CNC Machining Indy: How the Process Actually Works

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  What happens to metal during CNC machining Indy work
• 2
  Where tolerance stops being free
• 3
  How material choice changes the cut
• 4
  When a five-axis center is the right call
• 5
  How you know the part is right
• 6
  Machining sequence and finishing
• 7
  Which process fits which part

CNC machining removes material with a rotating cutter that follows a toolpath. Nothing is molded or cast. The tool presses into the workpiece and shears chips away, which means every cut puts load into the part. That load is the reason thin walls move, and it is the reason a drawing that looks fine in CAD can come back out of tolerance.

The cutting edge generates heat. Aluminum pulls heat away fast, so it machines cool and fast. Titanium and stainless hold heat at the edge, so the tool wears quickly and the cut has to slow down. This single difference explains most of the cost gap between a 6061 bracket and a Ti-6Al-4V bracket of the same shape.

Chip evacuation matters as much as spindle speed. Deep pockets trap chips, and a recut chip is a dull chip. Through-spindle coolant and peck cycles solve this, but they add cycle time. When a quote comes back high on a deep cavity, chip clearing is usually part of the reason.

Fixtures hold the part while all of this happens. A weak setup lets the part deflect under cutting force, and the finished dimension drifts even though the machine is accurate. We treat fixture design as part of the machining plan, not an afterthought.

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  Heat movesAluminum dissipates it; titanium and stainless do not.
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  Chips must leaveTrapped chips cause recutting and poor finish.
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  Setup is geometryA weak fixture shifts dimensions, not the machine.

A general tolerance block on a drawing is a starting point, not a machining plan. On a 100 mm aluminum plate, ±0.1 mm is routine. On a 300 mm steel shaft, the same number requires temperature control and a spring pass. Tolerance cost scales with part size, material hardness and the number of features that must stay in relation to each other.

Position tolerance is the harder problem. A hole with a ±0.05 mm diameter callout is easy on its own. Six holes that must all land within 0.05 mm of each other across a 400 mm bolt pattern is a different job. It needs one setup, or a fixture that repeats to better than the tolerance.

Surface finish and tolerance compete. A Ra 0.2–0.8 μm finish needs a light finishing pass, which takes time and a sharp tool. Push for both a tight finish and a tight corner radius in a deep pocket and the shop has to reach in with a long, thin tool that deflects. That is when a design should be reconsidered rather than quoted.

We hold ±0.005 mm on parts that genuinely need it, and we hold ±0.0002 in when a drawing is written in inches. Parts that do not need it should not carry it. Every extra zero on the drawing adds inspection time and cost without adding function.

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  Size drives costSame tolerance, longer part, harder to hold.
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  One setup winsRelated features should be cut without re-fixturing.
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  Don't over-specifyTolerance nobody measures is cost without benefit.

Aluminum 6061-T6 is the default for prototypes and housings. It cuts fast, takes anodizing well, and holds a good finish. 7075 is stronger but gummier at the tool edge, so it needs sharper geometry and a more careful feed. If a part does not need 7075 strength, 6061 saves cycle time.

Stainless 303 machines cleanly and is common for shafts and fittings. 304 and 316L work-harden, so a light pass that rubs instead of cuts will harden the surface and destroy the next tool. 17-4PH adds strength after heat treatment and is common in medical and aerospace parts, but the heat treat step has to be planned into the sequence.

Titanium TC4 (Ti-6Al-4V) and Inconel sit at the difficult end. They hold heat, they gall, and they need low cutting speeds with high coolant pressure. Cost per part is several times aluminum. That is acceptable for a flight or implant part. It is wasted money on a bracket that only carries a cable.

Plastics and copper alloys have their own rules. POM and PEEK need sharp tools and air blast to avoid melting and chip welding. C36000 brass machines faster than any steel and holds fine detail, which makes it a good choice for small fittings and manifolds.

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  Start with 6061Move up only when strength or temperature demands it.
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  Work-hardening alloys304, 316L and titanium punish a rubbing cut.
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  Brass machines fastGood for small fittings with fine detail.

A three-axis machine cuts from one direction. It is the fastest and cheapest option for plates, brackets and parts with features on a single face. If your part is essentially a prismatic block with holes, a three-axis mill plus a simple flip is the economical route.

A four-axis mill adds a rotary table, so the part can index to multiple faces without being unclamped. This is the practical answer for shafts with cross holes, or a part with features on four sides. Accuracy improves because the part is not moved between operations.

Five-axis machining tilts the tool as well as the table. It reaches undercuts, cuts complex contours in one pass, and keeps a short, stiff tool in the cut instead of a long one. GreatLight runs 16 simultaneous five-axis machining centers, with a Ø400 mm rotary table available. That capability matters most on impellers, medical instruments and aerospace housings.

Size decides the machine too. Our largest travel is 4,000 × 400 × 150 mm, with medium travels at 750 × 1,150 × 550 mm and 600 × 600 × 600 mm, and compact travels at 500 × 500 × 450 mm and 500 × 310 × 200 mm. A part outside the largest envelope has to be split or made another way. Check the envelope before the design is frozen.

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  Three-axisCheapest for prismatic parts and single-face features.
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  Four-axisIndexing without re-clamping; good for shafts.
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  Five-axisUndercuts, complex contours, short stiff tools.

Inspection is not a final step bolted onto production. It starts with the raw material certificate, continues with in-process checks at defined points, and ends with a final dimensional report. GreatLight inspects 100% of parts before shipment, and reports are available on request.

For a tight feature, the measurement method matters as much as the number. A caliper does not resolve 0.005 mm. That needs a micrometer, a bore gauge or a CMM. If a drawing calls for a tolerance that the inspection plan cannot verify, the tolerance is theoretical.

First article inspection is the usual control for a new design. It confirms the process before a run continues. For production volumes, in-process monitoring catches drift before parts are scrapped. Both add time to the schedule, and both are cheaper than a rejected shipment.

Qualification rate across our production is 99.99%. That figure comes from the inspection loop, not from hoping the machine stays in spec. When a process shows drift, we correct the process rather than sort the parts.

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  Material firstCertificates confirm the alloy before cutting.
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  Match method to toleranceA caliper cannot verify 0.005 mm.
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  Correct the processSorting parts hides the real cause.

Order of operations decides whether a part ends up straight. Roughing removes most of the material and puts stress into the workpiece. If the part is finished immediately, it can move after the clamps come off. A stress-relief step or a rough-then-finish sequence keeps the final dimensions stable.

Heat treatment belongs in the sequence, not after it. A 17-4PH part that is machined to final size and then aged will change dimensions and hardness. Machine oversize, heat treat, then finish. The same logic applies to stress-relieved aluminum.

Finishing changes dimensions slightly. Anodizing builds a surface layer; plating adds thickness on all sides. A shaft that fits a bore after machining may not fit after hardcoat anodizing. Call out the finish on the drawing and let the shop plan the pre-finish size.

We run anodizing in clear, colour, hardcoat and conductive types, plus electroless nickel, zinc, silver and gold plating, powder coating, black oxide, bead blasting, tumbling, brushing and polishing. Laser marking is available down to a minimum character height of 1.5 mm.

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  Rough then finishLets internal stress release before final cuts.
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  Heat treat in sequenceMachine oversize, treat, then finish.
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  Finishes add thicknessPlan pre-finish size for plated fits.