±0.005 mm CNC Machining: What to Demand from Your Supplier and How to Verify It
When you ask for ±0.005 mm, every shop says yes. The useful question isn't whether they can, but which process will hold the dimension and with what measurement uncertainty.
You have a drawing with a dimension at ±0.005 mm. You call five shops and all five say yes. That's the problem: at 5 microns, "yes" means nothing until the supplier names the process that will hold the dimension, the instrument that will measure it, and the uncertainty of that instrument. Almost everyone who says yes is thinking about the spec plate on their machining center, not about your part coming off the cycle in tolerance.
This article is for the engineer who has already decided they need high precision and is now qualifying who to buy the part from: what to ask, which answer disqualifies a supplier, and how to verify the dimension when the parts arrive.
In Summary
- ±0.005 mm is not a milling tolerance. It's the range of grinding, wire EDM, and honing. CNC leaves the part with controlled excess stock; the secondary operation closes the dimension.
- What you should demand isn't a capability, it's an executor: which process, on which machine, and who does it. "Our 5-axis center will hold it" disqualifies.
- Verification is decided by uncertainty, not by equipment. With a 0.010 mm band, the measurement system should contribute on the order of 1 micron and measure at 20 °C. Having a CMM is not having evidence.
- The error budget is spent before you start: 2 °C of shop drift consumes nearly half the band on a 100 mm aluminum part.
- Radii commits to ±0.007 mm on its high-precision CNC machining route. Below that, the requirement falls to grinding, wire EDM, or honing — processes the audited network already has, so the secondary operation has an assigned shop, not a promise to "find someone."
1. What 5 Microns Actually Are
±0.005 mm means a total band of 0.010 mm. A human hair measures 60 to 80 microns: you're asking the part to live inside a fraction of that through machining, cooling, measurement, and shipping.
Think of it as a budget: the 0.010 mm band is everything you have, and every error source takes its cut.
| Error source | Typical magnitude on a ~100 mm dimension | Comment |
|---|---|---|
| Part thermal drift — aluminum, 2 °C | ~4.7 µm | Coefficient ~23.6 µm/m·°C. Nearly half the band |
| Part thermal drift — steel, 2 °C | ~2.3 µm | Coefficient ~11.7 µm/m·°C |
| Machine positioning repeatability | 2–5 µm | A catalog figure for the machine, not for your part |
| Tool wear across the lot | 5–20 µm without active compensation | The most common cause of part-to-part drift |
| Tool deflection in the cut | 2–15 µm depending on length/diameter ratio | Critical in deep cavities |
| Distortion from clamping force | 1–10 µm on thin walls | Measures in tolerance in the fixture, out of it once released |
| Material stress release | 5–50 µm in rolled plate without stress relief | Shows up after the final pass |
| Measurement system uncertainty | Should be ≤1 µm for this band | If you don't know it, you don't know whether the part conforms |
Almost none of those sources is solved by a better machine: they're solved with operation sequence, thermal control, and a measurement plan. And measurement error is part of the budget — a supplier measuring with a 3-micron-uncertainty instrument ate 30% of the band before touching the part.
2. Which Process Holds the Dimension, and Why Milling Doesn't
The question that separates the supplier who read the drawing from the one who just wants the order: which process are you going to close this dimension with? Realistic ranges, in production and not on a demo part:
| Process | Realistic tolerance | Where it applies |
|---|---|---|
| CNC milling, 3, 4, and 5 axes | ±0.025–0.05 mm standard; ±0.01 mm fine | General geometry, cavities, complex surfaces |
| CNC turning | ±0.025–0.05 mm; ±0.005–0.01 mm fine on diameters | Turned geometries |
| Cylindrical and surface grinding | ±0.002–0.005 mm | Diameters and faces, typically in hardened material |
| Wire EDM | ±0.002–0.005 mm | Profiles in hard material, sharp internal corners |
| Honing | ±0.002–0.005 mm on diameter, with better roundness | Bores, liners, seal seats |
The read is direct: once the drawing drops below ±0.007 mm, it stopped being a milling or turning requirement and became a secondary-operation requirement. CNC is still indispensable — it produces the full geometry and leaves the critical dimension with controlled excess stock — but it isn't the process that closes it.
A consequence almost no supplier tells you: if the shop only has machining centers and lathes, your dimension is leaving their plant. The question isn't whether they subcontract it, it's whether they know to whom and whether they'll tell you. A shop that can't name the executor is transferring risk to you without saying so, because if that dimension fails they don't control the process that produced it.
