A Cutting Tool For The Milling A Cylinder Block Engineering Essay

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A cutting tool for the milling a cylinder block is likely to perform acceptably under extremely aggressive operating conditions. The required properties in a suitable tool material will certainly involve a degree of compromise and the emphasis to be place on one, in relation to another, will depend on the specific application. For instance to enhance the toughness of a material, hardness and its wear resistance properties are reduced and vice-versa. The requirement of the tool material are defined by a composite combination of mechanical, physical and chemical features such as

A large number of cutting tool material has been developed with a variety of properties and performance capabilities to meet the demands of high metal removal rates. The most suitable cutting material for the aluminium machining is shown in fig below.

The tools and cutters used for the machining of the cylinder block are made form the materials like High speed steel(HSS),Cemented Carbides, Sintered Tungsten carbide (WC) and coated carbide and Polycrystalline Diamond (PCD).This all tools and cutters can be used for the different types of milling operation depends on the Cy.block design geometry, feed, speed and other machining parameters.The machining of the High Al-Si Alloys is bit hard than standard aluminium alloys with the machines and tools available in the work shop,but it is possible with lower cutting speed. But lower cutting speed will raise the machining time, which has a negative result on the timing and cost.These machining problem could be solved by using diamonds cutting tools (Polycrystalline diamonds=PCD ).Only for drilling holes into billet block and for threading no diamond tools are available.

But the required PCD tools are not available in the workshop for the major operations like cylinder boring, main bearing hole boring of the cylinder block.

Generally aluminum alloys can be machined. They naturally produce reasonable cutting pressure and small machining temperatures. Machining of aluminium desires less electric power than steel, but the electric power required for soft metal machining with high speed is much greater than currently obtainable in traditional machines.

Machining Aluminium with High-Speed Steel

The majority of the tools and cutters in the workshop for cylinder block machining are made from HSS.

High-speed steels submit the following advantages:

High toughness,

High bending strength,

Ease of working,

Low price.

As the thin shape of cutting tool used for aluminium cutting has a higher tendency to break than tools with a negative geometry, toughness is very important there.

The HSS tools are relatively easy to form due to its property compared to the other tool materials. This property is very useful during the desigining of the tool shapes like Twist drill, form and profile cutters.

The main types of the tools are used for machining the Aluminium and alloys:

Twist drills ( S 12-1-1 )

Taps ( S 9-1-2 )

Milling cutters ( S 12-1-2 )

Form turning tools ( S 12-1-4 )

Profiling tools ( S 12-1-2 )

The described all tools and cutters are made from 12 percent tungsten with steel .This tools possess high hardness, good temperature stability and low bending strength.

Compared to the carbide-tipped and diamond tools HSS steel possess lower resistance to free surface abrasion. Machining of less silocon conatain aluminium alloys A319 is generally best with HSS tools and can be achieved satisfactory tool operating life.

The cutting parameters chosen here should be such that the temperature does not increase too high. This can be accomplish mostly by choosing restricted cutting speeds - mainly under 500 m/min .The speed range available here for the milling machine is 50 - 3750 rpm. So this is big disadvantages of the HSS tool that it can't use with high cutting speed during the machining of billet block. See the graph of machining aluminium with high speed steel below.

Fig:-Machining Aluminium with High-Speed Steel [2]

In case if casting is allowed for the bottom (Skirt) portion of the cylinder block, the cutting speed will be narrow due to casting process reasons, HSS tools can also be used to drill rough casting. See the graph of machining aluminium casting with HSS (Drilling) below.

Fig:-Machining Aluminium Castings with High-Speed Steel (Drilling) [2]

High Speed Steel have a density of 8.67x1000 kg/m3. This high density provide vibration and shock resistance and also provide its wonderful hardness and durability even at high temperatures while still permitting for its machinability into tools.

HSS run at a very low thermal expansion rate and also carries out heat at a very low velocity. Because of this two properties HSS tool can cut faster and can be use for a long time. Because of this low thermal conductivity, different types of the lubricants can be used for the HSS tools. Usually, upper the proportion of carbon in HSS, the lower the melting point of the steel, which will decrease its thermal properties as well.

Machining Aluminium Using Carbide-Tipped Tools

The use of carbide tipped tools is increasing in popularity. The alloys which have a highly abrasive resistance normally uses a carbide tipped tool for machining. HSS tools cannot be used with the High cutting speed which is characteristics for machining aluminium.

Carbide tipped tools is being more and more used for machining the less abrasive wrought alloys as well for economic reason. The high cutting speeds achievable, the tremendously long tool operating life, more than compensate for the higher price of Carbide tipped tools.

