The bore came in at 11:40 on a Thursday, dead nominal, and it had taken us nine days to get there.
There’s a rule in this trade nobody writes down. Cost has almost nothing to do with the metal. It has to do with the last thousandth.
Between ten thou and five thou, nothing happens. The machine holds it without anyone thinking about it. You’re not paying for that.
Below one thou the curve turns up hard, and it keeps turning.
At half a thou you’re managing the temperature of the part. Its temperature coming off the machine and its temperature on the CMM are different numbers, and that difference is a real fraction of your band.
So you slow the finish pass. You add a spring pass. You change tools on wear instead of on breakage. You let the part sit until it stops moving.
6061 housing, 12-off, for a customer that builds optical assemblies. One bore called at ±0.0005”.
We held it. All twelve, first pass, no rework. That’s the job and we’re good at it.
It cost nine days, a fixture we designed for this part and nothing else, and a CMM routine that took longer to run than the machining did. The customer paid for every hour, because it was on the print and we quote what’s on the print.
Three months later I was at their facility for something else. Their engineer had the assembly open on his monitor, and I asked him to scroll to where our housing went.
It took a dowel. Slip fit. Into a slot with twenty thou of clearance either side.
Twenty thousandths.
Nine days holding five ten-thousandths, on a feature with forty times that much room downstream.
I asked where the number came from. He pulled the drawing history. It came off a title block from a program that ended in 2019, into a template, into this part. Nobody chose it. Nobody had chosen it in seven years.
That’s the part I keep coming back to. It wasn’t a bad decision. It was the absence of one, inherited, and it spent real money in a building he’d never been to.
Every shop you send a drawing to will hold what you ask for. That’s what you’re paying us for, and we don’t get to decide you didn’t mean it.
He changed the block that afternoon. The next lot ran in four days.
For most of the history of this trade, a drawing was the only thing that travelled between the person who designed a part and the person who cut it. Everything the designer knew and didn’t write down stayed on their side. Everything we learned making it stayed on ours.
Which is a strange way to build things. Two people, one part, and a sheet of paper between them carrying numbers neither one has questioned in seven years.
The question was never whether we can hold your tolerance.
It’s whether anyone ever asked if you needed it.
