. Milling and turning reach Ra 3.2 (125 µin) easily and Ra 0.8 (32 µin) with a finishing pass; grinding, honing and lapping go finer. Casting, forging and cutting leave rougher surfaces that are machined only where a drawing asks for it.

The ranges are typical, not limits: a sharp tool, a rigid machine and a slow feed beat the lower end, and a worn tool misses the upper one. Each step finer costs time, so a designer who writes Ra 0.8 on a face that only needs Ra 3.2 adds finishing passes, and often a second process, to that face. Mark only the faces that need it and let the title block default cover the rest.

How do you read a surface finish symbol on a drawing?

The symbol is a check mark, √, whose point touches the surface or an extension line from it. The roughness value sits on the symbol: 125 on an inch drawing, Ra 3.2 on a metric one. A bar or circle added to the V says whether material must, or must not, be removed.

• Plain √. Any process may be used to reach the finish.
• √ with a bar across the top of the V. Material removal required: machine or grind the surface.
• √ with a small circle in the V. Material removal not allowed: leave it as cast, forged, moulded or rolled.
• A long horizontal line off the tall leg. Space for extra requirements: a process (“ground”, “polished”), a sampling length, or a second roughness value.
• A circle at the corner of the long leg. The finish applies all around the outline in that view.
• Two values stacked. An upper and a lower limit, for example 63 over 32: the surface must be between them.

On an ASME drawing the value is written above the V, left of the tall leg. On an ISO drawing it is written under the long line with its parameter, such as Ra 0.8. A symbol in or next to the title block, often with other symbols in brackets after it, sets the default finish for every surface not marked. On ISO drawings a single value is normally an upper limit that 16% of readings may exceed; “max” in the callout means none may.

What do the lay symbols mean?

Lay is the direction of the tool marks on a surface. A small symbol beside the check mark says which way they must run, measured against the line the symbol sits on in that view. It matters on sliding and sealing faces, where marks across the motion wear seals or leak.

Most drawings give no lay, which leaves it to the shop. When one is given, it usually sits on an O-ring groove, a seal face, a slideway or a part that will be lapped.

What do the triangle finish marks on old drawings mean?

Before the check mark was standard, many European and some older US drawings marked finish with inverted triangles. More triangles mean a smoother surface: one is rough machined, four is lapped or polished. A wavy line means the surface stays as it came, unmachined.

The triangles never had exact values, and different companies and countries mapped them to different numbers, so the right-hand column is a starting point. When a legacy print only shows triangles, agree a numeric Ra with the shop before the part is quoted. Faded and hand-drawn sheets carry other old conventions too; see mylar engineering drawings to CAD.

Which surface finish should you specify?

Specify the roughest finish that still does the job. Ra 3.2 (125 µin) suits most machined faces, Ra 1.6 (63 µin) mating faces and fits, Ra 0.8 (32 µin) bearing seats and static seals, and Ra 0.4 (16 µin) or finer only dynamic seals and sliding fits.

• Clearance and non-contact faces. Ra 6.3 (250 µin) or the title block default.
• Bolted joints and locating faces. Ra 1.6 to 3.2 (63 to 125 µin).
• Press fits and bearing bores. Ra 0.8 to 1.6 (32 to 63 µin), tighter for small fits; see press fit and snap fit design.
• Static O-ring faces. Around Ra 0.8 (32 µin); dynamic seals about Ra 0.4 (16 µin).
• 3D-printed parts. Printed surfaces are much rougher than machined ones; a finish callout on a printed part means post-machining or sanding.

Finish and tolerance go together: a tight tolerance on a rough surface can’t be measured reliably. See tolerance stack-up basics for the sizes, and the engineering drawing symbols chart for every other callout on the sheet.

Frequently asked questions

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