Shaft Machining: Material, Heat Treatment, Solution

Shaft is a critical component of machinery. It is connected to the frame via bearings, and the components mounted on it perform rotary motion around the shaft’s centerline. Together, these elements form a shaft assembly based on the shaft.

Overview of Shafts

A shaft is a rotating component of any machine, characterized by a circular cross-section, used to transmit power from one part to another or from a power source to a power consumer. To facilitate this power transmission, one end of the shaft is coupled to the driver while the other is connected to the machine. Shafts can be solid or hollow depending on the application; hollow shafts are advantageous for weight reduction. As a critical mechanical element, shafts support rotating components such as pulleys and gears. They are typically supported by bearings within a rigid housing, allowing gears and pulleys to transmit motion effectively.

Many other rotating elements are mounted onto the shaft via keys. Consequently, shafts are subjected to both bending moments and torque due to the reaction forces of the supported components and the torque generated during power transmission.

Based on the type of loading, shafts are categorized into the following three types:

• Spindle shaft: A shaft that only endures bending moments without torque. Spindle shafts can further be divided into rotating spindle shafts and non-rotating spindle shafts.
• Driving shaft: A shaft that only transmits torque without enduring significant bending moments, or the bending moments are negligible. For example, the shaft connecting the gearbox and the rear axle in an automobile.
• Transmission Shaft: A shaft that endures both bending moments and torque, such as the shafts in reducers. An example is the drive shaft connecting an automobile’s gearbox to the rear axle.

Based on shape, Shafts can be classified into several types. The table below shows different shaft types by their shape and design.

Machining Accuracy of Shafts

The surfaces of shaft components are generally divided into support journals and matching journals.

• Support Journals: These fit with the inner ring of a bearing to locate and support the shaft. They require high dimensional tolerance, typically IT5–IT7.
• Matching Journals: These interface with various transmission components. Their tolerance requirements are generally lower, typically IT6–IT9.

Geometric accuracy: Primarily refers to the roundness and cylindricity of critical surfaces like journal surfaces, external tapers, and tapered holes. Errors should generally be restricted within the dimensional tolerance zone. For precision shafts, geometric tolerances must be specifically defined on the technical drawings.

Positional accuracy: Includes the coaxiality of internal and external surfaces, radial runout, perpendicularity of critical end faces to the axis, and parallelism between end faces.

Surface roughness: All machined surfaces have specific roughness requirements determined by processing feasibility and cost-effectiveness. Support journals typically require Ra 0.2–1.6 μm, while matching journals for transmission components require Ra 0.4–3.2 μm.

Materials for Shafts

Shafts are primarily manufactured from carbon steel and alloy steel. 1045 carbon steel is the most common choice, usually requiring normalizing or quenching and tempering (Q&T) to improve mechanical properties. Alloy steels offer superior mechanical properties and heat treatment performance compared to carbon steel but are more sensitive to stress concentrations and carry a higher cost. Therefore, they are reserved for high-speed, heavy-duty, or special conditions (e.g., wear resistance, high/low-temperature environments). Note that since the elastic modulus of alloy steel and carbon steel is nearly identical at room temperature, replacing carbon steel with alloy steel will not significantly increase the shaft’s stiffness.

For shafts bearing heavy loads or requiring high strength, compact design, or better wear resistance, alloy steels like 5140, 5120, or 9255 are used. Because alloy steel is highly sensitive to stress concentrations, the structural design must minimize these effects. Proper heat treatment is mandatory to fully leverage the material’s properties.

Shaft blanks are produced from hot-rolled round bar or forgings. For complex shapes like crankshafts and camshafts, cast steel or ductile iron may be used, the latter offering excellent vibration damping, low sensitivity to stress concentration, and cost-efficiency.

Heat Treatment of Shaft

Medium carbon and medium carbon alloy steels (e.g., 1035, 1040, 1045, 1050, 5140, 3140, or 1541) are standard choices. These usually undergo normalizing or Q&T. If high wear resistance is required at the journal, surface hardening can be applied.

Material selection depends on load type, part dimensions, and hardenability. For shafts under bending and torsional loads, where stress decreases from the surface to the center, high hardenability is not strictly required. However, for shafts under tension or compression where stress is uniform across the cross-section, high-hardenability steel is necessary.

