Spec

A true story about human ingenuity.

We were manufacturing high reliability batteries for earth orbital satellites. Maximum quality, state-of-the-art design, very tight tolerances. We spent more time testing than manufacturing.

A full battery was typically around $2 million, depending on the particulars. It had to last 15 years in geosynchronous earth orbit. Satellites don't work with a dead battery. At least not when they are shaded from the sun by the earth.

The spacecraft manufacturer did further integration testing at the spacecraft level. All of that testing obviously took life out of the battery but that was included in the design.

The energy storage electrical capacity of the battery was highly dependent on the operating temperature, both charging and discharging. Capacity is how much energy the battery can store and deliver, usually measured in Ampere-hours or Watt-hours.

The battery would be charged to a certain capacity but the discharged capacity was always less due to electrical and electrochemical inefficiency.

Because of the temperature dependence, the temp during testing was tightly controlled within a few degrees. This required active temp control, especially when the battery was producing heat during discharge.

There was a minimum energy capacity that the battery was required to deliver in order to pass the test. Failure meant scrapping a very expensive battery and building a replacement. That was not only devastating from a cost point of view but it also negatively impacted schedule.

The highly skilled and experienced technicians who performed the testing came up with a perfectly legal “cheat”. When charging, they reduced the battery temp to the coldest end of the specification tolerance. This allowed the battery to accept more charge capacity.

Then during discharging, they warmed the battery up to the hottest end of the specification tolerance. The battery then produced more discharge capacity than it would at a colder temp.

All legal. All within specification tolerances. (Exceeding spec tolerances was considered a “failure” so this was serious business).

This wasn't a trivial matter. The battery could produce as much as 15% more energy by charging “cold” and discharging “hot”.

The spacecraft manufacturer had a quality representative who was stationed at our facility. His job was to monitor manufacturing, testing, etc.

He figured out what the techs were doing, playing with the temperature during testing to get maximum battery performance. He went and told his engineering staff. They went and told their management that the battery supplier was “cheating” on their testing.

It's hard to believe, but some astute manager was somehow able to figure out that this “cheating” was actually brilliant. Improving battery performance by 15% and it didn't cost them a nickel.

He not only took credit for it but he patented it and they started operating their spacecraft that way.

Charge “cold”. Discharge “hot”. It was so fucking obvious that it was genius. How do you get a patent on something like that? Be a multi billion dollar aerospace giant with good lawyers.

That manager got a bonus and a promotion.

Bonus lesson on specification tolerances courtesy of Grok:

Specification tolerances (often just called tolerances in engineering and manufacturing contexts) refer to the permissible limits of variation allowed in a dimension, property, or characteristic of a part, component, material, or system while still meeting the required specification (design intent, function, fit, performance, or quality).

In simple terms:

Nothing can be manufactured perfectly to an exact nominal (target) value every time — due to machine limitations, material behavior, tool wear, temperature effects, operator variation, etc. Tolerances define the acceptable range around that ideal value.

Core Definitions

Nominal size/value — The ideal or target dimension (e.g., 50 mm).

Upper limit — The maximum acceptable value (e.g., 50.1 mm).

Lower limit — The minimum acceptable value (e.g., 49.9 mm).

Tolerance — The total allowable variation = upper limit − lower limit (e.g., 0.2 mm).

Common ways tolerances appear on drawings/specifications:

Type Notation Example Meaning Typical Use Case

Bilateral (±) 25.00 ± 0.05 Can vary +0.05 or -0.05 from nominal Most common for general dimensions

Unilateral 25.00 +0.10 / -0.00 Only allowed to go larger (or only smaller) Press fits, shafts/holes

Limit dimensions 24.95 – 25.05 Direct min/max values (no nominal shown) Precision machining

General tolerance ISO 2768-m (medium) Applies to untoleranced dimensions (e.g., ±0.2 mm for 30–120 mm range) Sheet metal, less critical features

Why Tolerances Matter

Tolerances balance three key factors:

Function — Parts must fit, move, seal, carry load, etc. (too loose → failure; too tight → unnecessary cost).

Manufacturability — Tighter tolerances = more precise (and expensive) processes (e.g., grinding vs. saw cutting).

Cost — Every 0.01 mm tighter can dramatically increase price, lead time, and scrap rate.

Examples of Specification Tolerances

Simple linear dimension

A shaft specified as Ø20 ±0.02 mm

→ Acceptable range: 19.98–20.02 mm

→ Tolerance zone = 0.04 mm total

Hole & shaft fit (clearance fit)

Hole: 20.05 +0.04 / -0.00 mm

Shaft: 20.00 +0.00 / -0.03 mm

→ Maximum clearance = 0.09 mm, minimum clearance = 0.02 mm

Non-dimensional (other properties)

Material hardness: 58–62 HRC

Paint thickness: 80–120 µm

Temperature rating: ±5°C

GD&T (Geometric Dimensioning and Tolerancing) — advanced form

Instead of just size, controls shape/form/position:

⊥ (perpendicularity) 0.05 | A

→ The surface must be perpendicular to datum A within a 0.05 mm tolerance zone.

In summary, specification tolerances are the "rules of acceptable imperfection" written into technical drawings, product specs, or quality standards. They ensure parts can be realistically produced while still reliably performing their intended job when assembled or used. The art of good engineering is assigning just tight enough tolerances — no tighter, no looser.