Custom CNC Shaft Manufacturing Process

A well-machined custom shaft is a manifestation of a precise CNC (Computer Numerical Control) manufacturing work. Despite the seemingly simple cylindrical form, shafts are among the main components of such intricate mechanism as automotive powertrain – of which is comprised of an engine, a transmission, a driveshaft, axles, and a differential; all of them require properly-engineered shafts to transfer power and torque to the wheels. In nearly all internal-combustion vehicles, the driveshaft also has to bear the weight of the entire vehicle while acting as a bridge between the engine and the axles. As for the differential, its shafts must manage the delivery of power to allow some of the wheels to rotate at different speeds during cornering.

Considering their complex functions, meticulous planning and precise programming are of the utmost importance in CNC shaft manufacturing process. Every step throughout the workflow, from the early design concept to surface finishing, requires a borderline-obsessive attention to details.

Custom Shaft Manufacturing Workflow

CNC turning is likely the most common method to produce custom shafts, but it is also possible to use alternative processes such as CNC grinding or the more advanced EDM (Electrical Discharge Machining). Regardless of the method, however, the goal remains the same: to manufacture purpose-built custom shafts in accordance with the pre-determined specification in the most efficient way. Every method also follows more or less the same basic workflow.

Design

CNC shaft manufacturing process begins with understanding the specifications, intended applications of the products, operating conditions, and requirements for approval either by internal review team or independent third-party organization (if required). Once an agreement is put in place by all parties involved, the first step of the workflow revolves around design and programming. 

The machinist must determine the dimension, including length and diameter, as well as the material of choice. As part of the design step, an engineering team records and verifies the specifications. In case the client provides no technical drawing, the manufacturer should produce a CAD (Computer-aided Design) file of the custom shafts to be made. Depending on the project requirements, designers and engineers might have to work together to conduct simulated strength test and failure analysis on a computer.

If everything is verified and approved, the next step involves generating tool paths for the CNC machine using CAM (Computer-aided Manufacturing) software. The paths are essentially machine-readable instructions that govern how the CNC should operate. Such instructions are comprised of geometric codes to tell the machine how to move in a cartesian coordinate system (X, Y, and Z axes) and may include rotational speed, order of movements, start point, feed rate, stop point, angle, wait time, etc. A machinist will then load the codes into the CNC controller.

CNC Machine Setup

Apart from generating tool paths, the manufacturer also has to properly set up the machine itself. Very few things are more important than preparing the right machine type, for example lathe or milling, and cutting tools. The workpiece (the material to be transformed into a shaft) must be secured firmly to the appropriate fixtures. Unless the manufacturer uses new cutting tools, a machinist has to make sure they are all as sharp as they can be or at least sharp enough to shape the workpiece.

Machining

CNC turning is the standard option for custom shaft manufacturing. Either a machinist or an automated process makes sure that the workpiece is affixed to the machine. As the workpiece is rotating at a high speed, the cutting tools subtract/remove a set portion of material from it until the desired shape is achieved. A variety of specialized cutting tools might be implemented in succession and will make contacts with the workpiece from many different points and angles.  

The biggest advantage of CNC turning in shaft manufacturing is high efficiency and accuracy when working on cylindrical component. And just about every custom shaft is crafted from an already cylindrical metal bar. Successive precision cuts naturally come easy with CNC turning as the machine is in fact designed to handle any workpiece of cylindrical shape. Given accurate tool paths and correct cutting tools, the machine can produce at least near complete custom shaft in a single session. Constant monitoring throughout the cutting process is required to ensure quality.

Finishing

Several finishing methods (for custom shafts) are available such as anodizing, passivation, and bead-blasting. The main purpose of surface finishing is to improve physical appearance, but at the same it enhances functionality by smoothing out rough edges and removing any material leftover from the cutting process. Apart from those, finishing also involves a final check to ensure that the manufactured shaft is accurate to the required dimensions, down to the tiniest millimeter.

CNC Shaft Manufacturing Methods

The preferred method is usually CNC turning, but it is not the only option at manufacturer’s disposal. As a matter of fact, a custom shaft with complex design is likely produced using a few different processes to achieve quality surface finish and tight tolerance. Some of the most common shaft manufacturing methods include:

CNC Turning

As long as the workpiece is cylindrical in shape, CNC turning is the ideal subtractive manufacturing technique. During the process, a bar of solid workpiece is secured to the chucks and spun at high speed while specialized cutting tools precisely remove unnecessary materials from it. High speed rotation ensures that every cut is evenly made all across the circumference of the cylindrical workpiece. CNC turning is an efficient machine to achieve the specified diameter and smooth finish in one go. It is the preferrable option for high-volume production as well.

CNC Grinding

If CNC turning cuts a sizable volume of material from the workpiece, CNC grinding removes materials by utilizing abrasive wheel. Other than that, the basic processes in both methods are essentially the same. They are also equally reliable in terms of quality, although grinding is a bit behind when it comes to production speed. In most cases, CNC grinding is used for the surface finishing task of an already machined shaft.

