Dry Machining

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Dry Machining

1.1 Preface

According to traditional schoolbook information and research metalworking fluids

serve to cool the cutting tool and workpiece (cooling-effect), reduce friction between the

two (lubricating-effect), and remove chips (flushing-effect). Erdel (1998) reports that

manufacturing companies all over the world are currently examining the question whether

metalworking fluids are really needed in manufacturing processes and if so, to what

extent. According to Aronson (1995), the increasingly stricter environmental and health

regulations, along with their enforcement, are eliminating much of the flexibility in the

use of metalworking fluids. Pending Occupational Safety and Health Administration

(OSHA) and Environmental Protection Agency (EPA) regulations on metalworking fluids

have made dry machining a hot topic recently. While the need for dry machining may be

apparent, issues including the perceived inability to cut dry and the changeover costs,

result in dry machining being perceived as impractical by most manufacturers. However,

this is not the case, high-speed dry machining is possible in most manufacturing

processes. Integrated correctly, manufacturers can realize improved workpiece accuracy,

reduced manufacturing costs, and other related benefits associated with high-speed dry

machining.

1.2 Why Dry Machining

Heine (1996) states that the use of metalworking fluids in manufacturing

processes is viewed as undesirable for both economic and environmental reasons.

According to Erdel (1998), every year US manufacturers consume millions of gallons of

Dry Machining 4 metalworking fluids. Metalworking fluids have a considerable affect on

manufacturing costs and the environment. Even more important is the fact that OSHA and

the EPA consider metalworking fluids to be detrimental to the environment. These fluids

contaminate the air causing maintenance and employee health problems. Also, at the end

of the fluids useful life it must be disposed of properly. Machining parts with

metalworking fluids puts an enormous burden on manufacturing companies and our

environment. Manufacturing companies need to realize the costs and environmental

issues involved with the use of metalworking fluids and move to more environmentally

and cost conscious manufacturing practices.

Winkler (1998) reports that due to increasingly strict environmental laws aimed at

controlling health hazards and pollution, the costs of metalworking fluid use in

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manufacturing processes is rising substantially. The use of metalworking fluids in

manufacturing processes place an enormous burden on manufacturers to cover the

additional costs associated with their use to protect our health and environment. Some of

these costs include healthcare, facility maintenance, machine maintenance, procurement,

and disposal. The cost associated with the use of metalworking fluids is estimated to be

several billion dollars per year in the US alone. Several research studies (Erdel, 1998;

Heine, 1996) report that the use of metalworking fluids account for approximately 16%

of the total manufacturing costs. Therefore, the elimination of metalworking fluids in

manufacturing processes can be a significant economic incentive. To avoid these costs

and problems, manufacturers are beginning to explore dry machining. Considering the

high cost associated with the use of metalworking fluids and projected escalating costs

when stricter health and environmental laws are enforced, the choice of dry machining

seems obvious.

1.3 Dry Machining

Recent research (Daniel, Olson & Sutherland, 1997) reveals that the trend in

manufacturing is to minimize or eliminate the use of metalworking fluids in

manufacturing processes. Winkler (1998) reports that dry machining has the potential to

reduce environmental pollution, health hazards, and costs associated with the use of

metalworking fluids. However, to pursue dry machining, one has to compensate for the

several beneficial effects of metalworking fluids without using them. Heine (1996) states

that the removal of metalworking fluids in manufacturing processes can cause a variety of

machining problems related to heat, tool life, and chip removal. In dry machining, the

functions of metalworking fluids must be assumed by other alternative methods. The

challenge of heat dissipation without coolant requires a completely different approach to

manufacturing. Special tooling utilizing high-performance coatings, heat-resistant

materials and through-spindle air are required. A variety of new techniques are testimony

that new technology has rationalized further efforts to research and implement dry

machining in manufacturing processes. First attempts at these new techniques have

proven to be viable alternatives.

Approached incorrectly, changing over from wet to dry machining can be costly

and problematic. Optimum implementation requires machine tools designed for dry

machining with all the proper options. By examining the manufacturing processes capable

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of dry machining, it becomes apparent that the key is a balance between advanced metal

cutting strategies, special tooling and the machine tool specifications.

1.4 Methods of achieving Dry machining

1.4.1) Tool Materials

One approach towards achieving dry machining is to improve the properties of the

utting tool material by making them more refractory or take away the heat generated in

dry machining by some other means. High-temperature wear resistance and hardness are

prerequisites for tooling used in dry machining processes (Heine, 1996). There has been a

continuous development of tool materials over this century starting with high-speed

steels, cobalt alloys, tungsten carbides, ceramics, cubic boron nitride and diamond.

