How to Quote CNC Machining Without Surprises

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  Key takeaways
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  The seven variables that decide a CNC machining price
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  How material and machine choice change the same quote
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  Tolerance and surface finish: where cheap quotes break
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  Batch size, lead time, and the low-quote trap
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  What a good DFM review returns with the quote
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  How to quote CNC machining in 6 steps
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  Which quote detail changes what

Every machined part quote is built from the same seven inputs: part complexity, material, process and machine, batch size, finishing, tolerance, and lead time. Change one and the number moves. A single wall thickness change from 2.0 mm to 1.2 mm in an aluminum housing can add two extra setups and a soft-jaw fixture, which is real time on the floor.

Part complexity is about how many orientations the tool must reach. A flat bracket with through-holes is a 3-axis job. A part with undercuts, deep pockets, or features on five faces needs 5-axis work or multiple repositions. Each reposition adds setup time, and each setup adds a chance for stack-up error.

Material affects both the stock price and the cutting speed. Aluminum 6061 cuts fast with light tool wear. Stainless 316 and 17-4PH work-harden, so speeds drop and coolant management matters. Titanium TC4 and Inconel cut at a fraction of the aluminum rate and wear tools quickly. That difference shows up as cycle time, not as a line item.

Process and machine choice follows the part, not the budget. A 4,000 mm gantry job cannot run on a 500 mm machine. A Ø400 mm rotary table part may be cheaper on a mill-turn center than on a 3-axis mill with three fixtures. Batch size then decides how much fixture and programming cost each part carries.

Finishing and tolerance are the last two drivers, and they are the ones buyers most often underestimate. Anodizing, hardcoat, and electroless nickel add handling and sometimes a masking step. Tight tolerance adds inspection time as well as machining time. Lead time closes the loop: a 3–5 day ship date is normal capacity, while a rush date may mean overtime or a reshuffled schedule.

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  ComplexityCount the orientations and the undercuts, not the number of features.
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  MaterialMachinability index explains most of the cycle time gap.
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  ToleranceBelow ±0.01 mm, expect a finishing operation and more inspection.
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  Lead timeRush work costs more because it displaces scheduled work.

Start with machinability. Aluminum 6061-T6 machines at high surface speed with good chip evacuation, so cycle time is short and tool life is long. Stainless 303 is free-machining and a reasonable middle ground. Stainless 316 and 17-4PH need slower speeds and more rigid setups. Titanium TC4 (Ti-6Al-4V) and Inconel sit at the slow end, and they demand flood coolant and sharp, dedicated tooling.

The practical question is not which material is best, but which material meets the drawing at a cost the program can absorb. A 7075 aluminum bracket and a 17-4PH stainless bracket may both pass the load case, but their quotes can differ by a factor of two or more because of cycle time and tool consumption.

Machine selection follows geometry and size. GreatLight runs 16 simultaneous 5-axis machining centers, 12 four-axis mills, 27 three-axis machines, and 16 mill-turn centers, with a maximum processing size of 4,000 mm. Large travels of 4,000 × 400 × 150 mm handle long parts; medium travels of 750 × 1,150 × 550 mm and 600 × 600 × 600 mm cover most housings; compact travels of 500 × 500 × 450 mm and 500 × 310 × 200 mm suit small precision work.

A common quoting error is assuming 5-axis is always more expensive. For a part with features on four faces, one 5-axis setup often beats three 3-axis setups plus fixtures. The reverse is also true: putting a simple flat plate on a 5-axis center wastes money. Match the machine to the part.

Stock form matters as well. Plate, bar, and near-net forging each change the material price and the roughing time. If you can accept a forged or cast blank, you remove a large block of roughing from the cycle and the quote reflects it.

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  6061 vs 316Same geometry, roughly double the cycle time on 316.
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  Titanium and InconelPlan for lower speeds, more coolant, and faster tool wear.
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  Mill-turnBest when a part needs turning and cross-features in one setup.
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  Stock formNear-net blanks cut roughing time, not finishing time.

