What Stainless Steel Are Particularly Difficult in Precision Machining?

In the manufacturing of precision parts, stainless steel is widely adopted for its excellent corrosion resistance and strength. On the other hand, while broadly termed ‘stainless steel,’ machining characteristics differ greatly by specific material grade. For materials that are difficult to machine, problems such as reduced tool life, dimensional accuracy variations, and work hardening become apparent. This article organizes the characteristics and machinability of representative stainless steel grades (SUS303, SUS304, SUS316, SUS416, SUS420J2), comparing them with representative materials like S45C and C3604. It also explains, from the on-site perspective of a precision lathing manufacturer, the challenges that frequently arise during the lathe machining of high-precision automotive parts such as stainless steel injector nozzles and throttle shafts, along with approaches for their resolution.

Why Does Machining Difficulty Vary for Different Stainless Steel Materials?

Stainless steel is a general term for alloy steels where approximately 10.5% or more chromium is added to iron, and it exhibits high corrosion resistance due to a passive film formed on its surface. The presence and amount of elements added to enhance corrosion resistance—such as chromium, nickel, molybdenum—and free-machining elements like sulfur and lead, as well as differences in metallurgical structure (such as austenitic or martensitic), significantly affect machinability.

Machinability is evaluated comprehensively based on factors like chip fragmentation and evacuation, tool life, required cutting power, and finished surface roughness. Generally, stainless steels tend to have higher toughness and lower thermal conductivity compared to structural carbon steels. Consequently, heat generated during cutting tends to concentrate at the tool’s cutting edge, making it prone to tool wear and chipping. Additionally, many materials, particularly austenitic grades, are susceptible to work hardening. If lathing conditions are set incorrectly, cutting may just skim the hardened layer, leading to tool breakage and deterioration of dimensional and shape accuracy.

In the precision lathing of stainless steel parts, understanding the inherent mechanical properties and machinability of the material is a prerequisite for optimizing cutting tool material and geometry, cutting speed, feed rate, depth of cut, and cutting fluid or coolant supply method. Especially for components like automotive injector nozzles and throttle shafts, which require high roundness and stable surface roughness to ensure functional reliability, the accuracy of this process design and parameter setting determines product quality and mass production stability.

A Word from the Author:  “Even within the broad term ‘stainless steel,’ the cutting feel and tool life change drastically when the internal composition or structure changes. Without understanding the quirks of each material grade, even with setups intended to be the same, tool life and finish quality will not be stable.”

Comparison of Characteristics and Machinability of Major Stainless Steel Grades

Here, we organize frequently used representative stainless steel grades and other representative materials for comparison, presenting approximate machinability indices based on general reference data. A higher machinability index generally indicates easier machinability. While these values can vary depending on standards and measurement conditions, they serve as a useful indicator for material selection and process design direction.

Material Grade	Classification	Main Characteristics	Machinability Index (Approx.)
SUS303 (AISI303, ISO X8CrNiS18-9, DIN 1.4305)	Austenitic	Free-machining stainless steel with added sulfur, etc., for improved machinability. Slightly lower corrosion resistance than SUS304, but chips break easily, making it suitable for automatic lathe machining.	60–80
SUS304 (AISI304, ISO X5CrNi18-10, DIN 1.4301)	Austenitic	The most common stainless steel. Excellent corrosion resistance and toughness, but prone to work hardening and tends to produce tough, continuous chips.	40–50
SUS316 (AISI316, ISO X5CrNiMo17-12-2, DIN 1.4401)	Austenitic	Molybdenum addition improves pitting corrosion resistance. Higher toughness and lower thermal conductivity than SUS304, increasing heat load on tools.	35–45
SUS416 (AISI416, ISO X12CrS13, DIN 1.4005)	Martensitic	Free-machining martensitic stainless steel with added sulfur. Magnetic, with lower corrosion resistance than SUS304, etc., but good machinability.	65–80
SUS420J2 (AISI420, ISO X20Cr13, DIN 1.4021)	Martensitic	Higher carbon content provides excellent hardenability. Used for knives, tools, etc. Relatively easy to machine before hardening; after hardening, high hardness necessitates grinding or machining with carbide/CBN tools.	Before Hardening: 45–55 After Hardening: 20–30
S45C (AISI1045, ISO C45, DIN 1.0503)	Carbon Steel	General-purpose medium carbon structural steel. Can achieve a wide strength range through heat treatment; generally has better machinability than stainless steel.	55–65
C3604 (AISI C36000, ISO CuZn36Pb3, DIN CW511L)	Free-cutting Brass	Extremely easy to machine due to lead addition; high-machinability material that provides stable dimensional accuracy and surface finish.	100–140

From this comparison, it’s evident that among stainless steels, free-machining grades like SUS303 and SUS416 are relatively easy to machine, whereas common austenitic grades like SUS304 and SUS316 have low machinability indices, placing them in the category of difficult-to-machine materials for mass production cutting. Particularly, SUS316 is an essential material for applications requiring high corrosion resistance, such as marine, chemical plant, and medical equipment components, where tool selection and condition optimization expertise critically impact quality and cost.