However, the need to machine materials dry and at higher cutting speeds is imposing

pressure for the development of new tool materials. A formidable challenge, one that tool

manufacturers are actively pursuing to ensure their future. According to Heine (1996),

todays cutting tool materials like multiple layer coated carbides, ceramics, and cubic

boron nitride (CBN) are capable of combating the intense heat and achieving satisfactory

results without the use of metalworking fluids. Stolz (1996) reports that ultra fine grades

of carbide with multiple layer hard coatings currently offer an economical solution to dry

machining. Schneider (1999) states that research with ceramics and CBN also showed

promising results as a solution to dry machining. Ceramics and CBN stand out for their

hot hardness and resistance to high temperatures, which eliminates the need to reduce

temperatures at the cutting edge with metalworking fluids. According to Schneider

(1999), major opportunities will open up for all cutting materials that resist high

temperatures, but particularly for ceramics and CBN used in dry machining. In the

opinion of Aronson (1995), with the continued development of cutting tool materials, the

traditional roles of metalworking fluid may not be as important as they once were.

1.4.2) Tool Coatings

Another approach towards achieving dry machining is to improve the heat

resistance and hardness properties of coatings used on cutting tools made from ultra fine

grades of carbide (See Fig. 1). According to Winkler (1998), coatings separate the cutting

tool from the workpiece and offer the possibility to replace metalworking fluids in

manufacturing processes. Ten years ago, the only coating available was Titanium Nitride

(TiN). Now there are dozens of multiple layer coating combinations. Recent

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developments in multiple layer coatings on carbide cutting

tools have revived the pursuit of dry machining. The

hardness, lubricity, and thermal-resistance of these high-

performance coatings are said to provide the same benefits

of metalworking fluids in a manufacturing process. In the

opinion of Stolz (1996), titanium aluminum nitride (TiAlN)

has proven to be an effective coating in dry machining.

TiAlN has demonstrated the ability to run faster, longer, and

cooler in dry manufacturing processes. TiAlN possesses the

best price to performance value for high-temperature

operations like dry machining. Laser-Cut 964 is a new

coating that exhibits characteristics of extreme hardness and lubricity that surpass other

coatings available for dry machining. These features offer extended tool life and

productivity increases up to 25%. The one draw back to Laser-Cut 964 is that it carries a

25% to 30% cost increase over the other coatings. Another type of coating required when

dry machining is a soft, non-stick coating. These coatings are placed on top of the harder

multiple coatings mentioned above. Tools must have a lubrication coat on top of the hard

coating to reduce edge build up and help evacuate the chip. Stolz (1996) reports the

development of a new soft coating from Guhring called MOVIC that is playing a major

role in the rationalization of dry machining. Properly applied, coatings offer the same

benefits of metalworking fluids and lead to longer tool life at higher speeds and feedrates.

Tool life comparisons of the three different combinations of coatings can be seen in Table

1. At present, dry machinings effectiveness varies considerably from metal to metal. The

key will be the continued development of coatings, each one optimized for certain

applications.

Fig 1Special Tool Coatings

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Note. This research data was the result of a drilling test done in heat-treated steel with a

.3346 diameter drill and 1.0 depth of cut (395 sfm and .007 in/rev). From Stop Press:

Movic Rationalizes Dry Machining, by R. Stolz, 1996, Modern Cutting Tools

Technology, 16, p. 16. Dry Machining 9

1.4.3) Through Spindle Air

Of course, turning and milling processes are the easiest manufacturing processes

to convert to dry machining. In these processes, the cutting edges are exposed and chips

leave the cutting zone quickly, having little contact with the workpiece and tool.

Therefore, the chips serve as a medium to dissipate heat. On the other hand, chips are not

so easily flushed from a drilling process, allowing heat to build up quickly in the confined

depths of the hole. Special coated carbide tools to improve heat resistance and lubricity

and high pressure through spindle air must be utilized for successful high-speed dry

drilling. The drilling process uses precisely controlled high-pressure, through-spindle air

for chip disposal and to prevent heat buildup in the workpiece or the tool and prevent the

recutting of chips. To achieve the most aggressive metal removal in this process, the

through-spindle air requires a higher level of pressure than is available at most shops.

Boosting shop air pressure as high as 160 to 200 PSI provides superior cooling and chip

disposal. High-speed machining provides a high enough feed rate to reduce the

temperature rise of the workpiece by approximately 50 percent resulting in less thermal

expansion in the workpiece and accurate holes.

