Guides / Design
Taking weight out of a machined drone part without losing the part
September 7, 2026
Weight on a small unmanned aircraft is paid for twice: once in hover current and again in the battery you add to get the endurance back. So there is real pressure to hollow out every machined part. The limit is not the designer’s imagination, it is what survives the cutter and the clamping. The cost of each saved gram is worked through in material cost per gram saved on a drone part.
Wall thickness, realistically
A wall has two constraints. It has to be thick enough that the part is stiff, and thick enough that it does not deflect away from the tool while it is being cut.
- 0.6 mm is achievable in 6061 on a short wall with material around it.
- 0.8 mm is a safer floor in 7075.
- Once a wall is taller than roughly eight to ten times its thickness, chatter decides the outcome, not the material. Expect the shop to ask about it or to add a support boss.
A useful habit: pick the wall thickness you need for stiffness, then ask whether the wall needs to be that tall. A short thick rib usually beats a tall thin wall for both weight and cost.
Pockets, corners and cutters
Every internal corner in a pocket is a cutter radius. That is the single biggest lever you have over machining cost on a UAV plate.
| Internal radius | What the shop can use | Effect |
|---|---|---|
| 1 mm | 2 mm cutter, slow, fragile | Expensive, slow, more setups |
| 3 mm | 6 mm cutter | Reasonable |
| 5 mm and up | 10 mm cutter or larger | Fast, cheap, stiff tool |
Pocket depth matters for the same reason. A cutter can go roughly three to four times its diameter deep before it starts to ring. A 30 mm deep pocket with 2 mm corners needs a long fragile tool and a lot of passes. The same pocket with 5 mm corners is one tool and a fraction of the time.
Leave the floor thickness alone too. A 1 mm floor over a large pocket will drum and will not stay flat, whatever the drawing says.
Fastener bosses do the real work
Most of the load in a machined airframe part goes through the screws. A plate pocketed down to 2 mm with an unsupported M3 boss will pull through long before the plate itself fails.
- Keep full material thickness in a collar around each hole, roughly two times the screw diameter across.
- Blend the boss into the surrounding web with a radius rather than a step.
- In thin plates, use a threaded insert rather than a thread cut into 2 mm of aluminium.
Where the part moves
A part that has had seventy percent of its mass removed is not the billet it started as. Machining releases stress that was balanced in the stock, and a thin plate can lift off the surface plate a day after it came off the machine.
There is no drawing note that fixes this. It is a process answer:
- Rough both sides, leaving stock.
- Let the part rest.
- Finish with light passes and tabs holding it until the last one.
- Check flatness on a surface plate rather than assuming it.
If flatness is critical on your part, say so on the drawing, because it changes how the job is planned and it belongs in the quote.
What to skip
- Knife edges. A pocket wall that tapers to nothing chips on deburring and cuts the person assembling the aircraft.
- Lightening holes in a bending member. Holes in the middle of a web are fine. Holes near the surfaces of a beam remove the material that carries the bending.
- Organic optimiser output on a milled part. It bills like a sculpture. Print it or cast it, or simplify it into flat faces and radii.
- Chasing the last two grams. On a 40 g bracket, two grams is a rounding error next to one screw you did not need.
A sensible order of operations
- Work out the load paths and put material there.
- Pocket everything that is not a load path, with radii of 3 mm or more.
- Keep collars around fasteners at full thickness.
- Set the minimum wall at 0.6 mm in 6061 or 0.8 mm in 7075, and only if the wall is short.
- Send the model to the shop before you freeze the revision, and ask where it will move during machining.
That last step costs nothing. We answer it on every RFQ, within 24 business hours, and it is cheaper than finding out on the first article.
See also: part mass tolerance and weighing reports for setting and proving a mass limit.
Frequently asked questions
What is the thinnest wall you can machine in aluminium?
About 0.6 mm in 6061 and 0.8 mm in 7075 on a short wall with support around it. Tall thin walls are limited by chatter and deflection rather than by the cutter, so a wall ten times taller than it is thick is a different conversation from a short rib.
Does a bigger corner radius really save money?
Yes, and it usually saves weight too. A larger internal radius lets the shop use a larger cutter at a higher feed, so the pocket is cheaper, and it removes the stress concentration at the corner, so the web around it can be thinner.
Is topology optimisation worth it on a small drone part?
Rarely for a machined part. Optimiser output tends to be organic shapes that are cheap to print and expensive to mill. Simple pocketing with sensible ribs gets most of the weight at a fraction of the machining cost.
Need this part quoted?
Send the model and the quantity. Price and lead time come back within 24 business hours.