Wire and tube Drawing

Transcription

1 Wire and tube Drawing Drawing is an operation in which the cross-section of solid rod, wire or tubing is reduced or changed in shape by pulling it through a die. The principle of this procedure consist of reducing the thickness of a pointed,tapered wire by drawing it through a conical opening in a tool made of a hard material.the wire will take shape of the hole. Drawing improves strength and hardness when these properties are to be developed by cold work and not by subsequent heat treatment. This process is widely used for the production of thicker walled seamless tubes and cylinders therefore; shafts, spindles, and small pistons and as the raw material for fasteners such as rivets, bolts, screws. Drawing is classified as 1. Wire drawing 2. Tube drawing Wire Drawing:- Wire drawing is a metal-reducing process in which a wire rod is pulled or drawn through a single die or a series of continuous dies, thereby reducing its diameter. Because the volume of the wire remains the same, the length of the wire changes according to its new diameter. Various wire tempers can be produced by a series of drawing and annealing operations. (Temper refers to toughness.) (a)

2 (b) (c) Figure-(a)Wire Drawing sectional view (b) wire drawing set up (c) Enlarge View of Wire Reduction Process Characteristics Pulls a wire rod through a die, reducing its diameter Increases the length of the wire as its diameter decreases May use several dies in succession (tandem) for obtaining small diameter wires. Improves material properties due to cold working Wire temper can be controlled by swaging, drawing, and annealing treatments Tube Drawing When a hollow tube is drawn through a die, generally a mandrel or plug is used to support the inside diameter of the tube, this process is called tube drawing. The function of the plug is to effect wall reduction and to control the size of the hole. However, the mandrel may be omitted if it is not necessary to make a reduction in the wall thickness, or if the dimensions and surface of the inside are not important. The process to draw a pipe without any mandrel is known as tube sinking. In drawing tubes over a stationary mandrel, the maximum practical sectional area reduction does not exceed 40 per cent per pass due to the increased friction from the mandrel. If a carefully matched mandrel floats in the throat of the die, it is possible to achieve a reduction in area of 45 percent, and for the same reduction the drawing loads are lower than for drawing with a fixed plug. This style is called the drawing with floating plug. It is worth mentioning here that in this style, the tool design and lubrication can be very critical. Problems with friction in tube drawing

3 are minimized in drawing with a long mandrel. The mandrel consists of a long hard rod or wire that extends over the entire length of the tube and is drawn through the die with the tube. In this design, the area reduction can be 50 per cent. However, after drawing, the mandrel must be removed from the tube by rolling (reeling), which increases the tube diameter slightly and disturbs the dimensional tolerances. The drawing process discussed above has been illustrated in the figure 2.18 Figure-Tube drawing processes. (a) Sinking; (b) fixed plug; (c) floating plug; (d) moving mandrel Defects in drawing process: Figure (a) Wrinkling in the flange or (b) in the wall (c) tearing, (d) Earing, (e) surface scratches Basic press working Operation Sheet metal working is also a metal forming operation where the material being worked is in the form of sheets the working is usually at room temperature. The process is called the press working of sheet metal. For sheet metal working a set of die and punch assembly is normally used to obtain various components from a sheet metal. Die and Punch assembly

4 A simple die and punch assembly also called die-set is shown in figure. The parts of the die and punch system are labeled. This assembly is mounted on press. Figure-Die and Punch Assembly The press supports a ram (slide) that holds a punch and a bed that holds the die in a die holder connected to the bolster plate. In sheet metal working operation, the sheet thickness remains almost constant. The process has the following advantage: 1. The part fabricated are light in weight 2. High labour productivity 3. High efficiency of the process 4. Gives close tolerance The important characteristic of the sheet material is its formability i.e. its ability to stretch uniformly (resistance to thinning). Various tests are available to assess the formability of sheet. Some problem arise out of sheet used being thin, such as spring back. Buckling, and wrinkling. A proper die design and minimizing the unsupported length during processing may solve the problem. Cutting Process (sheet metal Working/ press working) A brief introduction of some important sheet- metal working operation is given below:

5 1. Blanking and Punching: In these processes a finite shape of sheet metal is removed by shearing the entire contour using a die and a punch. If the portion removed is the required product, the operation is called Blanking and the product is called blank. On the contrary, if the pierced sheet is final product, the operation is termed as punching. A simple blanking die and blanking operation is shown in figure. It is important to note that the punch and die corners are sharp and not provided with any radius as the process need to cause a rupture of the material. A clearance c is provided on die in punching operation and the punch is to the required size. Hence die diameter Dd =Dp+2c where Dp and Dd are the die and punch diameter. Figure Blanking and (b) punching 2. Shaving: The rough edges of a blanked part are removed by cutting (shearing) thin strip of metal along the edges on the periphery. 3. Trimming: trimming is the operation of cutting and removing unwanted excess material (flash, etc) from the periphery of previously formed/forged/cast component.