1.4.4) High-Speed Machining

While tooling challenges have been the immediate focus of dry machining, it is a

balance between tooling and machining strategies that make it reality. The key to dry

machining is the use of high-speed metal cutting techniques. Research has proven that a

combination of high feedrate and very high spindle RPM reduces, rather than increases,

the thrust force against the workpiece. This technique concentrates intense heat

immediately in front of the tool. This intense heat allows for high-efficiency machining.

The heat plasticizes the workpiece; greatly reducing its yield strength and increasing

metal removal efficiency substantially when compared to conventional machining

processes as shown in Table 2. Typically, this much heat would distort the part, but

because feedrates are so high most of the heat is retained in the chip and removed before

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the heat can soak into the workpiece. This makes the workpiece more thermally stable

and, as a result, more dimensionally accurate.

1.4.4. a) Principle of HSM

HSM is a relative concept in comparison to conventional machining processes. It

does not have a rigorous definition since the actual cutting speed depends on the work-

piece, tooling, etc. The most common definition involves the DN product of the spindle

bearing where D is the bearing bore diameter and N is the maximum rated revolutions per

minute (rpm) of the spindle. HSM processes typically possess DN values ranging from 2

million & onwards. Other definitions rely on the natural frequency of the system or ratios

of the spindle HP/ rotational velocity for example conventional machines have high

power at low rpm with values ranging from0.25-0.5 Hp/rpm whereas HSM machines

possess values less than 0.005 Hp/rpm. HSM can be classified according to process as

High speed milling, pocketing, contouring etc, according to machine tools as NC, CNC or

non CNC, and as per machine structure as Multiple axes, virtual axes and Parallel

structure, other classification includes categories as per interpolation & control.

1.4.4 b) Methods of achieving HSM

HSM can be effectively achieved by controlling the factors such as

1. Tool material

2. Machine tools

3. Feed rate & depth Cut

4. (Since it is a specialized topic onlt this much will be covered in this topic

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1.4.5 Special Chip Removal Systems

Due to HSM dry machining operations generate chip quantities ranging from 800-

5600 cm3/min. This places extreme demands on the coolant and chip removal systems.

Vertical carriages along with large automated conveyors and high pressure coolants of

400-1000 psi facilitate lubrication and chip removal. Fine filters are used in the lines to

remove dust, fine chips, and dirt which should minimize internal wear of the lines and

tool wear. In addition, chip management accounts for horizontal designs this design lets

the chips fall away. High pressure coolant sprays are used along with mist collector

installations to protect the health and safety of the operators.

1.5 Dry cutting Its advantages

Studies from western technical universities show that nearly 10 to 17 per cent of

production costs are connected with lubrication, cutting oils, handling of oils, storage, and

costs associated with environmental issues. Recommendations relating to ISO 14001

discuss about disposal of fluid waste and include hidden responsibilities. Enough has

been written about waterbased cutting fluids in the literature which have oil particles in

suspension in powerful chemicals termed as surfactants which cause skin related defects

when contacted for long. Extreme pressure additives often include chlorine, sulphur and

phosphorous which are very efficient in maintaining the cutting edge sharp but are feared

to be conducive to form carcinogenic compounds. Formulation of safe oils in all respects

is far away as of now. Cases such as mineral oils are under continuous focus as they

contain polycyclic aromatic hydrocarbons which can pose health hazards. There have

been trends to use hydro isomerised oils due to their low aromatic contents and volatility.

This has lead to looking into synthetic coolants such as polyalpha olefins, polyalkaline

glycols and so on. Waterbased metal cutting fluids contain micro organisms leading to

fungi, bacteria and harmful elements, which has made the formulators to recommend use

of bactericides such as S-triazine (alternate double bonded carbon and nitrogen).

On the other hand dry cutting completely, means removing coolants in cutting and one

would immediately think of the possible flank wear, the forces in hobbing, the resultant

surface quality, and heat dissipation needs. Nevertheless technological advances in tool

material, its coating technologies, machine tool design and control have made it possible

to machine a range of gears without coolant. The heat dissipation is removed through

quick chip disposal, which carries away the maximum heat. This is so that after the

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operation is complete one can feel the part by touching. The challenge of heat dissipation

in absence of coolant required a completely new approach to design, fixturing, chip flow

and so on. Special tooling from Mitsubishi (Refer case study) resulted in new tool

material, which took a few years of research.

