CNC Machining Tolerance Capability: ±0.005 mm and How It Is Held
CHX Metal holds plus or minus 0.005 mm on critical CNC features such as bores, pilots and seal faces, while general dimensions follow ISO 2768-m. The band is verified by CMM at first article, batch start and final release. Prototypes ship in 3 to 7 days and production in 7 to 15 days, with a minimum order of 1 piece.
Every RFQ eventually reaches the same question: what tolerance can you actually hold, and how do you prove it. This page states the number, shows how it is inspected, and explains which features can sit in a normal ISO 2768-m band so that the tight tolerance is spent where it changes function rather than everywhere on the drawing.
What ±0.005 mm Means on a Real Drawing
±0.005 mm is a two-sided band of 0.010 mm total. On a 20 mm bore, that is 0.05 percent of the nominal size, which is why it cannot be applied to every dimension: a part fully toleranced at that band would cost several times a part toleranced where it matters. We hold ±0.005 mm on diameters, bores, hole positions, shoulder depths and mating faces that the drawing marks as critical, and machine the rest of the part to ISO 2768-m.
The number is a capability statement for metal parts machined on our 220 machines, not an average of good days. It is verified by first article inspection and re-verified at batch start and at final release with CMM records that ship with the parts.
Comparator: ±0.005 mm Against ISO 2768-m
| Nominal length | ISO 2768-m (medium) | What we hold on critical features |
|---|---|---|
| 0.5 to 3 mm | ±0.1 mm | ±0.005 mm |
| over 3 to 6 mm | ±0.1 mm | ±0.005 mm |
| over 6 to 30 mm | ±0.2 mm | ±0.005 mm |
| over 30 to 120 mm | ±0.3 mm | ±0.005 mm or better |
| over 120 to 400 mm | ±0.5 mm | ±0.005 mm or better |
| over 400 to 1000 mm | ±0.8 mm | quoted per feature |
The gap is roughly 20 to 40 times tighter than general tolerancing, which is exactly why a drawing that marks every dimension ±0.005 mm is usually a drawing that has not been reviewed for cost. A part with eight critical dimensions and forty general ones machines faster and inspects cleaner than the same part fully toleranced.
Tolerance by Process
- 3-axis and 3+2 milling — pockets, slots, faces, hole patterns; ±0.005 mm on position and depth of critical features.
- Simultaneous 5-axis — contoured and angled features in one setup; the accuracy advantage is mostly in datum continuity, since fewer setups mean fewer stack-ups.
- CNC turning — diameters from 1 to 300 mm; ±0.005 mm on diameter and shoulder location.
- Swiss-type turning — long, slender parts where the guide bushing keeps deflection low; small diameters hold the same band without support from a tailstock.
- Surface grinding — used where flatness and parallelism decide function, such as seal faces and valve blocks.
How the Tolerance Is Inspected
Eight in-house inspection instruments back the number, with a coordinate measuring machine (CMM) as the reference. Critical dimensions are measured on the CMM against the drawing at first article, at batch start and at final release; a roughness tester covers Ra callouts, and micrometers, bore gauges and pin gauges handle in-process checks at the machine. Reports quote measured values rather than a pass stamp, so a customer can see the actual distribution and judge process capability.
For programmes that need statistical evidence, we supply SPC data on the critical characteristics: the mean and spread over the batch, not a single sample. That is what a receiving inspection department needs in order to accept a lot without re-measuring every part.
What Actually Limits Achievable Tolerance
Five things move a dimension, and only the first is about the machine:
- Fixture rigidity — a part that deflects under clamping is springing back after the cut, so fixture design is done in-house before the program is written.
- Tool wear and thermal growth — long runs drift; we compensate in the program and verify with in-process gauging rather than trusting the first piece.
- Material behaviour — thin walls in aluminium move after the part is released from the vise; titanium work-hardens and pushes back on the cutter; polymers relax for days after machining.
- Datum scheme — an unclear datum on the drawing turns a capable process into a debate at inspection.
- Feature geometry — a deep narrow slot or a long small bore is limited by tool reach and deflection long before the machine's positioning accuracy matters.
How to Call Out Tolerance So It Gets Held
- Mark the functional dimensions: bearing bores, seal faces, mating pilots, hole positions that set an assembly.
- Leave the rest under a general tolerance note (ISO 2768-m is the usual choice for machined parts).
- Use GD&T for what a plus/minus band cannot express — position, flatness, perpendicularity, concentricity.
- State the inspection method where it is disputed, for example “flatness verified on CMM over the full face”.
- Say which surface is the datum, and keep the datum reachable by the probe.
Send a drawing and we return a DFM note that flags any dimension we consider over-toleranced for its function, together with the cost difference of relaxing it. RFQ feedback arrives within 24 hours; prototypes run in 3 to 7 days and production in 7 to 15 days, with a minimum order of 1 piece.
Related Capability Pages
Frequently Asked Questions
Can you hold ±0.005 mm on every dimension? No, and no machine shop should claim that. ±0.005 mm is held on the critical features a drawing marks — bores, pilots, seal faces and hole positions — while general dimensions follow ISO 2768-m. Applying the tight band everywhere multiplies cost without adding function.
What is the difference between ±0.005 mm and ISO 2768-m? ISO 2768-m allows roughly ±0.1 to ±0.5 mm depending on nominal length, while ±0.005 mm is 20 to 40 times tighter. ISO 2768-m is the default for non-critical dimensions; the tight band is reserved for features that decide fit or sealing.
How do you prove the tolerance? First article inspection plus CMM records at batch start and final release. Reports list measured values, not just pass or fail, and SPC data on critical characteristics is available for programmes that require it.
Which processes do you use for tight tolerance work? 3-axis and 3+2 milling, simultaneous 5-axis, CNC turning, Swiss-type turning and surface grinding. The process is chosen by feature geometry, not by preference: a long slender diameter goes to a Swiss lathe, a seal face goes to grinding.
What is the smallest feature you can inspect? It depends on probe access more than on size. Deep narrow slots and long small bores are the usual limit, because the probe has to reach the feature without touching the wall. Send the drawing and we confirm probe access before quoting.
What lead time applies to tight tolerance parts? Prototypes in 3 to 7 days and production in 7 to 15 days, with surface treatment adding 3 to 7 days. Tight tolerance parts are scheduled with inspection time included rather than measured after the batch is packed.
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