Sequence matters too. On a steel part that will be ground, the correct order is soft machining, heat treatment, stress relief, and then grinding to final size. Heat treating after grinding moves the part and wastes the precision operation. If the supplier doesn't describe the sequence without being asked twice, they haven't defined it.
To lock in a dimensionally stable material before closing the drawing, the steel equivalences table solves the most common case: a drawing in a European standard, bar stock purchased in Mexico.
3. The Seven Questions That Disqualify a Supplier
This isn't a list to email and file. It's for hearing how fast and how specifically the person on the other end answers.
| Question | Answer that qualifies | Answer that disqualifies |
|---|---|---|
| Which process and machine will hold this dimension? | Names grinding, wire EDM, or honing, and the equipment | "Our 5-axis center" |
| Do you run the precision operation yourself or does someone else? | Shop name, location, and whether it's audited | "We handle it in house," with no grinder or EDM in their equipment list |
| At what temperature do you machine, and at what temperature do you measure? | 20 °C, metrology room, or part stabilization before measuring | Doesn't distinguish between the two questions |
| What is your measurement uncertainty on this dimension? | A figure in microns and the instrument that produces it | "We have a CMM" |
| How many parts will you measure, and on what trigger? | Control plan with frequency, re-verification events, and stop criteria | "We check the first one and the last one" |
| What do you do to the material before the final pass? | Stress relief, heat-treat sequence, and planned excess stock | Doesn't mention material as a variable |
| What Cpk will you demonstrate here, and on what sample? | Cpk ≥ 1.33 automotive minimum, ≥1.67 on critical features | Confuses Cpk with "the first part came out fine" |
If they answer all seven well, they can still fail. If they miss two, you already know what will happen at your receiving inspection.
4. How to Verify the Dimension When the Parts Arrive
At ±0.05 mm, a calibrated micrometer settles the argument. At ±0.005 mm the instrument becomes part of the problem and has to be treated with the same rigor as the part.
The instrument-to-tolerance ratio. Established industrial practice is that the measurement system consume a small fraction of the band: the traditional rule of thumb is 10:1, and automotive MSA criteria work with 4:1 as the minimum. With a 0.010 mm band, that means a system on the order of 1 micron. A digital caliper with 0.01 mm resolution isn't part of this conversation, not even as a screen.
Temperature isn't a detail. The international metrological reference for dimensions is 20 °C, under ISO 1. A part measured hot off the machine and accepted gets rejected the next day at your receiving inspection, and neither number is wrong: they were taken at different temperatures. Require the report to state the measurement temperature.
A CMM isn't always the best instrument at this scale. It's the right one for GD&T and complex geometry, and it's what a PPAP or FAI requires. But for a precision diameter in production, an air gage with a master ring can deliver better repeatability than a shop-floor CMM, because it compares against a physical master instead of reconstructing the diameter from probed points. The criterion isn't the most expensive equipment: it's the lowest uncertainty for that feature. What the dimensional report must contain is covered in the guide to CMM metrology and quality control in high-precision machining.
Align to the drawing datums. At 5 microns, aligning to a different datum produces deviations of the same order as the tolerance: you and your supplier will end up arguing over numbers you both measured correctly.
Ask for the repeatability study, not just the calibration certificate. The certificate says the instrument was fine the day it was calibrated. The R&R study says how much variation the complete system — instrument, operator, and method — introduces when measuring your part. Over a 10-micron band, that figure decides whether the report means anything.
5. When Not to Ask for ±0.005 mm
The honest position: most drawings that carry ±0.005 mm don't need it. They carry it because someone copied the general tolerance block from the previous drawing, or because the designer wanted a safety margin and put it in the tolerance instead of in the fit. That forces the shop to treat every dimension as critical and to quote the whole part at the cost of the tightest one.
Every micron you tighten gets paid for three times: in machining, with a full secondary operation; in verification, with part-by-part measurement in a controlled room; and in rejects, the cost nobody budgets. A dimension tightened beyond the capability of the available process doesn't produce worse parts — it produces rejected parts and rework.
The practical test is one line: if that dimension lands at the tolerance limit, what stops working? If the answer is "nothing," the tolerance is surplus. If it's an interference fit, a bearing seat, a sealing surface, or a locating interface in a precision assembly, the tolerance stays and gets paid for.
There's a middle case solved by redesigning rather than buying precision: splitting the tolerance across two parts of the assembly, converting a fixed fit into an adjustable one, or moving size control to geometric control. Ranges by process and the criteria for when to tighten are tabulated in the technical guide to tolerances in CNC machining.
