An Overview Of Factors Affecting In Blanking Processes

Transcription

1 An Overview Of Factors Affecting In Blanking Processes Amol Totre 1, Rahul Nishad 2, Sagar Bodke 3 1,2,3 B.E. student Abstract During the past decade, two clear trends have been observed in the production of metal components. Firstly, time-to-market needs to be shortened in order to introduce new products competitively. Secondly, ongoing miniaturization forces product dimensions to decrease. Of all forming processes employed in high volume production, blanking is one of the most widely used separation techniques. Still, analysis of blanking is mainly based on phenomenological knowledge. Since, also in blanking processes, requirements concerning product dimensions are becoming more severe so we focus on what are the factors that will affect in blanking process. Keywords Blanking, Burr, Clearance, Factors, Tool wear I. INTRODUCTION Blanking and piercing are both shearing operation. The difference is only in the scrap. In blanking what you cut out is of interest. In punching what you cut from is of interest. For example: You cut a hole in a sheet metal. If you are interested in the disc that is cut out, then the process is called blanking. The sheet metal with a hole through it is the scrap. If you are interested in the sheet metal that now has a hole through it, then the process is called piercing. The disc is the scrap shown in fig 1 Figure 1. blanking & piercing Blanking and piercing is used in almost all sheet forming operation. The size of hole can be vary from less than 1 to 100 mm or more. In this paper we focus on the blanking process and factors affecting the blanking product. A. Characteristics of the blanking process include: 1. Its ability to produce economical metal work pieces in both strip and sheet metal during medium or high production processes, The removal of the work piece from the primary metal stock as a punch enters a die, 3. The production of a burnished and sheared section on the cut edge, 4. The production of burred edges, 5. The control of the quality by the punch and die clearance, 6. The ability to produce holes of varying shapes quickly B. The blanking process has some downside effects. These include: Generating residual cracks along the blanked edges, Hardening along the edge of the blanked part or work piece, and Creating excess roll-over and burr if the clearance is excessive. The most common materials used for blanking include aluminum, brass, bronze, mild steel, and stainless steel. Due to its softness, aluminum is an excellent material to be used in the blanking processes II. BLANKING PROCESS Blanking process can be considered to include series of phases in which sheet metal undergoes deformation and separation as seen in fig 2 Contact of punch The punch first touches the fixed sheet.at impact a compressive stress rapidly builds on the punch and sends a shock wave through it Elastic and plastic deformation The punch penetrates into the sheet.first causing an elastic and then plastic deformation Shearing and crack formation When the stresses increase shearing occurs followed by fracture.fracture begins from both the punch end and die end of the sheet they usually meet and complete fracture of the material takes place Breakthrough If the sheet material has a high strength or is thick. A large force is required for blanking process. During fracture compressive forces are stored in the tool.

2 When complete fracture occurs there is an instant release of the compressive forces these generate shock, which can lead to breakage of the punch in some cases. Stripping The punch move down to the bottom dead centre and ejects the part. At the bottom dead centre direction of the punch motion is reversed. Due to the friction between the stock and the surface of the punch, the surface pressure intensifies. A stripper or blank holder strips the blank from punch Figure 2 Steps in blanking process A. Forces and stresses The cutting force do not act linearly along the cutting edge instead, the vertical force F v and horizontal force F H act in small area near the cutting edge as shown in fig 3 The distribution of those compressive forces are non uniform. The distance, l, between the forces F H and F V cause bending movement must be compensated for by counter bending moment that is created by bending stresses and horizontal normal stresses between work piece and tool fig also shows the resulting frictional forces µ.f H and µ. F V these frictional forces increases total blanking forces. The blanking process can be investigated by monitoring the change in the blanking force during the cutting process the force varies with the punch displacement, punch entry time or crank angle. Because part quality is evaluated in terms of region formed along the part edge,it is preferred to present. The load versus punch displacement. In addition the cutting work can be calculated by integrating the forces over the stroke.the theoretical load-stroke curve in a blanking process can be described schematically as seen in fig 5 Step 1: The sheet metal deforms elastically. Step 2: The limit of elastic deformation is reached and the material starts to deform plastically. The material flows along the cutting edge in the direction of the punch penetration and into the gap between the punch and die. The material floe causes strain hardening which result in an increase of the cutting force up to the maximum load. At this time the cross section is not reduced and shearing is not started. Step 3: the increase cutting force at cutting edge prevent the material from flowing and shearing start. Due to a decreasing cross section the blanking force decreases deposit the strain hardening of the material. Step 4: fracture starts after the formability limit of the material is exceeded. As soon as the initial cracks meet each other slug and skeleton are completely separated. The cutting force decreases rapidly during this phase. Figure 3 Stresses in blanking 391