A Word from the Author:  “Laying out the machinability indices makes ‘ease of cutting’ visible as numbers. SUS304 and SUS316 clearly fall into a more challenging range compared to carbon steel or brass, so it’s crucial to design tools and conditions specifically for them from the outset.”

Specific Challenges and Countermeasures for Stainless Steels with High Machining Difficulty

In the machining of stainless steels with low machinability indices like SUS304 and SUS316, the following challenges frequently become apparent on the shop floor.

1.  Work Hardening
The surface layer subjected to intense deformation during cutting hardens rapidly. If subsequent tool passes just skim this hardened layer, tool wear and chipping accelerate dramatically. This phenomenon becomes pronounced when finish cut depths are too shallow, compromising roundness and dimensional accuracy stability. Effective countermeasures include ensuring sufficient cut depth in finish passes to remove the hardened layer in one go and selecting machining parameter ranges that minimize work hardening.

2.  Continuous, Adhesive Chips
Austenitic stainless steels are highly ductile and tend to produce long, continuous chips. If chips wrap around the tool or workpiece, they can degrade surface finish, cause tool breakage, or lead to machine stoppage. Practical solutions include using inserts with optimized chip breaker geometry, employing high-pressure coolant for forced chip evacuation, and incorporating programmed chip-breaking actions.

3.  High Cutting Edge Temperature Due to Low Thermal Conductivity
Stainless steels have relatively low thermal conductivity among ferrous materials, causing generated cutting heat to concentrate at the contact point between the tool edge and workpiece. This leads to early degradation of tool coatings, built-up edge, and edge deformation due to plastic flow. Process design must incorporate heat management strategies, such as using carbide tools with heat-resistant coatings, optimizing coolant flow and nozzle position, and potentially utilizing tools with oil holes.

4.  Different Tool Wear Modes
While flank wear is often dominant in typical carbon steel machining, crater wear (due to the hardened layer and high temperatures) and micro-chipping at the cutting edge are more common in stainless steel machining. Therefore, predicting tool life based on conventional rules of thumb is difficult. Preventive replacement based on machining time/cumulative cutting length or anomaly detection via process monitoring becomes more effective.

For high-rigidity, sliding headstock-type CNC lathes like Swiss-type automatic lathing machines, systematically organizing tool materials, chip geometries, feeds and depths of cut, and coolant conditions into material-specific parameter tables helps ensure stable cycle times and quality even in mass production. Furthermore, incorporating subsequent processes like centerless or cylindrical grinding enables meeting tight dimensional and geometric tolerances for components ranging from shafts of a few millimeters to parts several tens of millimeters in diameter.

A Word from the Author:  “Stainless steel often presents a triple challenge: it ‘hardens,’ ‘produces unbroken chips,’ and ‘dissipates heat poorly.’ Therefore, at our company, we have databased combinations of tools and conditions for each material grade. On the shop floor, we fine-tune based on these ‘recipes,’ which enhances the stability of our mass production lines.”

Machining Example for High-Precision Stainless Steel Parts: Throttle Shaft

Taking the automotive throttle shaft as an example of a representative high-precision part made from the difficult-to-machine stainless steel, we outline key points in process design.

Throttle shafts, used as the rotational axis for throttle valves, are typically made from austenitic stainless steels like SUS304 or SUS316 due to exposure to corrosive environments and mechanical loads. They are often slender and long with compound shapes including keyways and mounting sections for throttle plates, making management of deflection and chatter during machining a critical challenge.

Key points in process design include using sliding headstock Swiss-type CNC lathes with guide bushing support to suppress deflection in slender workpieces and employing proprietary carbide turning tools or throw-away inserts tailored for the application to balance chip control and tool life.

A Word from the Author:  “Throttle shafts are components directly linked to engine performance and reliability. The ability to design a seamless process—from machining the difficult stainless steel material through heat treatment and grinding—and stably achieve micron-level precision in mass production is a key point that tests a supplier’s true capability.”

The Technical Foundation for Handling Difficult-to-Machine Stainless Steels in Mass Production

Stably mass-producing parts from stainless steels with low machinability requires a comprehensive manufacturing foundation that includes not only material understanding and cutting technology but also digital technologies.

