Milling

The first machine tools were designed and put into practice by J. Wilkinson in the XVIII century. Since then, there have been great technological developments. Especially the rapid development of computer technology has enabled its use in the machinery and manufacturing sector. Today, conventional machine tools have been replaced by computer aided machine tools (CNC).

With the help of the cutting tool rotating in a circular motion around its own axis, the process of removing chips with the smooth linear movement of the workpiece, which is tightly and securely attached to the workpiece, is called milling, the person who does this job is called a milling cutter, and the machines are called milling machines.

In response to the rotational movement of the cutting tool around its axis, the chip removal process that occurs with the progress of the workpiece is called milling. We can list the milling machine types as follows.

A- Cantilevered Milling Machines

a) Horizontal Milling Machines
b) Vertical Milling Machines
c) Universal Milling Machines

B- Manufacturing Milling Machines

a) Single Column Milling Machines
b) Double Column Milling Machines

C- Copy Milling Machines

D- Horizontal Hole Milling Machines (Bohrwerk)

E- Gear Wheel Milling Machines

a) Hobbing Gear Looms
b) Milling Machine (Fellow) Opening Gear Wheel with Vargeling Method 
c) Threading Machine with Rack-shaped Knife (Maag) 

F- NC Milling Machines 

G- CNC Milling Machines 

H- DNC Milling Machines

I- Robotic Controlled Milling Machines

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Chip Removal with Milling Method

With the help of a cutting tool, the process of separating the excess parts of the workpiece from the main part according to the technological rules in accordance with the dimensions and tolerances in the manufacturing drawing is called chip removal. The milling method is the removal of chips from the workpieces by rotating the cutting tool with inserts on it around its own axis and moving the table to which the workpiece is connected in the directions of x, y and z axes. The main feature that distinguishes the chip removal with milling method from other machining methods is the chip removal with more than one cutting edge at the same time. This shows the high efficiency of the milling method. 

The milling method is named according to the shape and form of the machining to be performed on the workpiece and the type of cutting tool used. 

• Face milling method, 
• Circumferential milling method 
• Form and circumferential milling.

Chip Removal with Face Milling Method 

Chip removal by face milling is the process of removing chips from the workpiece with a cylindrical milling cutter or a milling cutter with an inserted end mill. In face milling, the axis of rotation of the cutting tool and the machining surfaces of the workpiece are perpendicular to each other. 

Chip removal with a cylindrical milling cutter is provided by the cutting edges on the front and side edges of the cutting tool. Two cutting edges perpendicular to each other in the cylindrical milling cutter cut at the same time. Cylindrical milling cutters are usually manufactured as a whole from high quality steel. There are also types manufactured by attaching hard metal inserts around the cylindrical holder shaft. 

Insert cutting tools perform chip removal with more than one hard metal insert. You metal inserts are mechanically attached to the cutting tool shank with screws. 

Chip Removal with Circumferential Milling Method

Chip removal with a cylindrical milling cutter. The cutting tool is connected to the spindle by means of long spindles. The axis of rotation of the cutting tool is parallel to the chip removal surface. The milling method in which the chip removal process takes place by means of the cutting edges around the cutting tool with the rotational movement of the cutting tool is called circumferential milling method. In the peripheral milling method, the chip cross-section changes continuously during the chip removal process. 

Chip Removal with Same and Opposite Directional Milling

In milling operations, according to the rotation of the milling cutter and the feed direction of the table, it is possible to remove chips with two methods in peripheral milling. Same direction milling and opposite direction milling methods. Both methods have differences and superior sides and reasons for preference. 

Same Direction Milling

In the same direction milling process, the progress of the workpiece is in the direction of rotation of the milling cutter. Each tooth of the workpiece will remove a certain amount of sawdust, which enters from the top and makes a cut from more sawdust to less sawdust. During cutting, since the teeth grasp the chip from the top, it tries to press the work towards the table (vice, table, clamping mould, etc.) to which it is attached. It is good in terms of not removing the work. During normal feed, the amount to be lifted by the cutting teeth in one revolution is certain. The teeth remove this amount on the surface of the work, from the largest value to zero on the machined surface. Although the milling cutter tries to stretch the spindle to which it is connected at the beginning while going from many chips to few chips, the smoothly rotating milling cutter produces a smooth surface as the chip is reduced and finished. 

