This document summarizes new developments in gear hobbing machines and processes. It discusses how hobbing works to cut gears using a hob tool on a specialized milling machine. Modern developments include using powder metallurgical high speed steel tools which have higher toughness and lower costs than carbide tools. A new dry hobbing process has also been introduced to improve the working environment by eliminating coolant use. This dry hobbing process allows cutting speeds up to twice as fast as wet hobbing and provides benefits like reduced running costs and a safer environment.

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COURSE TITLE: MACHINE TOOLS
COURSE NO: MCE 4627
ASSIGNMENT TITLE:
NEW DEVELOPMENTS OF HOBBING MACHINE
NAME: Muhammad Awais
STUDENT ID: 131308
PROGRAMME: HDME
DAT OF SUBMISSION: I9 JULY 2016

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What Is Hobbing?
Hobbing is a machining process for gear cutting, cutting splines,
and cutting sprockets on a hobbing machine, which is a special
type of milling machine. The teeth or splines are progressively cut
into the workpiece by a series of cuts made by a cutting tool called
a hob. It is quite inexpensive process as compared to other gear
forming process. Hobbing is principle method for producing spur
gears and helical gears and is also used to produce many special
gear forms. But the main limitation of hobbing is that it can only be
used external gears.
Before we go into the details of hobbing new development
machining process, let’s have a little bit idea about gears. Gears
are mechanical component use to transmit power and motion
through this successive engagement of their peripheral teeth.
Gears perform certain key function within machine and the
samplings such as reversing the direction of rotation or altering the
angular orientation of rotary motion. Gears are also used to
convert rotary motion into linear motion and vice versa or to alter
gear speed ratios.
The methods of machining gears can be classified into two primary
categories.
Gear Generating:
Gear generating involves gear cutting through the relative motion
between rotating cutting tool and the generating motion or the
rotation of the workpiece. Hobbing and shaping is one of the
primary gear generating process.
Gear Form- Cutting:
It involves use of the form tooth cutters that have the desired gear
shape or profile. The primary gear form cutting process includes
broaching and milling. These both broaching and milling process
produces external and internal gears.
Hobbing Process:
One of the mostimportantconceptstounderstandaboutgearhobbingisthat
itisa generatingprocess.The termgeneratingreferstothe factthatthe shape
of the gear tooth that results is not the conjugate form of the cutting tool.
Rather, the shape of the tooth is generated by the combined motions of
workpiece andcuttingtool. Duringhobbing,boththe hob and the workpiece
rotate ina continual,timedrelationship.Foraspurgearbeingcutwithasingle

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start hob, the workpiece will advance one tooth for each revolution of the
cutter.When hobbinga20-tooth gear, the hobwill rotate 20 times,while the
workpiece willrotate once.The profileisformedbythe equallyspacedcutting
edgesaroundthe hob,eachtakingsuccessive cutsonthe workpiece,withthe
workpiece inaslightlydifferentpositionforeachcut.Several cuttingedgesof
the tool will be cutting at the same time. The figure given below depicts the
path of the cutting tool relative to the workpiece. [1]
HOB:
The hob is basically a won with gashes. Cut axially across it to produce
these cutting edges. Each cutting tooth is also relieved radially to
provide chip clearance behind the cutting edge. This also allows the hob
face to be sharpened and still maintain the original tooth shape. The
final profile of the tooth j created by a number of flats blending
together. The number of flats corresponds to the number of cutting
gashes which pass the workpiece tooth during a single rotation. Thus,
the greater the number of gashes in the hob, the greater the number of
flats along the profile whichimprovesthe "smoothness"of the toothprofile.
[2]
New Developments in Gear Hobbing:
In this section we are going to get ourselves familiar with the new
development in gear hobbing. Let’s start with the tool designs.
Modern Tool Design:

