Views: 0 Author: Site Editor Publish Time: 2026-06-09 Origin: Site
Stainless steel (304, 316, 17-4PH, etc.) is widely used due to its excellent corrosion resistance and mechanical properties, but it is also recognized as one of the “difficult-to-machine materials.” Compared to ordinary carbon steel, stainless steel has three key characteristics: high toughness, low thermal conductivity, and a high tendency for work hardening. These characteristics directly result in:
Difficulty in chip breaking, leading to the formation of built-up edge (BUE);
Concentration of cutting heat at the cutting edge, accelerating tool wear;
A work-hardened layer that makes subsequent machining more difficult and leaves tear marks on the surface.
Therefore, improving surface roughness in CNC machining of stainless steel cannot simply involve replicating the parameters used for ordinary steel; rather, it requires a systematic combination of methods. This article will provide a detailed analysis of eight proven, effective methods, breaking them down step by step—from cutting tools and parameters to cooling and machine tool rigidity.
The compatibility between the tool and the workpiece material directly determines surface quality.
Recommended Solution | Reason |
Carbide Grade: Select fine-grained, high-toughness grades (such as ISO Class K or M; refer to tool manufacturers’ specific recommendations for stainless steel, e.g., Sandvik GC1125, GC2030, Kennametal KC5025, etc.) | The high toughness of stainless steel requires the tool to possess both wear resistance and chip resistance |
Coatings: Preferably AlTiN (aluminum titanate) or TiCN/TiN composite coatings | AlTiN offers high heat resistance and lubricity, reducing chip adhesion; TiCN has high hardness and is suitable for finishing |
Avoid uncoated or standard TiN | Uncoated carbide is prone to adhesion with stainless steel, leading to surface scarring |
Practical recommendation: For finishing operations (passes intended to reduce surface roughness), use PVD-coated, sharp-edged inserts (such as grinding edges with chip-breaking grooves) and select geometries with large positive rake angles.
The “stickiness” of stainless steel requires a sharp cutting edge, but the edge must not be too brittle.
Geometric Parameter | Recommended Range | Function |
Rake Angle (γ) | 12°–18° (positive rake angle) for finishing | Reduces cutting deformation, lowers cutting forces, and suppresses built-up edge |
Rake Angle (α) | 6°–10° | Reduces friction between the rake face and the machined surface |
Edge Rounding Radius (R) | 0.02–0.05 mm (slight rounding or honing) Should not be too sharp (prone to chipping) nor too blunt (causes extrusion and heat generation) | A slight blunting treatment helps improve edge strength |
Chipbreaker | ide and shallow chipbreakers for precision machining | Ensures smooth chip evacuation and prevents chips from scratching the machined surface |
Key point: For precision machining of stainless steel, it is recommended to use ground or precision-ground inserts, which have a surface roughness 30% lower than that of sintered inserts.
Cutting parameters have a significant impact on the surface roughness of stainless steel. Improper parameters can cause built-up edge or vibration marks.
Recommended Range for Finishing Stainless Steel turning (e.g., 304/316)
Parameter | Recommended Value (Carbide, AlTiN Coated) | Remarks |
Cutting Speed Vc | 20–180 m/min (slightly higher than roughing to avoid chip sticking at low speeds) | Too low a speed tends to cause built-up edge; too high a speed leads to rapid tool wear and surface deterioration |
Feed Rate f | 0.05–0.12 mm/rev | Use lower values for finishing; Ra is proportional to the square of the feed rate |
Depth of Cut ap | 0.2–0.8 mm | The finishing depth of cut should not be too small (cut depths <0.1 mm may result in scraping rather than cutting), nor should it be too large |
Recommended Parameters for Milling Stainless Steel (Finishing)
Parameter | Recommended Value | Remarks |
Cutting speed Vc | 100–150 m/min | Moderate linear speed |
Feed per tooth fz | 0.03–0.08 mm/z | A small feed rate helps reduce Ra, but should not be less than 0.02 mm/z (otherwise, severe frictional heat will occur) |
Radial Depth of Cut ae | 0.1–0.3×D (where D is the tool diameter) | Primarily use down-cut milling to avoid surface scratches caused by up-cut milling |
Rule of Thumb: For requirements of Ra ≤ 1.6 μm, first estimate the theoretical roughness using a formula, then fine-tune the speed on-site. Generally, slightly increasing the cutting speed (near the upper limit) helps achieve a smoother surface.
Stainless steel has low thermal conductivity (approximately one-third that of carbon steel), making it difficult for cutting heat to be dissipated through the chips. Heat accumulates at the cutting edge and on the workpiece surface, leading to tool wear, workpiece surface softening or hardening, and worsened surface roughness.
