Introduction :-
The process of gear hobbing is one of
generation. The word "generating" alludes to the fact that the gear
tooth form cut is not the same as the hob, which is the conjugate form of the
cutting tool. Both the hob and the workpiece rotate in a continuous rotating
interaction during hobbing. The hob is normally fed axially during this
rotation, with all of the teeth eventually created as the tool passes the work
face. (Fig. 1a.)
Figure References : https://www.geartechnology.com/
To produce these cutting edges,
the hob is essentially a worm with gashes cut axially across it. To give chip
clearance behind the cutting edge, each cutting tooth is also eased radially.
The hob face can also be sharpened while maintaining the original tooth form. A
number of flats blend together to generate the tooth's ultimate profile. The
number of flats relates to the number of cutting gashes that pass during a
single rotation of the workpiece teeth. As a result, the higher the number of
gashes in the hob, the more flats along the profile, which increases the tooth
profile's "smoothness."
The Cutter :-
Figure 2 – Cutter
Figure References : https://www.geartechnology.com/
Cutter Modifications :-
To create changed tooth shapes,
you can design the shape of a cutting tool. This is done for a variety of
reasons. The hob tooth root can be designed to cut the gear tooth's outside
diameter. The tooth involute and the outside diameter of the blank will be
hobbed in one operation using this "topping" hob (Fig. 3). Finish
turning of the gear blank may be eliminated, resulting in fewer machine
operations.
With a "semi-topping"
or "tip chamfering" hob, sharp edges between the tooth flank and
outside diameter can be removed (Fig. 4). A hob like this could potentially
solve the problem of gear tooth bending under load if it's designed correctly.
Gears that will be completed
later, for as by skiving, shaving, or grinding, may need clearance in the gear
tooth fillet area for the finishing tool. A "protuberance" hob can be
used to cut this, resulting in an undercut (Fig. 5). The protuberance hob is used
to create a consistent stock for the finishing tool and to combine the hobbed
root area and the finished flank.
Figure 5
Figure References : https://www.geartechnology.com/
Gear Cutting :-
To transmit torque, a gear is a spinning machine
part with cut teeth that mesh with another toothed element. Gear is made from a
spherical blank with teeth running around the outside. Cutting gear is a
specialised task. Any machining operation that creates a gear is known as gear
cutting. Hobbing , broaching, milling, and grinding are the most popular
gear-cutting techniques. Forging, extrusion, investment casting, and sand
casting are examples of forming techniques that can be used after or instead of
cutting procedures.
Construction Of Gear
Hobbing Machine :-
1) The gear blank is rolled with a spinning
cutter called a hob in this procedure. Gear hobbing is accomplished with the
aid of a multipoint cutting instrument known as a gear hob. It resembles a worm
gear with a ring of straight flutes running parallel to its axis around its
periphery. These flutes are created with correct angles so that they can function
as cutting edges.
2) In a gear hobbing procedure, the hob is
turned at a sufficient rpm while the gear blank is simultaneously fed.
3) The gear blank is continuously spinning as
well. The rpm of both the gear blank and the gear hob are synced to the point
where each revolution of the gear bob causes the gear blank to revolve by one
pitch distance of the gear to be cut.
4) The motion of the gear blank and hob is kept constant and
consistent. Figure depicts a gear hob, whereas Figure depicts the process of
gear hobbing (Fig. 6).
5) The hob teeth have a defined helix angle and behave
like screw threads. The hob is tilted to a helix angle during operation to keep
the cutting edges square with the gear blank.
6) Gear hobbing is used to create a wide range of gears,
including spur gears, helical gears, hearing-bone gears, splines, and gear
sprockets, among others.
Figure 6 –
Process of Gear Hobbing
Figure
References :- https://learnmech.com/wp-content/uploads/2017/06/howgearhobbingprocessworks.png
Working Of
Gear Hobbing :-
•
Gear hobbing is a producing method that involves rotating the gear blank and a
cutter called a hob at the same time while maintaining a fixed gearing ratio
between the hob and the gear blank. The gear blank is fed into the rotating hob
in this procedure until the appropriate depth is obtained. The hob is fed
across the blank's face until all of the teeth have been completed.