The titanium carbide (TiC) types of tools are not suitable for machining aluminium block because the high affinity of titanium carbide for aluminium produce an unbearable diffusion out of the hard surface. For the machining of aluminium block,K01 and K10 ISO type with high WC content proven to be successful. It is possible to get smooth cut surfaces and edges with low roughness using K type tools because they have a very fine grained structure. This is actually one of the top main concerns while machining aluminium.

As a result of the above discussion, coated hard metals are not suitable for machining aluminium block.

Cemented carbide are not suitable for very low cutting speed operation because the chips have tendency to weld to the tool face and result in micro chipping of the tool. This is also not economically justified considering the cost of cemented carbides. The recommended cutting speeds for cemented carbides are in the range of 150 to 600 ft/min.

Cemented carbide is in many respects superior to HSS. Although they are more expensive than HSS tools they achieve longer tool lives and can be used at speeds up to six times higher than HSS tools.

The HSS and Carbide tipped tools available is the work shop shown in the fig below are used for the machining operation of the cylinder block.

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Machining Aluminium with Diamond

PCD the hardest of all tool materials, show brilliant wear resistance holds an enormously sharp edge, create small friction in the cut, offers high fracture strength, and has good thermal conductivity. These properties add to PCD tooling long life in conventional and high-speed machining of aluminium and aluminium alloys (A390, A356, and A319).

The high hardness of diamond makes it right material for cutting tools. This tools are used specially for machining hypereutectic Al-Si alloys (cylinder liner alloys).The hypereutectic alloys are very difficult to machine due to the coarse silicon inclusions present, which are extremely abrasive. Diamond tools have longer operating lives compared to carbide tipped tools as shown in the fig below. Also higher accuracy and better operating stability.

Fig:-Machining Aluminium with carbide -Tipped and Diamond Tools [2]

Because of better isotropic mechanical properties than monocrystalline (natural diamond, polycrystalline PCD) tools are broadly used in automotive industry. PCD tools can be used where interrupted cutting actions occur and they are less sensitive to impact loading as well .

The cutting properties of PCD tools are much superior than the carbide tipped tools for aluminium block machining. PCD tools are about 40 to 100 times more efficient than carbide tools depending on the cutting conditions. This means that the operating life of PCD tools can be up to 100 times longer than that of carbide tools so more than compensating for their higher price.

Because of its high cost and remembering the required less number of operation and also for one-off production of cylinder block, PCD tool is not the right choice of the tool material for this project.

Table shown below some Guide Values for Milling Aluminium Alloys [2]

Cutting parameter

Tool material**)

Casting alloy with 12% Si

Casting alloy with > 12% Si

Rake angle in °

HSS

HM

Diamond

12 to 20

8 to 20

2

-

8 to 20

2

Clearance angle

α in °

HSS

HM

Diamond

10 to 20

10 to 20

6

-

10 to 20

6

Cutting speed

v in m/min

HSS

HM

Diamond

up to 600

up to 1500

up to 2500

-

up to 300

up to 1000

Feed sz*) in

mm/tooth max

HSS

HM

Diamond

approx. 0.2

approx. 0.2

approx. 0.2

approx. 0.2

approx. 0.15

Cutting depth

a*) in mm

HSS

HM

Diamond

up to 6 ()

up to 0.8 ()

up to 8 ()

up to 0.8 ()

up to 2.5 ()

up 0.5 ()

-

-

up to 8 ()

up to 0.8 ()

up to 2.5 ()

up to 0.5 ()

Cooling

lubricant

HSS

HM

Diamond

(emulsion)

(emulsion)

(emulsion)

emulsion

emulsion

For shank-type milling cutters, the feed is also a function of the milling cutter diameter:

Up to 6 mm milling cutter diameter: 0.1 mm/t () or 0.08 mm/t ()

Up to 20 mm milling cutter diameter: 0.2 mm/t () or 0.12 mm/t ()

Up to 50 mm milling cutter diameter: 0.25 mm/t () or 0.17 mm/t ()

**) HM = carbide-tipped tools

HSS = high-speed tools

Insert Grade Tooling:

The factors that constraint cutting aluminium alloys at high speeds are the possibly risk of built-up edges, loss of chip control, flank wear, and inadequate surface finish. But using insert grades specially designed and made to machine aluminum reduce those risks. Inserts with 15 degree higher rake angle than cast iron insert rake angle, and the sharper, positive rake cutting edges provide no build-up on edges. And so the tool life will increase.

In workshop some insert grade tools are available which makes easy machining for aluminium cylinder block. For the main roughing and finishing operation face mill cutter use an insert for the smooth and fast machining operation.

