CNC Milling 4340 Alloy Steel is part of VT machining's grade-specific capability. 4340 Alloy Steel is high-strength and hardenable, and we combine controlled machining with material-aware setup to produce parts that meet your specification.
CNC Milling Capability for 4340 Alloy Steel
- Cr-Mo / Ni-Cr-Mo grades
- High strength & hardenability
- Tolerance to ±0.01 mm
- Heat-treatable to 30–55 HRC
4340 Alloy Steel Material Data
| Density | 7.85 g/cm³ |
|---|---|
| Tensile strength | 655–1020 MPa |
| Yield strength (0.2%) | 415–900 MPa |
| Hardness | 197–302 HB |
| Melting point | 1416 °C |
| Thermal conductivity | 42.6 W/m·K |
| Thermal expansion | 12.3 ×10⁻⁶/K |
| Electrical conductivity | 8% |
| Machinability (C36000 = 100%) | 65% (annealed) |
| Grades we run | 4140, 4340, 8620, 20CrMnTi, 42CrMo4, 20MnCr5 |
Published typical values for 4140 (Q&T). Other grades in the family differ — the certificate supplied with your order carries the actual heat-lot figures.
Heat treatment moves the part. Anything held tighter than ±0.02 mm is machined soft, hardened, then ground or hard-turned to final size.
Machining 4340 Alloy Steel with CNC Milling
When machining 4340 Alloy Steel, machinability is good in annealed condition; machined soft then heat-treated to final hardness. Our process plan reflects this so the finished part holds its tolerances without distortion or poor finish.
Documented parameters for 4340 Alloy Steel mean predictable cycle times and stable, repeatable results.
Achievable Tolerances for CNC Milling 4340 Alloy Steel
| Feature | Standard | Precision |
|---|---|---|
| Linear dimension (< 100 mm) | ±0.05 mm | ±0.010 mm |
| Linear dimension (100–300 mm) | ±0.10 mm | ±0.025 mm |
| Hole diameter (reamed) | H9 | H7 |
| True position of hole pattern | ±0.10 mm | ±0.025 mm |
| Flatness over 100 mm | 0.05 mm | 0.015 mm |
| Perpendicularity | 0.10 mm | 0.020 mm |
| Surface finish, as machined | Ra 1.6 µm | Ra 0.4 µm |
Standard is what a normal quotation assumes. Precision is available on nominated features and is priced separately, because it needs extra setups, in-process gauging and CMM verification. Call out only the features that need it — blanket-tightening a drawing is the single most common reason a quote comes back expensive.
Design Tips for CNC Milling 4340 Alloy Steel
Small design adjustments often cut cost and lead time. Generous internal radii, sensible pocket depth-to-width ratios and avoiding very thin unsupported walls all reduce cost and improve accuracy in 4340 Alloy Steel. If you send your model early, we will return practical DFM feedback before you commit to production.
Why 4340 Alloy Steel?
Customers select 4340 Alloy Steel for CNC Milling because it is high-strength and hardenable. We handle grade verification and any required heat treatment or finishing.
CNC Milling Tolerances & Surface Finish
For CNC Milling, dimensional and positional tolerances around ±0.01 mm are routine, tightening toward ±0.005 mm on precision setups. Surface finish on machined faces typically falls in the Ra 0.4–1.6 µm range depending on tooling and pass strategy.
Exact achievable figures depend on the alloy, the feature size and the part geometry. Tell us which dimensions are critical and we will advise feasibility up front.
Secondary Operations & Finishing
Most 4340 Alloy Steel parts need some finishing, which we manage in-house or through vetted partners. Options include deburring and edge-breaking, heat treatment, surface grinding, polishing, and plating or anodising for corrosion resistance and appearance. Give us the finished-part requirement and we will sequence every operation correctly.
CNC Milling Quality & Inspection
Quality is built in, not inspected on. We monitor key features during the run and confirm them at final inspection, with CMM verification available for tight tolerances. We provide the inspection and traceability records your quality system requires.
Applications of CNC Milling 4340 Alloy Steel
4340 Alloy Steel parts made this way are found throughout aerospace, medical, automotive, electronics and industrial equipment. Prototype, bridge and production volumes are all welcome.
How to Order CNC Milling 4340 Alloy Steel
Starting your project takes one email. Send your CAD or 2D drawing, the 4340 Alloy Steel grade and condition, the quantity and any finish or inspection requirements. We respond quickly with price, lead time and any DFM notes, in clear English.
Finishing Options for 4340 Alloy Steel
| Finish | Thickness / effect | Why it gets specified |
|---|---|---|
| Through hardening | — | 30–55 HRC, ground afterwards for tight sizes |
| Case hardening (carburise) | 0.3–1.2 mm case | 58–62 HRC surface, tough core |
| Nitride | 0.1–0.5 mm | 900–1100 HV, minimal distortion |
| Black oxide | 1–2 µm | Cosmetic + light corrosion protection |
| Zinc / nickel plate | 5–25 µm | Corrosion protection |
| Deburr / edge break | 0.05–0.3 mm | All parts unless the drawing says otherwise |
| Bead blast | Ra 1.6–3.2 µm | Uniform matt finish, hides tool marks |
| Laser marking | — | Part number, lot code, data matrix |
No drawing yet? Build one in the browser.
Set the metal, dimensions and features, watch the 3D model update as you type, then switch to the 2D sheet to add dimensions and tolerances. Download it or send it straight to us with your RFQ.
What We Need to Quote CNC Milling 4340 Alloy Steel
- GeometrySTEP or IGES for the 3D model, plus a PDF drawing if you have tolerance callouts. A dimensioned sketch is enough to start.
- MaterialThe exact 4340 Alloy Steel grade and condition — temper, heat-treat state or hardness. If you are not sure, tell us the function and we will suggest one.
- QuantityPrototype count and the expected annual volume. Both change how the part is made and therefore what it costs.
- TolerancesWhich features are critical. Everything else runs to our standard tolerance, which is what keeps the price sensible.
- FinishSurface treatment, cosmetic requirements, and any faces that must be masked.
- DocumentsWhether you need a material certificate, dimensional report, FAIR or PPAP with the shipment.
Missing something from the list is not a problem — send what you have. We will come back with the specific questions rather than a generic form.
4340 Alloy Steel in Other Processes
Related
Send the drawing, the 4340 Alloy Steel condition and the quantity — pricing and lead time come back the same or next business day.
Request a QuoteFrequently Asked Questions
Can VT machining make custom 4340 Alloy Steel parts to my drawing?
Yes. 4340 Alloy Steel 4340 Alloy Steel machining is made to order from your CAD/2D drawing. Share your geometry, material grade, tolerance and quantity and we will confirm feasibility, lead time and pricing.
What material thickness or size range suits CNC Milling 4340 Alloy Steel?
We machine 4340 Alloy Steel across a broad size range; share your part envelope for confirmation.
What is the typical lead time and minimum order for CNC Milling 4340 Alloy Steel?
CNC Milling prototypes for 4340 Alloy Steel can be produced quickly and scaled to series production. Send your drawing and quantity and we will advise lead time and any minimum order.
What is CNC Milling 4340 Alloy Steel and how is it done?
CNC Milling 4340 Alloy Steel refers to multi-axis milling cutters remove material to produce prismatic features, pockets, slots and contoured 3D surfaces. Applied to 4340 Alloy Steel — high-strength and hardenable — VT machining produces each part to your CAD drawing, selecting parameters from your tolerance, finish and quantity requirements.