CNC processing cost analysis: a buyer's guide

• 1
  What drives the number
• 2
  Where a CNC quote actually splits
• 3
  Five things to compare line by line
• 4
  Material and stock form set the floor
• 5
  Setup cost and volume: why 10 pieces hurt
• 6
  Tolerance, geometry, and the cost of tight
• 7
  Finishing, inspection, and paperwork
• 8
  Step by step: build your own cost model

Material is the easiest line to check and the hardest to argue about. A shop buys 6061-T6 plate at a published rate, then adds a cutting charge and a waste allowance. If your part is 80 mm × 60 mm × 25 mm, the blank might be 90 mm × 70 mm × 30 mm, so you pay for metal that becomes chips. In a CNC processing cost analysis, that buy-to-fly ratio is a real number, not a rounding error.

Alloy choice matters more than most drawings admit. 6061-T6 machines fast and is cheap. 7075 costs roughly twice as much per kilogram and cuts slower because it is harder. Stainless 316L is gummier still and will wear tooling faster than 303. Titanium TC4 (Ti-6Al-4V) can run five to ten times the cycle time of aluminum for the same geometry. If the drawing says 'stainless' without a grade, ask which one before you compare prices.

Stock form also moves the number. Plate is cheaper per kilogram than bar for flat parts, but bar is cheaper for turned parts because there is less waste. Castings or near-net forgings reduce machining time at the cost of tooling. For runs above a few thousand pieces, a die casting or investment casting blank can cut machine time by half, but the tooling amortization only works if the quantity is real.

One practical check: ask the shop to quote the same part in 6061-T6 and in 304 stainless. If the gap is under 20%, something is off. Stainless 304 typically runs 1.8–2.5× the aluminum price once you include slower speeds, shorter tool life, and heavier fixturing.

Setup is a fixed cost. It does not care whether you order 1 piece or 1,000. A 3-axis job with two setups might take 45 minutes of fixturing and probing. A 5-axis job with one setup can be faster to set up but slower to program. Either way, that cost is divided by the batch size. At 10 pieces, a 300 CNY setup adds 30 CNY per part. At 1,000 pieces, it adds 0.30 CNY.

This is the single biggest reason a prototype quote looks shocking next to a production quote. It is not price gouging; it is arithmetic. When you run a CNC processing cost analysis, separate the one-time charges from the per-part charges. Ask for a breakdown: programming, fixturing, first article, then unit price. That split tells you whether a higher unit price is real or just a small batch spread over few parts.

Volume also changes the process choice. Below 500 pieces, CNC milling is usually cheaper than die casting because tooling is near zero. Between 500 and 5,000 pieces, the two are close and the decision depends on geometry and finish. Above 5,000 pieces with stable geometry, casting plus light machining often wins. The crossover point moves with part size, wall thickness, and how much finishing is required.

There is a trap here. Buyers sometimes split a 1,000-piece order into ten 100-piece releases to manage cash flow. That destroys the setup amortization and can raise the total by 15–30%. If the design is stable, release in larger blocks. If it is not stable, keep the smaller runs and accept the higher unit price as an engineering cost.

Tolerance is where a quote can double without a single extra feature. A general tolerance of ±0.1 mm on a milled slot is routine. Tighten it to ±0.02 mm and the shop has to slow down, take spring passes, and check the part more often. Tighten to ±0.005 mm and you may need a temperature-controlled room, a specific machine, and a CMM operator. The machining time can go up 40–70% for the same shape.

Geometry adds cost in ways that are not obvious on a 2D drawing. Deep pockets with a depth-to-diameter ratio above 4:1 need long reach tools that deflect, so the shop must reduce feed and take lighter cuts. Thin walls below 1.5 mm vibrate and need support or a different workholding strategy. Undercuts and features on five faces may force a 5-axis setup, which is faster in labor but more expensive in machine rate.

Threads and surface finish also matter. A standard M6 tapped hole is cheap. A fine-pitch thread, a tapered pipe thread, or a thread that must be gauged adds time. A surface finish of Ra 0.8–1.6 μm is a normal machined finish. Ra 0.2–0.8 μm usually needs a separate finishing pass or a secondary operation. If the drawing calls for Ra 0.4 μm on a pocket floor that nobody will touch, question it.

The right question is not 'can you hold this tolerance' but 'does this feature need it.' A mounting boss that locates a bearing needs ±0.01 mm. A clearance hole for an M6 bolt does not. Mark critical-to-quality dimensions clearly and leave the rest at general tolerance. Shops price what they see, and a drawing with ±0.01 mm everywhere reads as 'this will be slow.'

Finishing is often quoted as a lump sum, which makes it hard to audit. Break it down. Anodizing is priced by surface area and by type: clear, color, hardcoat, or conductive. Hardcoat is thicker and costs more. Masking threads and bores adds labor and is frequently underestimated. Powder coating and black oxide are cheaper per part but add handling and cure time. Laser marking is fast, but minimum character height is 1.5 mm, so a tiny logo may need a different method.

Plating is another line that varies widely. Electroless nickel gives a uniform coating on complex shapes. Zinc plating is cheaper. Silver and gold plating are used for conductivity and are priced by thickness and area. If the drawing specifies a plating thickness in microns, the quote should state the same thickness. A quote that says 'plated' without a thickness is not comparable to one that says 'electroless nickel, 10–15 μm.'

Inspection is the last hidden cost. A standard job gets a dimensional check and a visual check. A job with a full CMM report, material certificates, and a first-article inspection per AS9102 costs more because someone has to write the report and hold the parts. For medical and automotive work, traceability and process validation add paperwork. That paperwork is real labor, and it should appear as a line item, not buried in the unit price.

Ask what inspection is included by default. A shop that does 100% inspection before shipment and provides reports on request is giving you something. A shop that ships on a sample check is cheaper, but you accept the risk. The right level depends on what happens if the part fails: a bracket in a prototype is not the same as a manifold on a safety-critical assembly.