What is the role of polishing in shafts? Introducing technologies and processing examples that achieve high-precision finishing.

Parts processing

For shafts used in rotating and sliding mechanisms, not only dimensional accuracy but also surface condition significantly impacts product performance.

Especially in bearing contact areas and sliding surfaces, even slight differences in surface roughness can affect vibration, wear, and lifespan, making polishing essential for maintaining stable performance.

This article will explain the role of polishing in shaft processing, how it differs from grinding, and the impact of surface finish on performance.This article will also introduce machining techniques, points to note, and examples of machining processes used in actual high-precision shaft design, so please use it as a reference.

table of contents

What is the role of polishing in a shaft?
- Reasons why the shaft needs polishing
- Division of roles between grinding and polishing in shaft manufacturing process design
Why polishing affects shaft performance
Main processing techniques when polishing shafts
Points to note when polishing the shaft
[Processing Example] High-precision shaft finishing of Sanwa Needle bearings
Frequently Asked Questions about Shaft Polishing
Summary | Shaft polishing is the final finish that determines performance.

What is the role of polishing in a shaft?

In the shaft manufacturing process, polishing is an extremely important process that determines the final quality of the product.Generally, cutting with a lathe or grinding with a grinding wheel shapes the "dimensions and form," while polishing plays the role of improving the "surface quality" to the greatest extent possible.

The surface condition of the shaft directly impacts the overall energy efficiency and quietness of the machine, making it essential to select the appropriate polishing treatment that aligns with the design intent.

This section details the necessity of polishing in shaft processing and the clear division of roles between polishing and grinding processes.

Reasons why shaft polishing is necessary
Division of roles between grinding and polishing in shaft manufacturing process design

Reasons why the shaft needs polishing

The primary purpose of polishing is to control surface roughness from nanometers to microns, thereby optimizing the contact between parts.In particular, for shafts used as sliding parts involving high-speed rotation or reciprocating motion, the smoother the surface, the less energy loss due to frictional resistance can be reduced, resulting in a longer lifespan for the entire device.

If the surface roughness of the shaft exceeds the acceptable limit, the risk of the following technical problems occurring during operation increases.

Progression of wear and seizure
The microscopic irregularities on the surface come into localized contact with the mating material, generating frictional heat.If this heat exceeds a certain limit, it can cause increased vibration and abnormal wear, ultimately leading to shaft seizure or breakage.

Functional deterioration due to insufficient oil film formation
If a smooth surface is not ensured, the lubricating oil will not be held evenly, hindering the formation of a stable oil film.Oil film breakdown not only accelerates wear on surrounding parts, but also significantly reduces rotational accuracy, ultimately compromising the overall reliability of the machine.

Division of roles between grinding and polishing in shaft manufacturing process design

In designing a process for efficiently manufacturing high-precision shafts, it is common to follow two steps: "establishing the shape through grinding" and "finishing the surface through polishing." Each of these steps has a distinct purpose.

The role of grinding: creating geometrically precise results.
Grinding is a process that uses a high-speed rotating grinding wheel to refine the outer diameter of a workpiece.The main mission at this stage is to ensure not only adherence to the outer diameter tolerance, but also precise geometric tolerances such as roundness, cylindricity, and coaxiality down to the micron level.
Grinding is a process that involves "removing" material, and can therefore be described as a process for completing the physical shape of the shaft.

The role of polishing: Optimization of surface function
Polishing, on the other hand, is a finishing process that smooths out the fine grinding marks and irregularities left on the surface after grinding, bringing the surface closer to a mirror finish.The amount of material removed by polishing is extremely small, and it does not have the power to significantly alter the dimensions.

However, dramatically improving the surface roughness (Ra/Rz) plays a crucial role in providing "functional added value" to the shaft, such as improved sealing performance and minimized sliding resistance.

Why polishing affects shaft performance

Shaft polishing is not merely a cosmetic finish; it's a crucial process that directly impacts the machine's performance.Here, we will summarize the main factors that influence shaft performance through polishing.

Rotational accuracy affects vibration, noise, and lifespan.
Diameter differences and sliding stability affect wear and seizure of parts.
The smaller the diameter of the shaft, the more difficult it is to finish.

Rotational accuracy affects vibration, noise, and lifespan.

In particular, in rotating and sliding mechanisms, surface roughness and contact conditions affect vibration, wear, and lifespan, so polishing quality can significantly impact product performance.

If the surface of the shaft is uneven, slight vibrations may occur during rotation.These vibrations not only cause noise and performance degradation throughout the machine, but in the long term, they also affect the lifespan of bearings and contact parts.

In particular, in high-speed rotating mechanisms, even slight surface roughness of the shaft affects vibration characteristics. By smoothing out the fine irregularities on the surface through polishing, the contact condition during rotation becomes stable, leading to a reduction in vibration and noise.

Diameter differences and sliding stability affect wear and seizure of parts.

Diameter difference refers to minute dimensional or shape differences between multiple diameter sections on the same shaft.If there is a large difference in diameter between parts, the contact condition with sliding parts will be uneven, which can lead to localized wear and seizure.

