A Comprehensive Collection of Factory Technical Specifications for CNC Programmers

Tolerance is the first line a programmer reads, and it decides almost everything downstream: tool selection, step-over, number of passes, and how long the part sits on the machine. A general machining tolerance of ±0.005 mm is achievable on our equipment, but it is not free. It usually means a finishing pass with a small radial engagement, a sharp tool, and a temperature-stable cut.

The practical split is this. Features that only locate or clear can run at ±0.05 mm and nobody notices. Bores that receive a bearing, dowel or press-fit pin belong at ±0.01 mm or tighter, and they should be single-point bored, not interpolated with an end mill. Slot widths and pocket walls that set a fit are the third group: hold them at ±0.02 mm and check them with pin gauges rather than calipers.

One more rule from the floor. Do not assign a tight tolerance to a feature that has no function. Every tight call adds an inspection step, and inspectors will flag the feature on the report. If a drawing shows ±0.005 mm on a non-critical edge, ask the designer before you program it.

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  ±0.005 mmFunctional fits, bearing bores, mating spigots. Boring or fine milling only.
• 2
  ±0.01 mmDowel holes, pin fits, seal grooves. Single-point boring preferred.
• 3
  ±0.02 mmSlot widths, pocket walls, rail steps. Pin gauge or CMM check.
• 4
  ±0.05 mmClearance holes, cosmetic edges, non-mating surfaces.

Finish is written as Ra, and Ra is set by step-over, tool radius and spindle speed, not by a magic pass at the end. On aluminium and mild steel we can reach Ra 0.2–0.8 μm with a fine finishing pass on a rigid setup. That range suits seal faces, sliding surfaces and hydraulic bores.

Ra 0.8–1.6 μm is the workhorse band. It covers most mating faces, housings and brackets, and it is where a 5-axis finishing pass with a ball or bull-nose tool lands naturally. Ra 1.6–3.2 μm is as-machined stock for clearance faces and surfaces that will be painted or anodized later.

Tell the programmer what the surface does before you name an Ra value. A cosmetic cover does not need a ground finish. A hydraulic bore does. When in doubt, specify the functional band and let the shop pick the strategy; we return a DFM note with the quotation if a call looks expensive for no benefit.

Harder materials change the arithmetic. Titanium and Inconel work-harden at the cut, so a light finishing pass with a worn tool can smear instead of cut. On those alloys we keep the finishing depth of cut above the work-hardened layer and accept Ra 0.8–1.6 μm as the practical limit.

• 1
  Ra 0.2–0.8 μmSeal faces, hydraulic bores, sliding surfaces. Fine finishing pass, rigid setup.
• 2
  Ra 0.8–1.6 μmMost mating faces and housings. Standard 5-axis finishing strategy.
• 3
  Ra 1.6–3.2 μmClearance and cosmetic faces, pre-paint or pre-anodize surfaces.

Aluminium 6061 and 7075 cut cleanly at high spindle speed, which is why they dominate prototypes and small runs. 7075 holds a better finish on thin walls but moves more after stress relief, so rough, rest, then finish. Stainless 304 and 316 work-harden quickly; climb milling with a constant chip load keeps the tool in the cut and avoids the rubbing that dulls edges.

Steel 4140 and 4340 are predictable at moderate speeds and take a good finish, but they need roughing allowance for heat treatment distortion if the part is hardened later. Titanium TC4 (Ti-6Al-4V) cuts at roughly a third of the aluminium surface speed and needs flood coolant and sharp edges. Inconel is slower still; we plan longer cycle times and more tool changes rather than pretend otherwise.

Plastics bring the opposite problem. POM and PEEK cut well but hold heat, so a deep pocket can close up on the tool. ABS and PC need sharp, polished flutes and generous clearance. Carbon fibre is abrasive; we use diamond-coated tooling and expect a shorter tool life.

If a part mixes materials in an assembly, the tightest tolerance should be set on the material with the best thermal stability, not on the one that is easiest to clamp.

• 1
  Aluminium 6061 / 7075Fast, clean cuts. Rough and rest before finishing thin walls.
• 2
  Stainless 304 / 316Work-hardens. Constant chip load, climb milling, no dwell.
• 3
  Steel 4140 / 4340Predictable. Leave allowance for heat-treat distortion.
• 4
  Titanium TC4, InconelLow surface speed, flood coolant, sharp edges, longer cycles.

A program is only as good as the setup it runs in. Our largest travel is 4,000 × 400 × 150 mm, which suits long rails and extrusion profiles. Medium travels of 750 × 1,150 × 550 mm and 600 × 600 × 600 mm cover most housings and brackets. Compact machines at 500 × 500 × 450 mm and 500 × 310 × 200 mm handle small, high-precision parts where rigidity matters more than envelope.

For parts with features on five sides, a single 5-axis setup removes the re-clamp error that shows up when a part moves between operations. That is the main reason to route a contoured part to a 5-axis center rather than three 3-axis operations. Fewer setups mean fewer datums to trust.

Thin walls are a clamping problem before they are a cutting problem. Below about 1.5 mm wall thickness on aluminium, we plan support material, a soft jaw or a vacuum fixture, and we take light finishing passes from both sides. On a 4,000 mm rail, support along the length matters more than the cutter choice.

If you cannot say how the part will be held, the tolerance on the drawing is a guess. Send the model and we will return a workholding note with the quotation.

• 1
  Long parts4,000 mm travel on the large mill. Support along the full length.
• 2
  Five-sided featuresOne 5-axis setup. No re-clamp error between operations.
• 3
  Thin wallsSupport, soft jaws or vacuum. Light passes from both sides.

Every part we ship gets 100% inspection before shipment. The sequence is raw material check, in-process monitoring, and final inspection, with reports available on request. That matters to the programmer because inspection points should be chosen while the program is written, not after the first article is cut.

Pick datums the machine can actually reach and the CMM can re-establish. A datum hidden inside a closed pocket is a problem at inspection time. Where a feature is bore-based, call out the bore as the datum and dimension the surrounding pattern from it. Where a part is turned and milled, agree on one primary datum rather than two competing ones.

Our qualification rate runs at 99.99%, and the small remainder is usually a setup or datum issue rather than a cutter problem. That is why first-article checks on the tight features are worth the ten minutes they take.

For regulated work, our quality system is certified to ISO 9001:2015, IATF 16949:2016, ISO 13485:2016 and ISO 27001:2022. Uploads stay confidential, and an NDA is available on request if the drawing needs one before we see it.

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  Raw material checkGrade and condition verified before the first cut.
• 2
  In-process monitoringDimensions tracked during the run, not only at the end.
• 3
  Final inspection100% before shipment. Reports on request.