CNC Surface Finish Options: From As-Machined to Anodizing
A working menu of CNC surface finish options: what each process does to Ra, which finishes change part dimensions, and how to specify them on a drawing without creating rework.
Key Takeaways
- Finish is a specification with three separate questions: how smooth, how it changes the size, and what it does to corrosion or wear.
- Mechanical finishes change geometry only at the edge; electrolytic and coating processes move the surface by a measurable amount per side, so tight features need masking or a pre-machining allowance.
- As-machined Ra 1.6 to 3.2 µm covers most internal parts. Going below Ra 0.4 µm needs a dedicated process and a cost conversation, not a note on the drawing.
- State the finish at quotation, name the standard, and say which features must stay untouched. That single line prevents most finishing rework.
Surface finish is where drawings go quiet. A part arrives with a tolerance block, a material callout and one line that says "anodize" or "as machined", and everyone downstream interprets it differently. The result is a batch that meets the dimensions but gets rejected on appearance, or worse, a bore that was measured before a coating went on.
This is the practical menu we use when quoting. It covers what each finish actually does, which ones change the size of the part, and how to write the callout so the shop and the finishing line read it the same way. For the full numeric tables behind this article, the surface finish reference page lists 28 processes with their dimensional effect and upper temperature limit.
1. Three questions decide the finish
Before choosing a process, separate the requirement into three questions. Buyers who answer them at the drawing stage get quoted accurately; buyers who answer them after the parts are made pay for a second batch.
- How smooth? A roughness value, usually Ra in micrometres. This is what most callouts mean.
- How much does the surface move? Mechanical processes remove material at the edge. Electrolytic and coating processes change size by a measured amount per side, and that amount interacts with your tolerance.
- What has to survive? Corrosion, wear, repeated cleaning, electrical contact, appearance, or nothing in particular. "Nothing in particular" is a legitimate answer, and it is the lowest cost one.
2. The Ra scale, and what as-machined really means
Ra is the arithmetic mean of the surface profile over a sampling length. It is a useful shorthand but not a complete description, because two surfaces with the same Ra can feel and behave differently. For CNC parts the practical scale runs like this.
| Ra (µm) | Typical route to get there | Where it is used |
|---|---|---|
| 12.5 to 6.3 | Roughing passes only, coarse tool marks | Non-functional clearance surfaces |
| 3.2 | Standard milling or turning finish | General machined faces, brackets |
| 1.6 | Finish pass, lighter feed, sharp tooling | Mating faces, most external surfaces on visible parts |
| 0.8 | Fine finish pass, small nose radius, stable setup | Sealing faces, sliding contact, o-ring grooves |
| 0.4 | Fine turning, grinding, or electropolishing after machining | Instrument surfaces, hydraulic and pneumatic sealing |
| 0.2 and below | Grinding, lapping, polishing, or electropolishing | Optical and bearing surfaces |
As-machined, meaning the part comes off the machine and is deburred, lands around Ra 3.2 µm on most surfaces and Ra 1.6 µm where the program uses a finishing pass. That is the right choice for the majority of internal parts, and it is also the lowest cost route, because no second operation is involved.
Two practical notes. Deep pockets and small internal radii cannot reach the same finish as an external diameter, because the tool that fits is not the tool that produces the finest surface. And Ra alone does not define a sealing surface: flatness and waviness matter as much as roughness, which is why we quote sealing faces with both a flatness value and a roughness value.
3. Mechanical finishes
These processes work by abrasion, impact or cutting. They change the edge and the surface texture, and they do not deposit anything.
| Process | What it does | Typical Ra | Dimensional effect |
|---|---|---|---|
| Hand or machine deburring | Removes sharp edges and burrs | No change | Edge break only, fractions of a millimetre |
| Barrel or vibratory tumbling | Blends edges, softens tool marks on small parts | Improves toward 1.6 | Removes 0.02 to 0.05 mm at edges; can round a chamfer more than drawn |
| Bead blasting | Uniform matte texture, hides tool marks | 1.6 to 3.2 | Negligible on flat faces, measurable on edges; not for sealing faces |
| Shot peening | Compressive residual stress for fatigue life | Coarser | Controlled by the specification; keep off precision bores |
| Polishing, mirror finish | Removes the profile mechanically | Below 0.2 | Removes material; do not call on a feature at final size |
| Mass finishing with media, ceramic or plastic | Consistent edge and surface condition across a batch | Process dependent | Consistent edge break, hard to mask selectively |
Edge condition is where these processes earn their place. A burr on a mating face or inside a bore is a functional defect: it changes the fit and it traps residue. Our deburring and edge finishing guide covers the edge callouts worth putting on a drawing.
