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Metal Cutting Processes 2 - Milling

 
Contents
  1. Introduction
  2. Types of Milling Machine
  3. 2.1 Horizontal Milling Machine
    2.2 Vertical Milling Machine
  4. Cutting Tools
    3.1 Cutting Tools for Horizontal Milling Machine
    3.2 Cutting Tools for Vertical Milling Machine
  5. Industrial Applications
  6. Milling Processes
    5.1 Spindle Speed
    5.2 Feed Rate
    5.3 Depth of Cut
    5.4 Direction of Cutter Rotation
  1. Typical Milling Operations
    6.1 Plain Milling
    6.2 End Milling
    6.3 Gang Milling
    6.4 Straddle Milling
  2. Milling Set Up
    7.1 Vice Alignment
    7.2 Work Holding Method
  3. Safety
 
1. Introduction

Milling machine is one of the most versatile conventional machine tools with a wide range of metal cutting capability. Many complicated operations such as indexing, gang milling, and straddle milling etc. can be carried out on a milling machine.

This training module is intended to give you a good appreciation on the type of milling machines and the various types of milling processes. Emphasis is placed on its industrial applications, operations, and the selection of appropriate cutting tools.

On completion of this module, you will acquire some of these techniques from the training exercises as illustrated in figure 1. However, to gain maximum benefit, you are strongly advised to make yourself familiar with the following notes before undertaking the training activities, and to have a good interaction between yourself and the staff in charge of your training.

Assessment of your training will be based on a combination of your skill and attitude in getting the work done.

Figure 1. Milling Products

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2. Types of Milling Machine

Most of the milling machine are constructed of °•column and knee°¶ structure and they are classified into two main types namely Horizontal Milling Machine and Vertical Milling Machine. The name Horizontal or Vertical is given to the machine by virtue of its spindle axis. Horizontal machines can be further classified into Plain Horizontal and Universal Milling Machine. The main difference between the two is that the table of an Universal Milling Machine can be set at an angle for helical milling while the table of a Plain Horizontal Milling Machine is not.

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2.1. Horizontal Milling Machine

Figure 2 shows the main features of a Plain Horizontal Milling Machine.

Their functions are :-

a. Column
The column houses the spindle, the bearings, the gear box, the clutches, the shafts, the pumps, and the shifting mechanisms for transmitting power from the electric motor to the spindle at a selected speed.

b. Knee
The knee mounted in front of the column is for supporting the table and to provide an up or down motion along the Z axis.

c. Saddle
The saddle consists of two slideways, one on the top and one at the bottom located at 90º to each other, for providing motions in the X or Y axes by means of lead screws.

d. Table
The table is mounted on top of the saddle and can be moved along the X axis. On top of the table are some T-slots for the mounting of workpiece or clamping fixtures.

e. Arbor
The arbor is an extension of the spindle for mounting cutters. Usually, the thread end of an arbor is of left hand helix.

Figure 2. Horizontal Milling Machine

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2.2. Vertical Milling Machine

Figure 3 shows a vertical milling machine which is of similar construction to a horizontal milling machine except that the spindle is mounted in the vertical position.

Its additional features are :-

a. Milling head
The milling head consisting the spindle, the motor, and the feed control unit is mounted on a swivel base such that it can be set at any angle to the table.

b. Ram
The ram on which the milling head is attached can be positioned forward and backward along the slideway on the top of the column.


Figure 3. Vertical Milling Machine

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3. Cutting Tools

3.1. Cutting Tools for Horizontal Milling

a. Slab Mills
For heavy cutting of large and flat surfaces.


Figure 4. Slab Mill

b. Side and Face Cutters
This type of cutters has cutting edges on the periphery and sides of the teeth for cutting shoulders and slots.


Figure 5. Side and Face Cutter

c. Slitting Saws
For cutting deep slots or for parting off.

Figure 6. Slitting Saw

Note: Horizontal milling cutters are specified by the name, the material, the width, the diameter, and the hub size of the cutter.

Example ---

Side and face cutter,
High Speed Steel,
Cutter size : 10 X ø 100
Hub size: ø 25

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3.2. Cutting tools for Vertical Milling

a. End Mills
Commonly used for facing, slotting and profile milling.


Figure 7. End Mill

b. Rough Cut End Mills
For rapid metal removal.


Figure 8. Rough Cut End Mill

c. Slot Drills
For producing pockets without drilling a hole before hand.


Figure 9. Slot Drill

d. Face Milling Cutters
For heavy cutting.


Figure 10. Face Milling Cutter

Note: Most vertical milling cutters are of end mill types and are specified by the material, the diameter, the length, the helical angle, the types of shank and the shank diameter. For face milling cutter, only the diameter of the cutter and the types of carbide inserts are required.

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4. Industrial Applications

Milling machines are widely used in the tool and die making industry and are commonly used in the manufacturing industry for the production of a wide range of components as shown in figure 11. Typical examples are the milling of flat surface, indexing, gear cutting, as well as the cutting of slots and key-ways.

When equipped with digital readout, the machine is capable of producing more precise work for the manufacturing of plastic moulds, tool & dies, and jigs & fixtures. Figure 12 shows a typical plastic mould produced by milling.


Figure 11. Components Made by Milling


Figure 12. Plastic Mould

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5. Milling Processes

Milling is a metal removal process by means of using a rotating cutter having one or more cutting teeth as illustrated in figure 13.

Cutting action is carried out by feeding the workpiece against the rotating cutter. Thus, the spindle speed, the table feed, the depth of cut, and the rotating direction of the cutter become the main parameters of the process. Good results can only be achieved with a well balanced settings of these parameters.


