CNC Machining Franklin: How the Process Actually Works

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  What CNC Machining Franklin Actually Removes
• 2
  Why Axis Count Changes the Part You Can Make
• 3
  How Material Choice Sets the Practical Limits
• 4
  Reading a Tolerance Callout Without Overpaying
• 5
  Where CNC Stops Being the Right Answer
• 6
  Matching Process and Material to the Job

CNC machining is subtractive. A rotating cutter removes material from a solid block until the remaining shape matches the model. Nothing is formed or cast, so the part's strength comes from the parent stock and the geometry you cut into it. That single fact drives every decision that follows: tool access, fixturing, wall thickness, and how much material you will pay to have turned into chips.

The cutting edge does the work, not the machine frame. A carbide end mill running at the right surface speed shears metal into chips. Run it too slow and the tool rubs, work-hardens the surface, and shortens tool life. Run it too fast and the edge breaks down from heat. On 6061-T6 aluminum, shops commonly run 300 to 500 m/min surface speed; on 316 stainless that drops to roughly 100 to 150 m/min.

Cutting generates heat, and heat moves the part. A block that measures 100.000 mm cold can grow several hundredths of a millimeter as the cut progresses. Good shops rough, let the part cool, then finish. That wait is not wasted time. It is the difference between a bore that measures ±0.005 mm and one that measures ±0.03 mm.

Geometry sets the boundary. Deep pockets need long tools, and long tools deflect. A tool with a 4:1 length-to-diameter ratio can still hold a tight tolerance; past 8:1, chatter shows up in the surface finish and the size drifts. That is why a five-axis machine helps: tilting the part lets a short, stiff tool reach a feature that a three-axis setup would need a long tool for.

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  Roughing removes bulkLeave 0.3–0.5 mm of stock for the finishing pass.
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  Finishing sets the toleranceLight depth of cut, high spindle speed, sharp tool.
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  Chip evacuation mattersRecut chips dull edges and scratch finished walls.

A three-axis mill moves the tool in X, Y and Z while the part stays put. It is fast, rigid, and cheap per hour. Most flat plates, brackets, housings with open faces and simple turned parts never need anything more. If your part has features on five faces, though, a three-axis machine forces you into multiple setups.

Every setup adds error. Clamp the part, cut one face, unclamp, rotate, reclamp, and you have introduced a new datum. Stack three setups and a ±0.005 mm feature on the first face may not line up with a hole on the fourth. Four-axis machining adds a rotary table, often Ø400 mm, so the part indexes around one axis without being re-clamped. That covers shafts, impellers and parts with radial features.

Five-axis machining tilts the tool or the table on two axes at once. The payoff is not speed. It is access. Undercuts, deep cavities, angled ports and blended surfaces can be cut in one setup with a short tool. Complex hydraulic manifolds, turbine blades and medical implants are the classic cases where five-axis is the only practical route.

The trade-off is programming time and machine rate. A five-axis program can take several times longer to prove out than a three-axis one, and the hourly rate is higher. If your part can be made in two three-axis setups, that is usually the cheaper path. Reach for five-axis when the geometry genuinely demands it, not by default.

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  Three-axisPrismatic parts, one accessible face, tight budget.
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  Four-axisShafts and radial features, indexed around one axis.
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  Five-axisUndercuts, blended surfaces, one-setup accuracy.

Aluminum is the default for prototypes and many production parts. Grades like 6061, 7075 and 6082 cut fast, hold a good finish, and cost less per kilogram than steel or titanium. They are also soft, so thin walls deflect under clamping pressure and threads strip if you tap too aggressively. A 0.8 mm aluminum wall is machinable; a 0.4 mm wall is a conversation with your machinist.

Stainless steel grades 303 and 304 machine very differently. 303 contains sulfur, which breaks chips and makes it the friendlier free-machining choice. 304 work-hardens the moment the tool rubs, so a dull cutter turns a soft surface into a hard one within a few passes. 316L and 17-4PH behave similarly and are common in medical and marine work where corrosion resistance matters.

Titanium and Inconel sit at the difficult end. Ti-6Al-4V conducts heat poorly, so the cutting edge absorbs most of it, and tool life is short. Inconel is worse. Both can be machined to ±0.005 mm, but expect slower removal rates, more tool changes, and a higher price per part. If a design can use aluminum instead, it usually should.

Plastics have their own rules. POM and PEEK cut cleanly and hold tolerance well. ABS and PC soften with heat and may need air blast rather than flood coolant. Carbon fibre is abrasive and eats tool edges, so it is often cut with diamond-coated tooling. Tell the shop the exact grade, not just the family.

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  AluminumFast, cheap, good finish. Thin walls deflect.
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  Stainless303 free-cutting; 304 and 316 work-harden.
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  Titanium and InconelMachinable but slow, with short tool life.

A tolerance is a cost statement. Asking for ±0.005 mm across every dimension on a part means every feature must be measured and controlled at that level. Most parts need that tightness on two or three critical features only. Everything else can live at ±0.05 mm or looser. Mark the critical dimensions on the drawing and let the rest float.

Surface finish follows the same logic. Ra 1.6–3.2 μm is a normal as-machined finish. Ra 0.8–1.6 μm needs a controlled finishing pass. Ra 0.2–0.8 μm usually means a secondary operation such as fine grinding or polishing. Each step down in roughness adds cost and time, so specify the coarsest finish that still does the job.

Inspection is where the promise gets checked. A shop that inspects 100% before shipment, checks incoming raw material, and monitors dimensions in process will catch a drifting tool before the whole batch is scrap. Reports are available on request. If your industry requires traceability, ask for the report up front rather than after the parts arrive.

Certifications tell you which systems are already in place. ISO 9001:2015 covers general quality management. IATF 16949:2016 is the automotive standard. ISO 13485:2016 applies to medical devices. ISO 27001:2022 covers information security, which matters when you send CAD files to an outside shop. Match the certificate to your industry, not to a marketing page.

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  Tight where it countsReserve ±0.005 mm for functional features.
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  Finish by functionSealing faces need finer Ra than cosmetic ones.
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  Ask for the reportTraceability should be agreed before the run.

CNC machining wins on accuracy, material choice and speed to a first part. It loses on unit cost at volume. If you need 50,000 identical small brackets, die casting or stamping will beat it on price once tooling is amortized. Machining is the right call for the prototype and the low-volume bridge run; the question is where the crossover sits, and that depends on part size and feature count.

Hollow parts are another boundary. A machined hollow sphere or a thin-shell enclosure wastes material and time, because most of the stock goes into chips. Vacuum casting or 3D printing can build that geometry additively or in a mould. Machining then handles the critical mating surfaces.

Very small features have a floor too. A 0.5 mm end mill exists, but it breaks easily and cannot cut deep. If your part is mostly features under 1 mm, consider whether the design can be relaxed, or whether a different process suits it better.

The honest summary: CNC machining is unmatched when you need a real material, tight tolerances, and a part in your hand quickly. It is the wrong tool when the geometry is hollow, the volume is huge, or the features are microscopic.

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  High volumeCasting or stamping wins after tooling cost.
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  Hollow shellsMost stock becomes chips. Consider moulding.
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  Micro featuresSmall tools break. Relax the design if you can.