Accurate OEM CNC Machining Solutions

Most OEM drawings carry a general tolerance block and a handful of tolerances called out tighter. Those tight callouts decide the process. A general note of ±0.1 mm on a 6061 bracket is routine on a three-axis mill. A Ø12 H7 bore with a 0.008 mm position callout against a secondary datum is not, and the cost difference between the two parts is mostly setup and inspection time, not cutting time.

We start by separating features that must be accurate from features that merely must fit. A mounting face that locates against a machined casting needs flatness control. A clearance hole for an M6 screw usually does not. When a drawing applies ±0.005 mm across every dimension, we flag it, because measuring all of it under temperature-controlled conditions adds cost without improving function.

The tolerance also sets the datum order. If datum A is a large face and datum B is a hole, the first setup is planned around A. If the drawing makes a small boss the primary datum, the part may need a sacrificial tab or a soft jaw cut to that boss so the feature survives clamping. This is decided before the first chip, not after.

• 1
  Tight callouts drive costSetup, fixturing and CMM time, not cycle time.
• 2
  General tolerances stay generalKeep ±0.1 mm notes off locating features.
• 3
  Datum order sets setup orderPrimary datum becomes the first clamping face.
• 4
  Functional vs cosmeticOnly restrict what the assembly actually touches.

Five-axis machining removes setups. On a housing with pockets on four sides and an angled port, three-axis work needs four or five fixture positions, and every reposition adds a stack-up error. Machining it on one five-axis setup keeps all faces in one coordinate frame, so position between the port and the side pockets depends on the machine, not on how well the operator reloaded the vise.

The gain is not automatic. A flat plate with holes and a perimeter profile runs faster on a three-axis mill, and putting it on a five-axis center only consumes spindle time. We route simple prismatic parts to the three-axis and four-axis machines and reserve the 16 simultaneous five-axis centers for contoured, angled, or multi-face work.

Tool access decides more than axis count. A deep cavity with a 3:1 depth-to-diameter ratio can be finished with a long reach tool on a three-axis machine if the wall is open. A closed impeller or a turbine blade with twist needs the tool tilted continuously, and that is where simultaneous motion is the only practical route. Short tools cut more accurately because they deflect less, so leaning the tool into a wall often improves the surface and the size at the same time.

Aluminium 6061-T6 and 7075 machine cleanly and hold size well, which makes them the default for accurate OEM parts. 7075 is stronger but moves more after heavy material removal, so we rough, stress-relieve where the schedule allows, and finish in a later operation. Thin walls below 1 mm on aluminium are prone to chatter and spring back, and the fix is usually a change to the geometry, not a slower feed.

Stainless 304 and 316L work-harden. A cutter that rubs instead of cutting will raise the surface hardness and dull the next pass, so we keep the feed per tooth up and avoid dwelling. 17-4PH in the H900 condition holds a fine finish and is common on medical and aerospace parts. Titanium Ti-6Al-4V and Inconel cut hot, conduct heat poorly, and need lower surface speed and more coolant, which raises cost per part.

Plastics behave differently again. PEEK and POM have a high thermal expansion coefficient, so a part measured right off the machine can be out of tolerance once it cools. We cut them with sharp, polished tools, keep coolant or air on the cut, and let the part rest before final inspection. Carbon fibre and glass-filled grades wear tooling fast and need diamond-coated or carbide tools changed on a schedule.

A tolerance is only meaningful with a measurement method attached. We check raw material certificates on receipt, monitor critical sizes during the run, and inspect 100% of parts before shipment. Final inspection uses calipers, micrometers, bore gauges, height gauges and a CMM, chosen by feature type. A deep bore is checked with a bore gauge or pin, not with calipers, because calipers cannot reach the bottom.

Inspection reports are available on request, and they list the drawing dimensions with the measured value and the instrument used. For first articles, we can supply a full dimensional report against the numbered drawing balloons. If your quality team needs a specific report format, send the template with the RFQ so it is built into the process instead of added at the end.

Traceability matters as much as the number. Material heat numbers are recorded against the job, and parts can be bagged and labeled by lot. For work under NDA, files stay inside the project folder and are not shared outside the team handling the job.