Recommended Cooling Methods:
High-Pressure Internal Cooling (20–70 bar): Directly sprayed into the cutting zone to forcefully flush away chips and dissipate heat. Particularly suitable for deep-hole drilling and turning of stainless steel.
External cooling nozzles: At least two nozzles, aimed at the front edge of the cutting tip and the rake face.
Cutting fluid selection: We recommend using extreme pressure (EP) emulsions or semi-synthetic cutting fluids at a concentration of 8%–12%. Oil-based cutting fluids containing sulfur and chlorine additives can significantly reduce friction during finishing operations, but environmental impact and workpiece cleanliness must be considered.
Important note: If chips are not flushed away promptly, they can cause scratches and secondary cutting on the machined surface, directly compromising surface finish.
When milling stainless steel, climb milling produces better surface quality than down-milling because the chip thickness decreases gradually, reducing friction and work hardening.
Additional Tips:
Use a ramp-in or helical interpolation to avoid impact marks caused by vertical plunging.
Ensure a uniform finishing allowance: The allowance reserved for finishing is generally 0.2–0.5 mm (in the diameter direction). An excessive allowance can cause chatter marks; if too small (<0.05 mm), the tool tip may “scrape” against the hardened layer, producing burrs.
For sidewall finishing, use unidirectional feed (rather than reciprocating feed), and retract the tool quickly after each pass to avoid surface damage caused by reverse cutting.
Stainless steel generates high cutting forces; any lack of system rigidity will result in vibration marks, directly worsening surface roughness by several times.
Checklist:
Workpiece Clamping: Use a sufficient number of clamping plates or precision vise jaws to avoid excessive overhang. Add auxiliary supports if necessary.
Tool Overhang: Turning tool overhang ≤ 1.5 times the shank height; keep milling cutter overhang as short as possible (L/D ≤ 3).
Tool Holder Selection: Prioritize hydraulic or heat-shrink tool holders (runout ≤ 0.005 mm) to avoid excessive runout from spring collets.
Spindle balancing: For high-speed finishing operations, tools must be dynamically balanced to G2.5 grade.
Vibration mitigation in cutting parameters: If low-frequency vibrations occur, try slightly reducing the feed rate or adjusting the spindle speed (±5%–10%) to break the resonance frequency.
Do not attempt to achieve a smooth surface from the raw blank in a single pass. A typical sequence is:
Process | Objective | Allowance | Feed Rate | Roughness Contribution |
Roughing | Remove most of the material | >1 mm | High | No concern for Ra |
Semi-finishing | Prepare a uniform allowance for finishing | 0.3–0.5 mm | Moderate | Ra ~ 6.3 μm |
Finishing | Achieve specified Ra | 0.2–0.5 mm | Small (0.05–0.1 mm/rev for turning; 0.03–0.06 mm/z for milling) | Ra 1.6–3.2 μm |
Polishing (optional) | Further refinement to Ra 0.4–0.8 μm | 0.05–0.1 mm | Very small (0.02–0.05 mm/rev) | Requires sharp cutting edges and excellent rigidity |
For high-precision requirements where Ra ≤ 0.8 μm is specified, post-processing steps such as rolling, brushing, or grinding may be added after finishing.
Modern CAM software offers specialized strategies for finishing stainless steel:
Trochoidal milling: For finishing grooves or cavities, use a small radial depth of cut combined with a large tangential feed rate to maintain a constant cutting load and reduce vibration.
Constant-pitch helical finishing: Used for cylindrical or conical surfaces to avoid entry and exit marks.
High-Speed Machining (HSM): Slightly increase spindle speed (but note that stainless steel is not suitable for excessively high linear speeds; generally do not exceed 200 m/min), combined with small cutting depths and feed rates, to achieve “light cutting,” thereby reducing cutting forces and heat generation.
Example: When machining curved surfaces on stainless steel molds, using parallel finishing with a 45° cutting angle results in better tool life and surface quality compared to 0° or 90° tool paths.
There is no single “magic parameter”; rather, it requires a combination of multiple approaches: the right tool + appropriate parameters + adequate cooling + stable rigidity + thorough step-by-step machining.
For practitioners, I recommend troubleshooting in the following order:
Check whether the tool is worn or clogged with chips (the most common cause)
Confirm that coolant is effectively reaching the cutting edge
Assess rigidity or cutting parameters by examining vibration marks
Gradually fine-tune speed and feed rate (adjusting by 10% at a time)
Finally, consider changing the tool grade or coating
After each improvement, measure and record the Ra value using a surface roughness meter. Once you have built your own database of stainless steel machining parameters, you will be able to consistently and quickly meet your customers’ surface quality requirements.
