Stainless Steel CNC Guide

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  Key takeaways
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  Which stainless grade fits which part
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  Why stainless steel cuts harder than aluminum
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  Tolerance and finish: what to ask for and what to drop
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  Machines, size limits and part features
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  Certificates and inspection reports
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  Lead time, MOQ and how the quote is built
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  Finishing, passivation and post-machining steps
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  Step by step: how to vet a stainless supplier

Stainless steel work-hardens as the tool rubs it. Aluminum shears cleanly and carries heat away fast. Stainless does neither. The cutting edge pushes the metal, the surface hardens under the tool, and the next pass meets a harder skin than the first. That is why a light finishing pass on stainless often fails where a heavier one succeeds.

Thermal expansion is the second problem. Austenitic grades such as 304 and 316 expand roughly 1.7 times as much as steel per degree. A part that measures Ø50.000 mm hot will not measure that after it cools. Shops that run stainless well keep coolant flowing, keep the part temperature stable, and measure after the part has settled.

The third problem is chip evacuation. Stainless chips are stringy and hot. They wrap around the tool, drag on the finished surface and break inserts. Through-tool coolant, peck cycles and interrupted feed help, but the setup has to plan for them.

For a buyer this matters in one practical way: a shop that quotes stainless at aluminum speeds will miss the tolerance, the finish or both. Ask what cutting parameters they start from. A straight answer is a good sign.

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  Work-hardeningDo not dwell in the cut. Keep the tool moving and the depth of cut above the hardened layer.
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  HeatFlood coolant, not mist, for 304 and 316 on any cut deeper than 2 mm.
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  ChipsThrough-tool coolant at 70–100 bar clears deep holes and keeps the finish consistent.

A stainless steel CNC guide that promises ±0.005 mm on every dimension is selling a story. Real tolerance is feature-by-feature. A bearing bore, a seal face or a mating spigot may need ±0.005 mm. A clearance hole for an M6 screw does not, and paying for it wastes money and inspection time.

Send a drawing with the critical dimensions marked. That lets the shop plan the setup, choose the right machine and quote the inspection honestly. When every dimension is critical, every dimension gets measured, and the price reflects that.

Surface finish follows the same logic. Ra 1.6–3.2 μm is what a normal milled or turned surface gives you. Ra 0.8–1.6 μm is a controlled finishing pass with the right insert and feed. Ra 0.2–0.8 μm usually means polishing or lapping after machining, and it applies to selected faces, not the whole part.

One more point on finish: stainless shows handling marks easily. A brushed or bead-blasted part hides small scratches that a bright polished part will show. If the part is visible, pick a finish that survives shipping.

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  Mark critical dimsRed circles on the print beat a note saying 'critical tolerances apply'.
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  Separate finish zonesSpecify which faces need Ra 0.8 μm and leave the rest as machined.
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  Avoid full-part polishingIt adds cost and risk on thin walls and sharp edges.

Stainless parts often need more than one setup. A turned body with cross-drilled ports, for example, needs a mill-turn center or a 4-axis mill with a rotary table. A part with undercuts or angled faces may need 5-axis work to avoid re-fixturing. Every extra setup adds a datum shift that has to be managed.

Size limits matter too. A shop with a 4,000 mm maximum processing size can handle long shafts and rails. A shop limited to 500 mm cannot, no matter what the quote says. Ask for the machine travel envelope, not just the maximum part size.

For small, complex stainless parts, 5-axis simultaneous machining pays for itself. It cuts the number of fixtures, holds one datum, and finishes more faces in a single pass. For a simple Ø20 mm turned pin, it is overkill.

Ask how the shop plans to hold the part. The answer shows whether they understand your geometry or just copied a number from a price list.

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  Cross-drilled partsMill-turn or 4-axis with a Ø400 mm rotary table.
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  Angled faces16 simultaneous 5-axis centers remove extra fixtures.
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  Long partsConfirm travel up to 4,000 × 400 × 150 mm before quoting.

Certificates are not decoration. They tell you which industries a shop already serves and which audits it passes. ISO 9001:2015 is the baseline for any serious supplier. IATF 16949:2016 points to automotive and EV work. ISO 13485:2016 points to medical devices. ISO 27001:2022 matters when your drawings and CAD files are sensitive.

Inspection practice matters more than the certificate on the wall. Ask three questions: is the material checked on arrival, is the process monitored during the run, and is every part inspected before shipment? A shop that answers yes to all three is doing what the certificate requires.

Reports should be available on request. That may mean a dimensional report on the critical features, a material certificate with the heat number, or a first article inspection report for a new part. None of these are free, but all of them are cheaper than a rework loop.

If your part goes into a regulated product, ask for the report format up front. It is easier to agree on a template than to argue about it after the parts are made.

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  Material certHeat number traced back to the mill certificate.
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  Dimensional reportMeasured values on the marked critical features.
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  FAI reportFirst article inspection on the first part of a new run.

A stainless steel CNC guide should be honest about timing. Quotation and DFM feedback in 12 hours is realistic for a shop with the right front-end. Production starting within 24 hours after approval is also realistic for simple parts. Shipping in 3–5 days applies to typical prototype and small-batch runs, not to a 10,000-part order with a hardening step.

MOQ is where many buyers get surprised. Some shops will not quote below 100 pieces. Others will run one prototype and then scale to 10,000+. If you are still validating a design, the second type is the one you want.

Read the quote line by line. Material, machining, finishing, inspection and shipping should be separate. A single lump price hides where the cost sits, and it makes it hard to compare two suppliers fairly.

Ask what happens if a dimension is out of tolerance. A shop that owns the rework without argument is worth more than one that shaves 5 percent off the price.

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  Quote breakdownMaterial, machining, finish, inspection and freight as separate lines.
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  MOQNo minimum order quantity means you can validate a design before scaling.
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  Rework policyAgree in writing who pays if a critical dimension is out.

Stainless parts usually need something after machining. Bead blasting and tumbling remove tool marks and edge burrs. Brushing gives a directional grain that hides handling marks. Polishing brings a part to a mirror finish but costs more and shows every scratch after assembly.

Passivation is often requested for 304 and 316 parts that see moisture or chemicals. It removes free iron from the surface and restores the chromium oxide layer. It does not fix a bad weld or a rough surface, so it should come after machining and finishing, not before.

Electroless nickel, zinc plating and black oxide are also common. Each one changes the final dimension slightly. If a plated part has a tight bore, the plating thickness has to be in the drawing allowance.

Laser marking is useful for part numbers and lot codes. Minimum character height is 1.5 mm for a clean mark on stainless. Smaller text on a curved surface is hard to read and may fail your incoming inspection.

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  Deburr firstTumbling or bead blasting before passivation gives a cleaner finish.
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  Allow for platingElectroless nickel adds 0.005–0.025 mm per side.
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  Marking sizeKeep laser text at 1.5 mm or larger for legibility.