Characteristics of the Same Directional Milling Method: 

- The type of chips produced is crumb and comma shaped. 
- The machined surface is clean.
- Since the rotating pocket knife has the same direction, it does not try to remove the workpiece. 
- The small horizontal force in automatic feed reduces energy consumption. 
- Since it cuts by pressing the part against the table, vibration is less and surface quality is higher. 
- In materials with hard surfaces, the cutting tool will be blunt quickly and tool life will be reduced. 
- In milling machines with a single spindle table, the amount of space between the nut and the table spindle, as much as the amount of space between the nut and the table spindle, suddenly pulls the table under the table with the workpiece, great dangers may occur as the knife will be forced due to excessive shavings. For this reason, milling in the same direction is not performed on this type of machine. 
- It is possible to give a larger feed rate and increase the chip depth compared to opposite-direction milling. This increases the production. 
- Chip removal is possible and economical by giving more chip depth.

Opposite Direction Milling 

Milling is a milling operation when the workpiece feed is against the direction of rotation of the cutter. Here the cutter tries to push the workpiece longitudinally and lift it upwards from the table. During cutting, the chip thickness increases uniformly towards the upper surface of the workpiece, which causes a strain on the milling cutter. This tries to flex the headstock spindle. The face of the work is therefore too wavy to be visible at first glance. Forcing the workpiece upwards from the jig to which it is attached requires it to be tightened securely.

Properties of Opposite Directional Milling Method: 

- The chip shape is comma-shaped. 
- As the penknife is forced into the material, the trowel flexes and this flexing causes a wave on the surface of the material. 
- The workpiece is forced out of the vice as the chips are torn out of the material. Therefore, it must be tightened securely. 
- The gap between the table shaft and the nut poses no danger. 
- Since the pocket knife forces the workpiece upwards continuously, the vibration is high. 
- On hard surfaces, the blade breaks off instead of cutting. This increases the life of the cutting tool. 

Especially in cast materials, the teeth of the pocket knife do not cut the shell. But it breaks it off. The good thing is that the pocket knife is not blunt immediately. Chip cutting is from less to more. The gaps between the machine table shaft and the nut do not affect chip removal and progress. The pocket knife, which rotates with force against force, makes a vibration-free cut. When the feed is stopped, the blade rotates where it is. At this moment, the process does not continue and there is no danger. However, the cutter must not be rotated idly on the workpieces. Otherwise it will affect the surface quality. The chips in opposite direction milling are comma shaped. For this reason, the most commonly used milling method in manufacturing is opposite direction milling.

Comparison of Same and Opposite Direction Milling Methods

In opposite direction milling, the cutting force is initially low and gradually increases. However, it tries to lift the workpiece off the table. This is also undesirable. In same direction milling, the cutting force is initially large. It tries to press the workpiece against the milling table. It is superior in terms of safety and surface roughness. Researches on the same and opposite direction milling methods have determined that the surface quality obtained in the same direction milling method is twice as much as that obtained from the opposite direction milling method. 

Same direction milling method is not preferred for cast materials. Because the layer on the outer surface of cast materials is harder than the inner surface. Since the cutting tool teeth will first penetrate this hard layer, it will wear quickly. It will also reduce the cutting tool life. 

Chip Removal with Symmetric and Asymmetric Milling Method 

In the symmetrical milling method, the axis of rotation of the cutting tool and the feed axis of the workpiece in the y direction coincide. In other words, the axis of the cutting tool must move from the centre point of the workpiece during machining. For symmetrical milling, the cutting diameter of the cutting tool must always be larger than the width of the workpiece to be milled. 

In the asymmetric milling method, the rotation axis of the cutting tool and the feed axis of the workpiece in the y direction do not coincide. In other words, if the axis of the cutting tool does not move from the centre of the workpiece during machining, this milling process is asymmetric milling method. For asymmetric milling, the cutting diameter of the cutting tool must be larger than the milling width of the workpiece.