Gear Cutting on the Milling Machine

Gear cutting is a fundamental process in mechanical engineering, essential for creating the toothed components that transmit power and motion in countless machines. While dedicated gear cutting machines like hobbers and shapers exist for high-volume and specialized production, the versatile milling machine, often found in workshops and fabrication facilities, can also be effectively employed for cutting gears, particularly for lower production runs, repair work, or custom applications.   

This article explores the process of gear cutting on a milling machine, outlining the types of gears that can be produced, the necessary tooling and attachments, the step-by-step procedure, and the inherent advantages and limitations of this method.

Types of Gears Cut on a Milling Machine

Milling machines are most commonly used for cutting spur gears, which have straight teeth parallel to the axis of rotation. With the appropriate setup and cutters, they can also be used to produce:

• Helical gears: These gears have teeth cut at an angle to the axis of rotation, offering smoother and quieter operation than spur gears. Cutting helical gears on a manual milling machine requires a more complex setup involving differential indexing or a helical milling attachment.   
• Worm gears and worm wheels: This pairing is used for high-ratio speed reduction. The worm, a screw-like gear, can be milled, as can the mating worm wheel, though specialized hobs are often preferred for the latter for better tooth form accuracy.   
• Bevel gears: These gears are conical in shape and used to transmit power between intersecting shafts. Cutting accurate bevel gears on a manual milling machine is challenging and often limited to simpler forms due to the complex tooth geometry.   

Tooling and Attachments

Successful gear cutting on a milling machine relies on specific tooling and attachments:

• Indexing Head (Dividing Head): This is a crucial attachment that allows for precise rotation and indexing of the gear blank. It ensures accurate spacing of the gear teeth around the circumference. Various indexing methods, such as simple, compound, and differential indexing, can be employed depending on the required number of teeth and the available index plates.   
• Gear Cutters: These are form cutters specifically designed to produce the correct tooth profile for a given gear system (e.g., involute profile) and pitch (module or diametral pitch). A set of involute gear cutters typically includes eight cutters for each pitch, as the ideal cutter shape varies slightly depending on the number of teeth to be cut. Disc-type form cutters are common for horizontal milling machines, while end mills can be used on vertical milling machines.   
• Arbors and Mandrels: These are used to securely hold the gear blank during the milling operation.
• Milling Vice or Clamping Kit: Essential for rigidly securing the indexing head and workpiece to the milling machine table.

The Gear Cutting Process

The process of cutting a gear on a milling machine generally involves the following steps:

1. Gear Blank Preparation: The gear blank, usually a cylindrical piece of material, is machined to the correct outside diameter, bore size, and face width according to the gear specifications.
2. Setup on the Milling Machine:
   • The indexing head is mounted and aligned on the milling machine table.
   • The gear blank is securely mounted on an arbor or mandrel, which is then placed between the centres of the indexing head and tailstock (if used for support).   
   • The appropriate gear cutter is selected based on the gear's pitch and number of teeth and mounted on the milling machine's arbor (for horizontal mills) or in the spindle (for vertical mills).
   • The cutter is carefully aligned with the centre of the gear blank.   
3. Calculating Indexing: The required indexing movement for each tooth is calculated based on the number of teeth to be cut and the capabilities of the indexing head. This determines how much the indexing head crank needs to be turned for each tooth space.
4. Cutting the First Tooth Space:
   • The gear blank is moved upwards (or the cutter downwards, depending on the machine type) to the correct depth of the tooth space. This depth is typically calculated based on the gear's pitch.
   • The milling machine table is then fed horizontally to cut the first tooth space along the face width of the gear blank.
5. Indexing to the Next Tooth: Once the first tooth space is cut, the cutter is retracted, and the indexing head is rotated precisely according to the calculated indexing movement to position the blank for the next tooth space.   
6. Repeat Cutting and Indexing: Steps 4 and 5 are repeated for each tooth until all the tooth spaces are cut around the entire circumference of the gear blank. It is often recommended to take multiple passes, gradually increasing the depth of cut, to reduce cutter load and improve accuracy and surface finish.
7. Deburring and Finishing: After all the teeth are cut, any burrs or sharp edges are removed from the gear teeth. Depending on the application and required precision, further finishing processes like filing or grinding may be performed.   

Advantages of Gear Cutting on a Milling Machine:

• Versatility: A milling machine can be used for a wide range of machining operations beyond gear cutting, making it a valuable asset in a workshop.   
• Cost-Effective for Low Volumes: For producing a small number of gears or one-off replacements, using an existing milling machine is often more economical than investing in dedicated gear cutting machinery.
• Flexibility: The setup can be adjusted to accommodate various gear sizes and pitches within the machine's capacity.
• Suitable for Repair and Custom Work: Milling is well-suited for creating replacement gears for older machinery or producing custom gears for specific applications.

Limitations of Gear Cutting on a Milling Machine:

• Lower Production Rate: Compared to dedicated gear cutting machines like hobbers, the tooth-by-tooth milling process is significantly slower, making it less suitable for mass production.
• Accuracy Limitations: Achieving high levels of accuracy and surface finish can be more challenging on a manual milling machine compared to specialized gear cutting equipment or CNC milling centers with dedicated gear cutting cycles. The accuracy is heavily dependent on the operator's skill and the precision of the indexing head and machine.
• Tooling Costs: A set of involute gear cutters for various pitches can represent a significant investment.
• Limited Complexity: Cutting complex gear forms like spiral bevel gears or internal gears is difficult or impossible on a standard manual milling machine.

Conclusion

While not the most efficient method for high-volume gear production, gear cutting on a milling machine remains a valuable technique for workshops, educational institutions, and for producing small batches or custom gears. With the right tooling, a solid understanding of gear principles, and careful execution, a skilled machinist can successfully produce functional gears using a milling machine, demonstrating the enduring versatility of this fundamental machine tool