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3.0 MILLING MANUFACTURING PROCESSMilling is the process of cutting away material by feeding a workpiece past a rotating

multiple tooth cutter. The cutting action of the many teeth around the milling cutter

 provides a fast method of machining. In the present climate many different configurations

of machine tool exist. Some machines have the table/workpiece stationary whilst the X, Y

and Z Axes move and others may be constructed to allow the workpiece/table to be the

moving part whilst the axes are fixed. In any condition the X, Y and Z axes directions arealways configured the same.

Figure 3.1: Axis of Milling Machine

The X axis is always considered as the longest axis, where X+ will be the table motioning

to the left and X to the right. The Y axis moves from front to back of the machine with the

table motioning towards the operator as the Y+ (positive) direction and away being the Y –  

(negative) direction. The Z axis where the tool normally is located, has the Z+ (positive)

axis motioning up and away from the workpiece/table and Z –  (negative) direction down

towards the workpiece/table.

The axis of rotation of the tool is perpendicular to the feed direction. The tool is called the

milling cutter and the cutting edges are called teeth. Mostly plane surfaces are created

through milling. It’s an interrupted cutting operation; the teeth of milling cutter enter and

exit workpiece during each revolution. So, the tool material and cutter geometry must be

chosen carefully to withstand cycles of impact forces and thermal shock. Different types

of milling operations are shown in Figure 3.2.

Figure 3.2:  Different Types Of Milling Operations.

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3.1 MILLING MACHINEThe milling machine removes metal with a revolving cutting tool called a milling cutter.

With various attachments, milling machines can be used for boring, slotting, circular

milling dividing, and drilling. This machine can also be used for cutting keyways, racks

and gears and for fluting taps and reamers. Milling machines are basically classified as

 being horizontal or vertical to indicate the axis of the milling machine spindle. These

machines are also classified as knee-type, ram-type, manufacturing or bed type, and planer-type milling machines. Most machines have self-contained electric drive motors,

coolant systems, variable spindle speeds, and power operated table feeds.

Most CNC milling machines (also called machining centers) are computer controlled

vertical mills with the ability to move the spindle vertically along the Z-axis. This extra

degree of freedom permits their use in die sinking, engraving applications, and 2.5D 

surfaces such as relief  sculptures. When combined with the use of  conical tools or a  ball

nose cutter , it also significantly improves milling precision without impacting speed,

 providing a cost-efficient alternative to most flat-surface hand-engraving work.

Figure 3.3: Five-axis machining center with rotating table and computer interface 

CNC machines can exist in virtually any of the forms of manual machinery, like horizontal

mills. The most advanced CNC milling-machines, the multiaxis machine, add two more

axes in addition to the three normal axes (XYZ). Horizontal milling machines also have a

C or Q axis, allowing the horizontally mounted workpiece to be rotated, essentially

allowing asymmetric and eccentric turning. The fifth axis (B axis) controls the tilt of the

tool itself. When all of these axes are used in conjunction with each other, extremely

complicated geometries, even organic geometries such as a human head can be made with

relative ease with these machines. But the skill to program such geometries is beyond that

of most operators. Therefore, 5-axis milling machines are practically always programmed

with CAM. 

3.2 MILLING OPERATIONS

The success of any milling operation depends, Before setting up a job, be sure that the to a

great extent, upon judgment in setting up the job, workpiece, the table, the taper in the

spindle, selecting the proper milling cutter, and holding the cutter by the best means under

the circumstances. Some fundamental practices have been proved by experience to be

necessary for and the arbor or cutter shank is all clean and good results on all jobs. Some

of these practices are mentioned be low... Before setting up a job, be sure that the

workpiece, table, the taper in the spindle, and the arbor or cutter shank are free from chips,

nicks, or burrs. Do not select a milling cutter of larger diameter than is necessary. Checkthe machine to see if it is in good running order and properly lubricated, and that it moves

freely, but not too freely in all directions.

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Consider direction of rotation. Many cutters can be reversed on the arbor, so be sure you

know whether the spindle is to rotate clockwise or counterclockwise. Feed the workpiece

in a direction opposite the rotation of the milling cutter (conventional milling). Do not

change feeds or speeds while the milling machine is in operation. When using clamps to

secure a workpiece, be sure that they are tight and that the piece is held so it will not

spring or vibrate under cut. Use recommended cutting oil liberally. Use good judgment

and common sense in planning every job, and profit from previous mistakes. Set up every job as close to the milling machine spindle as circumstances will permit. Examples include

die cavities, gas turbine blades, propellers, casting patterns, etc. 