Furthermore, in the case of sliding mechanisms, large differences in diameter can easily cause torque fluctuations and friction changes, affecting the overall operational stability of the device.By making the surface uniform through polishing, contact pressure can be distributed, thereby suppressing wear.

The smaller the diameter of the shaft, the more difficult it is to finish.

With small-diameter shafts, even a slight machining force applied by the tool can cause the part to flex.Furthermore, the shape accuracy and surface condition can change significantly depending on the workpiece support method and polishing conditions, making it difficult to ensure measurement accuracy.

Therefore, when polishing small-diameter shafts, it is necessary to properly design not only the equipment but also the processing conditions and measurement control.

Main processing techniques when polishing shafts

There are multiple processing methods for polishing shafts, and the optimal technique must be selected depending on the application, production volume, and required surface quality. Even simply "polishing" encompasses a wide range of techniques, from methods for processing large quantities of parts at once to methods that pursue surface roughness down to the micron level.

Here, we will summarize the characteristics and applications of three representative polishing techniques used in the final finishing of shafts.

Barrel polishing: For mass-produced shafts
Buffing: Improves appearance and reduces friction
Super-finishing: High precision contact surface

Barrel polishing: For mass-produced shafts

Barrel polishing is a polishing method in which a shaft (the object to be polished), media (polishing stones), and polishing aids (compounds) are placed in a special container, and the container is rotated or vibrated to polish the surface.

The biggest advantage of this method is that it can process hundreds or even thousands of shafts simultaneously by utilizing the force of friction between workpieces or media. This technique is extremely well-suited for removing minute burrs remaining after cutting or grinding, and for "R-shaping," which smooths out sharp edges.

On the other hand, it is not suitable for machining that requires precise dimensional accuracy for individual products, so it is widely used as a basic surface finishing process for mass-produced items.

Buffing: Improves appearance and reduces friction

Buff polishing is a processing method in which an abrasive is applied to a "buff" made of a flexible material such as cotton cloth, felt, or wool, and then pressed against the surface of a shaft while rotating it at high speed.Whether the process is done manually by an operator or by an automated machine, both methods are highly effective in achieving a beautiful, mirror-like shine.

In addition to improving the appearance quality, by removing surface irregularities to the absolute minimum, frictional resistance during sliding can be significantly reduced.

This technique is frequently used for finishing applications where decorative elements are required, or for areas where a smooth surface is needed, such as contact points with oil seals.

Super-finishing: High precision contact surface

Super finishing is a special finishing process in which an ultra-fine abrasive is pressed against the shaft surface with constant pressure, and the abrasive is vibrated at high speed while the workpiece is rotated.

The greatest feature of this method is that it can smooth out "grinding marks" (tiny bumps) that cannot be removed by general grinding processes, creating an ideal contact surface.

It is especially essential for parts that require extremely high sliding characteristics, such as bearing contact surfaces and engine crankshafts. Super-finished shafts dramatically reduce heat generation and wear due to friction, greatly contributing to improved energy efficiency and extended lifespan of the entire machine.

Points to note when polishing the shaft

Shaft polishing is an important process for improving surface quality,Processing conditions and process design may affect quality.Let's take a closer look.

Polishing conditions vary depending on the material and heat treatment state.
Over-polishing can affect dimensional accuracy.
The support method and processing conditions significantly affect the finished quality.

Polishing conditions vary depending on the material and heat treatment state.

The difficulty of polishing and the appropriate processing conditions vary greatly depending on the shaft material and heat treatment status.In particular, when working with highly hard materials such as hardened steel, adjusting the grinding pressure and processing time is crucial.

Also, depending on the material, polishing may cause microscopic scratches, so caution is advised.

Over-polishing can affect dimensional accuracy.

While polishing is primarily for surface finishing, excessive processing can affect dimensional accuracy.Therefore, for high-precision shafts, machining techniques that ensure dimensional accuracy during the grinding process are crucial.

Minimizing polishing makes it easier to control the quality of the surface roughness.

The support method and processing conditions significantly affect the finished quality.

In small-diameter shafts, deflection of the parts is likely to occur depending on the support method and processing pressure,This affects not only the surface condition but also the shape accuracy, so caution is necessary.

Depending on the specifications of the part, setting the support method and processing conditions for polishing can be quite challenging.

[Processing Example] High-precision shaft finishing of Sanwa Needle bearings

Shafts used in precision instruments and rotating mechanisms for next-generation mobility require sub-micron level precision control that exceeds conventional machining limits.

At Sanwa Needle Bearing, we have established centerless grinding as our core technology, and by combining our unique equipment development with a rigorous quality control system, we have achieved the manufacture of extremely small and ultra-precise shafts.

Through specific processing examples, we will explain the technical basis for the high-precision finishing provided by Sanwa Needle Bearings.

High-precision finishing of small-diameter microshafts
Submicron precision grinding finish
Stable quality control through integrated in-house production.

High-precision finishing of small-diameter microshafts

In the manufacturing of extremely small diameter micro-shafts with an outer diameter of Φ0.5 mm or less, the biggest challenge is controlling "deflection" and "runout" during machining.Because elongated workpieces are easily deformed by even slight pressure during grinding, it is difficult to maintain dimensional stability with conventional equipment.

