Introducing grinding process examples for SUS (stainless steel) shafts! Required precision and design considerations.

Stainless steel (SUS) shafts are used in a wide range of fields, including precision equipment, industrial machinery, and medical devices, due to their excellent corrosion resistance and wear resistance.On the other hand, for applications requiring high precision, such as roundness and surface roughness, it is common to combine turning with grinding.

In particular, for hardened materials and small-diameter shafts, the grinding process is crucial for stabilizing dimensional and shape accuracy, and the processing techniques used in each stage often significantly impact the quality.

This article explains the basic characteristics of stainless steel shafts, the reasons why grinding is chosen, the main processing methods, and the design and processing points necessary to achieve high-precision machining.

What exactly is a SUS (stainless steel) shaft?

SUS is the JIS standard designation for stainless steel, and it is widely used in machine parts due to its excellent corrosion resistance. In particular, when used in shafts, durability is often required for use in corrosive environments and as a sliding part.

Here, we will summarize the basic classification of SUS (stainless steel) and the performance characteristics that are important for shaft applications.

1. Definition of SUS and classification under JIS standards
2. Mechanical and physical properties of SUS
3. Main applications and performance of shafts made with stainless steel (SUS)

Definition of SUS and classification under JIS standards

SUS refers to an alloy steel whose main component is iron (Fe), with chromium (Cr) and nickel (Ni) added to improve corrosion resistance.The JIS standard defines it as follows:

Furthermore, SUS is classified into several categories depending on its composition and application.

Among stainless steels, austenitic stainless steels such as SUS303 and SUS304, and martensitic stainless steels such as SUS420J2 are commonly used for shaft applications.

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Mechanical and physical properties of SUS

Material properties and processing steps are closely related.When considering the machining of SUS shafts, it is necessary to understand the mechanical and physical properties of the material.

Main applications and performance of shafts made with stainless steel (SUS)

SUS shafts are used in the following applications:

• Rotating shaft of precision equipment
• pump and motor shafts
• Drive components for medical devices and food processing machinery
• Sliding parts for semiconductor devices and industrial machinery

In these applications, geometric tolerances such as roundness, cylindricity, and surface roughness, as well as dimensions, often directly impact performance.Therefore, for high-precision stainless steel shafts, there is a growing trend towards using grinding as the finishing process.

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Why grinding is chosen for stainless steel shafts

In the manufacturing process of stainless steel (SUS) shafts, "cutting" using lathes and milling machines is an essential basic process.However, for high-value-added components such as semiconductor manufacturing equipment, medical devices, and precision motors, "grinding" is generally specified as the final process.

Why is grinding necessary despite the increased costs and labor involved? The main reasons can be summarized into three points: "stable geometric tolerances," "handling high-hardness materials," and "controlling fine surface roughness," which go beyond the limitations of cutting processes.

From here, we will explain why grinding is chosen for SUS shafts.

1. To stabilize roundness and cylindricity
2. To accommodate high-hardness materials after heat treatment
3. To improve surface roughness and sliding properties

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To stabilize roundness and cylindricity

For a shaft that functions as a rotating axis (drive shaft), "runout" during rotation is a critical factor that can affect the product's lifespan.

While high-precision finishing is possible with machining, the load is concentrated on a single point of the cutting tool, making it unavoidable that the workpiece will deflect and dimensional changes will occur due to tool wear.

In particular, with long stainless steel shafts, a "drum-shaped" distortion is likely to occur due to relief in the central part.

Therefore, grinding, which involves making fine cuts with a high-speed rotating grinding wheel, minimizes workpiece deformation because the processing load (back force) is extremely small compared to cutting.This enables the stable achievement of micron-level roundness and cylindricity, significantly contributing to improved bearing fitting accuracy and reduced noise and vibration.

To accommodate high-hardness materials after heat treatment

Martensitic stainless steels such as SUS440C, which have been heat-treated to enhance wear resistance,It is extremely hard, and ordinary cutting tools cannot cut into it.

Furthermore, the unavoidable "distortion" and "dimensional changes" that occur during the heat treatment process can impair the accuracy of the final product.

Grinding using hard abrasive grains is an essential process in finishing these high-hardness materials.The ability to precisely correct distortion after heat treatment and reliably achieve geometric tolerances according to design drawings, even with difficult-to-machine materials, is a unique strength of grinding.

