Guide to Outsourcing CNC: What Moves Off Your Floor

• 1
  What a guide to outsourcing CNC really transfers
• 2
  How a CNC quote is actually built
• 3
  Which features decide the machine and the price
• 4
  Tolerance and surface finish: pick a band, not a number
• 5
  Where outsourcing CNC projects go wrong
• 6
  Which parts to outsource and which to keep in-house

You are not buying machine time. You are buying a process that turns a 3D model into a measured part. The supplier owns the setup, the tooling, the in-process checks and the scrap. That is the real transfer, and it is why the first question is never price. It is which operations you can hand over without losing control of the drawing.

A 3-axis part with one setup and loose tolerances is easy to move. A thin-walled housing that needs four setups and a fixture built for it is not. The difference is not the machine. It is how much process knowledge leaves the building with the part.

Most shops quote from a STEP file, a 2D drawing and a quantity. If the drawing disagrees with the model, the drawing usually wins, because that is what the inspector measures against. Send both, and say which one governs.

This guide covers the quote itself, the geometry that drives cost, the tolerance choices that matter, and the parts that should stay in-house. Read it before you send the first RFQ, not after.

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  Process, not hoursSetup, tooling, inspection and scrap all sit with the supplier.
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  Drawing governsIf model and drawing conflict, state which one is the authority.
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  Setup count is the cost driverEvery extra orientation adds fixture time and error stack-up.

A quote is a sum of hours and risk. Programming and setup are fixed costs spread over the batch. Cycle time scales with volume. Inspection and fixturing sit somewhere in between, and they are where two shops quoting the same part can land 40% apart.

Setup dominates small batches. Clamping a part, touching off tools, and proving the first article can take longer than the cutting. At one or two pieces, you are mostly paying for setup. At 500 pieces, cycle time takes over.

Material is quoted at bar or plate size plus a cut allowance, not at finished weight. A part that fits in a 100 × 100 × 50 mm block is not quoted at its final 60 g. Ask what stock size was assumed if the number looks high.

Tolerances drive cost faster than any other line item. Moving a bore from ±0.05 mm to ±0.005 mm can add a finishing pass, a temperature-controlled check, and a scrapped part or two during prove-out. The tighter the band, the fewer machines that can hold it repeatably.

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  Fixed vs variableSetup and programming are fixed; cycle time scales with quantity.
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  Stock, not finished weightMaterial is priced at the block it is cut from.
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  Tolerance is a multiplierTightening a single feature can add a whole operation.

Deep pockets, thin walls and features on many faces push a part toward 5-axis. A 5-axis center moves the tool or the workpiece along X, Y, Z plus two rotary axes, so an angled face can be cut in one orientation instead of three. Fewer refixtures means less stack-up error and better position repeatability.

Deep undercuts and internal channels that no straight tool can reach are the clearest case for 5-axis. If the feature cannot be reached from any single direction, or needs a long thin tool that will chatter, multi-axis is usually cheaper than a custom electrode or a split design.

Wall thickness matters more than most drawings admit. Below roughly 1 mm in aluminium, cutting forces start to deflect the wall and the finished part springs back. Add a light finishing pass, or thicken the wall and accept the weight.

Internal corners should carry a radius at least equal to the tool radius you expect. A square internal corner means either a broach, an EDM pass, or a redesign. Most shops will radius it for you and flag it in DFM, but asking first saves a revision round.

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  Multi-face featuresAngled faces and radial holes favour 4- or 5-axis.
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  Unreachable geometryDeep undercuts need multi-axis or a split part.
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  Corner radiiMatch them to standard cutter sizes to avoid EDM.

General tolerances belong on the title block, not on every dimension. A drawing where all 60 dimensions carry ±0.01 mm tells the shop nothing about which three actually matter. Mark the critical ones and let the rest sit at a general band.

Surface finish is called out as Ra, the arithmetic mean roughness. As-machined aluminium typically lands around Ra 1.6–3.2 μm. A good finishing pass reaches Ra 0.8–1.6 μm. Below that, you are buying lapping or polishing time, and the cost climbs quickly.

Function should set the number. A seal bore or a bearing seat needs a tight band and a fine finish. A bracket that bolts to a frame does not. Specifying tight everywhere is the most common way to double a quote without improving the part.

If a feature is measured, say how. A bore checked with a pin gauge and the same bore checked on a CMM can disagree at the edge of the band. State the method and the shop will hold the number the same way you will.

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  Title-block tolerancesKeep general bands general and flag only the critical dims.
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  Finish costs step upBelow Ra 0.8 μm you are paying for a secondary operation.
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  State the measurement methodPin gauge and CMM can disagree at the band edge.

The most common failure is an incomplete drawing. A STEP file with no tolerance callouts forces the shop to guess, and the guess will not match your intent. Send a 2D drawing with critical dimensions, datums and finish callouts.

The second is a late change. A revision after the first article has been proved can invalidate the fixture and the program. That cost lands somewhere, and usually on the next quote.

The third is a single-source plan with no second shop qualified. If your only supplier goes down for two weeks, you have no fallback. Qualifying a second shop on a low-risk part is cheap insurance.

Material certification is the quiet one. If the part goes into an aerospace or medical assembly, you need mill certs and traceability tied to the lot. Ask before the material is cut, not after the parts are finished.

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  Incomplete drawingsModel plus a 2D drawing with callouts, every time.
• 2
  Late revisionsChanges after first article can invalidate fixture and program.
• 3
  Single sourceQualify a backup on a low-risk part early.