4. Chemical and electrochemical finishes
| Process | Purpose | Dimensional effect per side | Notes |
|---|---|---|---|
| Passivation, ASTM A967 or A380 | Removes free iron and embedded contamination from stainless | Effectively none | Standard for 300 series and 17-4 PH parts |
| Electropolishing | Levels micro peaks, removes burrs, brightens | Removes 5 to 25 µm | Call tight features with an allowance, or mask them |
| Chemical conversion coating, MIL-DTL-5541 Type II | Corrosion protection and electrical conductivity on aluminium | Adds 1 µm or less | Good ground path; wipes off with aggressive handling |
| Black oxide, hot or cold | Colour and light corrosion resistance on steel | Adds under 1 µm | Needs an oil or wax after-treatment to perform |
| Zinc phosphating | Base layer before painting, wear-in for sliding surfaces | Adds 2 to 10 µm | Dimensionally meaningful; pre-size accordingly |
| Pickling and descaling | Removes heat tint and scale | Small removal, uneven on threads | Mask threads and precision bores |
5. Coatings and anodizing
Coatings deposit material, and deposited material adds size. This is the group where a 20 to 50 µm layer collides directly with a ±0.02 mm tolerance if nobody planned for it.
| Process | Use case | Per-side thickness | Upper temperature |
|---|---|---|---|
| Anodizing Type II | Colour, corrosion resistance, mild wear on aluminium | 5 to 15 µm | Around 200 degrees Celsius for the coating |
| Anodizing Type III, hard anodize | Wear surfaces on aluminium: guides, rails, valve bodies | 25 to 50 µm | Around 200 degrees Celsius |
| Electroless nickel | Uniform coverage on complex geometry, wear and corrosion | 10 to 25 µm | Around 400 degrees Celsius at high phosphorus |
| Hard chrome plating | Wear and rebuild of shafts and piston surfaces | 20 to 50 µm typical | Around 300 degrees Celsius |
| PVD coatings, TiN, DLC | Cutting performance and low friction on tooling and wear parts | 2 to 5 µm | Depends on the substrate temper |
| Powder coating | Outdoor appearance and corrosion protection on steel and aluminium | 60 to 100 µm | If this touches a precision bore, it will not fit afterward |
| PTFE or dry film lubrication | Low friction on wear surfaces | 10 to 25 µm | Avoid on threads and precision fits |
Anodizing deserves its own decision path, because Type II and Type III are not interchangeable: the hard coat wears longer but the thickness change is three times larger and the colour range is narrower. The differences, and the masking rules that go with them, are covered in the dedicated anodizing types article.
6. Which finishes move dimensions, in one view
This is the table worth pinning above the drawing board. Everything in the additive half needs an allowance or a mask on any feature at final size.
| Class | Processes | Effect on a tight feature |
|---|---|---|
| No dimensional change | As-machined, deburring, passivation | Safe at final size |
| Removes material | Tumbling, bead blasting, polishing, electropolishing, pickling | Add a pre-size allowance, or mask |
| Adds material | Anodizing, nickel, chrome, chem film, phosphating, powder coat, PVD | Mask, or pre-machine undersize by the coating thickness |
| Changes condition only | Shot peening, laser marking on a non-functional face | Keep away from sealing faces and precision bores |
7. Choosing by requirement
| Requirement | Sensible first choice | Watch out for |
|---|---|---|
| Corrosion resistance on aluminium | Anodizing Type II, or chem film where conductivity is needed | Anodizing is an insulator and breaks the ground path |
| Wear resistance on aluminium | Hard anodize Type III | Thickness change on bores; fatigue debit on sharp corners |
| Corrosion resistance on stainless | Passivation, plus electropolishing for a clean surface | Machining marks left by dull tooling stay visible |
| Electrical conductivity or EMI grounding | Chem film, or bare machined surface with a masked zone | Anodize and powder coat both isolate |
| Low friction | Fine machined finish, PTFE coating, or hard anodize with a sealer | Coatings wear through on high-load contact |
| Cleanroom or medical surfaces | Fine finish, electropolishing, passivation | Blind pockets and sharp internal corners trap residue |
| Appearance and branding | Bead blast plus Type II anodize in a chosen colour | Colour varies with alloy and coating batch |
| Traceability marking | Laser marking on a non-critical face | Do not mark a sealing face or a bore |
8. Cost and lead time
Finish rarely dominates part cost, but it dominates the schedule when it is added late. The pattern is consistent across the three groups.