Having said much about the potential for dry cutting of components the cost

benefit using carbide have often been inconsistent and this was also a reason for a lull in

the activity for sometime till the initial successes were explained by the pioneers. Perhaps

the best efforts of machine builders, cutting tool manufacturers, coating researchers have

not adequately resulted into the cost influentials, which the ultimate customer would look

into. It is not to say that the work done were depriving totally yet there was a scope for

further study on the economics.

In short to say -

1. Dry machining means no coolant hence resulting in cost reduction

2. No potential cost assisted with the coolant filtration system, equipments related to

it

3. The system greatly enhances environmental conditions by cutting down the

pollutants and thus termed as green machining

1.6 Typical Applications

1.6.1 Die Mould Manufacturing-In die mould manufacturing, Dry Machining involves

HSM milling with light depths-of-cut at high feed rates. Milling at lighter depths was

always possible, but a high speed makes it practical. As a result, the machining center can

do more. Through proper machining one can reduce the need for polishing & can even

eliminate EDM in some cases. Particularly when this last case is true, this can let a

machining center produce complex tooling competitively in a single setup.

1.6.2 General Production- Through HSM, special toolings & coatings in batch

production or high volume production, Here the speed in High Speed Machining is the

primary concern. Running fast, High Speed flexible machining centers become an

economical alternative to more dedicated systems for a variety of production parts.such as

gears, crank case, spindles, crankshaft etc

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2 Case study - Effectiveness of Dry hobbing

2.1 Introduction

The developments carried out on CNC hobbers in

areas arising from process technologies and associated

innovation in hob design and its material for appropriate

adaptation have lead to a partial and/or total renunciation of

cutting fluids. The process itself is environmental friendly due

to increased awareness of chemical effects of cutting fluids

besides saving in costs of manufacturing of gears in cutting

indirect expenses. The tool materials used have been sintered

or compounds of carbides with several advanced coatings.

The coatings themselves have been either carbon, titanium

nitrides, carbo-nitrides, titanium aluminium nitrides or as developed recently WCC

coatings all of which go to enhance the cutting ability by retarding wear growth

phenomenon. Nevertheless cost of carbides and its performance due to susceptibility on

impact loads to chip have posed a challenge to the users. Although cutting velocities of

carbides are higher than practiced by the industry there is a significant development in the

new HSS tool from Japanese researchers at Mitsubishi. Production performance of MHI

super dry hob of special coating with the entire terminology of the hob being called as

MACH7 has outstanding performance of a tool that is basically a highspeed steel tool.

The cutting strength and quality of hobbing, much above the conventional hob yet a bit

below that of carbide has evinced interest in the gear cutting industry and other

manufacturers are following suit. The details of such developments and the corresponding

dry cut hobbing machines are dealt in this paper.

Hobbers are achieved on account of their ability to

employ high rotary table speeds and the spindle speeds

without suffering from rigidity and working accuracy.

Experience on such machines in the production of automotive

gears is available in the industry and the advantages of

separate drives/digital

drives have been in use.

Maintenance of constant tooth depth measured in the form of

Fig.2 Improved axial guides

Fig.3 Improved Radial guides

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span value or diameter over pins to higher statistical values, [cpk] are common. Machine

designs have been adapted to minimise maintenance of mechanical assemblies or finding

areas of solutions to gearing problems in machine settings have reduced greatly. This is

the time when gear hobbers were given LM guides in their radial, axial and counter

column guide ways for improved running characteristics in all degrees of freedom. See

Fig 2 and 3 .

The quality of gears cut in these machines has been consistent and of higher grade

than what was possible on earlier machines using group concepts and gear trains. One can

say that indicating module as a cutting ability function was disregarded in the light of

larger feeds, higher number of hob starts to define hobbing capacity as a whole. These

machines have been useful in performing special processes like carbide hobbing and

skive hobbing as and when required. Basically carbide hobbing of soft gears and skive

hobbing of hardened flanks are different processes nevertheless the machines themselves

were capable.

2.2 Characteristics of carbide hobs

The use of carbide has been widely accepted in the industry for several years. The

stability of carbide at elevated temperatures allowed the manufacturers to take the

advantage of CVD coatings to enhance the cutting ability. The main reason for this use of

carbide lay in the desire to take advantage of high production rates possible. The gear

hobbing industry felt that the cost of tool could be offset by the production. Hence in the

early 90s, the era of using carbide hob in cutting soft gears began. Gaining the

experiences of inserted carbide use over the years has helped transformation to an extent

smoothly. However, a number of problems needed to be overcome to compete with the

best of HSS hob practice.

