Guides / Quality
Surface finish call outs that change the part, and ones that just cost money
July 30, 2026
Surface finish is the easiest thing to put on a drawing and one of the easiest ways to add cost without adding function. Anodise changes the surface you specify here; see our guide to drone parts finishing.
The value in the call out is real in three or four places on a typical UAV part and wasted everywhere else.
What Ra actually measures
Ra is the average roughness, the mean deviation of the surface profile from its centre line, in micrometres. It is a single number describing a surface that is not a single thing.
That matters in practice because two surfaces with the same Ra can behave very differently. A surface with occasional deep scratches can measure the same as one with even, shallow tool marks. For sealing surfaces this is why the direction of the machining marks matters as much as the number: marks running across a seal leak, marks running along it do not.
Typical values and what they cost
| Ra | How it is produced | Relative cost |
|---|---|---|
| 3.2 | As machined, roughing pass left on | Cheapest |
| 1.6 | Normal finishing pass | Standard, no premium |
| 0.8 | Light finishing pass or finishing tool | Small premium |
| 0.4 | Grinding, honing or a careful finishing strategy | Real premium, separate operation |
| 0.2 and below | Lapping or polishing | Specialist, avoid unless required |
The important step is between 0.8 and 0.4. Above that line, you are asking for a better cut. Below it, you are asking for another process, another setup and another handling step.
Where a finish call out earns its place
Sealing faces and o-ring grooves. Ra 0.8 to 1.6 on the groove walls and the mating face, with marks that do not run across the seal path. This is the classic case where the call out prevents a leak. See sealed UAV enclosures.
Bearing seats. Ra 1.6 or better so the outer race seats evenly rather than on the peaks of the tool marks.
Thermal interfaces. Ra 1.6 or better under a component, so the interface material is thin and even. Polishing does not help, as covered in heat sinks and cold plates.
Bonded joints. Adhesives generally want a slightly rough, clean, chemically prepared surface, not a polished one. Specify the preparation, not just the roughness.
Fatigue loaded parts. On a landing gear lug that sees thousands of cycles, a smoother surface in the highly stressed zone genuinely helps, because machining marks are crack starters.
Where it does not
- Outside profiles of plates and brackets.
- Pocket floors that do not mate with anything.
- Lightening holes.
- Anything that will be anodised for appearance, where the coating and the dye decide what it looks like.
Putting Ra 0.4 in the title block as a general finish is the most expensive line you can write on a drawing that did not need it. Call out the two or three surfaces that matter and leave the rest at the standard finish.
Cosmetic finish is a different conversation
Cosmetic requirements are not roughness requirements. A customer facing part may need:
- No visible tool witness lines on the show face.
- Bead blasting before anodise for an even matt appearance.
- A consistent grain direction.
- Scratch and handling limits.
None of those are captured by an Ra number, and all of them cost money. If a part has a show face, say so in words and, where possible, agree a boundary sample. Bead blast plus type II anodise is the normal route for an even matt finish, and it slightly softens the crispness of small features.
Deburring is the finish nobody specifies
Most parts do not need a better Ra, they need consistent edges. On a UAV part, edges matter for three reasons: they cut the people assembling the aircraft, they concentrate stress and they hold anodise badly if they are sharp.
Say what you want. A general note such as break all sharp edges 0.2 to 0.4 mm unless otherwise stated does more good than a tighter Ra on the faces, and it costs almost nothing.
Putting it on the drawing
- A general finish in the title block, usually Ra 3.2 or 1.6.
- Specific surfaces called out individually, with the Ra and any requirement on the direction of the marks.
- A general edge break note.
- Cosmetic requirements in words, with the show faces identified.
- Whether the values apply before or after coating.
Send the model with those notes through the RFQ page. Where we think a finish call out is doing nothing except adding cost, we say so in the quote, which comes back within 24 business hours.
Frequently asked questions
What Ra does a normal milled aluminium part come out at?
Around Ra 1.6 micrometres as machined with a sensible finishing pass, and Ra 0.8 is achievable with a lighter pass or a finishing tool. Anything below that on a milled surface starts needing extra operations.
Does a smoother surface make a part stronger?
Sometimes, on a fatigue loaded part, because machining marks are crack starters. On the average bracket it makes no difference and adds cost. Spend the money on radii in loaded corners instead.
Does anodising change the surface finish?
Slightly. The coating follows the underlying surface and adds a little texture, and hard anodise adds more than type II. A part that must be smooth after coating has to start smoother, or be finished after coating.
Need this part quoted?
Send the model and the quantity. Price and lead time come back within 24 business hours.