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Machine tools have also improved considerably in the last decades, but
their major impact on hobbing technology was related to tool
development. If we examine those past improvements, the focus was,
on the one hand, on the substrate materials and, on the other hand, on
the coating systems. Together, both developments led to much higher
cutting speeds and/or longer tool life. Even processes like dry hobbing
became a reality.
Coming from the conventional HSS substrates with TiN coating, the use
of carbide hobs seemed to be critical to dry hobbing applications. But
after initial success, problems with the process’s reliability regarding
tool life of the reconditioned carbide hobs (e.g., due to cobalt leaching
during stripping) stopped the trend. Then, the introduction of the more
heat-resistant TiAlN coatings—in combination with higher alloyed and
more homogeneous PM-HSS substrates—brought the dry cutting back
on track. Today, dry cutting with PM-HSS, as well as carbide, is a given.
And since the AlCrN-based coatings have recently been introduced
successfully in the gear hobbing market, speed and feed could be
increased even more in many applications. Besides their more
homogeneous structure, the main advantage of the powder
metallurgical HSS substrates is the ability to contain greater amounts of
alloys. [3]
The recent developed tool called powder metallurgical high speed steel
(PM-HSS) possess higher toughness and low cost compared to other
carbide tools. Due to the typically lower tool costs, PM-HSS is actually
the preferred substrate material for hobs, especially in the smaller
modules (e.g., automotive and truck industry). Characteristic of PM-HSS
is its reliable wear behavior in a widespread range of applications.
Carbide offers advantages, especially in the area of finishing and cutting
of high-strength workpiece materials (Rm> 900 N/mm2), due to its high
wear resistance. It is assumed that using PM-HSS leads to lower tool
costs (in this case, 16%). Carbide tools offer lower machining times
(22%) and thus lower machining costs. In the present example, both
advantages almost negate each other concerning cost per piece. This
clearly demonstrates that the decision regarding the right substrate
should always be made on an application-by-application basis. If no
significant advantages are present for carbide, PM-HSS is actually
favored in most applications due toitsmore reliableperformanceandlower
investment costs. [3]

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A New Design for Dry Hobbing Gears:
To improve the working environment and its protection new hobbing machining
process is introduced which is dry hobbing gears. Coolant-free metal cutting
methods are being devised and implemented in factories to ensure the safe
environment. In turning and milling applications, however, there are almost no
complete dry-cutting machines that do not use coolant at all, although some are
shifting from wet cutting to MQL (minimum quantity lubrication). Amid this, the shift
to complete dry-cutting machines is progressing in the field of gear cutting, and
especially hobbing.
The Advantages of Dry Cutting:
It's a fact that coolant produces oil dirt in factories and adversely
affects the environment as well. Dry cutting not only solves such
environmental issues, but it is also effective in improving cutting
efficiency and reducing the life-cycle cost due to its low running
costs.
The benefits of using dry hobbing from an environmental
standpoint have encouraged Mitsubishi's research into dry cutting
using high-speed steel hobs, which are more stable than
cemented carbide hobs. Our tests began at the same speed as
that used for ordinary wet hobbing with high-speed steel hobs. The
results were amazing, and it seemed like the ideal cutting method,
because increasing the cutting speed to twice as fast as the
ordinary cutting speed produced no abnormal wear or temperature
rise on parts. However, the absence of coolant also posed major
problems that prevent the practical use of dry cutting. The
problems included flying chips and the heat generated by chips,
which drove us to develop a new dry hobbing machine. The
following discusses the functionality and tools required for dry
cutting, taking our most recent machine as an example.
Main motor capacity is an important issue. In wet hobbing, parts
(carburized steel) are normally hobbled at cutting speeds of 80 to
120 m/min using high-speed steel hobs. However, dry hobbing
uses cutting speeds of 120 to 250 m/min, which are 1.5 to 2 times
as fast as those of wet cutting. Since the cutting power is directly
proportional to the cutting force and the spindle speed, dry-cut
hobbing machines require a main motor capacity that is 1.5 to 2
times as large as that of conventional wet hobbing machines. In
other words, converting conventional hobbing machines to
accomplish dry hobbing may be possible, but it does not contribute