Firstly, the machine itself had to be redesigned from the conventional ones to be

dynamically stable so as to withstand the cutting forces from carbides. The key factors to

be addressed were the rigidity and minimising the vibrations to control the carbides

inherent tendency to break at critical edges by strengthening the drives and torsion

stiffness. Even hob manufacturers who used early methods of carbide blade fixing on the

steel body and brazing and finish grinding started using adhesives to reduce the tendency

of heat. Now the techniques have been mastered and heat crack has been eliminated.

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Solid carbide blanks have the advantage of better design in that the difficulty of

contaminating the furnace with different material, adhesive etc is removed. Internal or

external corners are preferably avoided to contain the stresses and to prevent chipping.

Tooling is an area where unlike HSS hob carbide may not be taken for standardisation.

Maintenance in re-sharpening is to be done with diamond dresser and or special

polycrystalline structured material depending on the carbide grade and the angles

involved. Procedure used in grinding HSS and carbide remains more or less same except

the type of wheel, coolant, speeds and grinding feeds. For example plated super abrasive

180 grit, resin bonded Elgin diamond wheels are normal.

2.2.1 A practical example of adapting carbide hob

Assessment of carbide performance can be

done with two parameters. First is the cycle time in

production. A combined double gear 16MnCr5 gear,

one with Mn3, Z-28, root diameter 76.5mm, spur teeth

and second gear Mn3, Z-35, root diameter of

101.2mm, both being external were efficiently cut on

180mm CNC hobber to a target quality of Q7. The

hob used has been a Fette solid carbide hob, at spindle

speed of 990 rpm corresponding to a cutting speed of

280m/min, axial feed rate of 5mm/workpiece rev, for a total cutting depth of 6.75mm.

The hob dimensions have been 90x160x40 with basic profile to DIN 3972 hob material

being HM FC 60 N4-TiN (see Fig 4 ).

The total cutting time for both the gears in one set up was 0.95min without the use

of cutting oil of any nature. The test results are with the author. Polymer concrete bed and

thermal stability, quick disposal of heat from the chips with the help of compressed air

circulation were the points to note in this demonstration. The main motor power was

pegged at 5.5 KW. Short direct drive to the table, central hob head clamping via a

mechanical system using a large disk spring of approximately 300 mm diameter to exert

sufficient clamping force in hobbing, without vibration have been a few points of interest.

Of course the partial compression to a predetermined value in the disk spring assembly

ensures the magnitude of working force.

Fig.4 HOB

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2.2.2 Limitations of carbide

While it can be said that rigidity,

finish requirement based on the end

service condition of the gear itself, blank

conditions, work holding fixture design

are all important in carbide dry hobbing,

it is also important to note that feed rates

and hob speeds are to be related to the

materials hardness in rock well and the

technology of the hob itself. We find

that a variety of carbide grades are offered that need to be

examined for the toughness in entry and exit of heavy

cut due to shock loads and resistance

required. Majority of carbide grinding is with plated

diamond or resin bonded diamond wheels as standard

practice, which had limitations to get modified forms

accurately on the tooth flanks. This is being yet

optimised. The second consideration has been the

maintenance of hobs. One is handling during shop use and re sharpening due to its

susceptibility to chip and damage. It is very expensive for such occurrences. During re-

sharpening, the use of coolant as has to be taken care as chlorine or sulphur can lead to

enhance the leaching of carbide bond slowly. Crack detection need not be checked

through magna-flux method but any other chemical means using special compound such

as Zyglo, which is recommended by carbide hob experts for use. Though several carbides

are there the ones based on tungsten carbide and cobalt bonding is regular. The strength

and hardness while in use depends on composition and the uniformity of cobalt bond film

thickness around carbide particles. Although the increase in certain percentage of cobalt

enhances the transverse rupture strength (TRS) of carbide up to a level, it is at the loss of

certain hardness. The point at which the hardness and the TRS have to be compensating

would mean a satisfactory percentage of cobalt for the tungsten carbide as a whole, to

give consistently good performance. Fig 5 shows the relation between TRS vs. per cent

Fig.5 TRS graph

Fig.6 Compressive

strength Graph

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cobalt to show optimum hardness /TRS intersection. Another point to note is that cobalt

affords higher compressive strength at low percentage bonding (see Fig 6).

The reason to mention this is that in some cases, within a specific grade there can

be variation as far as the grain structure or size, to help tailor match to an application.

