COMMONLY USED TERMS & CALCULATION METHODS
Hob RPM
Depending upon the cutting speed requirement, RPM can be calculated by the following formula.
Cycle Time
Cycle Time (t) can be calculated by using the following calculations.
Hob Shift
The most important part of the Hobbing operation is to use the optimal life of hob by uniformly distributing the wear amount. This is achieved by shifting the Hob in the axial direction by using the following calculations, where micro shifting is not available on the hobbing machine.
Hob Setting Angle
- When the hand of Hob & Workpiece is same, then setting angle shall be β-γ.
- When the hand of Hob & Workpiece is opposite, then setting angle shall be β+γ.
- [ β: Gear Helix Angle γ: Hob Lead Angle ]
HOB MOUNTING ERRORS
SHARPENING GUIDELINES
Effect of Sharpening Errors on Gear Profile
- Positive rake angle produces Gear Tip plus (minus pressure angle)
- Negative rake angle produces Gear Tip minus (plus pressure angle)
- Convex rake angle produces thinner gear teeth on the middle of the profile
- Indexing error produces irregular profiles.
Hob Wear Amount
- For better cutting efficiency, hob wear should not be increased beyond following limits and should be sharpened quickly for better results.
| Module | Max. Wear Amount |
|---|---|
| 1 to 2 Module | 0.20 mm |
| 3 to 4 Module | 0.25 mm |
| 5 to 6 Module | 0.30 mm |
| 6 to 8 Module | 0.40 mm |
HOB DAMAGE ANALYSIS
HOBBING METHOD COMPARISON
| Climb Cutting | Conventional Cutting | |
|---|---|---|
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| Quick hob wear | Hob wear | Less hob wear |
| Prolonged comma | Basic shape of chip | Shorter comma |
| Lower | Cutting speed | Higher |
| Good | Hob performance | Poor |
| No | Backlash | Yes |
| Regular | Table feed | Jumpy |
| Poor | Component profile | Good |
| Lower | Productivity | Higher |
| Preferred | Helical Gear | - |
| Large Gears | Gear Type | Small Gears |
| Poor | Gear Finish | Good |
| Avoided | CNC Hobbing M/C | Recommended |
SHAVING METHODS & CHARACTERISTICS
Shaving Methods – Every shaving method differs according to the direction of the movement given to the cutter. The choice of method depends on the workpiece shape, machine characteristics, and volume of production.
Cross Axis Angle – The difference in the helix angle of the gear and the cutter is calculated along the PCD. Cross axis angle is an essential condition for the shaving process.
| Particulars | Conventional Shaving | Diagonal Shaving | Underpass Shaving | Plunge Shaving |
|---|---|---|---|---|
| Pictorial View & movement of Shaving method |  |
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| Gear and the cutter are moved parallel to the work piece axes. | The relative motion between cutter and gear takes place with an angle included in the range 5° to 45° w.r.t the gear axis. | Cutter feed in movement towards the gear is perpendicular to gear axis. | Cutter feed in movement towards the gear is radial to gear axis. | |
| Where to be used | Suitable for low and medium production operations. | Suitable for medium & high production operations. | Suitable for high production operations. | Suitable for high production operations. |
| Suitable for open gears | Suitable for open gears. | Suitable for shoulder type gears with cross axis angle limitation. | Used for open and shoulder type gears. | |
| Large width gears can be shaved. | Gear Face width is limited to 100mm as component face width should be necessarily more than cutter face width. | Max. Gear face width is limited to 55mm as cutter face width is more than component face width. | Max. Gear face width is limited to 55mm as cutter face width is more than component face width. | |
| Shaving time | Stroke length is long | Stroke length is relatively short. | Shaving stroke is extremely short. | No longitudinal movement of cutter. |
| Very long Shaving time | Shaving time is relatively short | Shaving time is very short. | Shaving time is least. | |
| Cutter Utilization | The cutter works only with a limited contact area at the center of teeth. | Better use of the cutter that can exploit all its length. | Cutter works progressively and partially along the whole face width. | Cutter works progressively and partially along the whole face width. |
| Guidelines for better results | Length of traverse should be 1/16” greater than face width of the component. | Sum of Traverse Angle & Cross Axis Angle should not be more than 55°. | It can be used with shoulder gears with minimum cross axis angle of 3°. | Radial in-feed should be carefully selected. |
| γ = Cross of Axes Angle, ε = Diagonal Angle, L1 = Shaving Cutter Tooth Length, L2 = Gear Tooth Length, L = Stroke Length | ||||