Owing to the variety of shapes possible and its high production rates, milling is one of the

most versatile and widely used machining operations. The geometric form created by

milling fall into three major groups:

  Plane surfaces: the surface is linear in all three dimensions. The simplest and mostconvenient type of surface;

  Two-dimensional surfaces: the shape of the surface changes in the direction of two of

the axes and is linear along the third axis. Examples include cams;

  Three-dimensional surfaces: the shape of the surface changes in all three directions

3.3 CNC MACHINING PROCESS 

RoughingThis is the first stage of machining where the object is to quickly remove the bulk of the

waste material, normally with the aid of a ripper cutter (see cutters below), this gives the

coarse stepped feature seen in the workpiece above.

Figure 3.4: Cam Roughing Machining

Semi RoughingThis stage of machining generally uses a smaller cutter than roughing, typically an end

mill, although the aim is still to remove the bulk of the waste material.

Figure 3.5: Illustrate CAM Semi Roughing

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Semi FinishingThe next stage, using a relatively large ball nosed cutter, is to start to form the final profile

of the workpiece, removing the steps generated in the two above procedures.

Figure 3.6: Illustrate Semi Finishing Machining

FinishingThe final stage, and the longest process of all, is the final cut to the desired size. A small

 ball nosed cutter traversing across the surface produces the finished shape.Although this is the final machining stage there is still much work to do in the form of

hand polishing and finishing before the article is complete.

Figure 3.7: Illustrate Finishing Machining

3.4 PROCESS PLANNING

The surface quality and dimensional accuracy achieved in different types of millingdepend on the type of milling operation. For rough cuts, the best surface finish is Ra

100~50 µm, while for finishing cuts much better surface finish of Ra 6.3~3.2 µm could be

achieved. These values are approximate and for machining of steel. When cutting gray

cast iron or non-ferrous materials, the surface finish is a grade higher. For mass

 production, the process plan is significantly changed.

The process plan for milling of a single prismatic part includes the following basic steps:

  Cut off the stock slightly larger than required;

  Cut the basic outside dimensions to size using a milling machine;

  Lay out the basic features of the parts (in manual setups, this involves coating thesurface with a blue stain, this is then cut and marked);

  Rough cut steps, radii, angles, grooves, etc.;

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  Lay out the holes to be drilled, and then drill them starting with a center drill andgradually

  Finish cut part features;

  Make internal threads and ream holes if required;

  Deburr the finished part.

3.5 THE MILLING OPERATION CAD CAM SOFTWARE

  The CAD/CAM process integrates the geometry, toolpath, and the G-code program to

create a part on the milling machine. Three steps are taken for the CAD/CAM process.

  First the geometry for the part is created as a CAD file.

  The second step is to create a toolpath by assigning a cutting option to a section of adrawing.

  In the final step the CNC program can be created using the selected postprocessor for

the specific milling machine which will be used to cut the part.

  There are five different toolpath options provided by CAD/CAM software to create a

 part program. They are Face toolpath, Contour toolpath, Drill toolpath, Pockettoolpath, and engrave toolpath.

  The face toolpath removes materials across the top surface of the work piece.

  Contour toolpath, removes materials to a certain depth along a specific line.

  Drill toolpath, identify selected points on a drawing to drill holes to a specific depth.

  Pocket toolpath, removes all materials within the outline of an enclosed geometry.

Pocket has the useful feature of island, which allows any shape within the enclosed

geometry stay intact, and remains, as an island.

  Engrave toolpath, creates toolpath to engrave art and letters.

  In creating drawings for CAD/CAM process layers are used to identify different parts

of the same geometry on different levels.

  Each layer or combinations of layers can be selected to be visible or invisible.

  This feature can be used to create part programs for complex parts by breaking down

the geometry into different sections and creating each section of a drawing on a

specific level according to its toolpath requirements.

  Each layer can be selected at a time for certain toolpath operation. Combination of all

toolpaths for each layer produces the part.

  Construction of different layers for a drawing is an innovative technique to simplifythe process of generating a toolpath for a part.