To improve surface roughness and sliding properties

For stainless steel shafts that come into contact with sealing components, oil seals, and bearings, surface smoothness directly impacts the "lifespan of the component."

In cutting processes, theoretically, the feed marks of the cutting tool (cutter marks) remain as tiny irregularities. These irregularities create frictional resistance on the sliding surface, leading to packing wear, oil leaks, and in the worst case, seizure.

Furthermore, because grinding polishes the surface with countless abrasive grains, it can achieve a much smaller Ra value (surface roughness) than cutting.By achieving a smooth, near-mirror finish, the compatibility with the mating material is enhanced, making it possible to extend the maintenance cycle of the entire device.

Main grinding methods and key points for selecting processing methods for stainless steel shafts

In the manufacturing of SUS shafts, inserting a grinding process after rough machining on a lathe is,This is a standard process for controlling dimensional tolerances to the micron level.

However, stainless steel has a high coefficient of thermal expansion and is prone to work hardening, making it essential to select the optimal grinding method according to the shape and production volume. From the selection of grinding wheels to the supply of grinding fluid and even the method of holding the workpiece, finding the right balance between processing accuracy and cost is key to producing high-quality shafts.

From here, we will explain in detail four representative processing methods commonly used for SUS shafts and the selection criteria for each.

1. Centerless Grinding
2. Cylindrical grinding
3. Screw grinding
4. step grinding

Centerless Grinding

Centerless grinding is a technique that finishes the outer diameter of a workpiece by holding it at three points—the grinding wheel, the regulating wheel, and the blade—without supporting it at a center.The greatest advantage of this method is that it boasts overwhelming productivity in mass production because workpieces can be easily attached and detached and continuous supply is possible.

Furthermore, due to its structure where the support section supports the entire circumference of the workpiece, even long, thin shafts are less prone to "bending," allowing for stable outer diameter accuracy.

However, because it lacks a center reference, it is not suitable for workpieces with stepped shapes or eccentricities.Setting the support conditions requires skilled techniques, and the quality of the setup directly impacts the roundness and finish quality. Therefore, it can be said that this is the most suitable selection for mass-produced items with simple straight shapes.

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Cylindrical grinding

Cylindrical grinding is the most standard method, in which the workpiece is held by center holes at both ends and a grinding wheel is applied while it is rotating.Because it uses a center hole as a reference, it can maintain extremely high roundness and cylindricity relative to the axis, making it ideal for machining precision rotating shafts with tight tolerances.

Its greatest strength lies in its high versatility, which allows it to flexibly handle single-piece production, prototypes, and even complex shape changes.

On the other hand, machining accuracy is strongly dependent on the quality of the center hole itself and the precision of the centering.Furthermore, when processing long stainless steel shafts, "bending" and "runout" are likely to occur due to their own weight and grinding resistance, making advanced support management using auxiliary tools such as shims essential.

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Screw grinding

Thread grinding is employed when pursuing the ultimate lead accuracy in shafts that function as feed mechanisms or lead screws.Compared to thread cutting using a lathe, this method can achieve a smooth thread surface and precise pitch even with high-hardness SUS materials.

In particular, for precision screw parts where smooth power transmission is required by minimizing frictional resistance, grinding is the standard finishing method.

However, screw grinding requires the use of grinding wheels shaped in a special way, which tends to increase processing time and thus the cost burden.This can be described as a specialized, high-value-added processing method that requires high hurdles in both equipment and technology, such as sharpening (dressing) grinding wheels and controlling machine temperature.

step grinding

Step grinding is performed when multiple different outer diameter dimensions exist on a single shaft, such as in bearing mounts or mating parts.The challenge of this machining process lies not only in matching the dimensions of each step, but also in simultaneously ensuring "coaxiality" and "positioning accuracy" at each step.

During the design phase, it is important to consider the shape with grinding performance in mind, such as by providing a "relief groove" at the base of the stepped section to avoid interference with the grinding wheel.

The decision of whether to grind multiple stages at once or to separate the process is crucial.Because it significantly impacts the final cumulative error and production efficiency, meticulous process design based on drawing specifications is required.