Tolerance and finish are separate requirements and they are priced separately. A dimension can be held to ±0.05 mm on a mill with a good setup. Holding ±0.005 mm on the same feature usually means a dedicated finishing pass, a temperature-stable environment, and a CMM check. That is added machine time and added inspection time.

Surface finish follows the same logic. As-machined Ra 1.6–3.2 μm comes off a normal finishing cutter. High finish Ra 0.8–1.6 μm needs a lighter stepover or a finishing tool change. Fine Ra 0.2–0.8 μm may require polishing or a specialized process, and it is harder to inspect consistently on a complex surface.

The mistake is calling out tight tolerance everywhere. If only a bearing bore and a sealing face need ±0.005 mm, say so and leave the rest at ±0.1 mm. Shops price the tightest callout they see, so a drawing covered in tight tolerances gets a price to match even if most of those dimensions are non-functional.

Inspection is part of the quote, not a free add-on. GreatLight inspects 100% of parts before shipment, with raw material check, in-process monitoring, and final inspection. Reports are available on request. If your industry needs traceability, such as automotive under IATF 16949 or medical under ISO 13485, say it in the RFQ so the inspection plan is priced in from the start.

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  Call out only what mattersTighten functional features, loosen cosmetic ones.
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  Finish is a process stepRa 0.2–0.8 μm is a different operation, not a setting.
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  Inspection has a costCMM checks and reports take time and belong in the quote.
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  Certification drives paperworkIATF 16949 and ISO 13485 add documentation, not just machining.

Setup cost is the same for one part and for one hundred parts. That is why the first piece is expensive and the hundredth is not. GreatLight has no minimum order quantity, so a single prototype and a 10,000+ part run are both normal work. Ask for the quantity bands and look at the slope. If the price barely drops from 10 to 100, ask why the setup is not being amortized.

Lead time is the last variable and the easiest to misread. Standard capacity ships parts in 3–5 days, and production can start within 24 hours after approval. Quotation and free DFM analysis come back within 12 hours. Those numbers assume the drawing is complete and the material is in stock. A missing finish spec or an exotic alloy can push the schedule out.

Now the trap. When one quote is far below the others, it usually contains an assumption: general tolerance instead of the critical callouts, as-machined instead of the specified finish, or a stock size that requires an extra weld. The part arrives, inspection finds the gap, and the rework or replacement costs more than the original difference.

A fair quote shows its basis. It names the material, the machine class, the finish, the inspection level, and what it assumed about the drawing. If a supplier will not explain the assumption, treat the number as provisional. GreatLight returns DFM notes with the quotation so you can see what was priced and what was flagged.

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  Ask for bands1, 10, 100, 1,000 pieces in one request.
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  Check the slopeFlat pricing across quantities hides setup assumptions.
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  Complete RFQ, faster quoteMissing data is the main cause of quote delays.
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  Read the assumptionsThey are where a low quote quietly removes scope.

A DFM review is not a sales document. It is a list of the features that cost money and the alternatives that do not. Typical notes cover wall thickness, corner radii, thread depth, and whether a pocket can be opened up to allow a larger cutter. Each note should come with an estimated effect on cycle time or setup count.

Two examples from everyday work. First, a deep pocket with a 3 mm corner radius forces a small cutter and a long, slow pass. Opening it to 6 mm lets a larger tool run at a higher feed and often removes an EDM step. Second, a part that needs features on four sides may be quoted cheaper as a 5-axis job than as three 3-axis setups with fixtures.

Good reviews also flag what not to change. If a tolerance is functional, the note should say so rather than suggest loosening it. If a surface finish is required for sealing or wear, it stays. The point is to find cost that carries no function, not to weaken the part.

GreatLight reviews CAD files with every quotation and returns the analysis within 12 hours. Uploads are handled as confidential, and an NDA is available on request. If your program needs one, ask before sending the files.

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  Radius and depthSmall internal radii force small tools and slow passes.
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  Face countFour-sided features may suit one 5-axis setup.
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  Functional limitsA good review protects tolerance and finish that matter.
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  ConfidentialityNDA available on request before files are shared.