Custom CNC Ordering Guide: From RFQ to Inspected Parts

• 1
  What a Custom CNC Order Really Contains
• 2
  Why Part Geometry Decides the Machine, Not the Other Way Around
• 3
  How Tolerance Choices Change the Price
• 4
  Material and Finish: Where Hidden Cost Sits
• 5
  Inspection, Documentation and What to Ask For
• 6
  When Custom CNC Is the Wrong Process
• 7
  Step by Step: Running a Custom CNC Order
• 8
  Matching Machine Class to Part Type

A custom CNC order is not a file upload. It is a set of decisions: material, geometry, tolerance, finish, quantity and inspection. Get those six right and the quote is clean on the first pass. Miss one and the shop either asks three questions or prices in a cushion you never see.

The engineer's job at RFQ stage is to tell the shop what the part must do. The shop's job is to say how it will be cut. When both sides mix those up, the result is a cheap quote that becomes expensive at first article, or a high quote for a part that never needed ±0.005 mm.

Most delays we see come from drawings that describe intent but not limits. A model shows nominal geometry. A drawing says which surfaces actually matter. Send both, and state which one governs where they disagree.

Quantity changes the process, not just the price. One prototype and a 10,000-part run may use different fixturing, different stock, and sometimes a different machine class. Tell us the annual volume early so the process is chosen once.

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  ModelSTEP, IGES, X_T, SLDPRT or Parasolid for 3D geometry.
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  DrawingPDF or DWG for 2D callouts, datums and notes.
• 3
  Critical featuresList the fits, sealing faces and bores that carry function.
• 4
  VolumePrototype count plus expected annual usage.

A 3-axis mill cuts from one direction. Every new face needs a new setup, and every setup adds a datum shift. For a plate with holes on one face, that is fine and cheap. For a housing with ports on four sides, the same part on a 3-axis machine can need five or six operations.

A 4-axis machine adds rotation around one axis. Shafts, bushings and parts with features indexed around a bore become single-setup work. This is often the most cost-effective upgrade in a custom CNC order, because it removes handling rather than adding capability.

A 5-axis machine moves the tool and the part together. Undercuts, compound angles and contoured pockets can be cut without re-fixturing. We run 16 simultaneous 5-axis machining centers, and they earn their cost on parts where setup count, not cutting time, is the bottleneck.

The trade-off is real. Five-axis programming takes longer, and the machine hour is higher. If your part is a flat bracket with a bolt pattern, a 3-axis machine will make it faster and cheaper. Complexity is what you pay for, so only buy it where geometry demands it.

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  3-axisPrismatic parts, one dominant face, simple holes and pockets.
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  4-axisRotational features, slots and holes around a bore.
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  5-axisUndercuts, compound angles, deep contoured cavities.

Tolerance is the single largest cost lever in a custom CNC ordering guide, and it is the one most often set by habit. Copying ±0.005 mm across every dimension on a drawing tells the shop nothing about which features matter. It also forces slower passes, more probing and more scrap risk everywhere.

A practical approach is to split the drawing into functional and non-functional dimensions. Sealing faces, bearing bores, mating spigots and press fits get the tight callout. Clearance holes, outer profiles and non-mating surfaces get the general tolerance block.

We hold ±0.005 mm (±0.0002 in) where a part needs it. That number is a capability, not a default. Applying it to a 300 mm aluminium plate with no thermal control on the shop floor is a different risk than applying it to a 40 mm stainless bushing.

Temperature matters at this level. Aluminium moves roughly 23 μm per metre per degree Celsius. A 400 mm part measured at 25 °C and used at 40 °C will not match its inspection report, no matter how good the machine is. State the working temperature when it counts.

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  General blockUse for non-mating features; keeps cycle time down.
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  Tight calloutsReserve for fits, sealing faces and bearing bores.
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  GD&TPosition and profile callouts need datums that match how the part is held.

Material choice sets cutting speed, tool wear and the risk of movement after machining. Aluminium 6061 and 7075 cut fast and hold tolerance well. Stainless 316L and 17-4PH work-harden, so light passes and rigid setups matter more than spindle speed. Titanium TC4 (Ti-6Al-4V) needs sharp tooling and generous coolant, and it will move if you remove stock unevenly.

Plastics behave differently again. POM and PEEK hold dimension better than ABS or PP, but all of them can creep under clamping pressure. Thin walls in plastic are a fixturing problem, not a cutting problem.

Finish is usually quoted as a separate line and applied after machining. Anodizing adds a build layer of a few micrometres, which matters on a threaded or press-fit feature. Hardcoat anodizing builds more and cuts into the tolerance budget. Say which features must stay bare.

Laser marking has a floor. Our minimum character height is 1.5 mm, so a serial number designed at 0.8 mm will not read cleanly after marking. Fix that at drawing stage, not at inspection.

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  AnodizingClear, colour, hardcoat or conductive; check build on fits.
• 2
  PlatingElectroless nickel, zinc, silver and gold for wear or conductivity.
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  Bead blasting and polishingCosmetic and Ra control from 0.2 to 3.2 μm.
• 4
  Black oxide and powder coatCorrosion and appearance on steel parts.

Inspection should match the drawing, not a generic checklist. A part with three tight bores needs dimensional reports on those three bores. A cosmetic panel needs surface and finish verification. Ask for the data that proves the features you called out.

Our standard flow is a raw material check, in-process monitoring, and a final inspection before shipment. Every part is inspected. Reports are available on request, and the qualification rate we record is 99.99%.

For regulated programs, the paperwork matters as much as the part. We hold ISO 9001:2015, IATF 16949:2016, ISO 13485:2016 and ISO 27001:2022. That covers general quality, automotive, medical devices and information security. Tell us at RFQ which certificate your program requires so the documentation is built in parallel with production.

Confidentiality is part of the order, not an add-on. Uploads are secure and confidential, and an NDA is available on request before you send files.

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  Dimensional reportRequest for critical callouts and fits.
• 2
  Material certTraceability to the mill lot for regulated parts.
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  First articleUseful on new geometry before a full run.

CNC machining removes material. That makes it accurate and material-flexible, but it wastes stock and takes time proportional to volume. If a part is a thin-walled enclosure with no tight features, sheet metal fabrication will be cheaper and faster. If it has internal channels and a complex shell, casting or 3D printing may win on cost.

Deep pockets are a common boundary. A pocket deeper than about four times its cutter diameter needs long, slender tooling that deflects. The shop can still cut it, but cycle time and scrap risk climb. Redesigning the corner radius to suit a larger cutter often saves more money than any tolerance relaxation.

Thin walls are the other limit. Below roughly 0.8 mm in aluminium, clamping and cutting forces start to distort the part, and the finished wall may not match the model. This is a design decision, not a machine limitation.

The honest answer is that CNC is the right call for prototypes, low to mid volume, tight tolerance and parts where material properties matter. For a 50,000-piece simple bracket, it is usually the wrong one.

• 1
  Consider sheet metalUniform thickness, bends, no tight machined features.
• 2
  Consider castingHigh volume with complex internal geometry.
• 3
  Consider 3D printingVery complex shells at low volume, looser tolerance.