Principle Elements of Metal Machining:

1: Cutting Speed:

The cutting speed can be defined as the relative surface speed between the tool and the job. It is a relative term since either the tool or the job or both may be moving during cutting. It is expressed in m/min.

2: Feed:

It may be defined as the relatively small the cutting tool relative to the work piece in a direction which is usually perpendicular to the cutting speed direction. It is expressed in mm/rev or mm/stroke.

It is more complex element as compare to the cutting speed. It is expressed differently for various operations.

3:Depth of cut:

The depth of cut is the thickness of the layer of the metal remove in one cut or pass measured in a direction perpendicular to the machine surface. The depth of cut is always perpendicular to the direction feed motion.

 

Sheet metal Cutting Operations:

1: Blanking:

Blanking is the operation of cutting a flat shape from sheet metal. The product punched out is called the “blank” and the required product of the operation the hole and the metal left behind is discarded as waste.

2: Punching or Piercing:

It is a cutting operation by which various shaped holes are made in sheet metal. Punching is similar to blanking except that in punching, the hole is the desired product. The material punched out from the hole being waste.

3: Notching:

This is cutting operation by which metal pieces are cut from the edge of the sheet , strip or blank.

4: Perforating:

This is a process by which multiple holes are very small and close together are cut in a flat sheet metal.

5: Trimming:

This operation consists of cutting unwanted excess of material from the periphery of a previously formed component.

6: Shaving:

The edge of a blanked part are generally rough, uneven and un square. Accurate dimensions of the part are obtained by removing a thin strip of metal along the edges.

7: Slitting:

It refers to the operation of making incomplete holes in a work piece.

8: Lancing:

This is a cutting operation in which a hole is partially cut and then one side is bent down to form a sort of tab. Since no metal is actually removed and there will be no scrap.

9: Nibbling:

The nibbling operation , which is used for only small quantities of components, is designed for cutting out flat parts from sheet metal. The flat parts from simple to complex contours. This operation is generally substituted for blanking. The part is usually moved and guided by hand as the continuously operating punch cutting away at the edge of the desired contour.

 

 

 

Forming Operations:

 

 

1: Bending:

In this operation; the material in the form of flat sheet or strip is uniformly strained around a linear axis which lies in the neutral plane and perpendicular it’s the length wise direction of the sheet or metal.

2: Drawing:

This is a process of forming a flat work piece into a hollow shape by means of a punch which cause the blank into a die cavity.

3: Squeezing:

Under the operation, the metal is caused to flow to all portions of a die cavity under the action of compressive forces.

Types of chips

The three common types of chip from a single point tool are.

1;:Discontinuous or segmental chip:

Discontinuous chips is formed by a series of rupture occurring approximately perpendicular to the tool place face’ each chip element passing off along the tool face the chip element’ in the form of small segment may adhere loosely to each other and becomes slightly longer.

Since the chips break up into small segments the friction between the tool and the chips reduces’ resulting in better surface finish. These chips are convenient to collect’ handle and dispose off. Discontinuous chips tends to be formed when one or more or the following conditions exist:

<!--[if !supportLists]-->1. <!--[endif]-->Brittle material , such as cast iron and bronze.

<!--[if !supportLists]-->2. <!--[endif]-->large chip thickness

<!--[if !supportLists]-->3. <!--[endif]-->low cutting speed

<!--[if !supportLists]-->4. <!--[endif]-->small rack angle

Discontinuous chips are also produced when cutting more ductile material with the use of a cutting fluid.

 

2: Continuous Chips:

Continuous chips are formed by the continuous plastic deformation of metal without fracture in front of the cutting edge of the tool and is formed by the smooth flow of the chip up the tool face. Mild steel and copper are considered to be most desirable materials for obtaining continuous chips. The chips obtained have same thickness throughout. This type of chip is the most desirable. Since it is stable cutting, resulting in generally good surface finish. On the other hand these chips are difficult to handle and dispose off.

Continuous chips tend to be formed when the following condition exist:

<!--[if !supportLists]-->1. <!--[endif]-->ductile material

<!--[if !supportLists]-->2. <!--[endif]-->high cutting speed

<!--[if !supportLists]-->3. <!--[endif]-->small chip thickness

<!--[if !supportLists]-->4. <!--[endif]-->large rack angle

<!--[if !supportLists]-->5. <!--[endif]-->minimum friction of chip on tool face by :

<!--[if !supportLists]-->· <!--[endif]-->polished tool face

<!--[if !supportLists]-->· <!--[endif]-->use of efficient cutting lubricants.

<!--[if !supportLists]-->· <!--[endif]-->Use of tool material with low-coefficient of friction.

3: Continuous Chip with Built up Edge:

This type of chip is very similar to the continuous chip. With the difference that it has a built up edge adjacent to tool face and also it is not so smooth. It is obtained by machining on ductile material, in this condition of high local temperature and extreme pressure in the cutting and high friction in the tool chip interference, may cause the work material to adhere or weld to the cutting edge of the tool. Successive layers of work material are then added to the built up edge. When this edge becomes larger and unstable , it breaks up and part of it is carried up the face of the tool along with the chip while the remaining is left over the surface being machined, which contributes to the roughness of the surface. The built up edge changes its size during the cutting operation. It first increases , then decreases, then again increases etc.