Casting Surface Finish Guide: As-Cast, Machined and Coated Areas

Every casting drawing asks the same quiet question, even when the buyer does not say it out loud: what will the surface look like, and does it matter? “Casting surface finish” sounds like one specification. It is really three. The as-cast areas, the machined areas, and the coated areas of the same part behave differently, cost differently, and fail differently. A buyer who understands which is which gets better parts for less money. A buyer who does not over-pays or under-specifies.

At Waking, we have cast and machined in the same factory in Qingdao since 2002 — investment casting for stainless, carbon and alloy steel; low pressure and gravity casting for aluminum alloy; then CNC machining, finishing and inspection all in-house. This guide is built from what we see on the shop floor.

As-cast surfaces are a process output, not a defect

Investment casting, also called lost wax casting, reproduces the texture of the ceramic shell that formed around the wax pattern. The result is a smooth, fine-grained surface — far smoother than sand casting — but it is not flat and it is not mirror-like. Run a fingernail across it and you feel a faint grain. That is normal, and it is not a flaw.

The as-cast surface also carries honest evidence of the process. Gate and riser locations leave small marks after cutoff. Gravity and low pressure aluminum castings may show a parting line where the die halves met. Deburring cleans the edges, but it does not turn a cast surface into a machined one.

Where is as-cast finish acceptable? Most places. Internal passages in a valve body. Non-sealing faces. Surfaces nobody will ever see. The irregular micro-texture even gives coatings and sealants something to grip.

The mistake buyers make is treating as-cast as a defect. It is not. It is the base condition of the part, and it is the cheapest condition. Cost starts when you ask for something better.

Machined surfaces reset the rules

Machining is how you take control. When a surface must seal against a gasket, mate with another housing, carry a bearing, or hold a dimension tighter than casting tolerance allows, you machine it. Machining removes the cast skin and generates a new surface you can actually measure.

That sounds obvious. The practical consequences are not. Machining exposes the material under the cast skin, which is where hidden problems live. If porosity or shrinkage sits just below that skin, the cutter finds it. This is why X-ray inspection matters for pressure-containing parts. At Waking we run an XTH320 X-ray system to screen for internal porosity before a casting ever reaches the CNC line. A part that looks perfect on the outside can hide defects that machining will expose later. Better to find them in inspection than in your customer’s pressure test.

Machined finish is also verified differently. Cast surfaces are judged by comparison. Machined surfaces are measured — surface roughness, roundness, profile. Waking runs more than 400 CNC machines in-house, and the inspection room carries COORD3 and ZEISS SPECTRUM CMMs, roundness and profile instruments, and hardness testers. First article inspection and sample approval happen before production, not after.

Here is the buyer’s side. Every machined surface costs money. Each one adds setup time, tool wear and inspection. Put machining symbols only where they are truly needed. A flange face, a bore, a sealing seat — yes. A large panel that nothing ever touches — no. Ask your supplier to itemize machined areas in the quote. That line item is where the real cost shows up.

Coated surfaces inherit everything underneath

A coating does not fix a surface. It copies it. If the substrate is burred, scratched or contaminated, the anodizing, plating or coating reproduces those problems, sometimes making them more visible. Surface prep is the coating.

That is why finishing steps are ordered the way they are. Deburr first. Clean second. Then treat. At Waking those steps happen in-house on parts we cast and machine ourselves. The treatments we run cover the common requests: polishing, anodizing for aluminum, plating, passivation for stainless steel, and coating where a protective layer is required.

Each treatment has its own demands. Passivation restores the oxide layer on stainless after machining has cut through it. It is not a visual finish; it is corrosion protection. Anodizing grows a hard oxide layer on aluminum, but the surface below decides how uniform that layer looks. Plating adds dimension. If the coating thickness is not planned for, a precision bore can come out of tolerance wearing a beautiful finish.

Two rules. Tell the foundry which areas get coated and which stay bare — masked areas exist for a reason. And ask whether coating thickness counts inside the dimensional tolerance. A good engineering review raises that question before you do.

Put all three on the drawing

Most surface finish disputes start with an under-documented drawing. The fix is simple discipline. Add a general note: “All surfaces as-cast unless otherwise specified.” Then mark only the exceptions with machining symbols and roughness values. Then locate coated areas with clear boundaries.

That one note changes everything. It tells the foundry that as-cast is acceptable by default, so they do not quote polishing or machining you do not want. It also tells them which surfaces are critical, so they plan inspection around those features.

For cast surfaces, a physical reference sample beats a number every time. A roughness value means little on a complex cast shape. An approved sample means everything. Get one during the sample approval stage and keep it as the production standard.

What a serious foundry should do about it

A real casting factory will not just accept your drawing and quote it. They will read it, ask which surfaces are functional, and tell you where you are over-specifying or under-specifying. That conversation is the engineering review. At Waking it works the same way for every customer: send the drawing and requirements, engineering reviews and quotes, then casting, machining and delivery — with heat treatment if specified, inspection at every critical step, traceability tied to material certificates and inspection reports, and quality systems certified to ISO 9001 and IATF 16949.

Casting surface finish is not a cosmetic detail. It is a cost driver, a quality signal and a functional requirement at once. Split it into its three parts, mark your drawing clearly, and you will get exactly the finish you pay for — and no more.

Frequently Asked Questions

What does an as-cast surface from investment casting actually look and feel like?

It reproduces the fine-grained texture of the ceramic shell, so it is smooth with a faint grain but not flat or mirror-like. Small marks from gate/riser cutoff or a parting line on aluminum castings are normal and not defects. As-cast is the base condition and is acceptable for internal passages and non-sealing surfaces unless the drawing says otherwise.

How do I decide which surfaces on my casting drawing need to be machined?

Machining is needed only where the surface must seal, mate with another part, carry a bearing, or hold dimensions tighter than casting tolerance allows. Functional faces like flange faces, bores, and sealing seats should be marked; large non-contact panels should stay as-cast. Ask your supplier to itemize machined areas in the quote so the cost is visible.

Why is X-ray inspection performed before CNC machining on cast parts?

Machining removes the cast skin and exposes material underneath, which can reveal hidden porosity or shrinkage that was not visible on the outside. X-ray inspection such as Waking's XTH320 system screens for internal defects before the part reaches the CNC line. This avoids discovering defects later in your customer's pressure test.

If I specify anodizing or plating on a machined casting, do I need to account for coating thickness in tolerances?

Yes. Coatings add dimension, and if the thickness is not planned for, a precision bore can go out of tolerance even with an attractive finish. You should mark which areas get coated and which stay bare, and confirm with the foundry whether coating thickness counts inside the dimensional tolerance.

Key Parameters at a Glance