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

In precision mechanisms and small devices, small-diameter shafts with diameters of a few millimeters or less are sometimes used. In recent years, with the miniaturization and increased functionality of machinery, there has been a growing demand for high-precision small-diameter shafts that are not only thin but also have strict control over geometric accuracy, including roundness and surface roughness.

However, small-diameter shafts present many technical challenges, such as reduced rigidity of the components, so it is important to consider the manufacturing process from the design stage.

This article will clarify the definition and applications of small-diameter shafts, explain why high-precision machining is difficult, and discuss design points based on actual machining examples.

What is a high-precision small-diameter shaft?

A small-diameter shaft generally refers to an extremely thin shaft with an outer diameter of φ3mm (a few millimeters) or less.However, for components incorporated into precision mechanisms such as medical devices, semiconductor manufacturing equipment, and micromotors, simply being able to "cut into thin pieces" is not enough.

To truly be called a "high-precision small-diameter shaft," it's necessary not only to meet the specified outer diameter dimensions, but also to have its "geometric precision," such as roundness (how perfectly round it is), coaxiality, and surface roughness, strictly controlled to the submicron (1/1th of a millimeter) level.

Incidentally, the reason why high-precision machining of small-diameter shafts is extremely difficult is that the thinner the shaft becomes, the more significantly the rigidity (hardness and strength) of the material itself decreases.In particular, with shapes that have a large L/D ratio (length-to-width ratio) where the outer diameter is small relative to the overall length, "flexing" or "chatter (vibration)" occurs when the cutting tool is applied, causing the material to slip away.

It's the same principle as when you try to sharpen a mechanical pencil lead with a cutter; it breaks or bends easily.

Therefore, unlike conventional shaft machining, it requires extremely advanced precision machining techniques and know-how, including controlling cutting heat, applying the cutting tool, and supporting the material (rust prevention).

As a general guideline, the dimensional classifications and precision levels for small-diameter shafts are as follows:

Applications of small-diameter shafts requiring high-precision machining.

Small diameter shafts are used in medical devices, semiconductor manufacturing equipment, small robots, micromotors, etc.It is an extremely important part that is incorporated into the "heart" of state-of-the-art precision machinery.In these small mechanisms, even a dimensional error of just a few microns in the shaft or a slight irregularity in surface roughness can directly lead to a fatal defect in the entire product (abnormal noise, malfunction, reduced lifespan).

Specifically, high-precision, small-diameter shafts play a vital role in the following demanding applications and roles:

Precise positioning function (semiconductor manufacturing equipment, optical instruments, etc.)
In semiconductor devices and lens drive systems that require nano-level control, even a slight deviation in the straightness or dimensional accuracy of the shaft will prevent it from stopping in the correct position.

Achieving precise positioning down to the micron level absolutely requires meeting extremely stringent dimensional tolerances.

Improved sliding stability (medical devices, small cylinders, etc.)
It is used in areas where parts rub against each other and move smoothly (slide), such as the internal components of catheters and endoscopes, and tiny pistons.If the surface roughness (Ra) is rough, movement becomes stiff and wear particles are generated due to friction, so a smooth finish close to a mirror surface is required.

Reduction of "backlash (play/looseness)" (small robot joints, precision gear mechanisms, etc.)
When used in precision gear shafts, even a slight gap (backlash) between the shaft and the hole can cause delays or looseness in movement.To minimize this gap to almost zero, extremely high precision is required for the outer diameter dimensions and roundness.

Suppression of "rotational runout" during high-speed rotation (micromotor spindle, etc.)
This is the shaft of a super-high-speed rotating motor used in drones, dental drills (turbines), and other devices.If the shaft's "roundness" or "coaxiality" is poor, significant vibration (rattling) and abnormal noise will occur during rotation, causing the motor itself to break down quickly.

Exceptional geometric precision is essential to minimize runout to the absolute minimum.

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Reasons why machining small-diameter shafts with high precision is difficult.

Because quality control is difficult for small-diameter shafts, various challenges must be overcome in order to process them with high precision.

This section explains the main difficulties in machining small-diameter shafts.

1. A large L/D ratio reduces rigidity.
2. Distortion is likely to occur due to heat treatment and processing stress.
3. Grooving and eccentric machining can easily disrupt rotational balance.
4. Selecting the appropriate support method and ensuring measurement accuracy are difficult.

