The starting point
The customer builds electronic smart locks sold across Western Europe. The drive cam inside the mechanism transfers motor torque to the bolt carriage, so it needs hardness, dimensional stability and a clean pivot bore. It was being turned and milled from 17-4PH bar on a European subcontractor's machines.
Three things made that painful. First, material: the finished cam weighs 11.4 g and the bar blank weighed 35 g, so 68% of an expensive precipitation-hardening stainless was going into the swarf bin. Second, the part needed a milled profile and a cross-drilled hole after turning, which meant a second setup. Third, at 600,000 pcs/year the machining capacity was becoming a constraint on the customer's whole assembly line.
Why the part suited MIM
Our DFM review flagged it as a strong candidate almost immediately. It sat in the sweet spot for metal injection molding: under 50 g, geometrically busy enough that machining wasted time, high enough volume to amortise tooling quickly, and — critically — only two dimensions on the drawing were tighter than ±0.05 mm.
Those two dimensions were the pivot bore and the face that indexes against the motor housing. Everything else could run as-sintered at ±0.3%, which is where MIM is comfortable. That let us plan a process where the moulded part is essentially finished apart from one bore operation.
Design changes we asked for
A drawing written for turning is rarely optimal for moulding, and this one was no exception. We proposed four changes and the customer's engineering team accepted all of them after review:
- Wall thickness equalisation. The original had a 5.8 mm hub next to a 1.6 mm web. Uneven sections shrink unevenly during sintering, so we cored out the hub to bring it to 2.8 mm. This also removed 1.9 g of material.
- Radii on internal corners. Sharp internal corners are stress risers and they make ejection harder. We added 0.4 mm radii throughout.
- Gate location agreed up front. Placing the gate on a non-functional boss meant the witness mark never touched a mating surface.
- Datum scheme rewritten. The turned version referenced the outside diameter. For MIM the pivot bore is the natural primary datum, which made the tolerance stack tighter, not looser.
Tooling and validation
We cut a 4-cavity production tool in 31 working days. The T0 trial produced parts within 0.04 mm of nominal across the board; one cavity showed a slight shrink deviation on the indexing face, which we corrected by adjusting the sinter support fixture rather than re-cutting steel.
First-article samples went out nine working days after T0, with a full CMM report, material certificate and hardness verification in the H900 condition. The customer ran them through their own life test rig — 100,000 lock cycles — before releasing the tool for production.
Results after two years of production
The program has now run continuously for over two years at roughly 50,000 pcs/month. The measured outcomes:
- Unit cost down 43%. The saving came roughly half from material utilisation and half from eliminating the second machining setup.
- Two operations removed. Milling and cross-drilling are now moulded features. Only the pivot bore is reamed after sintering.
- Tooling paid back in four months at the customer's actual monthly volume.
- Hardness spread narrowed. The bar stock supply had shown 31–38 HRC across lots; our controlled H900 cycle holds 33–36 HRC.
- Lead time to 25 days from order to shipment, against 40 days previously.
What we would tell a similar buyer
The decisive factor was not that MIM is cheaper in the abstract — it is not, for many parts. It was that this specific part had a bad match between its geometry and its manufacturing process. Once you are throwing away two thirds of an expensive alloy and paying for a second setup, the arithmetic tends to favour a net-shape process.
The honest counterpoint: if this part had run at 40,000 pcs/year instead of 600,000, the tooling payback would have stretched past two years and we would have advised against it. Volume is what makes the conversion work, and we say so before quoting rather than after.
Talk to the engineer who ran this program
If your part sits in the same territory — similar size, similar volume, similar frustration with the current process — the fastest route is a drawing and your current unit cost. We will come back with a process recommendation, a tooling estimate and a break-even volume, usually inside 48 hours.