CNC machining tolerance: what the number really buys you

• 1
  What CNC machining tolerance actually controls
• 2
  Where the achievable tolerance comes from
• 3
  How material and geometry shift the limit
• 4
  From drawing to first article: the tolerance chain
• 5
  Typical tolerance bands and what they cost

A CNC machining tolerance is the band a dimension is allowed to land in after cutting. If a bore is called out at Ø25.000 ±0.020 mm, any bore from 24.980 to 25.020 mm passes. The drawing is not asking for 25.000 mm exactly, because no machine holds one number forever. It is asking for a window wide enough to be made repeatably and narrow enough that the part still functions.

That window is a contract between design intent and process capability. A shaft that runs in a bushing needs clearance, so the tolerance is set by the fit, not by pride. A bearing seat that must hold preload may need ±0.005 mm. A mounting hole that only passes a bolt can sit at ±0.2 mm and never cause a problem. Same shop, same machine, very different cost.

Tolerances show up in three forms on a drawing. Bilateral limits give a plus and minus around a nominal, such as ±0.05 mm. Unilateral limits put the whole band on one side, such as +0.02 / 0 mm for a press-fit pin. Limit dimensions state the two extremes directly, such as 10.00 to 10.20 mm. All three describe the same thing: the boundary of acceptable.

The number alone is not enough. Datum structure decides what the tolerance is measured from. A flatness callout on a face that also serves as datum A means the face must be made flat before any other feature is located from it. Skip that order and the inspector will reject a part that measures fine in the machine but not on the granite plate.

Machine geometry sets the floor. A 3-axis mill with linear guides and ground ballscrews may position within ±0.010 mm over its travel. A simultaneous 5-axis machining center with a Ø400 mm rotary table adds two more axes of error, but modern thermal compensation keeps the stack tight. GreatLight runs 127 high-precision CNC machines, including 16 simultaneous 5-axis centers and 16 mill-turn centers, so the tolerance we quote depends on which machine the job fits.

Spindle and tooling add the next layer. A 12 mm carbide end mill deflects under load. Push it too hard and the wall bows, even if the machine is perfect. Tool runout of 0.010 mm TIR will show up as a 0.010 mm size error on every pass. Tool wear moves the cut progressively, which is why we monitor in-process and change inserts on a count, not on a hunch.

Workholding is where many jobs quietly fail. A vise clamped on a thin wall springs the part open; after unclamping, the wall relaxes and the dimension moves. A vacuum plate on a 1.5 mm aluminum panel holds it flat enough for ±0.05 mm, but not for ±0.005 mm. Soft jaws bored in place, or a dedicated fixture, are the answer when the tolerance is tight and the part is flexible.

Thermal drift is the slow variable. A spindle running for two hours grows a few microns. A shop that swings from 18 °C at night to 30 °C in the afternoon moves more than that. We hold ±0.005 mm (±0.0002 in) on parts where the geometry, material and fixturing allow it, and we say so when they do not, rather than quoting a number the process cannot repeat.

Material decides how much the cut pushes back. Aluminum 6061 and 7075 cut clean and hold tight dimensions with sharp tooling. Stainless 316 and 17-4PH work-harden, so a dull tool rubs instead of cutting and the size creeps. Titanium Ti-6Al-4V and Inconel generate heat at the edge, and that heat goes into the part, so a finishing pass may need to be lighter and slower to stay in band.

Plastics behave differently again. POM and PEEK machine well but move after cutting as internal stress releases. A tolerance of ±0.05 mm on a PEEK part is reasonable; ±0.005 mm on the same part is a gamble unless the stock is stress-relieved and the part is allowed to settle. ABS and PP are softer and tend to smear, so the surface finish and the size both suffer.

Geometry sets the aspect ratio problem. A bore 10 mm deep and 10 mm wide is easy. The same bore at 100 mm deep needs a long tool that deflects, and the tolerance at the bottom will be looser than at the top. Thin walls below 1 mm deflect under clamping and cutting force. Deep pockets with small corner radii force a small tool, which limits how much material can be removed per pass and how tight the floor can be held.

Surface finish and tolerance travel together. A Ra 0.8–1.6 μm finish on a sealing face usually comes with a tighter size callout, because the same tool and pass control both. Our fine finish range is Ra 0.2–0.8 μm when the application needs it. As-machined at Ra 1.6–3.2 μm is fine for brackets and covers, and it costs less.

The chain starts at DFM. Before we cut, we look at the callouts and flag the ones that will be expensive or impossible. A ±0.005 mm tolerance on a feature 300 mm from the datum is a different job than the same tolerance on a feature 20 mm away. We send a quotation and a free DFM analysis within 12 hours, so the conversation happens before the metal is committed.

Setup is next. For tight work we rough, stress-relieve if the material allows, then finish in a separate operation. Roughing leaves 0.3–0.5 mm of stock; finishing takes it in one or two light passes. Cutting the part free in the same setup that made it is the goal, because every reclamp adds a new error source. A 5-axis center helps here, since it can reach five faces without the operator moving the part.

Inspection closes the loop. A caliper is not enough for ±0.005 mm. We use micrometers, bore gauges, height gauges and CMM checks on critical features. We inspect 100% of parts before shipment, with raw material checks, in-process monitoring and final inspection. Reports are available on request. If a dimension is drifting, we see it in-process rather than at the end of the run.

The historical late-delivery probability across our production is below 2%, and parts typically ship in 3–5 days once production starts. Those numbers come from keeping the tolerance chain visible at every step, not from rushing the last operation.