CNC Processing Cost: What Drives the Number

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  Key takeaways
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  What Moves CNC Processing Cost Most
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  Material choice and how it lands on processing cost
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  Quantity, setup and why unit price falls
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  Cycle time, tolerance and the cost of holding a number
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  Finishing, inspection and what the quote leaves out
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  Step by Step: Checking a Processing Cost Quote

Material is the first line a shop writes and often the largest. The price per kilogram matters, but the machinability rating matters more. Aluminium 6061 cuts quickly with standard carbide and produces short chips. Stainless 316 work-hardens under a dull tool, so feeds drop and the same pocket takes two to three times longer. Titanium TC4 (Ti-6Al-4V) and Inconel sit at the far end: low cutting speeds, high tool wear and more frequent tool changes.

Stock size is the quiet cost. A part cut from a 100 mm plate when a 60 mm plate would do pays for metal that becomes chips. On a 6061 bracket the blank can be 30 to 40% of the material line. Ask your supplier to quote near-net stock, or a casting or forging blank, before you accept the first number.

Not every feature needs the expensive alloy. A common pattern is a 7075 aluminium housing with a 17-4PH insert. The housing machines four times faster and the insert carries the wear surface. Split the part by function, not by habit.

One caution: material substitution changes stiffness, thermal expansion and corrosion behaviour. Swap a stainless bracket for aluminium and the deflection under load changes even if the drawing looks the same. Check the load case before you chase the lower number.

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  Free-cutting first
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  Watch work-hardening grades
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  Blank close to net shape

Setup is a fixed block of work: programming, fixture build, first-article check and machine warm-up. On a single prototype that block is charged against one part. On a 500-piece run it is spread thin. This is why the same bracket can quote at one level for a prototype and a small fraction of it at volume. There is no minimum order quantity at our shop, so a single part is fine, but the buyer should understand what they are paying for at quantity one.

Fixturing is where a good design review pays back. Parts with a flat face, two clear datum holes and parallel clamping surfaces are quick to hold. Parts that are all curved surfaces need soft jaws, custom nests or a five-axis setup that machines five faces in one clamping. That single clamping often beats two or three separate operations because every re-clamp adds a position error and a queue step.

Programming time also amortises. A family of parts that shares geometry can reuse CAM templates and tool libraries. Ten variants of the same housing cost far less to program than ten unrelated parts.

Practical rule: if the annual volume is above roughly 200 pieces, spend an hour on DFM before the first cut. On low volume, keep the design simple and accept the setup cost.

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  Ask for the setup line separately
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  One clamping beats three
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  Reuse part families

Cycle time is the number the spindle actually runs. It is driven by the volume of metal removed, the tool reach needed to get there, and the surface finish on the walls. A pocket 4 mm wide and 30 mm deep needs a long, thin tool that must run slowly and cannot take a heavy cut. Widen it to 6 mm and the same pocket can be cut with a stiffer tool at a higher feed. Radii matter in the same way: an internal corner radius of 1 mm forces a small cutter, while 3 mm lets a larger one in.

Tolerance adds cost in a step, not a slope. A general machining tolerance of ±0.1 mm is normal work. Tightening a bore to ±0.005 mm means slower finishing passes, temperature control and a CMM check. If only two features locate a bearing, tighten those two and leave the rest at general tolerance. Blanket tightening of every dimension on a drawing is one of the most common ways buyers overpay.

Surface finish follows the same logic. As-machined Ra 1.6–3.2 μm is standard. Ra 0.8–1.6 μm needs a finishing pass. Ra 0.2–0.8 μm usually means a separate operation on a fine machine or a polishing step. Specify the finish only where a seal, a bearing or a sliding surface needs it.

A useful check before you release a drawing: list every dimension with a tolerance tighter than ±0.02 mm and write down why it is there. If the answer is habit, loosen it.

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  Open internal radii
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  Keep walls rigid
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  Tighten by function

Post-processing is usually quoted separately because it leaves the machine shop. Anodizing is priced by surface area and by type: clear, colour, hardcoat or conductive. Hardcoat is thicker and wears better, and it costs more. Electroless nickel, zinc plating, silver and gold plating each carry their own rate and their own lead time. Powder coating, black oxide, bead blasting, tumbling, brushing and polishing all sit in the same group.

Masking is a hidden line. Every area you do not want coated must be masked by hand. A part with six masked bores costs more to finish than a part with one. Design the finish so the functional surfaces are grouped and the masked area stays small.

Inspection is the other quiet line. General inspection samples a run. When a drawing calls for a report on every part, or for full dimensional data, the shop has to hold the part on a CMM instead of moving it. That time appears in the quote. Our standard is 100% inspection before shipment, with raw material check, in-process monitoring and final inspection, and reports on request.

Laser marking and engraving is a small item with a hard limit: minimum character height 1.5 mm. Smaller text fills in or reads badly, so it is worth checking the marking layout early.

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  Group functional surfaces
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  Match finish to wear
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  Say what inspection you need