Surface Finish Chart - Ra, Rz, N-Grade Conversion for Cast and Machined Parts

"Ra 3.2," "63 finish," "N8" - engineering drawings express surface finish in three different languages, and suppliers quote in a fourth ("as-cast," "bead blasted"). This chart converts between all of them and maps each value to the process that actually produces it, so you can specify a finish once and get the same surface from any supplier.

Bookmark this page. It is the reference our own engineers use when reviewing drawings.

---

The Master Conversion Chart

Ra (arithmetic mean roughness) is the global default. One micrometer equals approximately 40 microinches (1 um = 39.37 uin), and ISO N-grades map to preferred Ra values:

N-Grade	Ra (um)	Ra (uin)	Typical process	What it looks like
N3	0.1	4	Lapping, superfinishing	Mirror
N4	0.2	8	Fine grinding, honing	Near-mirror
N5	0.4	16	Grinding, fine turning	Smooth, no visible marks
N6	0.8	32	Fine milling/turning	Marks barely visible
N7	1.6	63	Standard CNC machining, good die casting as-cast	Visible but fine marks
N8	3.2	125	Standard machining, typical die casting as-cast	Clearly visible, uniform
N9	6.3	250	Rough machining, investment casting as-cast	Obvious marks
N10	12.5	500	Rough machining, sand casting as-cast	Coarse
N11	25	1000	Rough casting, flame cut	Very coarse

The lookups engineers search most:

• 32 surface finish = Ra 0.8 um = N6 (fine machining)
• 63 surface finish = Ra 1.6 um = N7 (standard machining / good as-cast die casting)
• 125 surface finish = Ra 3.2 um = N8 (standard machining / typical as-cast die casting)
• 250 surface finish = Ra 6.3 um = N9

Ra vs Rz - The Conversion That Is Not One

Drawings (especially European ones) often specify Rz (ten-point height) instead of Ra. Rz measures peak-to-valley height and is always larger than Ra for the same surface. There is no exact conversion - the ratio depends on the surface profile shape - but a working approximation for machined surfaces is:

Rz is roughly 4 to 7 times Ra. Ra 3.2 um corresponds to about Rz 12.5-25 um.

If a drawing says Rz 25 and you quote Ra capability, convert first. Mixing up Ra and Rz on a spec review is a classic way to promise a finish four times better than asked - or reject a good part.

RMS (root mean square), still found on older US drawings, runs about 10-11% higher than Ra on the same surface. Close enough to treat as equal for quotation purposes; not close enough for inspection reports.

What Each Process Actually Delivers

This is where finish specifications meet manufacturing reality:

Process	Typical Ra range (um)	Notes for buyers
High-pressure die casting, as-cast	1.6-3.2	New dies on zinc beat this; worn dies on aluminum run rougher
Investment casting, as-cast	3.2-6.3	Ceramic shell texture
Sand casting, as-cast	6.3-25	Always needs machining for functional faces
CNC milling/turning	0.8-3.2	N6-N8; 3.2 is standard shop default
Grinding / honing	0.1-0.8	Bearing and sealing surfaces
Bead blasting	~1.5-3.2 (fine media)	Satin cosmetic texture; specify by sample
Polishing	0.1-0.8	Pre-plating prep on zinc castings
EDM	1.6-6.3	Depends on finishing passes

Two lessons from this table:

1. A good die casting needs no machining for cosmetic surfaces. As-cast N7-N8 plates, coats, and anodizes directly - that is the economics of the process.
2. Overlapping ranges mean "process capable," not "guaranteed." A worn die drifts from Ra 1.6 toward 3.2 over its life. If a surface must stay fine for the whole program, say so - and tie it to die maintenance or a machining op.

How to Specify Surface Finish on a Casting Drawing

The symbol itself (per ISO 1302) is a check-mark with the Ra value inside. Getting the symbol right is the easy part. The mistakes happen around it:

1. Specify function, not habits. Defaulting every face to Ra 1.6 forces unnecessary machining on a part that casts at 3.2 for free. Ask what the surface does: sealing face (needs flatness + fine finish), cosmetic face (needs appearance + sample), neither (leave as-cast).

2. Separate "cosmetic" from "roughness." A bead-blasted housing can hit Ra 2.5 and still be rejected for blotchy color. Cosmetic surfaces need an approved boundary sample; roughness values alone do not capture appearance. See bead blasting for how to write that callout.

3. Remember coatings change the number. Powder coat adds 60-100 um of thickness and its own texture; chromate conversion coating adds nothing measurable. Specify whether the Ra applies before or after finishing.

4. Do not mix systems. One drawing, one parameter (Ra or Rz), one unit system. Mixed drawings generate the most RFQ clarifications we see - and the most quiet assumptions when suppliers do not ask.

5. State the evaluation basics on critical surfaces. Sampling length and cutoff matter when comparing inspection reports between your metrology lab and your supplier's profilometer.

For how surface specs interact with cast tolerances, see die casting tolerances.

---

Frequently Asked Questions

What is the difference between surface finish and surface roughness?

In practice, nothing - shops use both terms for the same Ra values. Strictly, "surface texture" is the umbrella term covering roughness (fine, closely spaced), waviness (wider spacing), and lay (direction). Drawings almost always mean roughness.

What surface finish can die casting achieve without machining?

Typically Ra 1.6-3.2 um (N7-N8) as-cast on aluminum and magnesium, with zinc running finer - especially on fresh dies. Cosmetic bead blasting evens out the appearance without changing the roughness class much.

Is Ra 3.2 the same as 125 finish?

Yes. Ra 3.2 um equals Ra 125 uin, which is N8. It is the default "as-machined" finish on most shop drawings and the typical as-cast finish on die castings - which is why many cast parts go straight to coating without machining.

How is Ra actually measured?

Usually with a contact profilometer - a diamond stylus dragged across the surface - set to a defined sampling length and cutoff. Non-contact optical methods exist but read differently on reflective or blasted surfaces. When a value is disputed, agree on the instrument and settings before arguing about the number.

Does a lower Ra always mean a better part?

No. Over-specifying finish adds cost with zero function: machining a face that could stay as-cast, or polishing a surface heading under powder coat. Some surfaces even want texture - coating adhesion and gasket sealing can prefer a defined roughness. Specify to function.

---

Related Reading

• bead blasting - the most common cosmetic finish on castings
• die casting tolerances - dimensional capability alongside surface capability
• die casting surface finishing guide - finishes that go on top of the surface
• die casting secondary machining - when and where to machine castings
• die casting design guide - design decisions that affect finish

---

Reviewing a drawing with finish callouts? Send it to yaoqingpu1983@gmail.com - we will flag any finish that is over- or under-specified before it costs you money | +86 138 1403 4409 | No.6, Rungu Road, Nanjing, China