Usually, it is always better to plan the use of a hob based on experience gathered in a gear production shop. This, no doubt, is very important from effective utilization of hob, which ultimately results in lowering the tool cost per piece. The hob could be a new one or a re-sharpened one. However, it has to be coated for better performance.
There are two ways to look at while estimating the hob life. One could be using the hob for the targeted batch production where the quantity of parts to be produced is a known figure. Once the target is achieved, the hob is offered for re-sharpening and coating. The second could be to estimate the production volume with reference to the performance of each hob-tooth in terms of length of gear teeth can be cut.
Below is a simple methodology developed to estimate the hob life.
Hob life is the expressed in terms of lineal inches / meter cut per hob tooth.
Knowing the hob parameters and specifications and utilizing life factors that have been developed through experience, it is possible to calculate tool life estimates with a certain degree of accuracy.
The first step is to calculate the number of usable teeth in the hob. To do this, it is necessary to determine the usable length of hob as follows:
Usable Length = HL – HB – NCP – RZ – GZ/2,
Where,
HL = Hob length
HB = Hub length (total of both sides)
NCP = Normal Circular Pitch
RZ = Roughing Zone = SQRT ((Gear OD – Whole Depth)*Whole Depth)
GZ = Generating Zone = (2*ADD)/TAN (PA)
Usable number of teeth = ((Usable Length)/NCP) * No of gashes
Once the usable number of teeth has been calculated, determine the total number of linear meter that can be cut per sharpening. The following example shows how above methodology can be used.
Hob OD: 100
Length: 200
No of Gashes: 14
Hub width: 5
NCP: π * Mn
Work piece: 40 Teeth
Module: 4
Outer Diameter: 168
Whole Depth: 9
Face width: 40
Pressure Angle: 20
Usable Length = 200 – (2 * 5) – (π * 4) – √ ((168-9)*9) – (2 * 4)/TAN (20) = 161.6
Usable no of teeth = (161.6 / 12.56) * 14 = 180
For a coated tool of 3.0 m length tooth life, hob life between re-sharpening will be = 540 m length = 338 parts
Knowing the usable length of hob, the hob shift amount will be 161.6 / 338 = 0.478 mm amount / piece.
With the modern CNC machines it is always possible to decide the strategy how to put hob in use so that there is uniform wear and tear across its face width, targeted production volume is achieved with consistent accuracy and planned numbers of resharpening are achieved thereby giving the least tool cost per piece over the total estimated life of the hob.
Let us implement above scientific approach for better effective utilization of hob.
