Personal CNC Processing Service Guide

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  Key takeaways
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  What to compare across personal CNC processing service quotes
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  Tolerance in a personal CNC processing service guide
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  Material and finish choices that change the part
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  Why one part costs more than ten
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  Certifications, NDA, and inspection reports
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  How to run a small-order job end to end

Tolerance is the first thing engineers compare, and the easiest to misread. A shop that states ±0.005 mm is describing what its machines and inspection can hold on a controlled feature, not a promise that every dimension on your part lands there. On a 200 mm aluminum bracket, thermal drift and fixturing deflection eat into that budget before the cutter touches metal.

The practical question is which features carry the tight callout. A bearing bore, a dowel pin hole, and a sealing face need the tight tolerance. An outer profile that bolts to a frame usually does not. When you send a drawing where every dimension is ±0.01 mm, you pay for inspection time you do not need.

Ask the supplier to mark up your drawing with achievable tolerances per feature. A good shop returns that markup with the quote. That document is worth more than the price, because it tells you where the risk sits before chips are made.

Geometry matters as much as the number. Deep pockets, thin walls under 0.8 mm, and features with a high depth-to-diameter ratio all push cost up. If a 5-axis setup removes the part in two operations instead of five, tolerance stacks shrink and so does the price.

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  Critical features firstList the 3–5 dimensions that decide fit and function. Everything else can be looser.
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  Wall thicknessBelow 0.8 mm in aluminum, deflection and chatter become the limit, not the tool.
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  Depth-to-diameterPast 4:1 in a small pocket, expect a longer cycle and a higher price.
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  Datum choicePick a datum you can measure on the finished part, not one that disappears after the first op.

Material selection drives strength, weight, corrosion behavior, and machinability. Aluminum 6061-T6 is the default for prototypes and fixtures because it cuts fast and holds tolerance well. 7075 gives higher strength for aerospace brackets but costs more and machines slower. Stainless 303 is the free-machining grade; 316L is chosen when corrosion or biocompatibility matters, and it work-hardens if the feed is too light.

Plastics behave differently. POM and PA machine cleanly and hold dimensions. PEEK handles heat and chemicals but is expensive and abrasive on tooling. Carbon fiber composites cut into abrasive dust, so the shop needs dust control and carbide tooling.

Finishing is where quotes diverge most. As-machined surfaces sit around Ra 1.6–3.2 μm. Bead blasting or tumbling brings that to Ra 0.8–1.6 μm. Fine cosmetic finishes reach Ra 0.2–0.8 μm and require more hand work. Anodizing, plating, and powder coating are separate line items with their own lead time.

Ask whether finishing happens in-house. When a shop sends parts out for anodize, your lead time now depends on a third party, and rework loops get longer. In-house finishing keeps one party accountable for the final surface.

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  Pick the grade, not the family6061-T6 and 6082 are not interchangeable on a fatigue-loaded part.
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  Match finish to functionSealing faces need a controlled Ra. Cosmetic panels need color consistency.
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  Laser marking limitsMinimum character height is 1.5 mm. Smaller text will not stay legible.

Small-volume machining is priced by setup, not by material. Programming, workholding design, tool selection, and first-article inspection happen once whether you order one part or fifty. That fixed block is why a single prototype can cost several times the unit price of a ten-piece run.

Material has a minimum buy size. If your part needs a 100 mm × 100 mm × 50 mm block of 7075, the shop buys a plate and machines one part from it. The leftover is scrap on your invoice unless you order more pieces from the same stock.

Cycle time scales with complexity, not size. A part that fits in your hand with twelve tapped holes and two tight bores can take longer than a plate four times its size with simple profiles. When you compare quotes, compare the operations listed, not the part dimensions.

Volume changes the right process. Below roughly 100 parts, machining usually wins. Past a few thousand, die casting or vacuum casting starts to compete on unit cost, and the tooling amortizes. A supplier that offers both can tell you where the crossover sits for your geometry.

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  Setup is fixedOne-time cost that does not shrink with quantity.
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  Stock minimumsYou often pay for the plate, not the part.
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  Complexity beats sizeFeature count drives cycle time more than envelope.
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  Process crossoverMachining below ~100 parts, casting above a few thousand.

Certificates tell you what a shop's quality system covers. ISO 9001:2015 is the baseline for process control and traceability. IATF 16949:2016 matters if your parts go into automotive production. ISO 13485:2016 applies to medical devices. ISO 27001:2022 covers information security, which is relevant when you upload proprietary CAD files.

Do not treat a certificate as a substitute for an inspection plan. Ask what is measured, on what equipment, and how often. A shop that inspects 100% of parts before shipment and offers reports on request gives you something to check against your own incoming inspection.

Confidentiality deserves a written answer. Uploads should be handled as confidential, and an NDA should be available on request. If a supplier hesitates on an NDA for a prototype with novel geometry, that is a signal about how they treat customer data.

Capacity is the last check and often the deciding one. Machine count and axis configuration determine what geometry is possible. A shop with 16 simultaneous 5-axis centers and a 4,000 mm maximum processing size can take work that a three-axis-only shop has to turn down or split across setups.

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  Ask for the scopeA certificate covers a site and a process, not every part you send.
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  Inspection frequency100% before shipment is a stronger claim than sampling.
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  NDA on requestGet it signed before you upload production CAD.
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  Machine fit5-axis access and work envelope decide feasibility, not marketing pages.