Reference pages for engineers who have to pick a grade before the drawing is released. Compare stainless, aluminum, titanium and plastics on strength, machinability and finish, then send the part out for a DFM check.
Most of these are decided at the desk, before any chip is cut.
316 resists chlorides, but it work-hardens fast and galls on the tool. A bracket that needed no corrosion pass gets a higher cycle time, more tool changes and a rougher as-machined surface. If the part never sees salt or acid, 303 machines about three times faster per cut.
17-4PH in the H900 condition machines cleanly, but the same part in the annealed condition can move 0.05 mm during aging. If the tolerance is tight, the sequence matters more than the grade. We cut oversize, age, then finish to size. Skip that order and you scrap the lot.
Ti-6Al-4V conducts heat poorly, so the cutter keeps the heat and the wall deflects. A 1.5 mm wall run at full radial engagement will chatter and come out tapered. Trochoidal paths, lower radial cut and a rigid setup fix it. The toolpath is the fix, not a slower spindle.
POM and PA move with moisture and temperature. A ±0.05 mm callout on a 200 mm POM part is not realistic. PEEK holds better but costs many times more than POM. Match the grade to the actual service temperature, then set the tolerance band around that.
Each guide answers the same three questions: what the grade does, how it cuts, and when to avoid it.
Austenitic grades (303, 304, 316, 316L) do not harden by heat treatment. They harden by cold work, which is why a heavy finishing pass on 304 can raise the surface hardness under the cutter and dull the edge. 303 adds sulfur for chip breaking, so it machines cleanly but welds poorly and is not the grade for a pressure boundary.
Martensitic and precipitation grades behave differently. 420 and 440C reach high hardness after heat treatment and are common for shafts, molds and wear plates. 17-4PH gives high yield strength with better corrosion resistance than 440C. The trade-off is the aging step and the dimensional change that comes with it.
6061-T6 is the default for machined aluminum: good strength, good finish, predictable chips. 7075-T6 is stronger but less corrosion resistant and more prone to stress corrosion cracking in the wrong environment. 5052 and 3003 bend well and are common for sheet parts, though 3003 is soft and dents easily. 201 is strong and machines well but has poor corrosion resistance unless it is anodized.
Titanium is a different problem. Ti-6Al-4V has roughly twice the strength of 6061 at similar density, but the thermal conductivity is low and the material galls. Cutting speeds drop to a fraction of aluminum, and tool life is measured in minutes, not hours. Use titanium only when the strength-to-weight ratio or the corrosion resistance is the actual requirement.
Use this as a first filter, then read the full guide for the grade you pick.
| Grade | Best for | Watch out for |
|---|---|---|
| 303 stainless | Shafts, bushings, fasteners | Poor weldability, low corrosion resistance |
| 304 / 316 stainless | Food, medical, marine hardware | Work hardening, gummy chips, tool wear |
| 17-4PH | High-strength valve and pump parts | Dimensional change during aging |
| 420 / 440C | Wear plates, molds, knife edges | Must be machined before hardening |
| 6061-T6 | General machined housings | Lower strength than 7075 |
| 7075-T6 | Aerospace, molds, high load | Stress corrosion if not anodized |
| Ti-6Al-4V | Aerospace, medical implants | Low speed, short tool life, high cost |
| POM / PEEK | Insulators, seals, wear pads | Thermal growth, moisture absorption |
The grade only matters if the process can hold the tolerance. These are the routes we run in-house.
16 simultaneous 5-axis centers for contoured surfaces, impellers and single-setup complex parts. Fewer setups means less accumulated error on tight features.
27 three-axis machines, 12 four-axis mills and 16 mill-turn centers. Turned parts up to Ø400 mm on the rotary table, milled parts to 4,000 mm.
One-off parts for fit checks and test rigs. No minimum order, so a single prototype goes through the same inspection as a production run.
Bent and formed parts in 5052, 3003 and 304. Good for enclosures and brackets where a machined block would waste material.
Anodizing, plating, bead blasting, brushing and laser marking. Finish callouts change the final dimension, so they belong in the drawing review.
For geometry that cannot be machined or for early ergonomic checks. Use it before committing to a machined alloy.
