A medical pump builder in Massachusetts sent us a shaft drawing with a ±0.005 mm diameter callout. His previous supplier measured every piece with a hand micrometer on the shop floor and shipped a report that came back entirely green. His own CMM, in a 20 °C room, rejected 60 of 400 pieces.
That is the part nobody warns you about. Micron precision machining is not only about whether a shop can cut to five microns. It is about whether anyone in the chain can prove it.
Why The Measurement Decides The Part
Start with one number. Aluminum grows about 23.1 µm per meter per degree Celsius. On a 300 mm part, one degree of temperature difference moves the material 0.0069 mm. Your whole tolerance band is 0.01 mm.
Buyers chasing micron precision machining usually spend their time on spindle accuracy and axis counts. Those matter, but they are not where projects die. Projects die after the part is made, when two people measure the same piece and get two different answers.
The rule to carry around is simple. Measurement uncertainty should be no more than one tenth of the tolerance. For a ±0.005 mm callout, your measurement system has to be good to ±0.0005 mm. Almost nothing on a normal shop floor is.
The Twenty Degree Rule Nobody Writes On The Drawing
ISO 1 sets 20 °C as the standard reference temperature for dimensional measurement. Every micron precision machining drawing in the world inherits that assumption. Almost none of them state it.
So walk the arithmetic. A 300 mm aluminum part measured at 26 °C reads 0.042 mm longer than it will at 20 °C. That is four times the whole tolerance band, and it is not a machining error. The part is fine. The room is not.
Steel behaves better at roughly 11.5 µm per meter per degree: the same 300 mm part shifts 0.021 mm over the same 6 °C, still twice the band. Titanium sits near 8.6, brass near 19.
So a supplier who runs micron precision machining properly will not let a part be gauged straight off the machine. It has to soak, in a controlled room, before anybody writes a number down.
What Each Gauge Can Actually Prove
Ask what will be used to inspect the part, and listen closely. This is where most micron precision machining conversations turn vague.
A vernier caliper is good to about ±0.02 to ±0.05 mm. It cannot see your callout at all.
A hand micrometer reads to 0.001 mm, which is not the same as being accurate to 0.001 mm. Held in a warm hand against a warm part, real uncertainty drifts to a few microns. Inside micrometers, bore gauges and air gauges do better on diameters, and they do it fast, which matters at volume.
A bridge CMM is specified in the low single microns plus a length-dependent term, and it needs its own temperature-controlled room to deliver that.
Optical systems handle edges, small features and soft parts that a touch probe would deflect. Surface finish needs its own instrument entirely, because a CMM cannot tell you the Ra.
A gauge proves what it was built to prove, in the conditions it was calibrated for. Anybody quoting micron precision machining should be able to name the instrument and the room without being asked twice.
What It Costs To Hold A Micron
Nothing here is free, and the price jump is not linear. You can see it as a step change in custom cnc machining quotes rather than a slope.
A 20 ±1 °C inspection room. Granite plates and a CMM with a calibration certificate. Roughing, a stress relief or ageing cycle, then finishing, with the part resting in between. Lighter depths of cut so the tool and the workpiece stay cool. SPC charts with a Cpk target instead of a pass stamp. First article reports that carry the measurement uncertainty, not just a green tick.
Go from ±0.05 mm to ±0.005 mm and you have not made the job ten times harder. You have changed which question you are answering. Below about ±0.01 mm, micron precision machining stops being a cutting problem and becomes a metrology problem with a machine tool attached.
What To Put In The RFQ So The Argument Never Happens
State the reference temperature. Write 20 °C on the drawing next to the tolerance. It costs nothing and it settles arguments later.
Name the inspection method for each critical dimension. CMM for position, air gauge for bore, surface tester for Ra. Left unnamed, the supplier picks the cheapest one.
Ask for gauge R&R, or at least the measurement uncertainty on the report. Under 10% GRR is good, 10 to 30% is marginal, and above that you are measuring noise.
Agree the arbitration instrument in advance. If there is a dispute, whose CMM decides? Settle it before the purchase order.
Send the functional requirement too. A bore that carries a bearing and a bore that only passes oil have different critical dimensions, and a supplier doing micron precision machining will plan them differently once you say which is which.
Quick Answers On Micron Precision Machining
What tolerance can actually be held in production?
On a 300 mm aluminum part, ±0.01 mm is routine with a controlled process. ±0.005 mm is achievable on selected features with a temperature-controlled room and a real inspection plan. Below that, talk to us first.
Do parts need to sit before inspection?
Yes. A 300 mm aluminum part needs hours to reach 20 °C. Gauging a warm part cold is the most common cause of a false rejection we see in micron precision machining.
What do you use for critical features?
A CMM in a temperature-controlled room for geometry, air gauging for high-volume bores, and a separate instrument for surface finish. Reports carry the uncertainty.
How do you handle a dispute over a rejected lot?
We re-measure both sides on the same instrument, in the same room, at the same temperature. It usually ends there.
Does cheap micron precision machining ever pay off?
Only when the tolerance is not really micron level. If a ±0.005 mm callout is priced like a ±0.05 mm job, the measurement step was left out of the quote.
Send the drawing, the function of each critical feature, and the reference temperature for judging the parts. If your micron precision machining quote never says how the part will be measured, that is the gap worth asking about first. Reach us through our inquiry page, or start from the precision cnc machining service we run in house on CNC machined parts.