At Sanwa Needle Bearing, we utilize our in-house developed processing equipment and expertise specializing in small-diameter machining to precisely polish even extremely thin shafts with low rigidity without compromising their shape.

This technological capability enables us to achieve both high yield and stable quality in micro-sized rotating shafts, which are essential for medical devices and precision electronic components.

What is high-precision machining of small-diameter shafts? We introduce precision and design tips from manufacturing examples.

Submicron precision grinding finish

The most distinctive feature of Sanwa Needle Bearings lies in its grinding technology, which achieves world-class dimensional and geometric tolerances of ±0.0001 mm (0.1 μm).

In particular, the extreme specifications such as a roundness of 0.0001 mm and a surface roughness of Ra 0.02 are achieved through a unique grinding process that is unmatched by other companies.

By creating perfect geometric tolerances in the grinding process and then applying ultra-precise surface finishes according to the application,We can provide shafts for high-precision rotating mechanisms that minimize friction loss to the absolute minimum.

A system of 100% control using ultra-high-precision measuring instruments with a resolution of 0.00001 mm supports the reliability of this submicron accuracy.

【Related article】
Sanwa Needle Bearing's technological prowess

Sanwa Needle Bearing's pursuit of ultra-high precision

Stable quality control through integrated in-house production.

In the manufacturing of high-quality shafts, a lack of coordination between processes is a major factor in variations in precision.
For example, if cutting, heat treatment, and grinding are outsourced to different companies, there is a risk that the control of distortion and material removal at each stage will become ambiguous, resulting in inconsistent final quality.

Sanwa Needle Bearing has established an "integrated production system" in which all processes, from cutting and plastic deformation to heat treatment, grinding, and surface treatment, are completed in-house.

Because we can immediately proceed to precision grinding after optimal heat treatment using our in-house vacuum furnace, we can minimize heat treatment distortion and manufacture even shafts requiring complex processes with consistent quality standards.

This system allows us to achieve both quick turnaround times and rigorous quality assurance that does not tolerate even one defective unit out of a million.

【Related article】
Sanwa Needle Bearing's commitment to quality

Frequently Asked Questions about Shaft Polishing

Regarding the shaft polishing process, numerous questions arise during the design and processing planning stages regarding precision limits and criteria for selecting the manufacturing method.

This section provides answers from a practical perspective to typical questions that designers and engineers often encounter.

Q1. To what extent can the precision of the shaft be improved by polishing it?
Q2. Can shaft polishing be used to adjust dimensional tolerances?
Q3. Is it possible to achieve a high-precision polished finish even on small diameter shafts?

Q1. To what extent can the precision of the shaft be improved by polishing it?

The primary role of polishing is to "improve surface roughness," and the basis for geometric tolerances such as shaft dimensional accuracy, roundness, and cylindricity is determined by the preceding grinding process.

While it is difficult to dramatically improve geometric tolerances through polishing alone, by combining extremely high grinding precision at the ±0.0001mm level with polishing techniques such as superfinishing to remove minute irregularities, it is possible to achieve surface quality at the submicron level.

Sanwa Needle Bearings achieves the ultimate smoothness required for high-precision rotating mechanisms through the advanced coordination of grinding and polishing.

Q2. Can shaft polishing be used to adjust dimensional tolerances?

Generally, the process of adjusting the dimensional tolerance of a shaft to within microns is performed in the grinding process, not the polishing process.

Polishing is a finishing process that removes minute ridges from the surface, resulting in very little material removal. Attempting to make large dimensional corrections through polishing not only increases processing time but also risks compromising geometric accuracy such as roundness and cylindricity.

Therefore, the optimal process design is to use grinding to create dimensions according to the design drawings, and to dedicate polishing solely to optimizing the final surface function.

Q3. Is it possible to achieve a high-precision polished finish even on small diameter shafts?

Small-diameter shafts with a narrow outer diameter are prone to workpiece deflection due to the pressure applied during machining.Because controlling the support method and pressure conditions is extremely difficult, the difficulty of polishing increases dramatically.

However, by combining specialized support jigs to compensate for the lack of rigidity in the workpiece, ultra-precision polishing equipment that can control minute vibrations, and processing know-how based on many years of experience, high-precision polishing is entirely possible even for small-diameter shafts.

Sanwa Needle Bearing has established a production system that achieves uniform surface quality while suppressing deflection, even for extremely small diameter parts in the Φ0.5mm class.

Summary | Shaft polishing is the final finish that determines performance.

Shaft polishing is not merely a cosmetic finish; it's a crucial process that affects rotational and sliding performance.By appropriately dividing the roles between grinding and other processes, we can achieve high-precision shafts that balance dimensional accuracy and surface quality.

At Sanwa Needle Bearing, we have established an integrated in-house production system that includes cutting, plastic deformation, and heat treatment, with grinding as our core technology.If you are having trouble machining small-diameter shafts or high-precision parts, please feel free to contact us.

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