• The
hob teeth are placed parallel to the blank's rotating axis while hobbing spur
gear. The axis of the hob is set over an angle to provide the suitable helix
for helical teeth bobbing. Worm gears, on the other hand, are made with the
hob's axis placed at a right angle to the gear blank.
• In
most cases, two methods are used in the hobbing process -
1)
Conventional Hobbing
2)
Climbing Hobbing
Video Reference : https://youtu.be/vGPuDHCybx4
• When
compared to other gear-making techniques, the rate of output is fairly high.
The procedure can be used to make spur, helical, worm, sprocket, and spline
gears, among other things. Cutting up to the shoulder is not possible with the
rotary cutter.
• The
functioning concept of the gear hobbing process is depicted in the diagram
below.
Figure
7 - Gear
hobbing process: (A) schematic representation; (B) actual photograph.
Gear Hobbing Parameters:-
In the operation of gear
hobbing, three critical parameters must be controlled: indexing movement, feed
rate, and the angle between the axis of the gear blank and the gear hobbing
tool (gear hob).
Hobbing Process Types:-
Gear hobbing is divided
into distinct types based on the direction in which the hob is fed for gear
cutting. The classification is described in the following paragraphs.
1. Using
Axial Feed in Hobbing
In axial hobbing, the gear blank is first brought toward the hob to obtain the necessary tooth depth, and then the rotating hob is fed parallel to the gear blank's axis. This is how spur and helical gears are machined.
2. Using Radial
Feeding in Hobbing
The hob and
gear blanks are placed with their axes parallel to each other in this way. The
rotating hob is fed against the gear blank in a radial or perpendicular
direction to the gear blank's axis. Worm wheels are made using this process.
3. Hobbing with Tangential Feed
Video
Reference : https://youtu.be/8UvXY2bT-o8
The Benefits
and Drawbacks of the Gear Hobbing Process -
The following
are some of the benefits of the gear hobbing process:-
(a) Because
gear hobbing is a quick and continuous process, it is more cost effective than
alternative gear generation methods.
(b) A shorter
production cycle time, resulting in a higher production rate.
(c) When compared to other gear machining methods, the procedure has a higher variability in the following of sense.
- Capable of producing a wide range of gears, including spur gears, helical gears, worms, splines, and sprockets, among others.
- The required indexing (called such) method is fairly simple, and the module is capable of producing any number of teeth with consistent accuracy.
- Gear hobbing is used specifically to create a special form of gear known as herringbone gear cam.
- This technique can handle a wide range of batch sizes (from tiny to huge volume).
(d) Several gear blanks can be machined at the same time if they are installed on the same arbour.
(e) A hob is a multipoint cutting tool with
multiple cutting teeth or edges. Because only a few cutting edges are active at
a time, there is plenty of time to dissipate the created heat. There is no
cutting tool or overheating.
Gear Hobbing Machine Drawbacks :-
1. It isn't used to make internal gears.
2. Restricted adjacent shoulders that are
greater than the gear's root diameter.
3. Hobbing is not suitable for splines or
serrations.
Gear hobbing's uses
include:-
1 ) It's commonly used to make spur and helical gears, as well as worms and worm wheels.
Figure 8 –
Spur gear , Helical gear , Worm gear
Figure References : https://en.wikipedia.org/wiki/Gear
1) Aayush Surawar ; Mechanical Engineering Department , VIT Pune.
2) Samarth Takbhate ; Mechanical Engineering Department , VIT Pune.
3) Devashish Tambade ; Mechanical Engineering Department , VIT Pune.
4) Umesh Wanare ; Mechanical Engineering Department , VIT Pune.
5) Viren Wanekar ; Mechanical Engineering Department , VIT Pune.
Very Informative 💯👍
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