Corrected Lead Hobs-understanding The Need & Machine Settings

During our interactions with customers, we find the common challenge faced by customers about getting incorrect component parameters using Corrected Lead Hobs.

To address the challenge, we will talk about what are corrected lead hobs, why it is necessary to design such hobs and how to set up the hob on machine depending upon type of hobbing machine; CNC or Manual.

What is Corrected Lead Hob: Module & Pressure Angle of Corrected Lead Hob is different from Module & Pressure Angle of component.

Why it is necessary to design Corrected Lead Hob: When True Involute Form (TIF) diameter & Fillet Radius of component are not achieved with standard design, then Pressure Angle of Hob is reduced or increased to obtain the correct TIF & Fillet Radius of component. This can be explained with the below example. 

Standard Hob Design: Principally, TIF Shaving should be below Required TIF Diameter. However, when hob is designed with standard method, TIF required & TIF shaving are almost at the same values. Please refer Component Generated Profile in Picture 1.

Corrected Lead hob Design: To achieve the principal requirement of getting TIF Shaving below TIF required, Pressure Angle of Hob is reduced from 20° to 18°. Please refer Component Generated Profile in Picture 2.

 Hence, the corrected lead hobs are necessitated to be designed due to limitations in achieving the component data.

Machine Settings

Corrected lead type designed hobs bring new challenges at the time of usage like understanding the correct machine settings depending upon the type of gear & machine, considering the wide presence of Manual & CNC Hobbing machines.

Marking Guidelines on Hobs: For Spur Gears, only Lead Angle will be marked on Hob and for Helical Gears, Set Angle is also marked on Hob.

  1. Machine Setting for Manual Hobbing: For Spur Gears, Hob Lead Angle as marked on machine to be set manually. For Helical gears, Hob Set angle to be set on machine.
  2. Machine Setting for CNC Hobbing Machines: Input Original Module & PA in Component Data File and Input Corrected Module & PA in Hob Data File. No input of Set Angle is required.

Since any mistake in machine setting and/or feeding the data will lead to incorrect component parameters, the article will help in bringing the clarity about the necessity to design the Corrected Lead Hobs and understanding machine settings based on type of gear and machine.

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