Usually,
normal module and normal pressure angle of gear and its respective hob will be
the same.
Some
times, it is necessary to of hob due to the following change module and
pressure angle reasons.
1.0
To obtain required active profile length on gear with protuberance type hob,
when usual hob (module & pressure angle of hob & gear are same) not
able to generate.
2.0
To change the shape of the fillet radius and increase the active profile
length in hobbing.
3.0
Some time to increase the tip width of the hob, which will also help to
increase the life of the hob in such applications.
In
case of short lead hob, the base pitch of the hob will be the same to base
pitch of the gear.
Ie Gear normal module
x Cosine (nor pr angle of gear) = Hob normal module x Cosine (nor pr angle of
hob).
Hob
designer will select suitable module and pressure angle for the hob based on
gear data. Occasionally designer selects different module & pr angle to
meet any one of the above three requirement. Such type of hob is called as
short lead hob.
While
short lead hob is used, user has to set the machine as below.
For manual machine
1.0
Set index change gears (to suit gear number of teeth and number starts on hob)
on hobbing machine as usual.
2.0
Set differential change gears (to suit gear helix angle, hand of helix and
number of starts) on hobbing machine as usual.
3.0
Usually hob head setting angle = (Helix angle of gear - lead angle of hob) when
hand of helix angle on gear and hob are same. But while using short lead hob,
manufacturer marks hob setting angle on hob. The same is to be set on machine.
Don't follow the above formula in such case.
How
user can identify short lead hob from cutter marking?
Example:
Usual
hob is marked as: 2.5M, 20°PA, 2°30'LA, RH,
Short
lead hob is marked as: 2.4321(2.5)M, 15°(20°)PA, 2°15'LA, RH, Hob set
Ang=27°10', .