For high impact loads or extreme toughness and wear resistance requirements, alloy carburizing steels like 5120 or 4320 are used, followed by carburizing, quenching, and low-temperature tempering. Low-importance shafts with minimal loading can use general-purpose carbon steels like 1015 and 1025.

Heat treatment in shaft machining is shown in the figure.

1. Normalizing or annealing: Arranged before machining to refine grains, eliminate forging stress, and improve machinability.
2. Quenching and tempering (Q&T): Usually performed after rough turning but before semi-finish turning to achieve optimal comprehensive mechanical properties.
3. Surface hardening: Arranged before final finishing to allow for the correction of any quenching-induced deformation.
4. Low-temperature aging: Required for high-precision shafts following local quenching or rough grinding.

Selection of Positioning Datums

The common positioning datum for shaft parts is the dual center holes. Since coaxiality and perpendicularity are the primary accuracy requirements, and the design datum is usually the shaft centerline.

However, center holes cannot be used if:

• Rough machining requires higher rigidity, in which case the outer diameter surface is used.
• The shaft is hollow with through hole.

Methods for hollow shafts:

1. Small holes: Chamfer a 60° internal cone (width ≤ 2mm) at the hole opening.
2. Cylindrical holes: Use a taper plug (1:500 taper).
3. Large taper holes: Use a mandrel with a taper plug.

Machining Process Routes

The machining focus is the external cylindrical surfaces of the journals. The process route is designed around the sequence of external diameter machining, with secondary features like threads, splines, keyways interspersed.

(1) Carburized Steel Shafts

Material Prep → Forging → Normalizing → Drilling Center Holes → Rough Turning → Semi-finish/Finish Turning → Carburizing → Quenching & Low-temp Tempering → Rough Grinding → Secondary Feature Machining → Finish Grinding.

(2) Standard Precision Q&T Steel Shafts

Material Prep → Forging → Normalizing (Annealing) → Drilling Center Holes → Rough Turning → Q&T → Semi-finish/Finish Turning → Surface Hardening & Tempering → Rough Grinding → Secondary Feature Machining → Finish Grinding.

(3) Nitrided Steel Shafts

Material Prep → Forging → Normalizing (Annealing) → Drilling Center Holes → Rough Turning → Q&T → Semi-finish/Finish Turning → Low-temp Aging → Rough Grinding → Nitriding → Secondary Feature Machining → Finish Grinding → Lapping/Polishing.

Machining Example: Stepped Shaft

This component is a stepped shaft consisting of cylindrical surfaces, shaft shoulders, threads, and a square head section.

(1) Material and Blank Manufacturing

The shaft is made of 45 steel (AISI 1045 equivalent). Although the production volume is small, the diameter of the journals varies significantly (ranging from a maximum of ø50mm to a minimum of ø13mm). To optimize material utilization, a forging is used as the blank, followed by a normalizing treatment.

(2) Technical Requirements

Dimensional Accuracy: The ø25mm journal must meet an IT7 tolerance grade. The surface roughness is specified at Ra 0.8μm. Using the ø50mm shoulder as the datum, the perpendicularity tolerance of the ø25mm axis is 0.02mm, and the parallelism tolerance between the two end faces of the shoulder is 0.02mm.

(3) Positioning Datums

To ensure the coaxiality of the various journals, the center holes at both ends are used as the fine datum. This positioning datum remains consistent throughout the turning, milling, and grinding processes to minimize setup errors.

(4) Machining Methods

As the shaft consists primarily of surfaces of revolution, turning and grinding are the main forming processes. Because the ø25mm journal and the ø50mm shoulder faces require high geometric precision in addition to dimensional accuracy, grinding is performed after turning. The processing sequence for the external surfaces is:Rough turning → Semi-finish turning → Grinding

(5) Machining Route

For components with high precision requirements, roughing and finishing stages must be separated to ensure quality. This shaft’s machining is divided into three stages: Rough Turning, Semi-finish Turning, and Grinding.

(6) Heat Treatment

The ø25mm × 35mm section and the square head surfaces require hardening. This is scheduled after semi-finishing and before the final grinding process. Normalizing is performed after the blank is forged to relieve internal stresses, refine the grain structure, and improve machinability.

How Getzshape Can Help

Getzshape delivers high-quality custom CNC machining, sheet metal fabrication, electrical discharge machining, die casting and more. Our CNC machining capabilities for shaft machining are listed below.