CNC Drilling

Some shafts are more than mere cylindrical bar of metal, but equipped with holes and grooves so they can be fitted to gears and other rotary components. Drilling creates grooves of varying depths and widths to accommodate gears of different dimensions, too. Either the machined shaft or the drilling tools may rotate and move depending on the angles, shapes, and position of the grooves.

CNC Milling

When a machined shaft needs to have keyways, splines, or flats in certain positions, CNC milling is the go-to option to fabricate the features. The cutting/milling tools rotate at high speed and remove materials from the shaft to produce the desired shape. During the process, the shaft may remain stationary or move and tilt slightly to adjust position.

EDM

There is no cutting, drilling, and milling tool in EDM. Inside the system, the workpiece is secured and attached to a positive terminal (anode), whereas the “shaping” tool is wired to a cathode. When they are placed in proximity to each other, the produced electrical current creates sparks. Extreme heat generated by the sparks can melt and evaporate materials from the workpiece. EDM is most often used when the machined shaft requires intricate small cuts.

Depending on the types and intended use, a custom shaft might be manufactured using two or more machining methods. Metallurgic analysis can determine which methods work best for certain materials.

Common Types and Applications of Custom Shafts

The main idea behind custom manufacturing is to produce all the different types of shafts for a range of specific purposes. Among the most common types and their applications are as follows.

Hollow Shafts

As the name suggests, hollow shafts are essentially cylindrical tubes to allow air, water, oil, wires, and other components/objects to pass. Thanks to the void in the center, they are lighter compared to their solid counterparts of the same diameter and length. Typical applications include medical devices, conveyor systems, plumbing systems, and pumps. If manufactured from high-strength materials, hollow shafts might be used to build components for performance cars and aircraft to help reduce weight.

Splined Shafts

One distinctive characteristic of splined shafts is that they feature longitudinal grooves or cut along their length. The grooves fit to gear, pulley, or any other corresponding components to form a sturdy connection and ensure smooth transfer of rotational motions. Such an assembly is observed in applications that require precise positioning like automotive gear couplings and transmission.

Threaded Shafts

Some shafts require external or internal threads to form strong connection with other mechanical components like bolts or screws. Threaded shafts are used in applications that require easy yet safe attachment and detachment of multiple parts.

Tapered Shafts

Most commonly seen in automotive pulley system, wheel hubs, and gears, tapered shafts feature gradual reduction in diameter along their length. The difference in diameter between one end and the other ensures secure mounting when the connected points of a mechanism – bridged by the shaft – are of varying sizes.

Keyed Shafts

The groove in a keyed shaft corresponds to a key (or tooth) of other components, preventing the shaft from slipping out of position or rotating independently. It is usually found in mechanism with sprockets, gears, and pulleys. Keyed shafts are often used for applications in which maintaining firm connection between components in high-torque mechanism is required, for example pumps and motors.

CNC Shaft Material

Aluminum and stainless steel are likely the most common materials for CNC shaft manufacturing, but other metals or even alloys also work for certain purposes.

Titanium

It is an excellent material of choice for precision shafts to be used for heavy-duty applications. Thanks to its superb corrosion-resistant property and exceptional strength-to-weight ratio, titanium shafts are most frequently used for performance cars and in the aerospace industries.

Stainless Steel

Arguably the most widely-used material for CNC shaft manufacturing, stainless steel is an all-around reliable metal with good corrosion resistance, durability, and strength. The material comes in a variety of grades and quality designed for a wide range of applications from chemical processing and medical devices to automative sector and marine environments.

Carbon Steel

The 1045 grade carbon steel is an exemplary material; soft enough for efficient machining, yet it can withstand heat treatment to increase hardness. Carbon steel might not be as corrosion-resistant as stainless steel but it still makes for an exceptional material for motor shafts or pump shafts.

Aluminum

A lightweight and corrosion-resistant metal, aluminum is a relatively soft and therefore highly machinable. It is the ideal shaft material when weight reduction is crucial to the intended applications.

Alloy Steel

First thing first, alloy steel and steel alloy are two different things. The composition of alloy steel has about 5% of alloying elements which may include chromium, nickel, tungsten, vanadium, and manganese. If corrosion resistance and machinability are indispensable, few materials are better than either 4340 or 4140 alloy steel.

Nickel Alloys

With exceptional corrosion resistance and remarkable strength, nickel alloy is the suitable material for precision shafts to be utilized in high-temperature applications and heavy-duty turbines.

The shaft’s eventual performance is greatly affected by at least three different factors including weight, corrosion resistance, and tensile strength. Hardness also plays a factor but most metals are generally hard enough for every application, although some are more malleable/machinable than others. While titanium and carbon steel are superior to aluminum and stainless steel in terms of weight reduction and strength/durability, they are also more expensive.

End Notes

CNC shaft manufacturing involves a series of detail-oriented processes from initial design concept to surface finishing and quality control. Therefore, it is best to have good understanding of the project requirements, working conditions, and intended applications of the finished shaft in the first place, because they will ultimately affect the choice of material, machining method, and overall quality of the finished product. Bear in mind that each material has its own strong and weak points, so does every machining method. As such, the general rule of thumb is to obtain good balance between quality and budget.