Drone Parts from Prototype to Series, with the Right Process for Each Part

Hydroforce machines the first drone parts from solid metal and then moves each part to the process that suits its series: high-pressure die casting for finned housings, powder metallurgy and MIM for gears and latches, CNC machining for structural fittings. One workshop, one quality plan and one set of documents from the first prototype to the series batch.

A drone programme goes through two very different phases. While the airframe is being developed, parts change after every test flight, and nobody wants to pay for a die that will be scrapped next month. Once the design is frozen and the first series is ordered, the question turns around: which parts are still worth cutting from a solid block, and which should be cast or pressed so that the thousandth piece costs less and matches the first one.

We run machining, die casting and powder metallurgy in the same workshop, so a builder of multirotor, agricultural, fixed-wing, ground or marine drones can start with us at the prototype and stay for the series. This article explains how that move is planned part by part.

Stage 1: Prototypes Machined from Solid

A machined prototype needs no tooling. When the design changes, the CAD model changes and the next part comes off the machine with the new geometry. For early airframes that is the deciding argument.

• Materials: 6xxx and 7xxx aluminium alloys for arms with an integrated motor seat, centre plates, motor mounts, arm-folding joints and brackets; titanium and stainless steels 1.4301/304 and 1.4404/316 for shafts, marine housings and parts in salt water.
• Accuracy: ±0.05 mm and IT6-IT7 fits on bores and mounting faces, with geometric control to ISO 2768-fH, ISO 286 and ISO 1101.
• Machines: Haas CNC milling centres with a 1270 x 660 x 635 mm work envelope and DMG MORI turning machines up to Ø500 mm and 1000 mm length.
• Inspection: the first piece is measured on the Zeiss CMM, and the program proven on it is reused for the next parts.
• Finishing: anodizing or powder coating to the specification, so a prototype can be flown and tested in the same finish the series will have.

One point is worth settling at this stage. If a housing will later be die cast, its machined prototype should already carry the draft angles, wall thicknesses and fillet radii of the casting. Test results from a prototype with square pockets and thick walls do not carry over to a thin-walled cast housing. We review the drawing for the series process while the prototype is being made, so the flight tests are done on the geometry that will go into production.

Gear stages for servo actuators, sprayer pumps and winches are cut and ground to ISO 1328 for the prototype. A loaded stage usually needs grade 7. The same gear can later be pressed and sintered, but only once its module, width and profile are final.

Stage 2: A Series Process for Each Part

When the design is frozen, each part is looked at on its own. The table shows the usual routes on a drone or a ground robot. The last column matters as much as the others: a cast or sintered part still has surfaces that are finished on a machine.

Structural fittings often stay machined in series. A motor mount carries thrust and vibration, it is small, and 7xxx aluminium machined and anodized is the right material for it. In series we machine these parts in batches of identical units on one setup and one CMM program, which brings the piece price down without changing the part.

Ground robots with a boom or a blade are the exception to the whole scheme. Their hydraulic cylinders are made to order from a single unit, so the prototype cylinder and the series cylinder go through the same route, with the same pressure test at 1.5× working pressure.

Die Casting for Housings: What Changes in Series

A finned housing is the part that gains most from the move to die casting. Machining cooling fins out of a solid plate takes many minutes per housing and turns most of the plate into chips. On our press a production cycle runs 30 seconds to 2 minutes, depending on part size, and the fins, mounting bosses and cable ports are formed with the body in the same shot.

• Walls and weight: thin-wall sections of 0.5 to 1.5 mm, with ribs placed where the housing needs stiffness.
• Accuracy: ±0.1 to 0.3 mm as cast, surface Ra 1.6 to 6.3 μm.
• Heat: aluminium alloys conduct heat at 90 to 160 W/m·K, so the housing itself carries heat away from the power electronics.
• Tooling: dies are machined in our own tool room from H13 or P20 steel and trialed on a production press before the first article is signed off.
• Sealed or heat-treated housings: vacuum-assisted die casting where a housing must hold a seal at a gasket face or be solution treated to T6. Porosity-sensitive zones are checked by X-ray.
• Finishing: CNC pass on mating faces, bores, threads and seal grooves, then powder coating, e-coating, chromating or anodizing to the specification.

For small parts that need very thin sections and fine detail, zinc alloys are cast on hot-chamber machines. Magnesium can be discussed for a specific part.

Sintered and MIM Parts: Gears, Latches and Small Levers

Gears for sprayer pumps, winches and servo drives are the second group that changes process in series. A pressed and sintered gear gets its tooth profile from the die, so every gear from one tool has the same geometry.

• Powder metallurgy (PM): compaction up to 800 MPa, final density up to 97%, tolerance ±0.05 mm as sintered and tighter after sizing. Powder goes into the part, so up to 30% less material is used than when the same gear is cut from bar. The residual porosity damps vibration and makes the gear run quieter.
• Metal injection molding (MIM): for small parts under about fifty grams with cross-holes, pockets and undercuts, such as folding hinges, lock hooks, detents, micro gears and gimbal brackets. Steels include 17-4 PH and 316L, walls go down to 0.3 mm and tolerance to ±0.02 mm. A shot takes 15 to 45 seconds.

Sintering pays off from thousands of parts upward, because it needs a compaction or MIM tool. Below that quantity the gear stays cut. Many drives combine both routes: sintered gears for the fast, lightly loaded stages and a machined gear for the loaded output stage, all from one supplier and under one quality system.

Heat treatment follows the drawing: through-hardening and tempering, case carburizing or nitriding, and solution plus ageing for 17-4 PH. Bores and bearing seats that need a closer fit are sized or machined after sintering.

How the Change of Process Is Managed

Moving a part from the milling machine to a die or a press tool is the point where a series can go wrong. We plan it in the same way for every tooled part.

1. Drawing review before tooling. Draft angles, parting line, gate and overflow positions, and the stock left for machining are agreed before the die or mould is made. The bearing bores and sealing faces are marked as zones where porosity is not accepted. For a MIM part the review also covers even wall thickness, datums for the pivot and mounting holes, large flat areas that may warp during sintering, and a gate position away from functional surfaces; the mould cavity is made about 1.18× the final part size to allow for shrinkage.
2. Tool trial and first article. The first shots or first pressings are measured completely, every dimension on the drawing, and compared with the CAD model and with the approved prototype.
3. Control plan. Critical characteristics and check frequencies are written before the series is released. On request we share the plan and agree witnessed tests or third-party inspection.
4. Same baseline for repeat orders. Tooling drawings, process parameters and sintering programmes are kept, so a repeat order is made against the same baseline as the first article.

The documents do not change with the quantity. A prototype and a series batch leave with the same material certificate, dimensional report and coating record.

What to Send Us

To quote both stages at once, we need:

• the 3D model or drawing of each part, with the material and finish;
• the prototype quantity and the expected yearly series quantity;
• the surfaces that carry load, seal or locate other parts;
• the environment the drone works in: dust, spray chemicals, salt water or temperature range.

We answer with the machined prototype price, a recommended series route for each part, and the tooling and piece price for that route. Send your files to office@hydroforce.ee or use the contact page.

Related reading: aluminium die-cast housings for LED lighting, sintered gears by powder metallurgy and MIM, quality planning by technology. Technology pages: CNC machining, die casting, powder metallurgy and MIM, hydraulic cylinders. Industry page: Drones and Unmanned Systems.