6 Figure (a) shaving and (b) trimming 4. Notching: Notching is a cutting operation in which metal sheet, stripsor blank are cut (sheared) at the edge as shown in figure. Ting 5. Perforating: perforating is a process used to make multiple holes which are small in diameter and close together, in flat work material. 6. Slitting: Slitting refers to the operation of making incomplete holes from s slit (an opening) in the sheet metal as shown in figure. 7. Lancing: Lancing is a operation of cutting on one side and bending on other side to form a sort of tab or louver. Figure (a) Notching, (b) Slitting, (c) Lancing and (d) Nibbling 8. Parting: parting is a operation of shearing a sheet into two or more pieces. 9. Nibbling: Nibbling is the operation used to punch overlapping small holes along a contour to cut out contoured parts from sheet metal. This operation is generally a

7 substitute for blanking if the number of pieces required is small. The part is usually guided by hand continuously. 10. Deep Drawing: This is also known as cup or radial drawing as it produces cup shaped objects. Depth produced in deep drawing is more than the diameter. Shallow parts can also be produced by this method. Typical products made by this process include domestic pots, pans, food container, automobile fuel tanks, etc. In deep drawing a flat sheet metal blank kept under a blank holder is forced into a die cavity by means of a punch. Force on blank holder should be sufficient to prevent wrinkling of sheet but should allow the blank to slide into the die cavity as it is drawn in. Figure- Deep drawing 11. Bending: In bending operation the metal is stressed in both tension and compression at values below the ultimate strength of the material without appreciable change in its thickness. In designing a rectangular section for bending, the material allowed for the bend must be determined, since the outer fiber are elongated and the inner ones shortened, the neutral axis of the section moves in towards the compression side which throws more of the fibers in tension. The total thickness is slightly decreased and width increased on the tension side and narrowed on the other. The bent part retains some of its elasticity which is recovered after the punch is removed as shown in figure. This is known as spring back. Spring back may be corrected by over bending by an amount that brings the part to return to the correct shape.

8 Figure-bending operation Application of sheet- metal working: Sheet metal is widely used for numerous industrial and non-industrial applications including: Aircraft: Fuselages, wings, body panels, trim parts, etc. Automotive: body panels, bumpers, doors, chassis, trim parts, brackets etc. Construction: roofing, home building and structural applications. Other applications: appliances, food and beverage containers, boilers, kitchen equipment, office equipment etc. Defects in sheet- metal working: Defects in sheet-metal formed parts originated from the following basic sources: 1. Defective raw materials (sheet-metal stock) 2. Defective die- design 3. Defective processing (forming) technique Defects due to raw materials and die-design can be tackled at source by purchasing quality raw material and good design of die and checking its quality before starting production and taking into consideration the factors affecting quality die design. Defects due to forming technique will be discussed with sketches in the following paragraph.

9 1. Stretcher strain marks: During drawing or forming, as stretching begins, yielding is localized in a thin visible band (luders lines). As stretching continues further, families of these lines criss-cross and deform on yielding. Figure-Stretcher strain marks 2. Earing: Earing is a defect that occurs due to uneven drawing as shown in figure. To eliminate Earing excessive deformation in deep drawing should be avoided. Figure-Earing due to uneven drawing and part failure 3. Orange peel effect: when a coarse grained material is drawn, the grain will manifest themselves into a rough surface due to re crystallization in areas with maximum strain hardening and not contact with die surface. This roughness resembles the surface of an orange peel. Figure -Orange and peel effect 4. Sinking: sinking refers to small depression on the surface of a formed part. It gives rise to unbalanced internal stresses which may cause failure.

10 5. Burr and Bend: Due to improper clearance burrs are formed on the cut edge at periphery. Figure- Burr and Bend due to excessive clearance 6. Part failure: As shown in figure 7. Strain hardening: Occurs when a metal is strained to plastically deform below recrytalization temperature (cold working). Strength and hardness increase and ductility reduced. 8. Wrinkling: flanges and wall wrinkle due to lose punch/blank holder clearance as shown in figure Figure -Wrinkling on flanges and wall. Part failure may also occur