Technical strengths can be summarized in the following elements:

Design and In-house Manufacturing of Dedicated Tools:
By designing and manufacturing dedicated tools optimized for each workpiece material—incorporating feedback from the machining floor to refine cutting edge geometry, rake angles, chip breakers, and coating specifications—and producing them using in-house tool grinding equipment, it becomes easier to achieve both long tool life and excellent surface finish, which are often difficult with standard off-the-shelf tools. This approach, conscious of the work hardening characteristics and chip forms of the material from the tool design stage, is particularly effective for difficult-to-cut materials like SUS316.

High-Precision Cutting at Mass Production Scale:
Production lines comprising numerous automatic lathes and grinding machines, coupled with accumulated machining data from years of experience, enable handling mass production volumes reaching millions of lots, even for precision parts like shafts and pins requiring sub-micron tolerances.

Manufacturing Involvement from the Design Stage:
Involving manufacturing expertise from the early product design phase allows for material selection advice, shape proposals considering manufacturability, and cost-reduction suggestions through process integration. This facilitates designs that balance mass productivity and reliability even when using difficult-to-machine stainless steels and can also shorten lead times from prototyping to mass production ramp-up.

Risk Mitigation through Digital Technology and Multi-site Production:
Visualizing operational and machining data collected from machine tools and analyzing machining conditions and tool life enables continuous improvement in reducing line stoppage risks and optimizing parameters. Furthermore, operating equipment with the same concepts and quality standards across multiple factories enhances the overall resilience of the supply chain.

If you face challenges in balancing mass production and quality stability with difficult-to-machine stainless steels or other hard-to-machine materials, an approach combining tool and process design based on material properties with digital technologies can be highly effective.

A Word from the Author:  “For stainless steel, it’s only by considering material selection, tools, process design, and equipment configuration as an integrated whole that it becomes possible to ‘run smoothly’ in mass production. Precisely because it’s a difficult material, total optimization combining on-site data with design capability is crucial.”

Frequently Asked Questions

<Q1>Which is more difficult to machine, stainless steel SUS304 or SUS316? And what are the main reasons?

Generally, SUS316 is considered more difficult to machine than SUS304. The main reasons are that SUS316 contains molybdenum, making it tougher and more prone to work hardening compared to SUS304. Additionally, its lower thermal conductivity causes cutting heat to concentrate more readily at the tool edge. As a result, chips tend to be more adhesive, and tool wear progresses faster. Therefore, the selection of tool material and coatings, and the optimization of cutting speed, feed, and coolant conditions become even more critical.

<Q2>Are there countermeasures to suppress work hardening in stainless steel?

While completely eliminating work hardening is difficult, it is possible to minimize its impact. There are three key points. First, ensure that the depth of cut, especially in finish passes, is not set too shallow, so the hardened layer is removed in a single pass. Second, avoid using worn tools and maintain a sharp cutting edge. Third, review the combination of cutting speed and feed to avoid conditions where the tool edge merely skims the hardened layer. Managing these aspects through systematized condition tables for each material helps reduce variability on the shop floor.

<Q3>What are the key points for machining small-diameter (around 1 mm) stainless steel parts with high precision?

For small-diameter stainless steel parts, even slight deflection or vibration directly translates to dimensional/geometric errors, making the following points crucial. First, suppress deformation in slender workpieces using sliding headstock lathes with guide bushing support. Second, select tool materials like ultra-fine grain carbide that ensure edge strength even with micro cross-sections, and use tool geometries that allow intentional control of chip shape. Third, combine high-pressure coolant and programmed chip-breaking actions to prevent chips from wrapping around the fine workpiece. Designing the process with these points in mind enables mass production of small-diameter parts while maintaining high roundness and straightness free from deflection.

With operations in Japan, Thailand, and India, E&H Precision, Asia’s Largest CNC Lathe Turned Parts Manufacturer has over 1,000 automatic lathing machines, produces more than 1 million precision-machined parts daily. We provide stable deliveries to customers across a wide range of industries, including automotive, electronics, medical, and aerospace, regardless of lot size, and ship to Asia, Europe, North America and South America. 

With over 50 years of experience and a proven track record of producing approximately 1,500 product items annually, we deliver high-quality, cost-competitive precision turned parts. 

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About the Author: This article was created based on the expertise of E&H Precision’s manufacturing engineers, quality control staff, and sales engineers, and shares information on actual machining case studies, quality improvement initiatives, machining techniques, material properties, drawings, and standards, all based on experience from the manufacturing shop floor. Prior to publication, the information presented here is reviewed by our in-house engineers to ensure accuracy and practicality. 

** The images on this blog are for illustrative purposes only. Some were created using AI and They may differ from the actual situation.