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to an increase in cutting performance in many cases because the
cutting conditions are restricted by the main motor capacity. [3]
Dry Hobbing Gear Working Process:
Dry hobbing a gear using a conventional TiN-coated hob causes
abnormal wear, but a TiAIN-coated hob causes almost no wear.
This is due to the following. First, using a coating film with excellent
abrasion resistance. In dry cutting, the hob's teeth are exposed to
an extremely high temperature because of the lack of coolant. In
TiN coating, the Ti composition of the coating film is oxidized and
transformed into a brittle composition of TiO2, which prevents
maintaining its original abrasion-resistance characteristics. On the
other hand, in the case of the TiAIN film, AI is selectively oxidized
at the depth of approximately 0.5 µm from the surface and is
considered to be a rigid film. It has been found that the TiAIN
coating has high abrasion-resistance characteristics thanks to the
effect of this film.
Secondly, a protective film is produced by the deposition of
hobbing chips. In wet hobbing, an extreme-pressure additive
contained in coolant prevents the deposition of hobbing chips. In
dry hobbing, however, hobbing chips are easily deposited to
protect the tool surface, reducing the amount of wear. [3]
Recent Developments:
The maximum speed of high-speed steel dry hobbing used to be
200 m/min. However, it has been increased to 250 m/min thanks
to the development of a new coating film having better high-
temperature oxidation resistant characteristics than TiAIN.
Development of Gear Hobbing Fixture Design for Reduction
in Machine Setting Time:
For the hobbing on gear hobbing machine properly designed hobbing
fixtures are needed. Design of such fixtures is complex and numerous
plausible designs are possible for a single work piece. The fixture design
should aimat restraining unwanted movement of the work piece under
the action of cutting forces through- out machining. Fixtures are work
holding devices, which are used in most of the manufacturing
operations such as, machining, assembly, inspection, etc. Featuring
systems, usually consisting of clamps and locators must be capable to

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assure certain quality performances, besides of positioning and holding
the workpiece throughout all the machining operations.
TIME CONSUMTION IN HANDLING AND FIXING THE MACHINE:
Fixture forms an important factor in traditional and modern
manufacturing systems, since fixture design directly affects
manufacturing quality and productivity. Since the total machining time
for a workpiece includes work-handling time, the methods of location
and clamping should be such that the idle time is minimum. The design
of fixture should allow easy and quick loading and unloading of the
workpiece. This will also help in reducing the idle time. During
manufacturing of gears on Hobbing machine, the time taken for the
setting of the hobbing fixture was very high. Whenever a gear of a new
Root Diameter is to be manufactured, old fixture has to be removed &
the relevant new fixture has to be installed. For changing of the fixture
on the gear hobbing machine the time required was found out to be
approximately 145 Minutes. This includes the time required in setting
up of a new fixture and unloading an old fixture. The data was collected
for 6 months pertaining to the number of settings done in each month
on 8 hobbing machines. The number of settings came out to be 578. This
was considerable amount of wastage of time and money for the
company. This wastage was due to the changing of fixture every time a
gear of new root diameter was to be manufactured. The type of fixture
used and redesigned had locating mandrel, face clamping for disc type
of gear blanks having bore.
New Design of Locating Mandrel:
The design of the locating mandrel was such that it had to be removed
along with the base. The gear blanks’ resting was fixed with the locating
mandrel and clamping cup was removable. In the new design resting
was made removable by making it like a bottom cup and clamping cup
i.e. top cup was already removable. The base of the fixture was made
common as during the loading of the fixture on machine its center has
to be aligned with the machining center, the communization has
reduced this time also. [4]

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REFERENCES:
[1]:http://www.geartechnology.com/articles/0313/How_Gear_Hob
bing_Works
[2]:http://www.geartechnology.com/issues/0194x/gimpert.pdf.
Writer name: Dennis, Gimpert
[3]:http://www.gearsolutions.com/article/detail/5654/a-new-design-
for-dry-hobbing-gears
Writer name: Hirofumi Kage
[4]: http://www.ijsrp.org/research-paper-1012/ijsrp-p1068
Writer name: Amar Raj Singh Suri*, A.P.S. Sethi**