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Fig:- Set of work shop Tools and cutters

Cutting force

Machining of Aluminium block need only approx. 30 % of the cutting force required for steel. About the cutting force, rake angle is the main concerns. The cutting force is reduced nearly 1.5% by increasing 1° of rake angle.[1]

Required important characteristics of inserts for aluminium block machining.

rake angle > 25°

sharp, uniform cutting edge

extra-smooth rake face

2) Tool life

During the cylinder block machining time the cutting edge can be used until it is worn out. In the theme of aluminium machining this condition is determined by three wear forms as shown in the fig below .Built-up edges and the flank wear is more common with silicon content aluminium alloys (A390 cylinder liners, 356 cylinder block) machining. Both of these situation result in inadequate surface quality and lesser tool life. And this is not acceptable for the cylinder block requirements. [1]

crater wear flank wear built up edges and

adhesion to the cutting edge

Reasons for wear:

Thermal And Mechanical Stress

Diffusion Processes, Friction

Abrasive Alloy Elements

Tendency To Stick

Required important characteristics of inserts for aluminium block machining.

extremely wear resistant hard metal grades

optimised good coating

extra-smooth rake face

3) Surface finish

Surface finish during the machining of billet block is depending on kinematic roughness, surface roughness and roughness caused by disturbances. The kinematic roughness is a function of the relative movement between tool and billet block and the size of the corner radius.

The surface roughness is depend on the aluminium alloys properties, cutting material and data, geometry, and the surface quality of the rake face. Disturbances, e.g. lack of strength of the machining arrangement, outcome in surface imperfection such as chatter marks.

The required characteristics of inserts for aluminium block machining.

extra-smooth rake face

optimised micro-geometry of the cutting edge

fine grain substrates

4) Chip formation

Poorly formed chips disturb the block machining process significantly. They interfere with both the billet block and the milling machine, and are difficult to remove them from the machining atmosphere. Precisely machined surfaces show scratches; the quality of the block is reduced.

The control of the chip thickness is very important during milling of aluminium block. When running at high speeds, slow feed rates direct to excessive rubbing instead of cutting. This generate thick chips that can produce overheating, discoloration and poor tool life.

Normally, thickness of chip depends on how a cutter is offered to a billet block. A 45° lead angle produce a thinner, longer chip, and place fewer load on the cutting edge permitting for higher feeds per tooth. This geometry forced in to optimization stage specially for aluminium inserts top face and then smash these chips in to normal forms and sizes. Also, the insert's more open rake angle offer extra clearance for easier chip migration.

The required characteristics of inserts for aluminium block machining.

Have aluminium specific chip grooves

Should reduced friction between chip and rake face

Requirements for Efficient Machining of Aluminum cylinder block

Particular geometries and quality of a carbide end mill are essential for economical machining due to the soft and gummy nature of the aluminium. So for this reason the End mill tools, which is going to be use in block machining should be specially designed for the aluminium.

To separate a chip from the billet block material, in tool design high rake angle and pointed edges are required .Positive rake angles Up to 20° axial and 25 ° radial positive rake angles are usual.

Generally around 45°helix angle also beneficial. The helix assist travel chips up and away from the cutting area and also creates an outstanding surface finish. The angle also helps to make softer the force at entry of cut, and this result in a smoother and quieter cut (see Figure 1).

Key end mill geometries.

Fig;-Key End mill geometry [3]

For milling of aluminum cylinder block, a two- or three-flute end mill perform best because this permits for bigger flute areas. A core diameter of a bit less than 1/2 percent of cutter diameter is most excellent for the same reason. For effortless chip motion away from the cutting region, open flute design is important. Surface finish on the flute is crucial as well. Low silicon aluminum alloys (A319) with long chipping have a tendency to attach to cutting tools. Since a machined heated chip runs over the flute it will try to attach on to the tool face. The flute face must be very smooth to cancel out this tendency.

Coolant is essential for chip migration and maintaining the tool cool. When chip organization turn into a problem consider using a coolant-through end mill. Coolant pipe exit in the flute region and help to shift the chips out of the cutting region. Roughing tooth profiles are also supportive in dropping the size of the chip, making it easier to handle and blow out of the way.[3]

All of these basics help reduce the chance of built up edge (BUE), which is ordinary failure form when end mill is used in aluminum cylinder block machining. Built up edge is the growth of cylinder block material on the cutting edge. Once this take place the cutting action turn into a tearing action. Surface finish fall straight away and spindle load will increase considerably. During the full width of slotting operation of Crankcase and Cooling Water jacket, tool breakage is most probable to occur before get to the feed hold control.[3]

Some dead sharp corner designs of cylinder block like bulkhead (crankshaft passage) tend to break down quickly so it is advisable to shun using a dead sharp corner. Tool life will be increase using a corner radius or chamfer type end mill .

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