Examples of high-precision machining of stainless steel shafts and key points of machining technology

The difficulty of machining a SUS shaft varies dramatically depending not only on the material's properties but also on the combination of its shape and required precision.Especially considering the thermal expansion and work hardening unique to stainless steel, it is essential not only to machine according to the drawings, but also to design the process to predict distortion and select the optimal machining conditions.

Here, we will present four representative processing examples and explain in detail the technical points required to achieve high-precision stainless steel shafts.

1. Straight Shaft
2. Grooved shaft (fixing mechanism component)
3. Stepped shaft (fitting section/bearing component)
4. end face precision machined shaft

Straight Shaft

For straight shafts, which are the heart of a rotating mechanism, pursuing geometric precision such as roundness and cylindricity is of utmost importance, as is not just the outer diameter (size).Even slight shape errors can cause significant vibration, noise, and even abnormal wear of bearings during high-speed rotation.

To achieve a highly precise straight shape, the key is to minimize "escape" during machining.

Especially with thin-diameter or long SUS shafts,Because grinding pressure can easily cause minute bends, thorough centering, the use of steady rests, and multiple micro-grinding passes are necessary to ensure linearity and roundness down to the micron level.

Processing example

• Straight shaft (rotating mechanism component)
• Shaft for automotive motors
• Precision shafts for home appliances

Sanwa Needle Bearing Machining Technology

• Supports outer diameter accuracy of 0.5 μm, roundness of 0.1 μm, and cylindricity of 0.1 μm.
• We can achieve a surface roughness of Ra 0.02 or less for cylindrical surfaces.
• Optimizing grinding conditions and processing sequence suppresses variations during mass production.
• With a process configuration centered on centerless grinding, we can support mass production on a scale of 100 million units per month.

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Grooved shaft (fixing mechanism component)

Shafts equipped with retaining ring grooves and keyways are widely used as components in fixing mechanisms.However, groove machining is prone to stress concentration, and depending on the machining method, it carries the risk of degrading the runout accuracy of the entire shaft.

Austenitic stainless steel (such as SUS304) is particularly prone to work hardening, so applying excessive load during grooving can result in unstable dimensions during subsequent finish grinding.

Therefore, heat treatment and stress relief are performed during the rough machining stage.Process control that involves thoroughly understanding material properties is crucial, such as minimizing distortion while fine-tuning the final groove width and positional accuracy through grinding.

Processing example

• shaft with retaining ring groove
• Shaft with keyway
• Shaft for fixing mechanism

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• Austenitic stainless steel can be hardened by machining and heat treatment, while martensitic stainless steel can be hardened by grinding; the appropriate processing method can be selected depending on the conditions.
• It allows for groove machining by grinding and can handle high-hardness materials after heat treatment.
• Even when the rigidity balance changes due to groove machining, it is possible to maintain outer diameter accuracy by combining cutting and grinding processes.

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Stepped shaft (fitting section/bearing component)

In stepped shafts with bearing and mating sections, the most difficult aspect to control is the "coaxiality" of the multiple outer diameter shafts.Even if each stage is individually highly accurate, misalignment of the axis will significantly impair the overall assembly accuracy of the device.

The key to avoiding this problem is to grind multiple stages in a single chucking (single fixing) whenever possible.

Properly dress (shape) the grinding wheel for stepped grinding,By simultaneously managing positional accuracy from the reference plane (shoulder tolerance) and axial runout, it becomes possible to maintain a high level of correlation accuracy for each part, even in complex shapes.

Processing example

• Shaft with bearing fitting
• Stepped shaft for gear mounting
• Shaft for precision rotating mechanism

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• The coaxiality of the fitting portion of the rotating mechanism is controlled to a level of 0.001 mm.
• It is possible to create a stepped shape using a lathe and then finish the mating part by grinding.
• The dimensional accuracy of the stepped outer diameter is controlled to 0.004 mm.
• Supports multi-tiered structures such as double tiers and intermediate tiers.

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Explanation of the uses and manufacturing process of stepped shafts, as well as points to note when designing and processing them

end face precision machined shaft

For shafts used in motor shafts and the ends of precision mechanisms, the precision of the "end face (edge)" as well as the circumferential surface affects the performance of the device.If the perpendicularity of the end faces is insufficient, the assembled parts will tilt, which directly leads to an imbalance in rotation and positioning errors.

