CNC Processing Quotation Online: What the Number Is Built From

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  How a CNC processing quotation online turns geometry into cost
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  Why the same CAD file quotes differently in aluminium and 17-4PH
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  Where tolerance stops being free and starts being cost
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  Five geometry features that make an instant quote unreliable
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  What the file must contain before the quote means anything
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  What happens between upload and the 12-hour reply
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  When an instant online quote is enough, and when it is not

An online quote engine does not look at your part the way a machinist does. It reads the file, measures features, and applies rules. If the rules do not match the real cutting process, the number is wrong in one direction or the other.

The first input is volume removed. A part that starts as a 100 × 100 × 50 mm block of 6061 and ends at 40 g of material carries far more cost than a near-net shape. Material is priced per kilogram, and the block you pay for is the block that gets cut away.

The second input is the number of setups. Every time the part moves to a new face, someone has to re-fixture it, re-zero it, and prove the position. A part with features on four sides needs four setups on a 3-axis machine. On a 5-axis machine it may need one. That single difference often swings a quote by 30 percent or more.

The third input is tolerance spread. A general tolerance of ±0.1 mm is routine. Tightening a bore to ±0.005 mm means slower passes, smaller stepovers, temperature control and more inspection time. The quote engine should flag that, not hide it.

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  Material removedStock size and shape drive both material cost and cycle time.
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  Number of setupsEach new face adds fixturing, zeroing and proving time.
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  Tolerance bandTight bands need slower passes, temperature control and inspection.
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  Feature typeDeep pockets, thin walls and small holes are harder than they look.

Material changes the quote more than most engineers expect. Aluminium 6061 cuts fast, holds tolerance well, and is cheap per kilogram. Stainless 316 is roughly three times slower to cut, work-hardens if the feed is wrong, and needs more coolant and more tool changes.

Titanium TC4 (Ti-6Al-4V) is worse. It conducts heat poorly, so the cutting edge stays hot while the chip carries little heat away. Tool life drops, feeds drop, and the cycle time climbs. A bracket that takes 20 minutes in 6061 can take 90 minutes in Ti-6Al-4V.

Plastics behave differently again. POM and PEEK cut cleanly but move with temperature. ABS and PC can gum up if the chip is not cleared. The quote engine may treat them as easy because they are soft, but a thin wall in POM will deflect and scrap the part.

The practical rule: ask for a quote in the material you will actually use for production. A prototype in 6061 tells you the geometry works. It does not tell you what the part will cost in 17-4PH.

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  Aluminium 6061 / 7075Fast, stable, good for prototypes and production parts.
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  Stainless 303 / 316LSlower cutting, work-hardening risk, more tool wear.
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  Titanium TC4Poor heat conduction, short tool life, long cycle times.
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  Plastics POM / PEEKClean cuts but thermal movement and thin-wall deflection.

Most online quote engines price a default tolerance band. At GreatLight the working tolerance is ±0.005 mm, but not every feature needs it. A mounting hole at ±0.1 mm and a bearing bore at ±0.005 mm are not the same job.

The cost jump happens when tolerance approaches the machine's thermal and geometric limits. At ±0.005 mm, the shop has to control room temperature, use a probe, and inspect on a CMM. At ±0.05 mm, a caliper and a good setup will do.

Surface finish follows the same logic. Ra 1.6–3.2 μm is as-machined. Ra 0.8–1.6 μm needs a finishing pass. Ra 0.2–0.8 μm needs slower feed and a sharper tool, and it raises scrap risk on thin features.

If the drawing marks every dimension as tight, the quote will be high and the shop will spend time proving features that do not matter. Mark the critical ones. Leave the rest at general tolerance.

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  General tolerance±0.1 mm, standard inspection, lowest cost.
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  Close tolerance±0.05 mm, probe check, moderate cost.
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  Precision tolerance±0.005 mm, temperature control and CMM, highest cost.
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  FinishRa 0.2–0.8 μm costs more than Ra 1.6–3.2 μm.

An instant quote is reliable when the part is prismatic, has open faces, and uses standard tool sizes. It becomes unreliable when the tool cannot reach the feature without a special holder or a different machine.

Deep pockets with a depth-to-width ratio above 4:1 need long, thin tools. Those tools deflect, so the shop has to slow down and take lighter passes. The quote engine may not see that until a machinist reviews the file.

Thin walls below 0.8 mm in aluminium or 0.5 mm in stainless are a different problem. The wall moves during cutting, so the shop has to leave material, stress-relieve, and finish in a second pass. That is a manual decision, not an algorithm.

Undercuts, internal channels, and features on five or six faces push the part toward 5-axis work. GreatLight runs 16 simultaneous 5-axis machining centers, so this is routine, but it is not the same price as a 3-axis part.

Small holes below Ø1 mm and sharp internal corners below the tool radius are the last group. They need EDM or a smaller tool, and both add time.

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  Deep pockets over 4:1Long tools deflect; slower passes and more checks.
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  Thin walls under 0.8 mmDeflection risk; may need a second finishing pass.
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  Undercuts and 5-face featuresPushes the job to 5-axis; higher setup but one fixturing.
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  Holes below Ø1 mmNeeds small tools or EDM; slower and more fragile.

A STEP file with a clean solid model is the best input. It carries exact geometry, so the engine can measure features and volume. IGES works but can leave gaps. STL is a mesh, so curved surfaces become faceted and the volume estimate drifts.

The model alone is not enough. The shop needs a 2D drawing or a 3D annotation for tolerances, datums, surface finish, and material. Without that, the quote assumes general tolerance and as-machined finish.

Threads, knurls, and engraving should be called out. Laser marking has a minimum character height of 1.5 mm, so a 0.5 mm logo will not reproduce cleanly. Better to catch that before the quote than after the first part.

If the file is under NDA, say so at upload. GreatLight keeps uploads secure and confidential and can sign an NDA before the file is opened. That does not change the price, but it changes who sees the drawing.

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  STEP preferredExact solid geometry; best for automatic feature measurement.
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  IGES acceptableWorks, but check for surface gaps before quoting.
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  STL for referenceMesh format; curved surfaces are approximated.
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  Drawing requiredTolerances, datums, finish and material must be stated.

The upload triggers two things at once: an automated cost estimate and a DFM check. The DFM check looks for features that cannot be machined as drawn, tool accessibility, and tolerance conflicts.

Within 12 hours, GreatLight returns a quotation and a free DFM analysis. The DFM report may suggest a small change, such as opening a corner radius to match a standard tool or relaxing a tolerance that does not affect function.

If the part is simple, the quote is final and production can start within 24 hours. If the DFM report finds a problem, the shop waits for your decision before cutting metal. That is deliberate. Cutting a part that will be redesigned is the most expensive mistake in the process.

For production runs, the same file goes through a first-article inspection. The quote should state whether inspection reports are included or available on request.

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  Automated estimateRuns at upload; gives a first number.
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  DFM checkFinds features that cannot be machined as drawn.
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  Manual reviewEngineer confirms setup, tooling and tolerance.
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  Production releaseStarts within 24 hours after approval.