A large L/D ratio reduces rigidity.

Small-diameter shafts tend to have a larger L/D ratio. Parts with a large L/D ratio have lower rigidity because their diameter is small relative to their shaft length. In other words, it is important to note that they are more susceptible to the effects of tool load and support conditions during machining.

Even slight force applied to the shaft during turning or grinding can cause deflection, potentially affecting the outer diameter and roundness. Therefore, for small-diameter shafts, it is necessary to optimize machining conditions and determine appropriate support methods.

Distortion is likely to occur due to heat treatment and processing stress.

Because small-diameter shafts have a small component diameter, even slight internal stresses can easily manifest as bending or dimensional changes. Therefore, the effects of distortion due to heat treatment tend to be greater.

Furthermore, changes in stress balance during subsequent processes such as groove machining or hole drilling can result in shaft distortion after processing. Therefore, process design that incorporates distortion countermeasures, such as performing grinding after heat treatment, is required.

Grooving and eccentric machining can easily disrupt rotational balance.

If the cross-sectional shape is not consistent due to keyways or D-cuts, the rotational balance of the small-diameter shaft may be disrupted, potentially leading to rotational runout and vibration.

Especially in high-speed mechanisms, even slight eccentricity can affect rotational accuracy and noise, making finishing and precision control after groove machining crucial.

Selecting the appropriate support method and ensuring measurement accuracy are difficult.

As the diameter decreases, the rigidity decreases, so small-diameter shafts are prone to slight deflection even when tools or grinding wheels come into contact with them during machining.

For example, when using centerless grinding for the outer diameter finishing of a shaft, the workpiece is supported by being sandwiched between the grinding wheel and the regulating wheel. As a result, the smaller the diameter, the more susceptible it becomes to the effects of the support conditions. Therefore, for small-diameter shafts, it is crucial to maintain roundness and bending accuracy by carefully controlling the support conditions and machining conditions.

Furthermore, measuring small-diameter shafts becomes difficult. When evaluating accuracy at the micron level, the results can vary depending on the resolution of the measuring equipment and the measurement method. Therefore, for high-precision machining of small-diameter shafts, securing a suitable measurement environment and method is just as important as machining technology.

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Precision and machining points for high-precision small-diameter shafts

Because small-diameter shafts have low rigidity, adding shapes such as grooves or holes can easily cause bending or distortion due to changes in load (cutting resistance) and stress during processing.

Here, we will explain the mechanism behind why accuracy tends to decrease for each typical shape, and the key points for processing and design to maintain high accuracy.

1. Grooved shaft
2. Perforated Shaft
3. Shaft with anti-rotation

Grooved shaft

This is a shaft with keyways, D-slots for positioning, and grooves for snap rings.

Factors contributing to decreased accuracy
When a groove is formed by machining a portion of a small-diameter shaft, the sectional rigidity of that section is drastically reduced. Furthermore, the cutting resistance during machining and the balance of internal stress release are disrupted, making the entire shaft more prone to bending into an arc or experiencing runout during rotation after machining.

Key points for processing and design
It is important to anticipate deformation caused by grooving, perform rough machining on a lathe, then groove, and finally incorporate heat treatment (stress relief).

Centerless grinding is often used for finishing the outer diameter, and our company's expertise and meticulous process sequence ensure a high-precision finish without distortion.

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Click here for grooved shafts for Sanwa Needle Bearings.

What is a grooved shaft? Explaining types, uses, machining accuracy, and design considerations

Perforated Shaft

This is a shaft that has through holes, blind holes, and tapped holes (internal threads) on its end faces.

Factors contributing to decreased accuracy
Because the drill bit is very thin and requires drilling into a small-diameter part with an outer diameter of only a few millimeters, the drill bit is prone to slipping during the drilling process, leading to serious misalignment (deterioration of coaxiality). In addition, the hole makes the wall thickness thinner and uneven, so the distortion (thermal deformation) during heat treatment is significantly greater than that of a solid shaft.

Key points for processing and design
Precise center hole machining to prevent misalignment and optimization of step-by-step drilling (a method of drilling holes while gradually removing chips) are required. It is also important to consider the balance from the design stage to avoid creating areas that are extremely thin.