In end-face grinding, the key points are controlling the pressure in the axial direction and selecting a grinding wheel that suppresses burr formation.

Furthermore, when using the end face as the reference surface, the Ra (surface roughness) should be finished to the absolute minimum.This enhances adhesion with the mating material and eliminates minute play, resulting in a highly accurate assembly.

Processing example

• end face precision finished shaft
• Shaft for positioning mechanism
• Precision assembly shafts

Sanwa Needle Bearing Machining Technology

• Supports perpendicularity of 0.003 mm between the end face and outer diameter.
• End face finishing allows for machining to a surface roughness of Ra0.03.
• Supports finishes of Ra 0.02 or less on cylindrical surfaces.
• The end face shapes are compatible with F-face, R-face, and T-face.

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Key points for stabilizing accuracy when grinding stainless steel shafts.

To stably mass-produce and manufacture high-precision stainless steel shafts, not only is technical expertise at the processing site essential, but consideration of "ease of processing (machinability)" from the design stage is also indispensable. Designing without regard for the physical properties and shape limitations of the material will directly lead to decreased yield and increased costs.

Here, we will explain three key points essential for minimizing quality variations and maximizing processing stability.

1. Do not specify unnecessarily strict tolerances.
2. The process design will involve determining the final dimensions after heat treatment.
3. Select a processing method according to the L/D ratio.

Do not specify unnecessarily strict tolerances.

In the design of precision shafts, it is often assumed that setting tighter tolerances will increase product reliability,Specifying excessive precision (over-specifying) will exponentially increase the difficulty and cost of machining.

Stainless steel has a high coefficient of thermal expansion, and even slight temperature changes during processing can cause dimensional fluctuations of several microns. Therefore, determining the truly necessary precision for functionality and setting appropriate tolerance ranges ultimately leads to improved processing stability and optimized cost performance.

Optimizing the balance between on-site processing limits and the tolerances required for the product is the first step to achieving true high quality.

The process design will involve determining the final dimensions after heat treatment.

In SUS shafts (such as SUS440C) that undergo heat treatment to improve wear resistance,Process design that takes into account the occurrence of "distortion" due to heat treatment is essential.

If the final dimensions are finished before hardening, there is a high risk that the final geometric accuracy will be compromised due to internal stress during heat treatment, which can cause the shaft to warp or the dimensions to expand or contract. To prevent this risk, rough machining is performed with sufficient grinding allowance (material removal) before hardening, and the process flow of "achieving the final dimensions by grinding after hardening" is strictly followed.

This makes it possible to completely eliminate minute distortions caused by heat treatment and ensure extremely high dimensional accuracy.

Select a processing method according to the L/D ratio.

For small-diameter, long shafts, the rigidity decreases as the diameter (D) becomes smaller relative to the length (L), i.e., the "L/D ratio" increases, and the difficulty of machining increases dramatically.When the L/D ratio exceeds a certain level, the shaft "bends" due to the pressure of the grinding wheel, making it easier for dimensional errors such as the central part becoming thicker, and surface chatter due to minute vibrations to occur.

For such complex shapes, it is necessary to devise support methods that are appropriate to the shape characteristics, such as selecting centerless grinding, which supports the workpiece all around, instead of cylindrical grinding with support at both centers, or, if cylindrical grinding is used, placing appropriate "vibration dampers".

Selecting the optimal construction method that takes the L/D ratio into consideration is key to ensuring the straightness and surface quality of long shafts.

Summary | Process design is crucial for high-precision grinding of stainless steel shafts.

SUS shafts are used in many machine parts due to their excellent corrosion resistance and wear resistance.In particular, to achieve stable and high-precision machining, it is necessary to comprehensively consider material properties, heat treatment processes, and the selection of machining methods.

Sanwa Needle Bearings utilizes shaft processing technology centered on centerless grinding to manufacture parts with precision ranges of 0.5 μm for outer diameter accuracy, 0.1 μm for roundness, and 0.1 μm for cylindricity.Furthermore, we can propose material selections and processing methods tailored to your application and quantity, from small-lot production to mass production systems of up to 100 million units per month.

If you would like to optimize the machining process for SUS shafts based on their component specifications and precision requirements, please feel free to contact us.