Absolutely accurate CNC machining: what it takes to hold a number

A drawing note of ±0.005 mm is a system requirement, not a machine requirement. The machine has to be capable of roughly half that, so the process window can absorb everything else. At GreatLight the working tolerance is ±0.005 mm, which is ±0.0002 in. That number only means something when it is tied to a feature, a material, and a measurement method.

Thermal drift is the first thing that eats a tight tolerance. A spindle warms up over the first hour of cutting. A 100 mm aluminium part can grow 2–3 μm for every 1 °C of temperature change. So on a ±0.005 mm feature we rough, let the part cool, then finish. Skipping the cool-down is the most common reason a first article passes and the third one does not.

The second factor is stock. A feature cut from 0.3 mm of remaining stock behaves differently from one cut from 3 mm. Heavy stock means more cutting force, more deflection, and more spring-back in thin walls. When a wall is under 1 mm, we plan the tool path around it before the first cut.

• 1
  Feature, not blanketApply ±0.005 mm to the 3–5 features that matter. Blanket tolerances raise cost with no benefit.
• 2
  Datums firstA tight tolerance on a feature with no clear datum cannot be inspected, so it cannot be held.
• 3
  Material mattersAluminium 6061 and 7075 hold tighter than 316L stainless or Inconel under the same cut.
• 4
  Measure the same wayAgree on CMM vs. micrometer and on clamping state before production starts.

Every time a part is unclamped and moved to another machine, its position is re-established. Each re-clamp adds error. On a 3-axis machine a part with features on five faces may need three or four setups, and each one stacks its own deviation. Five-axis work holds the part in one fixture and rotates the table or the head, so the same datum carries through the whole job.

That is the real argument for simultaneous 5-axis, not speed. GreatLight runs 16 simultaneous 5-axis machining centers with a Ø400 mm rotary table, plus 12 four-axis mills, 27 three-axis machines, and 16 mill-turn centers. The choice between them is driven by how many faces carry tolerance, not by how complex the part looks.

A part with tight features on two opposing faces is often a better fit for a 4-axis mill than a 5-axis center. A part with compound angles, deep pockets reachable only from one direction, or an undercut that needs the tool tilted, needs the extra two axes. Matching the machine to the feature count keeps cost sane.

• 1
  Best for 5-axisCompound angles, contoured surfaces, deep cavities, features on four or more faces.
• 2
  Better on 3 or 4 axisPrismatic parts with tolerance on two faces. Cheaper and just as accurate.
• 3
  Mill-turnShafts and housings with turned and milled features in one cycle, no second datum.

A tolerance without a finish call is incomplete. Two parts can both measure within ±0.005 mm and behave completely differently in a seal, a bearing bore, or a sliding joint. GreatLight works to three finishing bands: Ra 0.2–0.8 μm for fine finishes, Ra 0.8–1.6 μm as the standard high finish, and Ra 1.6–3.2 μm as-machined.

Getting below Ra 0.8 μm usually means a separate finishing pass with a smaller stepover or a slower feed, not a different machine. That adds cycle time. If a bore only needs to clear a pin, Ra 1.6 μm is enough. If it is a hydraulic sealing surface, the call should be Ra 0.8 μm or finer, and the drawing should say so.

Post-processing can move the finish too. Bead blasting and tumbling even out tool marks but can round a sharp edge. Anodizing adds a few microns of build-up, which matters on a thread or a press fit. Hardcoat anodizing can add more. Tell us the finish before we set the final cut, not after.

• 1
  Ra 0.2–0.8 μmSealing faces, bearing bores, optical mounts. Extra finishing pass required.
• 2
  Ra 0.8–1.6 μmGeneral precision fit. Default for most tight-tolerance parts.
• 3
  Ra 1.6–3.2 μmClearance holes, brackets, non-contact surfaces. No extra cost.

Aluminium 6061 and 7075 are the easiest to hold at ±0.005 mm. They cut clean, carry heat away, and do not work-harden much. 7075 is stiffer and holds a thin wall better than 6061, but it is more prone to stress relief movement when a lot of stock is removed. On a part with heavy material removal, we rough, stress-relieve if needed, then finish.

Stainless 303 and 304 machine reasonably well but tend to work-harden at the cut. 316L is worse. The fix is a constant feed that stays in the cut, not a light rub that hardens the surface. 17-4PH in the H900 condition is strong and holds tolerance well, but it is tough on tooling and the cycle is longer.

Titanium TC4 (Ti-6Al-4V) and Inconel are the hard cases. Both hold heat at the cutting edge, so thermal growth and tool wear dominate. Tolerances are still achievable, but the process needs more passes, more coolant, and more inspection. If a part is Inconel and the tolerance is ±0.005 mm on a thin wall, that is a conversation to have before quoting.

• 1
  Easy to holdAluminium 6061, 6061-T6, 7075, brass C36000, steel 1018 and 1045.
• 2
  ModerateStainless 303, 304, 17-4PH, steel 4140 and 4340, titanium TA2.
• 3
  Hard cases316L, Inconel, TC4, magnesium AZ31B on thin walls.

A tolerance claim is worth nothing without a measurement plan. GreatLight inspects 100% of parts before shipment, covering incoming raw material, in-process checks, and final inspection. Reports are available on request. For a ±0.005 mm feature, the inspection method has to be at least four times more capable than the tolerance, so a CMM or a high-resolution micrometer, not a caliper.

Fit and function still drive most acceptance decisions. We keep a sample center for first articles and process validation, so a customer can see a physical part before a production run. Historical qualification rate on shipped parts is 99.99%, and the historical late-delivery probability is below 2%, but the useful number for an engineer is the one on the inspection report for their part.

Documentation matters in regulated work. GreatLight holds ISO 9001:2015, IATF 16949:2016, ISO 13485:2016, and ISO 27001:2022. For automotive, medical, and aerospace programs the inspection report, material cert, and process record travel with the parts. Uploads stay confidential, and an NDA is available on request.

• 1
  IncomingMaterial grade and condition verified before the first cut.
• 2
  In-processCritical dimensions checked during the run, not just at the end.
• 3
  FinalFull inspection before shipment, reports on request.
• 4
  First articleSample center validation for new parts and new processes.