Figure 13. Milling Process

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5.1. Spindle Speed

Spindle speed in revolution per minute (R.P.M.) for the cutter can be calculated from the equation :-

where --

N = R.P.M. of the cutter
CS = Linear Cutting Speed of the material in m/min. ( see table 1 )
d = Diameter of cutter in mm

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5.2. Feed Rate

Feed rate (F) is defined as the rate of travel of the workpiece in mm/min. But most tool suppliers recommend it as the movement per tooth of the cutter (f). Thus,

F = f . u . N

where --

F = table feed in mm/min
f = movement per tooth of cutter in mm ( see table 1 )
u = number of teeth of cutter
N = R.P.M. of the cutter

where

C.S. and feed rate for some common material :-

Tool Material
High Speed Steel
Carbide
Material
Cutting Speed
Feed (f)
Cutting Speed
Feed (f)
Mild Steel
25
0.08
100
0.15
Aluminium
100
0.15
500
0.3
Hardened Steel
---
---
50
0.1
Table 1

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5.3. Depth of Cut

Depth of cut is directly related to the efficiency of the cutting process. The deeper the cut the faster will be the production rate. Yet, it still depends on the strength of the cutter and the material to be cut.

For a certain type of cutter, a typical range of cut will be recommended by the supplier. Nevertheless, it should be noted that a finer cut is usually associated with a better surface finish as well as a long tool life.

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5.4. Direction of Cutter Rotation

a. Up Cut Milling

In up cut milling, the cutter rotates in a direction opposite to the table feed as illustrated in figure 14. It is conventionally used in most milling operations because the backlash between the leadscrew and the nut of the machine table can be eliminated.


Figure 14. Up Cut Milling

   

b. Down Cut Milling

In down cut milling, the cutter rotates in the same direction as the table feed as illustrated in figure 15. This method is also known as Climb Milling and can only be used on machines equipped with a backlash eliminator or on a CNC milling machine. This method, when properly treated, will require less power in feeding the table and give a better surface finish on the workpiece.

If you move the wrong direction, you may break the cutter as shown.


Figure 15. Down Cut Milling

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6. Typical Milling Operations

6.1. Plain Milling

Plain milling is the milling of a flat surface with the axis of the cutter parallel to the machining surface. It can be carried out either on a horizontal machine or a vertical machine as shown in figure 16.


Figure 16. Plain Milling

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6.2. End Milling

End Milling is the milling of a flat surface with the axis of the cutter perpendicular to the machining surface as shown in figure 17.


Figure 17. End Milling

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6.3. Gang Milling

Gang milling is a horizontal milling operation that utilises three or more milling cutters grouped together for the milling of a complex surface in one pass. As illustrated in figure 18, different type and size of cutters should be selected for achieving the desire profile on the workpiece.

Figure 18. Gang Milling

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6.4. Straddle Milling

In straddle milling, a group of spacers is mounted in between two side and face milling cutters on the spindle arbor as shown in figure 19. for the milling of two surfaces parallel to each other at a given distance.

Figure 19. Straddle Milling

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7. Milling Set Up

Correct use of holding device and a good set up are of crucial importance in achieving a safe, accurate, and efficient operation of the machine. Large workpiece can be mounted directly onto the machine table by means of tenons and screws while small workpieces are usually held by machine vice as shown in figure 20. In either case, a dial indicator is used for alignment checking.

Figure 20. Machine Vice

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7.1. Vice Alignment

In the setting up of the vice onto the machine table, the fix jaw of the vice must be set parallel to the machine table using a Parallel Bar and a Dial Indicator as illustrated in figure 21. Adjustments can only be made by using a hide face hammer to correct its position such that a near zero indicator movement is achieved at all positions along the parallel bar.

Figure 21. Machine Vice Set-up

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7.2. Work Holding Method

In the machining of a complex component, it is usually started off with the milling of a rectangular block. To ensure that each surface of the rectangular block is perpendicular to its neighbouring surfaces, the following points should be noted:-

  • The vice jaws and the workpiece must be free from burrs, chips, and cutting fluid.
  • Smaller workpiece should be supported by parallel bars to provide the supporting datum.
  • Round bar must be placed between the workpiece and the movable jaw to ensure that the workpiece is in perfect contact with the fix jaw.
  • The vice handle should be tightened by hand to avoid over clamping of the workpiece as well as the vice. Hide face hammer should be used to assure that the workpiece is in perfect contact with the supporting base.
  • On completion of the milling of the first face, the workpiece should be unloaded, deburred, and cleaned before the next operation.
  • To machine the second and the third faces, the workpiece should be clamped with its preceding machined surface facing against the fix jaw of the vice.
  • Similar clamping method can be applied in the machining of the fourth face.
  • Yet it can also be clamped on the vice without the round bar.
  • Both ends of the workpiece can be machined with the periphery flutes of the cutter using up cut milling as shown in figure 23.


Figure 22. Holding Method by Using a Machine Vice


Figure 23. End Surface Milling

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8. Safety

Safety practices of a machine shop should be followed. A complete understanding of the Safety Rules would enable the students to identify potential hazards that may occur under different working conditions such that appropriate preventive actions can be taken to avoid the happening of accidents. Emphasis should be given that the eyes of the machine operator must be protected by wearing a face shield (figure 24) to prevent accident that may be caused by chips, cutting fluid, and tool breakage.

Machine operators must also take care of their body such as fingers which keep out of any moving parts, especially the rotating cutter of the machine, to prevent any unnecessary accident hurt. The milling machine must be stopped immediately when any accidence occurred, so the operator must stand near by the control panel of the machine and pays more attention on the operation. Remember the Chinese phrase 'carefully can be driven the boat in thousand year' that you will enjoy the benefits provided by milling.

Figure 24. Face Shield

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