Views: 200 Author: Zhe Publish Time: 2026-11-14 Origin: www.hardware-sp.com
A surface finish callout can protect sealing, sliding, appearance, or a coating result. It can also add unnecessary machining time if it is applied across the whole part. This CNC surface finish chart explains common Ra values, the processes behind them, and how you can choose a requirement that matches the function of your custom part.
Ra is the arithmetic average roughness of a measured surface profile. It is commonly stated in micrometres (µm) or microinches (µin). A lower Ra number describes a smoother measured profile, but it does not automatically make the part better. The right finish depends on what that surface needs to do after machining.
Ra is just one piece of the spec. A sealing face might also need flatness. For bearing or sliding surfaces, performance can hinge on the material, hardness, lay direction, lube, and geometric tolerances. If these things count, get them on the drawing — don’t just lean on a roughness value.
Check out the handy chart below—your go-to starting point for all engineering talks! These are just indicative ranges, not a factory’s guaranteed capability. Real results shift depending on stuff like material, part geometry, rigidity, tooling, cutter path, machine setup, and even which spot you measure. Always lock in the required surface finish with your supplier before finalizing drawings!
Typical Ra | Approx. µin | Common Process | Where It May Fit | Buyer Note |
12.5 µm | 500 | Rough milling or rough turning | Hidden or non-functional areas | Tool marks are usually visible. |
6.3 µm | 250 | General milling or turning | Many non-mating machined faces | Useful when appearance is not a functional driver. |
3.2 µm | 125 | Controlled as-machined finish | General machined surfaces | A common reference point; validate for your material and geometry. |
1.6 µm | 63 | Fine milling, turning, or boring | Mating faces and moderate functional contact | May need a finishing strategy and more cycle time. |
0.8 µm | 32 | Fine finish pass, reaming, or controlled boring | Selected fits and smoother contact surfaces | Apply only where the function needs it. |
0.4 µm | 16 | Grinding, fine turning, honing, or polishing | Critical sliding or sealing interfaces | Often involves tighter process and inspection control. |
0.2 µm Or Finer | 8 Or Lower | Fine grinding, honing, lapping, or specialized polishing | Specialized functional surfaces | Review necessity and measurement method early. |
Ask what the surface contacts and how it works. A gasket seat, sliding diameter, cosmetic cover, grounding point, welded area, or hidden bracket face does not need the same finish. Define the surface by its job. That keeps the callout connected to a real engineering need instead of a copied note from an older drawing.
A common cost problem is applying one fine finish to all faces of a part. If only a sealing land or mating face needs a controlled Ra, identify that surface directly on the drawing. Other surfaces can remain as-machined when their texture does not affect fit, appearance, corrosion protection, or later processing.
Coating, anodizing, plating, welding, bonding, heat treatment, and assembly may change what the surface needs. For example, a bond area may need preparation rather than a polished appearance. A coated interface may need a final dimensional condition after finishing. State whether the requirement applies before or after the secondary process.
Milling and turning create directional tool marks. Tool geometry, feed, speed, step-over, rigidity, coolant, and the part material all influence the result. A finishing pass can improve the surface, but a fine result on a deep pocket, thin wall, or flexible feature may need a different setup or an additional operation.
Boring and reaming are often considered when a hole needs a more controlled surface and size relationship. Grinding can be appropriate for selected surfaces where both geometry and a fine finish matter. These are process choices, not default upgrades. Your supplier should compare them against the feature function, batch quantity, and inspection plan.
Bead blasting gives a nice even look, but it doesn’t take the place of a required Ra for sealing or sliding surfaces. Polishing can clean up the look or get rid of small blemishes, but if you don’t watch it, it can round edges and shift dimensions. Anodizing and plating bring extra things to sort out: which surfaces get masked off, and which measurements we take after the coating goes on.
When your part needs machining plus a secondary operation, Hardware-SP can review the manufacturing route through its custom CNC machining service.
A smoother requirement can mean slower feeds, a separate finish pass, extra tooling, more stable workholding, secondary processing, and additional inspection. The cost change depends on where the finish is required and how difficult the feature is to reach. A fine finish on one accessible diameter is different from the same finish inside several deep pockets.
The lowest Ra is not always the lowest project risk. Over-specifying surface finish can increase quote cost and make first-article approval harder. Under-specifying a sealing, bearing, or contact surface can create assembly issues later. The practical target is the least demanding finish that supports the part function and the rest of the process chain.
Here’s a casual version for talking with your foreign counterparts:
When you add notes on a drawing, make sure to list the parameter, value, unit and exactly which surface it applies to. If the requirement depends on the process stage — like before coating, after plating, or along the sealing contact face — include that too. For appearance critical surfaces, use an approved sample or reference standard whenever you can. Just giving an Ra number won’t cover things like color, shine, or what scratches are acceptable.
· State Ra in µm or µin and keep the unit consistent across the drawing.
· Point the symbol or note to the exact functional face, bore, diameter, or groove.
· Include related flatness, geometry, or coating conditions only when the function requires them.
· Clarify the inspection method or sampling expectation for a critical surface.
If finish acceptance must be tied to part quality records, review Hardware-SP’s quality control and inspection approach when preparing the RFQ.
It depends on the surface function. A non-mating face can often use an as-machined result, while a seal, sliding surface, or controlled fit may need a finer value. The drawing should specify the least demanding Ra that still supports the assembly and service condition.
Ra is one common roughness parameter used to describe surface texture. Surface finish can also include direction of lay, waviness, visual appearance, coating, and other conditions. For a functional surface, do not assume one Ra number describes every requirement.
No. A lower Ra is smoother, but smoothness only adds value when it supports the part function. It can add process and inspection cost, and in some bonding or coating situations a polished surface may not be the desired condition.
State the condition that matters to your part. If the final coated surface controls fit or function, identify that in the drawing and discuss it with the supplier. If the requirement applies to the machined substrate before coating, write that clearly as well.
Choose surface finish by function, not by habit. Send Hardware-SP the drawing, material, finish areas, and any post-process requirements for a DFM-based review before you issue the RFQ. Contact Hardware-SP to discuss your custom metal part.