CNC Processing Bowling Green: How the Process Actually Works

• 1
  What happens inside a CNC machine
• 2
  What ±0.005 mm really means on a drawing
• 3
  Material choice drives the whole plan
• 4
  Where part geometry pushes back
• 5
  Finishing and inspection close the loop
• 6
  Which process fits which part

Every CNC machine does the same basic thing: a controller reads a toolpath, converts it into axis commands, and moves a cutting tool through metal along that path. The tool spins at a set surface speed. The table or spindle moves in X, Y and Z. A three-axis mill can only cut from one direction, so any feature on the side of the part needs a second setup. That second setup is where error creeps in.

A five-axis machine adds two rotary axes. The tool can tilt and the table can rotate, so the cutter reaches five sides of a part in a single setup. For a housing with bores on four faces, that removes three refixturings. Each refixturing you remove removes a stack of positional error. This is the main reason five-axis work holds ±0.005 mm on features that sit on different faces of the same part.

The cutting itself is simple mechanics. A flute shears material. Heat goes into the chip, the tool, and the part in that order. If the chip carries the heat away, the part stays cool and the finish stays clean. If the tool rubs instead of cutting, heat goes into the workpiece, and you get a shiny but dimensionally drifting surface. Feed and speed charts are a starting point, not a rule.

The controller matters less than people think. A good CAM programmer who understands stock removal, tool engagement, and workholding will beat a better machine with a careless setup almost every time. We see this on the floor constantly: same machine, two programmers, two very different cycle times and scrap rates.

• 1
  One setup, five facesFewer refixturings means less positional error stacking up.
• 2
  Heat follows the chipIf the chip is thin and blue, the part stays cool.
• 3
  CAM beats ironToolpath strategy moves the tolerance needle more than spindle specs.

A tolerance is a promise about where a surface can sit. On a drawing, ±0.005 mm looks like a small number. In the shop, it is a constraint that decides which machine, which tool, and how many setups the job needs. A feature held to ±0.005 mm on one face is routine. The same tolerance across two faces machined in separate setups is a different job entirely.

Temperature is part of the tolerance budget. Aluminum expands roughly 23 μm per meter per °C. A 300 mm aluminum part that warms 5 °C during roughing grows about 35 μm before you even take a finish pass. Good shops rough, let the part cool, then finish. If a supplier measures hot parts straight off the machine, the numbers will drift by the time the part reaches you.

Surface finish has its own scale. As-machined aluminum usually lands around Ra 1.6–3.2 μm. A finer finishing pass gets you to Ra 0.8–1.6 μm, which is what most sealing faces and bearing bores need. Below Ra 0.2–0.8 μm you are into polishing or lapping territory, and the cost curve turns steep. Ask yourself what the surface has to do before you write a finish callout.

Tolerance and finish are not the same requirement. A part can be dimensionally perfect and still leak because the surface is too rough for the seal. It can also be mirror-smooth and out of position. Separate the two on the drawing and the shop can plan the operations properly.

• 1
  Same setup, tight toleranceEasy. Split setups, tight tolerance, hard.
• 2
  Watch temperatureRough, cool, then finish. Do not measure hot.
• 3
  Finish is functionalMatch Ra to the job the surface does, not to a habit.

Aluminum is the default for most prototypes and light production parts. 6061-T6 machines clean, holds tight tolerances, and anodizes well. 7075 is stronger but gummier, so it wants sharper tools and lighter depths of cut. 2024 is common in aerospace brackets. If your part sees no load, 6061 will usually be the cheapest path to a good part.

Stainless tells a different story. 303 is the free-machining grade and cuts with a clean chip. 304 and 316 work-harden if the tool rubs, so the operator has to keep the feed up and never dwell. 17-4PH gives high strength after heat treatment and is common in medical and aerospace hardware. If you need corrosion resistance plus strength, 17-4PH is often the right call, but it costs more per part than 303.

Titanium, Inconel and magnesium sit at the hard end. Ti-6Al-4V has low thermal conductivity, so heat stays in the cutter. Tool life drops and cycle times climb. Inconel is worse on both counts and is usually reserved for high-temperature parts where nothing else survives. Magnesium AZ31B and AZ91D cut fast and light, but the chips are flammable and need dedicated handling.

Plastics machine in a different regime. POM and PEEK hold dimensions well. ABS and PC move with temperature and clamp pressure, so light passes and sharp tooling matter. Carbon fiber eats tool edges, so you plan for tool changes. The material you pick sets the machine, the tooling, the feed rates, and the finishing options. Pick it before you finalize the geometry.

• 1
  Default to 6061-T6Cheapest reliable path for most aluminum parts.
• 2
  303 for stainlessFree-machining grade. Use 304/316 only when the spec demands it.
• 3
  Titanium and Inconel cost timeBudget for slower cutting and more tool wear.

Deep pockets are the classic problem. A cutter has to reach the bottom of the pocket, and the deeper the pocket, the smaller the tool shank relative to its length. Long, thin tools deflect, chatter, and leave witness marks. If a pocket is more than four times deeper than it is wide, expect the shop to slow down and take lighter passes, or to EDM the corner detail instead.

Thin walls are next. A wall under 1 mm will move when you clamp it and again when you release it. The finished part can be in tolerance on the machine and out of tolerance on the bench. Good shops leave stock, add support ribs, or flip the part and take light finishing cuts from both sides. The drawing rarely says any of this, but the part knows.

Undercuts and internal features need the right tool geometry. If the tool cannot reach a feature from any angle, the feature cannot be cut by milling. Sometimes a mill-turn center solves it: the part rotates and a turning tool reaches the inside. A Ø400 mm rotary table can also open up angles a three-axis machine cannot reach.

Sharp internal corners are a frequent request and a frequent cost driver. A cutter has a radius, so every internal corner has a radius too. If the drawing calls for a sharp corner, someone has to broach, EDM, or hand-finish it. Tell the shop the smallest internal radius you can live with. It usually saves real money.

• 1
  Deep pocket ruleOver 4:1 depth-to-width, expect slower cutting and chatter risk.
• 2
  Thin walls moveClamping and releasing shift the part. Support or flip it.
• 3
  Corners have radiiState the smallest internal radius you actually need.

A machined surface is rarely the final surface. Anodizing, plating, powder coating, and bead blasting all change dimensions slightly. Hardcoat anodizing builds up on the surface and can shift a bore by a few micrometers. If a bore is held to ±0.005 mm and also hardcoat anodized, the shop needs to know the finish before it cuts the bore. Tell the supplier the finish callout at quote time, not after.

Inspection is where the promise gets checked. A shop that measures every part before it ships is telling you the tolerance is real and not a hope. Raw material certificates, in-process checks, and a final inspection report are standard for regulated work. If you need a report with the shipment, ask for it at quote time so it goes into the plan.

Laser marking is often mistaken for a finish step you can add later. It is, but the minimum character height is around 1.5 mm. If your part number has to be legible after plating, plan the marking after plating and give the shop enough flat area. Tiny marks on a curved surface will not read.

We run 100% inspection before shipment on regulated jobs, with reports on request. That is not a marketing line, it is how the tolerance claim stays true at the customer's dock. The inspection plan is part of the process plan, not an afterthought.

• 1
  Finish changes sizeTell the shop the finish before it cuts the bore.
• 2
  Inspect before ship100% inspection keeps the tolerance claim honest.
• 3
  Plan the marking1.5 mm minimum character height, flat area, after plating.