Numbers below are the working envelope, not the marketing ceiling.
| Parameter | Range | Notes |
|---|---|---|
| Tolerance | ±0.005 mm (±0.0002 in) | On critical features with a stable setup |
| Surface finish | Ra 0.2–0.8 μm fine | Ra 0.8–1.6 μm standard, Ra 1.6–3.2 μm as machined |
| Maximum part size | 4,000 mm | Travel 4,000 × 400 × 150 mm on large machines |
| Materials | Aluminum, stainless, steel, copper, titanium, plastics | Inconel and magnesium on request |
| Inspection | 100% before shipment | Raw material, in-process and final checks; reports on request |
| Order size | 1 to 10,000+ parts | No minimum order quantity |
Every guide on this page is written against the machines and inspection we actually run.
Fifteen years of production work across aerospace, automotive, medical and robotics parts. The grade advice comes from jobs that shipped, not from a handbook.
Held on critical features with verified setups. If a feature cannot hold that, we say so in the DFM analysis instead of quoting it anyway.
16 simultaneous 5-axis centers, 16 mill-turn centers, 12 four-axis mills and 27 three-axis machines. Hard materials get the rigid machine they need.
Quotation and free DFM analysis within 12 hours. Production can start within 24 hours once the drawing and material are confirmed.
Measured across production runs. Raw material check, in-process monitoring and final inspection before anything ships.
Parts ship in 3–5 days for standard work. Historical late-delivery probability is below 2%.
7075-T6 and Ti-6Al-4V parts where strength-to-weight decides the grade. Thin walls, tight profiles, anodized where corrosion matters.
316L and titanium components with no cross-contamination between grades. Clean deburring, documented inspection, ISO 13485 process control.
17-4PH and 4140 parts machined oversize, heat treated, then finished to the final tolerance. IATF 16949 process discipline.
6061-T6 and 7075 frames, 303 stainless shafts and POM insulators in the same assembly. Mixed-material runs without mixing tolerances.
Choose by the environment. 316 adds molybdenum, which holds up better in chlorides, salt spray and some process chemicals. 304 is fine indoors and costs less.
On the machine, both work-harden and produce stringy chips. For a part with no corrosion exposure, 303 cuts faster and leaves a better as-machined finish than either.
Aging precipitates a copper-rich phase in the matrix, and the part shrinks slightly as the structure rearranges. The amount depends on the aging temperature and the part geometry.
The practical answer is sequence. Machine oversize, age, then finish to the final tolerance. We plan that in the DFM analysis so the finished part lands inside the band.
Yes on rigid setups with the right cutter and coolant strategy, but the cost is different. Cutting speeds for Ti-6Al-4V are a small fraction of 6061, and tool life is much shorter.
For thin walls below about 2 mm, chatter becomes the limiting factor. Trochoidal toolpaths and lighter radial engagement control it. If the feature is very thin, we will say so before quoting.
Above roughly 45 HRC, carbide starts to struggle and the setup has to be more rigid. 440C in the hardened state is usually ground, not milled.
So the sequence is set by the hardness target. Machine 420 or 440C in the annealed state, heat treat to the required hardness, then finish by grinding or hard milling where the geometry allows.
POM absorbs moisture and moves with temperature. A ±0.05 mm band on a long POM part is optimistic. PEEK holds dimensions better and takes higher temperatures, but the material cost is far higher.
Tell us the service temperature and the mating part, and we will set a realistic band instead of copying a metal tolerance onto a plastic drawing.
It can. Anodizing builds a thin oxide layer, and plating adds thickness on every surface. Both move the part toward the upper limit of the tolerance band.
So the finish callout belongs in the drawing review, not in a separate purchase order line. We adjust the pre-finish dimension so the coated part lands inside tolerance.
No minimum order quantity. One prototype and a 10,000-part run go through the same inspection process.
For a first trial, send the drawing with the material and the critical dimensions marked. You get a quotation and free DFM analysis within 12 hours.
Inspection reports are available on request, covering raw material certification and final dimensional checks. Every part is inspected before shipment.
If you need an NDA before sharing drawings, one is available on request. Uploads are kept secure and confidential.
Upload the part with the grade you have in mind. We check machinability, tolerance and finish, then quote the process that holds it.
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CNC Metals 13 grades
CNC Plastics 10 grades
Machines & processes 12 options
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