P & P WRESS RESS ORK

Press Work:

Press Work may be defined as a chipless manufacturing process by which various metal components are made by cold stamping. The machine used for press work is known as a press.

Process:

A press has a frame which supports a ram or a slide and a bed, a source of mechanism for operating the ram in line normal to the bed. The ram is equipped with suitable punches and a die black is attached towards the bed. A stamping is produced by the downward motion of the ram, towards the die, and applying impact loading and shear force to the sheet of metal. Press work operations are generally done at room temperature.

The sheet metal operations in press work are usually grouped into two types: cutting operations and forming operations. In forming operations the shear force applied are below the ultimate shear strength that leads only to the contouring of the work piece. For metal cutting operations, the forces applied are of an impact type, and shear the metal as they exceed the ultimate shear strength.

Elastic Recovery / Spring-Back effect:

In metal working processes the deformation imparted to the work piece is the sum of the plastic deformation, and elastic deformation. The elastic deformation is recoverable, whereas the plastic deformation is permanent. So, at the end of a metal stamping process, when the pressure on the metal is released, there is an elastic recovery and the total deformation will get reduced a little. This phenomenon is known as “spring-back”, for which the yield strength is a primary deciding factor. Although the spring-back is proportional to the yield strength, the amount of spring-back is very difficult to predict. It has been standardized for only a few alloys for specific configurations, so the trial-error method usually is employed for determining the amount. To compensate for the spring-back, cold deformation must always be carried out beyond the desired limit by an amount equal to spring-back.

Press Operations:

Blanking:

Blanking is cutting up a large sheet of stock into smaller pieces suitable for the next operation in stamping, such as drawing and forming. Often this is combined with piercing. The whole and metal left behind are usually discarded as waste. The process and machinery are usually the same as that used in piercing, except that the piece being punched out in the piercing process is scrap.

Punching:

Punching in metalworking is the process of using a punch press to push a punch through the material and into a die to create a hole in the work piece. A scrap slug from the hole is deposited into the die in the process. Depending on the material being punched this slug may be recycled and reused or discarded. The hole walls will show burnished area, rollover, and die break and must often be further processed. Punching is often the cheapest method for creating holes in sheet metal in medium to high production. Punching is the most cost effective process of making holes in strip or sheet metal for average to high fabrication and it is able to create multiple shaped holes. Punches and dies are usually fabricated from conventional tool steel or carbides that create a burnished region roll-over, and die break on sidewall of the resulting hole.

Piercing:

Piercing is a shearing process where a punch and die are used to create a hole in sheet metal or a plate. The process and machinery are usually the same as that used in blanking, except that the piece being punched out is scrap in the piercing process.

Lancing:

Lancing is the manufacturing process where a piece of material is sheared and bent in order to make tabs, vents, and louvers. A key part of this process is that there is not reduction of material, only a modification in its geometry.

There are three key characteristics to the process of lancing: The material is only modified, nothing is removed or one or more cuts are made, with the

remainder being bent to the necessary angle or curve or ability to make a special feature in a part using only one cut in the material.

Lancing can be used to make partial contours and free up material for other operations further down the production line. Along with these reasons lancing is also used to make tabs (where the material is bent at a 90 degree angle to the material), vents (where the bend is around 45 degrees), and louvers (where the piece is rounded or cupped).

Notching:

Notching is a metal-cutting process used on sheet metal or thin bar stock, sometimes on angle sections or tube. A shearing or punching process is used in a press, so as to cut vertically down and perpendicular to the surface, working from the edge of a work piece. Sometimes the goal is merely the notch itself, but usually this is a precursor to some other process: such as bending a corner in sheet or joining two tubes at a tee joint, notching one to fit closely to the other.

Notching is a low-cost process, particularly for its low tooling costs with a small range of standard punches.

Perforation:

Perforation refers to the puncturing of a material with a harder (usually sharp) object to create a hole or aperture. A press die is used to form multiple holes on a metal sheet usually giving it in a specific pattern. Die and punch sets can be used for thicker materials, or materials that require large gauge holes, while smaller holes are usually not made in an industrial press except for large work pieces.

Nibbling:

The nibbling process cuts a contour by producing a series of overlapping slits or notches. This allows for complex shapes to be formed in sheet metal up to 6 mm (0.25 in) thick using simple tools. Nibbling can occur on the exterior or interior of the material; however interior cuts require a hole to insert the tool.

The process is often used on parts that do not have quantities that can justify a dedicated blanking die. The edge smoothness is determined by the shape of the cutting die and the amount the cuts overlap; naturally the more the cuts overlap the cleaner the edge. For added accuracy and smoothness most shapes created by nibbling undergo filing or grinding processes after completion.

Slitting:

It refers to the operation of making incomplete holes in a wor kpiece. Usual stamping dies or cutters are placed at an offset to the work piece so that an incomplete cut is made than a usual complete hole.

Bending:

Bending is a common metalworking technique to process sheet metal. It is usually done by hand on a box and pan brake, or industrially on a brake press or machine brake.

Usually bending has to overcome both tensile stresses as well as compressive stresses. When bending is done, the residual stresses make it spring back towards its original position, so we have to over bend the sheet metal keeping in mind the residual stresses. When sheet metal is bent, it stretches in length. The bend deduction is the amount the sheet metal will stretch when bent as measured from the outside. A bend has a radius. The term bend radius refers to the inside radius. The bend radius depends upon the dies used, the metal properties, and the metal thickness.

Shaving:

The edges of a blanked part are generally rough and uneven. Accurate dimensions and smoother edges are obtained by removing a thin strip of metal along the edges. This operation is termed as shaving.

Squeezing:

Under this operation, metal is caused to flow in all portions of a die cavity under the action of compressive forces. This form of stamping usually gives desired size and shape to the sheet metal directly, and no further operations are required. The sheet metal takes the shape of the die into which it is made to squeeze.