Finally actual application results can vary differently. A detailed study by manufacturers

like Carboloy, Sandvik, Wendt, Firth Stirling, Adamas or Kennametal all can be guiding

and their recommendations are to be sought for proper grade selection. This is analogous

to the experience with grinding wheels. Therefore if we compare carbide hobs for dry

cutting with other HSS hobs it is about three times costlier. Cost of sharpening is greater

than for HSS hobs. Recoating is always not readily possible and lower performance due

to running with absence of coating at cutting face may have to be expected in reuse.

Nevertheless machining speeds in the order of 300-350 m/min are possible.

. 2.3 Specific features

Optimum high speed hob drive system.

Table drive and work holding system for

stiffness and accuracy.

Optimum hob mounting system to control axial

and radial run outs.

Optimum machine configuration for dynamic

rigidity and thermal stability.

The above procedures followed by MHI can be explained in the following paragraphs

covering horizontal machine design, hob head, table drive and more recently newly

adapted direct drive configuration on machine like GN-D10A which was shown in IMTS

and JIMTOF 2000, linear guide ways, chip disposal system, savings in power, machine

types that are offered to various industry segments including tractors where large modules

are encountered and a few working examples of components and the cycle time/ tool wear

data for reference to the shop floor engineers.

Fig.7 Machine configuration

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2.3.1) Developments in machine tools

a) Structure -Basic machine structure is designed to be strong

with respect to forces emanating from dry cut hobbing and the

thermal deformations that are induced by the process. As said

earlier the process heat must be taken away from the working

zone, more precisely from the work piece itself rapidly. The

radial slide, the axial slide and the tailstock assembly have been

provided with specially ground guides to dampen the excitations from

the tough cutting conditions practically over the range of harmonics.

The machines themselves have been made of horizontal construction instead of

vertical type, as the latter is prone to gradual thermal stress accumulation. The

temperature differential along the vertical axis, towards the column free ends can lead to

deformations, in the form of involute function caused by the bed deformation. Some

manufacturers have still kept the design as vertical, as the original design was

contemplated to take care of the thermal contingencies yet many

have already redesigned to horizontal construction. Fig 7 shows

the possible deformation of a vertical construction of dry cut

hobber. Fig 8 shows the horizontal construction of the machine

where the free fall of hot chips is directly into the conveyor that

is independent below, can be seen for easy and quick transportation outside the machine.

The heat source in the hot blue chips is not kept in the

working zone at all. As MHI builds machines for coarse

pitch dry hobbing also it can be seen as to how a GN 20 dry cut hobbing machines bed is,

by the FEM analysis for the rigidity of bed structure in Fig 9 .

In the meantime MHI demonstrated the successes of higher module dry hobbing

and went ahead with new designs to upgrade smaller

series machines like GN-D 10A, to directly drive the

hob spindle by the motor. The new version is shown in

Fig 10. In the earlier version there used to be a pinion

driving a second stage helical gears to run the main gear on the

spindle although all have been high quality ground gears. Bevel or

spiral bevel gears were not used due to their critical TCA concept and fatigue

Fig.8

Fig.9 FEM analysis of BED

Fig.10

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phenomenon by their complex log load-log life

relationship. Hob spindle and counter support

bearing are built in rigid housings in the hob head.

The hob arbor has tapered ends to suit international

standard to enable mount the hob accurately in the

spindle. High clamping force and driving torque are ensured. Typical arrangement is

shown in Fig . In combination of slant bed configuration to the adjustable tail stock

position and automatic clamping one can use conveniently the minimum quill position for

fixturing, tailor made to specific work piece. The

arrangement is shown in Fig The work spindle

consists of adequately dimensioned rigid housing

with preloaded bearings and highly precise master

wheel mounted positioned at strategic point along

the work head axis. The single reduction pinion gear

has the exact precision in terms of angular transmission accuracy. Besides the mounting

sequences are simple unlike complicated compound gearing in the critical area of table.

The arrangement is shown in Fig . The new smaller version of GN -D 10A has the direct

drive to the table owing to the higher rotational speeds required to be encountered with

advanced tools/hob steels today, to the tune of 650 rpm. This can pave the way for more

rigorous and productive needs, as the corresponding hobs are being made available to

machine dry.

