Steel turning: a guide to grades, machinability, and points to consider

What you must remember :

• Turning produces large series of parts of revolution with high dimensional accuracy.
• Automation steels (11SMn30, 11SMnPb37…) are optimized for high production rates.
• Austenitic stainless steels (303, 304, 316L) require specific cutting conditions.
• The alloy steels (42CrMo4, 16MnCr5) are machined before heat treatment, with tolerance management.
• Bar diameter (Ø 3–65 mm) or billet (up to Ø 150 mm): the format determines the process.

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Steel turning, i.e. the machining of steel by material removal on numerically controlled lathes, makes it possible to produce in large series parts of revolution with high dimensional accuracy: screws, shafts, rings, fittings, inserts, connectors.

However, not all steels behave the same way in this process. Between a high-speed, automated steel designed for high production rates and an alloy steel intended for heat treatment, the machining, lubrication, and dimensional control requirements are radically different. This article presents the main families of steels machined in bar turning, their specific characteristics, and the technical parameters to master to guarantee compliant and repeatable parts.

Steel machining: principles and applications

Steel turning refers to the machining of steel by material removal, performed on CNC lathes. This process enables the high-volume production of parts of revolution: screws, shafts, bushings, fittings, inserts, and connectors, with high dimensional accuracy and repeatability suitable for industrial volumes. Mastering the process relies on the appropriate combination of the chosen steel grade, cutting parameters, and equipment: this combination guarantees part conformity and high production productivity.

The main families of steels machined by screw machining

Automation steels (or sulfur steels)

Designed for high-speed machining, automated steels incorporate sulfur, and depending on the grade, lead, bismuth, or tellurium. Sulfur forms manganese sulfide precipitates that promote chip fragmentation and prevent stringy chips. Lead, with its low melting point, generates a lubricating layer that reduces tool-workpiece friction and extends tool life.

Note: Lead (Pb) in automation steels is gradually being replaced by bismuth (Bi) or tellurium (Te) for environmental and regulatory reasons (REACH Regulation). So-called "lead-free" grades offer comparable machinability performance.

These steels are suitable for connectors, valves, screws, and instrumentation where dimensional requirements are high but mechanical stresses are moderate. They are supplied in the form of drawn or rolled bars, an ideal format for continuously feeding automatic lathes.

Austenitic stainless steels

Essential whenever parts need to resist corrosion or meet hygiene requirements, stainless steels (303, 304, 316L) present more demanding machining characteristics. Austenitic stainless steel tends to stick to cutting edges and produce long chips that are difficult to remove.

Mastering these processes requires specific cutting conditions: positive geometry tools, efficient lubrication, and optimized speeds and feeds. Grade 303, with added sulfur, offers significantly improved machinability compared to 304 or 316L: it is the preferred choice for bar turning when chemical resistance is not critical.

Heat-treated steels (alloyed and case-hardened)

For applications requiring high mechanical strength or significant surface hardness, alloy steels such as 42CrMo4, 16MnCr5, or 100Cr6 (bearing steel) are used. These materials are generally machined in the annealed or normalized condition, then subjected after machining to heat treatment (quenching, tempering, case hardening, nitriding) to achieve their final properties.

Machining these steels requires particular attention to tolerances, as heat treatments induce dimensional variations. Defining any necessary grinding allowances and controlling dimensions before treatment are integral parts of the machinist's expertise.

The question of diameter and supply method

The form in which the steel is supplied directly influences the production method and the equipment used.

• The bars (Ø 3 to 65 mm): Continuously fed on automatic lathes with fixed or sliding headstocks, they enable uninterrupted high-volume production. The bar diameter directly determines the maximum diameter of the workpiece and the type of lathe used.
• The plots (up to Ø 150 mm): These are sections of bar cut to length, allowing for the machining of larger diameter or longer parts on CNC lathes. The operator loads the billets manually or via a magazine. This approach is preferred for large cross-section parts, prototypes, and medium production runs.

Key parameters for machining steel in bar turning

Regardless of the grade of steel being processed, several technical parameters are crucial for obtaining conforming parts and optimal productivity:

• Cutting speed and feed They must be adapted to the grade and the desired surface finish. A high-performance steel like 11SMn30 can withstand cutting speeds of around 200 to 300 m/min, whereas an austenitic stainless steel requires a speed of 80-120 m/min to avoid premature tool wear. The feed rate, combined with the speed, directly determines the surface finish and the tool life.
• The geometry of cutting tools Each steel family requires a specific tool geometry. Automated steels tolerate neutral or slightly positive geometries. Austenitic stainless steels require pronounced positive cutting angles to prevent surface work hardening and edge bonding. Hard alloy steels require specialized coatings to withstand the high temperatures generated during cutting.
• Lubrication and cooling Lubrication serves two distinct functions: dissipating the heat generated during cutting and facilitating chip evacuation. For automated steels, standard lubrication is generally sufficient. For austenitic stainless steels, high-pressure lubrication (up to 80 bar in some configurations) is necessary to break up long chips and prevent chip buildup on the tool.
• Dimensional control during production Steels expand differently depending on their composition and thermal conductivity. A temperature difference of 10°C during production can induce dimensional variations of a few microns in a stainless steel part, enough to exceed a tight tolerance. Regular dimensional monitoring is essential to guarantee conformity across the entire production run.

Jacquemoux Décolletage's expertise: from raw material to finished part

Working with steel in bar turning is not simply a matter of programming a lathe. It is a chain of skills that goes from the choice of the grade to the metrological validation of the finished part, including the definition of machining ranges, the setting up of the tools and the management of non-conformities.

Jacquemoux Décolletage intervenes from the consultation phase to guide the choice of the grade according to functional constraints and production volumes, including when material optimization makes it possible to improve machinability without degrading the mechanical properties of the part.

With a capacity ranging from Ø 3 mm in bar to Ø 150 mm in billet, the machine park covers a particularly wide spectrum: micro connector components as well as massive structural parts are produced on a single site, without recourse to subcontracting.

Each batch is inspected according to a defined monitoring plan, with complete traceability of materials and controls from steel receipt to delivery. Measuring equipment is adapted to the required tolerances, whether for online inspections or offline sampling.