6. What Radii Holds, and Who Executes What Falls Below
The tightest tolerance Radii commits to on its machining route is ±0.007 mm. That's the floor of the CNC route with a finishing operation — 3-, 4-, and 5-axis milling and turning — with a traceable dimensional report. It isn't a brochure figure: it's the commitment.
For your ±0.005 mm dimension that means something concrete: the requirement isn't solved on the machining center, it's solved with grinding, wire EDM, or honing, depending on the geometry. And those three operations exist inside Radii's audited shop network.
| Without an identified executor | With the operation inside the network |
|---|---|
| The shop quotes and figures out later who does it | The process is assigned at quoting time |
| Subcontracting is invisible in the lead time | The secondary operation is in the plan, with its own time |
| If the dimension fails, nobody controls that process | The executor is audited against the same criteria |
| The dimensional report arrives from two sources | One dimensional package per part |
In practice: feasibility analysis runs before quoting, so features requiring a secondary operation are flagged when the CAD enters the platform and not at final inspection. The critical dimension drives routing — a project below ±0.007 mm goes to the combination of shops that includes the precision operation, not to the fastest shop. The dimensional report ships with the parts. And the network is audited for ISO 9001 and AS9100 compliance, with production under IATF 16949 systems: the documentary framework a Tier 1 or aerospace customer will require from you when they audit your supplier.
What you won't read here is that Radii "does ±0.002 mm, no problem." The sites ranking above this search say yes to everything, and that yes is worthless to the engineer who has to defend the purchasing decision the day the part fails.
Frequently Asked Questions
Can a ±0.005 mm dimension be machined on a CNC machining center?
Not reliably with milling alone. That is 5 microns (0.0002 in): less than the thermal drift of a 100 mm aluminum part when the shop temperature moves 2 °C. That dimension is closed with a secondary operation — grinding, wire EDM, or honing — on a part the CNC left with controlled excess stock. A supplier who claims it comes straight off the machining center did not read the drawing.
What tolerance does Radii commit to in CNC machining?
Radii commits to ±0.007 mm. That is the floor of the machining route with a finishing operation, backed by a dimensional report. Below ±0.007 mm the requirement is no longer a milling or turning requirement — it falls to grinding, wire EDM, or honing. All three processes exist inside the audited shop network, so the dimension is closed by an identified executor.
How do I verify a supplier actually holds ±0.005 mm?
Ask for three things in writing: the uncertainty of their measurement system on that dimension (a figure in microns, and by industrial practice no more than one tenth of the tolerance band), the temperature at which they measure (the reference is 20 °C under ISO 1), and the gage repeatability and reproducibility study. If the answer to all three is "we have a CMM," you don't have evidence: you have a machine.
How much more does ±0.005 mm cost than ±0.05 mm?
Cost doesn't rise proportionally, it rises in steps. Going from ±0.1 mm to ±0.01 mm can multiply machining by 2x to 4x; going below ±0.01 mm adds a full extra operation, with its own setup, tooling, and inspection, plus part-by-part measurement. If the dimension doesn't define a real function, that cost buys nothing.
Which materials can hold a ±0.005 mm dimension?
Dimensionally stable ones: quenched and tempered steels, tool steels, precipitation-hardened stainless such as 17-4 PH, and bar or castings that were stress relieved beforehand. Aluminum and brass can reach that dimension, but their thermal expansion is high enough that the part only conforms in the controlled environment where it was measured. Rolled plate without stress relief moves as the material releases.
Conclusion: At Five Microns, the Supplier Who Says "Yes" Knows the Least
Qualifying a high-precision supplier isn't asking whether they can. It's asking how, and listening for whether the answer contains a process, a machine, a temperature, and an uncertainty figure. All four or none.
- ±0.005 mm is the range of grinding, wire EDM, and honing — CNC prepares the part, the secondary operation closes the dimension.
- The error budget is spent on thermal drift, tool wear, and material stress before the machine has any say.
- With a 10-micron band, the measurement system has to contribute on the order of 1 micron and measure at 20 °C; ask for the R&R study, not just the calibration certificate.
- If the dimension doesn't break a function at its limit, take it off the drawing: that's the largest saving in the whole project.
- Radii holds ±0.007 mm on the CNC route and executes what falls below it with grinding, wire EDM, or honing inside the network, with the executor assigned from quoting onward.
→ Upload your CAD or drawing at app.radii.com.mx — feasibility analysis flags the features that require a precision secondary operation before they reach production.