Guides / Materials

Anodising drone parts, and the dimensions it quietly changes

September 13, 2026

Anodise is not paint. It is an oxide layer grown out of the aluminium itself, which is why it does not chip off and why it changes the size of your part. This is one decision in our guide to drone parts finishing, which also covers chemical film, powder coat and marking.

On an unmanned aircraft the choice is usually between type II and type III, and most of the trouble comes from picking one without thinking about the dimensions it touches.

The two you will actually specify

	Type II, sulphuric	Type III, hard anodise
Typical thickness	5 to 25 microns	25 to 75 microns
Hardness	moderate	much harder, wear resistant
Colour	dyes well, wide range	dark grey to black, dyes poorly
Dimensional growth	small, usually ignorable	must be planned for
Corrosion resistance	good	better
Cost	lower	higher
Best on	plates, housings, covers, cosmetic parts	clamps, wear faces, agricultural and coastal aircraft

There is a third option worth knowing. Chemical conversion coating, sometimes called chromate or by a trade name, is a very thin coating that stays conductive. It is the answer for a part that needs corrosion protection and an electrical path, at the cost of almost no wear resistance. For more on a thin film that stays conductive, see chemical film on drone parts. For powder coat as the other option, see powder coat vs anodise for drone parts.

Where the dimensions go wrong

Anodise grows in both directions from the original surface. As a working rule, half the coating thickness grows outward and half converts material that was already there.

• A bore gets smaller by about the coating thickness on diameter.
• A shaft or boss gets larger by about the same.
• A flat face rises by about half the thickness.
• A thread loses clearance on both flanks, which is why a fine thread in a hard anodised hole can stop accepting its screw.

On a type II part at 10 microns you can usually ignore this. On a type III part at 50 microns you cannot. A 25 H7 bore in a boom clamp that goes out hard anodised will be roughly 0.05 mm tighter than the drawing, which is most of the tolerance band gone.

Three ways to handle it, all decided before the part is cut:

1. Machine undersize or oversize by the growth and let the coating bring it to size. Say so on the drawing, with the post coating dimension as the one that matters.
2. Mask the feature so it stays bare. Good for bores, bearing seats and ground paths, and it costs a little more.
3. Finish after anodising. Ream or hone the bore once the coating is on. Removes the coating exactly where it was masked, which is fine as long as the exposed ring is not the corrosion risk.

Threads, ground paths and bonded faces

Threads. Anodise both flanks and a class 6H hole can bind. Common practice is to mask tapped holes, or to tap after anodising, or to use a helical insert sized for it. Say which one you want.

Ground paths. The oxide is an insulator. If a plate is part of the electrical return for a motor, an ESC or an antenna, the contact area has to be masked or conversion coated. This is a very common field failure on a new airframe and it is invisible on the drawing unless someone writes it down.

Bonded faces. Adhesives generally like anodise, particularly a thin type II, because the porous surface gives the glue something to hold. Hard anodise sealed to the maximum is smoother and can bond worse. If a face is structural bonding, call it out and leave the sealing decision to someone who knows the adhesive.

Colour, and the thing nobody warns you about

Type II takes dye well, so black, red, blue and gold are all routine. Two points matter for a fleet:

• Batch to batch colour varies. Two lots of black parts made six months apart will not match exactly. If cosmetic match across an assembly matters, say the parts must be anodised in one batch, and expect to pay for it.
• Alloy changes the shade. 7075 comes out with a different cast to 6061 in the same tank. A frame built from both alloys will never look uniform in a light colour. Dark grey and black hide it best.

Hard anodise on 7075 tends to come out darker and more olive than on 6061. Nobody can fix that, it is the copper in the alloy.

What to put on the drawing

A finish call out that a shop can price without guessing looks like this:

• Process and type: hard anodise, type III.
• Thickness: 40 to 50 microns.
• Colour: black, or natural.
• Masked features listed by number: bore A, tapped holes B, ground pad C.
• Whether dimensions on the drawing are before or after coating.
• Sealing: sealed, or unsealed for bonding.

Six lines. They remove almost every anodising question that comes back on a quote.

One more practical note

Anodising is a placed process almost everywhere, including here. The parts leave the shop, go to the anodiser, come back and get inspected before they ship. That adds several days to a lead time, and it is the reason a first article schedule should include it rather than quoting the bare machining time. The capabilities page says which processes are in house and which are placed.

For the corrosion side of the same decision, see aluminium and carbon fibre joints. For the alloy choice underneath, see 6061 or 7075.

Frequently asked questions

Keep reading

• Copper busbars and brass contacts on a drone: sizing, plating and joints
• 2024 aluminium for UAV fittings: fatigue strength against corrosion risk