"Custom only pays off above 10,000 units."
The sentence sounds like experience, and it comes up early in every project — usually before anyone has written down a number. That is what is wrong with it: it replaces a calculation with a gut feeling. And the calculation works out differently today than it did ten years ago.
The calculation has four inputs
Whether a drive development carries itself depends on four numbers:
The one-off development effort. Dimensioning, prototypes, validation, tooling where required. That is the amount to be earned back.
The unit-cost difference. What the custom drive costs per unit compared with the catalogue part. This figure is surprisingly often negative — the custom drive is cheaper, because nothing is oversized and no function is paid for that the product does not need.
The annual volume.
The remaining life of the product.
From these comes a simple division: development effort divided by the annual saving gives the payback time.
And here is the first flaw in the rule of thumb. It states a volume as the threshold, although the result is a time. At 2,000 units a year and five years of remaining life, the product lifetime adds up to 10,000 units — the same number as the rule of thumb, but now with a reason behind it. For a product that will be built for another ten years, half the annual volume is enough.
The term that never appears
Up to this point the calculation is standard. What it almost always leaves out is the counter-calculation: what the catalogue compromise costs elsewhere.
A drive that is eight millimetres too long makes the housing grow — more material, a larger tool, more shipping volume. A drive that is thirty percent oversized pulls a larger power supply along with it. A drive that is three decibels too loud puts damping material on the bill of materials. Waste heat nobody planned for needs cooling. And a standard drive running permanently at its limit fails earlier than the service plan assumes.
None of these items shows up in the price of the drive. They sit in other bills of materials, in other cost centres, often in other departments. They play no part in the drive decision — even though the same decision created them.
Add them in, and the break-even moves down noticeably.
Why the rule of thumb has aged
The ten thousand comes from a time when custom meant everything from scratch. Your own design, your own tooling, a manufacturer who first had to be found and qualified.
Today a considerable part of the development sits in proven building blocks — motor topologies, gear stages, electronics platforms, test concepts — that are adapted rather than reinvented. What remains is the application-specific dimensioning and the validation. That lowers the one-off effort substantially, and the threshold with it.
So the rule of thumb is not calculated wrongly. It is calculated with numbers that no longer apply.
When it still does not fit
Three situations in which the catalogue remains the right answer:
The remaining life is short. If the product runs for another one or two years, hardly any development pays back.
The volume is uncertain. A calculation with an uncertain annual quantity is not a calculation. Better to start with the catalogue part and set up the development once the series is established.
Timing decides. A drive on the shelf is available immediately; a development takes months. When market entry outweighs everything, that is a legitimate argument — as long as the decision is recorded as a deliberate compromise instead of quietly becoming a constraint inside the product.
And the simplest case stands: if a catalogue drive genuinely fits, use it. That is the fastest and cheapest route, and nobody should talk you out of it.
Run the numbers
Four figures, ten minutes. Once you have them, you no longer need a rule of thumb.
If what you are missing is the development effort for your specific application, that is exactly what the feasibility check is for. You get a defensible figure instead of an estimate — and with it the one input you cannot work out on your own.