The use of gantry loaders to change parts from the fixture and load fresh blank

and remove the hobbed gear to be downloaded to the end bin or magazine to carry over to

the next operation, simply add to the productivity. The part

change time has been reduced to a second. This is made

possible by the gantry loader in conjunction with the twin

arm indexing system that loads the work piece to the fixture

in the work head/tailstock. For heavier type work pieces the holding

arms are designed differently considering the torque and inertia. (See fig

13)

b) Chip Removal system- Fig shows the

arrangement of how the chip disposal is handled. The

Fig.13

Improve

axial

guides

Fig.14 Chip Removal System

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cross section of the portion in between the bed sections act as separator to the rest of the

structure and allows the chips fall freely down to the conveyor. The sheathing is of

different material than the rest and facilitates the chips fall quickly and smoothly. The

discharge chute and the wide gap are shown. Fig 14 is the open structure for the

evacuation of hot chips. Among the leading machine manufacturers of dry cut hobbers

who have contributed to the technology in cooperation with coating specialists firms for

the hobs, it is MHI who have developed the process for higher module through their

GN25 series machines. It is the HSS hob material or the treatment to the substrate or the

primary protection to the cutting face due to the hot chip flow or creation of successive

coating layers of various compounds with different wear resistance levels that have

dominated the challenges to overcome, which kept many manufacturers busy.

2.3.2 Developments in Cutting Tools

Mitsubishi realised that environmentfriendly process will eventually be mandated

globally by all gear manufacturers though the catch up may take sometime in the

beginning. Besides if one looks very carefully at the machine designs developed in the

early part of 90s they still lacked some improvements which were necessary for hot chip

disposal and the structural rigidity for dry hobbing. More time was needed to blend the

requirements from all points of view to spearhead towards commercial penetration

including alternate tool material lest the demanding gear industry would not show interest.

It took four years for MHI to come out with-

The new tooling

Equipment

Process optimisation

High Speed Steel solution that would be ideal as the base material and coatings

could be improved to handle the temperatures and speeds, unlike carbide, which is

expensive, brittle and requires care in sharpening and edge preparation.

a)Hob configuration: The use of shell type hobs was found to give

the best performance due to increased stiffness. The use of large

Fig.15 FEM HOB

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tapers on both the ends helped reduce axial run outs with respect to rotation. Tool

diameters and length limits were optimised using FEM and set for maximum rigidity. The

number of allowable flutes, chip clearances, and sharpening were also critically examined

(see Fig 15- FEM analysis).On machines with work piece capacity of 100mm and

module up to 4, a hob length of 140mm and in its neighbourhood is preferred to contain

radial stiffness and special chip relief features to eliminate chip welding. Typical regrinds

up to 28 have been made possible.

Mach7 super dry hob characteristics

include optimum cutting speeds up to 200

m/min while super dry coating operating range

can be as high as 440 m/min. This results in

twice faster cutting than standard HSS TiN

coated hobs.Multiple start hobs are easily

produced on steel body cutters, which is another

reason in favour of HSS. Average tool life has been

found to be five times longer compared to wet

cutting TiN coated hobs.The relationship between

cuttingdistance expressed as m/no of shift vs. flank wear in hob showing HSS wet, super

dry cutting performance on SCM415 /SCM420/SCr420 are shown in Fig 16. The actual

cutting features will be shown later with component examples and the machine used.

On account of the care taken in the

total machine construction, the dynamic

rigidity deployed, and the other structural

stiffness thought about and executed, MHI are

able to guarantee the hobbed gears

dimensional stability in the form of production

measure of consistency in the span value or OBD value as a

function of time. This is shown in Fig .The tests relate to a gear of 1.49mod, number of

teeth 17, Helix angle 20 degree, face width 18mm, on a material of 180 Bhn. The hob

speed is 1,530 rpm Axial feed 2mm/w.p rev Cycle time 10 Quality of the part required

DIN 8

Fig.17

Fig.16 Graph Flank Wear

Dry Machining

18

2.4 Results-

2.4.1 Hob edge wear After 15,400 pieces flank wear

was found to be 0.045mm/crater wear was 0.044mm.

The wear has been symmetrical with respect to chip

flow vectors and the position of crater on hob face

supported by even wear along the flanks. Such

phenomenon is also useful in regrinding as the material

removal is even. In terms of residual stresses induced in

the gear tooth flanks and at the tooth root there may not be

any asymmetrical peak to reckon for the purpose of eventual deformation during HT.

2.4.2 Hob wear After 1,400 work pieces the flank wear was found to be only 0.07mm.

The performance on the conditions of cutting has been very good and encouraging.