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What is a Hollow Shaft? The Basics of Hollow Shaft Design that Achieves Both Lightweight and Rigidity

Shaft with anti-rotation

This process involves adding a special shape to the surface of the shaft to prevent "free spinning" (slipping) when combined with parts such as gears and pulleys. Because the cross-sectional shape changes and strong pressure is applied, the following points should be noted when using small-diameter shafts.

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D-cut
This process involves flattening the ends or middle sections of a shaft (creating a D-shape), and is frequently used as a simple anti-rotation mechanism.

Key points in machining and design include the fact that when milling a flat surface, the lateral force from the cutting tool causes the thin shaft to "bend." This poses a risk of the machined surface becoming tilted or eccentric, making robust workpiece holding (steady truing) during machining and strict control of finishing accuracy essential.

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Rounded edges
This process involves creating a mesh-like or flat pattern of fine irregularities on the surface to prevent the part from coming loose during press-fitting and to provide a non-slip grip when turning it by hand.

Regarding the processing and design, a key point is that typical "roll knurling" transfers a pattern by crushing (plastically deforming) the metal with strong pressure. This can cause the outer diameter of the processed area to expand on small-diameter shafts, leading to the entire shaft bending.

When high precision is required, measures such as performing finish grinding of the outer diameter after knurling or adopting "cutting knurling" (a method of creating patterns by cutting) which does not put stress on the material are necessary.

Knurling
Similar to knurling, this process creates fine irregularities (such as spline-like serrations) on the surface.

In terms of processing and design, the basic processing principle is the same as knurling, involving plastic deformation, so expansion of the outer diameter and bending are unavoidable in small-diameter shafts. It is important to take measures to limit the impact on accuracy to a localized area, such as increasing the distance between the "functional part to be knurled" and the "high-precision fitting part where bearings etc." are inserted, during the design stage.

Swell
This is a retaining process that involves plastically deforming a portion of the shaft by hammering or pressing it from the outside, causing it to become locally thicker (expanding into a flange-like shape).

A key point in processing and design is that small-diameter shafts have less room for material to swell, so strong impacts and stresses from swell processing spread throughout the entire shaft, causing serious bending and eccentricity.

When applied to high-precision, small-diameter shafts, it is necessary to either design the system to minimize stress transmission or incorporate a distortion relief process after deformation.

Frequently asked questions regarding high-precision machining of small diameter shafts

When considering the design and manufacturing of small-diameter shafts, many questions arise regarding the feasibility of manufacturing and precision control.

Here are some frequently asked questions from designers and procurement personnel.

Q1. What are some common overlooked considerations when designing high-precision small-diameter shafts?
Q2. Why is it difficult to machine small-diameter shafts with high precision using centerless support?
Q3. What are the advantages of having a small diameter shaft with low surface roughness?

Q1. What are some common overlooked considerations when designing high-precision small-diameter shafts?

In small-diameter shafts, poor roundness or excessive curvature can cause rotational runout and wear.Therefore, caution is advised if only the outer diameter tolerance is specified and geometric tolerances such as roundness and cylindricity are not specified.

Furthermore, for small-diameter parts, the final accuracy can vary significantly depending on the order of processes, so it is necessary to consider finishing processes after groove machining and D-cutting.

Q3. What are the advantages of having a small diameter shaft with low surface roughness?

Shafts with a smoother surface have lower sliding resistance and therefore a longer wear life.Furthermore, the reduced friction variation in the rotating mechanism leads to improved operational stability and quietness.
In precision mechanisms, differences in surface roughness directly affect operational accuracy, making the finishing quality achieved through grinding crucial.

Summary | For high-precision machining of small-diameter shafts, focus on those with extensive machining experience.

Small-diameter shafts become more difficult to machine as their diameter decreases, and are more susceptible to issues such as reduced rigidity, runout during machining, and distortion due to heat treatment.Therefore, by considering the processing method from the design stage and collaborating with experienced processing manufacturers, it is possible to prevent quality problems and rework during mass production.

Sanwa Needle Bearings has a proven track record in precision micro shafts and precision metal parts.We can handle complex machining processes such as grooving and D-cutting, and we also offer machining consultations from the design stage. If you are considering high-precision machining of small-diameter shafts, please feel free to contact us.