Mechanical finishes are usually handled in-house and fold into the machining operation, so the cost is time in the deburring or finishing cell rather than a separate purchase order. Chemical and electrochemical finishes are outsourced by most machine shops, including us, and add between two days and a week depending on the process and the queue. Coatings sit at the slower end because they often require racking, masking and a cure or anodize cycle, and hard anodize plus a sealer is slower than Type II.
The cost lever buyers miss is area. Finishing is priced largely by surface area and by racking complexity, so a part redesign that removes a large pocket or an inaccessible internal face does more for the finishing cost than any negotiation. The second lever is masking: every masked zone is hand work, so a drawing that masks six features costs more than one that masks two.
9. How to specify a finish so it survives the shop
Write four things on the drawing and the argument disappears.
- The process and the standard. "Anodize per MIL-A-8625 Type II, Class 2, black" beats "black anodize". Standards exist so both sides measure the same thing.
- The roughness value where it applies. State Ra with a limit, in the unit your inspection team will use, on the surfaces where it matters. Do not put a blanket Ra on a whole drawing, because it prices the part as if every face were a sealing face.
- The masking or allowance note. "Mask threads" or "pre-machine bores 0.03 mm undersize before hard anodize" turns a rework risk into a line of text.
- The inspection expectation. If finish is checked on a sample, say so. If a certificate of conformity for the finish is required, say so at quotation, because the finishing supplier has to issue it.
One more note for transferred parts. If a part ran fine at another shop and the surface is now out of tolerance after coating, the drawings are usually not the problem. The previous shop pre-machined to a different allowance. Ask what the pre-coat dimensions were before you re-quote, and the transfer usually goes smoothly.
Frequently Asked Questions
What is the standard surface finish for CNC machined parts?
As-machined and deburred, which lands around Ra 3.2 µm on general faces and Ra 1.6 µm where a finishing pass runs. It is what most non-functional surfaces need. Tighter values are quoted as a requirement on specific faces.
Does anodizing change the size of my part?
Yes. Anodizing Type II adds roughly 5 to 15 µm per side and Type III hard anodize 25 to 50 µm. On a feature at final size that difference matters, so either mask it or pre-machine it undersize by the coating thickness.
Can you bead blast and still hold a sealing face tolerance?
Yes, if the sealing face is masked or finished after blasting. Bead blasting is a texture process that also touches edges, so leaving final-size sealing surfaces exposed to it is a rework risk rather than a cost saving.
Which finish should I specify for an aluminium part that needs electrical grounding?
Chemical conversion coating per MIL-DTL-5541 Type II, or a bare machined zone. Standard anodizing is an electrical insulator and will break the ground path, which is a common miss on enclosure and bracket drawings.
How much does surface finish add to lead time?
Mechanical finishes normally add nothing beyond the machining lead time because they run in-house. Outsourced chemical finishes add two days to a week, and coatings with anodize or cure cycles sit at the slower end. If finishing is on the critical path, say so and we sequence the routing around it. The service-side view, which finishes run in-house and which are subcontracted, is on the surface treatment page.
Can you advise on finish choice before I finalize the drawing?
Yes. Send the drawing with the functional requirement stated and we will come back with a finish recommendation, the dimensional allowance if one is needed, and the lead time it carries. It is part of quoting, not a separate service, and it is cheaper than finding out after the parts are made.
This article is general engineering guidance, not a binding manufacturing guarantee. Finish specifications, dimensional allowances, lead times and pricing are confirmed per part based on drawings, materials, quantities and process review. Request a formal quotation for part-specific terms. (c) 2026 KGL. All rights reserved.
Related Articles
More from the Design for Manufacturing series.
Have a project in mind?
Send drawings to kgl@kglmfc.com or request a quote. We respond within 24 hours.
Request a Quote