Another example of a heavy truck gear 6DP 55 teeth, face width 40mm, spur teeth,

OD 227mm dry hobbed on GN25A at a cycle time of 2.5. Test results are there at the

machine try out department at MHI where a final drive ring gear having 77 teeth,

10.45DP, 32mm face width, 28 deg Helix RH, SCR 420h material 180 BHN could be

demonstrated with super dry cut, in one pass climb hobbing to produce within a cycle

time of 56. The gear has to be preshave hobbed.

2.4.3 Details on the types motors employed and the power saving

Compared to normal CNC

hobbers as well as the various

models of MHI hobbers for dry cut

are shown in Fig 19 . Developments

have also been made in respect of

combined machine operations for

mass production of parts. The

concept of combining operations

were witnessed for quite sometime now, as the user industry appreciates the concept for it

reduces the machine installation costs and saves the space required for several stand

Fig.18

Fig.19

Dry Machining

19

alone machines. One such example is the GT 06 machine, which can be either with a

shaving operation as finishing or rolling operation as finishing. In the latter case it is

called GT 06R. Horizontal dry hobbing, chamfering and finishing operation such as

shaving or rolling can be done in one installation. It is also possible to integrate a turning

CNC four spindle (two loading station extra during machining) chucker to start with the

blank and finish the soft stage gear at the other end.

Dry Machining

20

3 Conclusions

Dry cut hobbing: A solution to Indian needs

While all these developments are explained it is required to have a service facility

to re-coat the hob once it is due for regrinding. At the moment this facility has been in

countries like US, Europe outside Japan on account of the extent of such technology

prevalence. In our set up or situation it may take some time to have the facility here as the

technology is beginning to grow in the wake of costs cuttings, energy savings,

environmental issues and many other factors that have pushed this technology forward.

Till such time it is worthwhile to check the economics of the new technique to ones

circumstances as the global tool technology is getting more concentrated towards higher

cutting conditions. Any situation where the component variety is going to be looked upon

for greater productivity while the costs have to be low in the long run, it is the faster dry

hobbing method that can only survive. With the difference between the cost of dry cut

hobbing machine and that of the equivalent size normal CNC hobber tending to narrow

down due to technology adaptation worldwide and the competitive market, the deciding

factor may be the dry cut tools vs. productivity as a function of quality. The opportunity

for export in the new millennium, the exposure to new technology and the associations

with global manufacturers can all be accelerators. With the changes at the threshold ahead

dry cut hobbing can answer the needs.

At last say, the technology of Dry machining has been embraced and many

companies are aggressive in research and development of such processes. What else we

can put, the future is bright for High Speed Machining.

Dry Machining

21

4 Reference List

1. Aronson, R. B. (1995, January). Why dry machining? Manufacturing

Engineering, 114, 33-36.

2. Daniel, C. M., Olson, W. W., & Sutherland, J. W. (1997). Research advances in

dry and semi-dry machining (Technical Paper No. 970415). Society of

Automotive Engineers.

3. Emuge-Franken. (1997). High performance cutting tools for dry machining in

action [VHS Tape]. (Available from Emuge Corp., 104 Otis St., Northborough,

MA 01532)

4. Erdel, B. P. (1998). The road to precision near dry machining [On-line].

Available: www.abpi.net/T2007/papers/adv/road/road.htm

5. Heine, H. J. (1996). Environmentally conscious manufacturing: Dry machining-A

promising option. (NTIS No. PB96-129093KZO)

6. Kibbe, R. R., Neely, J. E., Meyer, R. O., & White, W. T. (1995). Machine tool

practices. New Jersey: Prentice-Hall.

7. Schneider, J. (1999, January). Ceramics and CBN. Manufacturing Engineering,

122, 66-73.

8. Stolz, R. (1996, Summer). Stop press: Movic rationalizes dry machining. Modern

Cutting Tools Technology, 16, (22) 11-17.

9. Winkler, J. (1998). Dry drilling [On-line]. Available: www.ifw.unihanover.

de/industrie/ind_main_e.htm

10. Winkler, J. (1998). Ecologically improved manufacturing processes [On-line].

Available: www.ifw.uni-hanover.de/industrie/ind_main_e.htm

11. Dry Machining 14 Walker, J. R. (1997). Machining fundamentals: From basic to

advanced techniques. Illinois: Goodheart-Willcox.

12. High Speed Machining In Aerospace Application Kevin Luer

13. High Speed Milling Of Aluminum Theory & Analysis Cincinnati Machinery

Machining Centers for High Speed Machining Modern Machine Shop

Websites

www.machinicst.com

www.mmsonline.com

www.coromant.sandvik.org