3 Figure 6 zones of the edge Figure 5 load Vs time digram B. Different zones of part edge The shear edge is made of different zones based on the method of material that has occurred. The ratio of the different zones is influenced by different parameters, such as the punch corner radius, to name a few, in general it is preferred to have large shear zone and smaller burr the zones and deformation modes of the blanked part edge are given as follows and in fig 6 o Rollover zone (z r ): caused by plastic material deformation o Shear zone (z s ) : smooth and shiny area created during material shearing o Fracture/rupture zone (z f ) : rough surface, result after the material cracks o Burr zone (z b ) : caused by plastic deformation o Depth of crack penetration (D cp ) : angle of fracture zone depends mainly on clearance o Secondary shear : created if cracks do not run toward each other and material is sheared again III. FACTORS AFFECTING IN THE BLANKING PROCESS A. The effect clearance Clearance c is the space (per side )between the punch and the die tool and die producers enjoy some kind of mystique related to their work as being both an art and science proper clearance between cutting edges enable fractures to start ideally at cutting edge of the punch and also at the die. The fracture will proceed towards each other until they meet and the fractured portion of the sheared edge then has a clean appearance. For optimum finish of a cut edge, correct clearance is necessary and is function of the kind, thickness, and temper of the material. The upper corner of the cutting edge of the strip stock and the lower corner of the blank acquire a radius where the punch and die edge respectively make contact with the work material. This edge radius is produce by plastic deformation taking place and is more pronounce when cutting soft materials. Excessive clearance will also cause large radius at this corner as well as a bur on opposite corner. When clearance is not sufficient, additional layers of the material must be cut before complete separation is accomplished. With correct clearance, the angle of fractures will permit clean break below the burnish zone because the upper and lower fracture will extend toward one another. 392

4 Excessive clearance will result in tapered cut edge because for any cutting operation, the opposite side of the material that the punch enters after cutting, will be the same size as the die opening. The width of the burnish zone is an indication of the hardness of the material. Provided that the die clearance and material thickness are constant, the softer the material the wider will be the burnish zone. Table 1 Value of clearance as the percentage of the thickness of material Harder metals require large clearance and permit less penetration by the punch than ductile materials; dull tool (punch and die) create the effect of too small a clearance as well as bur on the die side of the stock. Clearance is generally expressed as a percentage of the material thickness, but some authorities recommend absolute values. C = D m -d p /2 from fig 7 MATERIALS MATERIAL THICKNESS T(MM) < TO TO TO TO 7.0 LOW CARBON STEEL COPPER AND SOFT BRASS MEDIUM CARBON STEEL 0.2% TO 0.25% CARBON HARD BRASS HARD STEEL 0.4% TO 0.6% CARBON Table illustrate the value of the shear clearance in percentage depending on the type and the thickness of the material Figure 7 punch and die B. The effect Punch geometry Punch geometry affects the punch stresses and temperature as well as punch life. Figure shows the maximum forces by using different punch shapes when blanking a round part. The punch forces differ because the area of contact with the sheet at given instant in the penetration length is not the same. Figure 8 force Vs time digram Shear angles/chamfers on the punches are also used for easy stack up of the slugs. The slugs, when bent, become neatly stacked over and one another fig 8 shows the various punch shapes 393

5 D. The Effect of the Sheet Thickness For a given material, the energy requirement in blanking is influenced by the sheet thickness. It has been observed that: 1. The blanking energy decreases with increasing clearance-to-sheet thickness ratio c/t and increases with increasing sheet thickness. 2. The proportions of the different depth characteristics of the sheared profile are affected by the thickness. E. The Effect of Material The part edge quality also depends on the mate-rial being blanked. Materials with large ductility, low yield strength, and homogeneity will have better blanked edge qualit y, dimensional tolerances, and longer tool life Figure 9 effect of tool wear and blanking force on part edge quality as predicted by simulation.58mm thick copper alloy C. The effect of tool wear Tool wear leads to the Formation of burrs and increases burr length. Burr length is generally an important criterion in the industry to evaluate part quality. Burr length indicates when the tool should be reground to obtain the sharp die-and-punch radius. It has also been observed that the effect of tool wear is more pronounced at higher blanking clearances. The effect of tool wear on part edge quality is significant. Tool wear leads to the formation of buns and increases burr where the effect of tool wear was simulated by assuming different punch corner radii in simulations. It has also been observed that the effect of tool wear is more pronounced at higher blanking clearances from fig 9 &10 Figure 10 tool wear IV. CONCLUSION In the present paper we see the factors affecting in the blanking process like the A. Clearance B. tool wear C. Sheet Thickness D. Material E. Punch geometry Also by the help of the various fig we see the what are importance of this factors in the blanking process out of which clearance, thickness & tool wear is important factors clearance value of the various material is given in the table 1 like brass, steel, carbon steel etc. REFERENCES [1] R. Hambli, (2002), Design of Experiment Based Analysis for Sheet Metal Blanking Processes Optimization. The International Journal of Advanced Manufacturing Technology, Vol.19, Page No [2] F. Faura, A. Garcia, and M. Estrems, (1998), Finite element analysis of optimum clearance in the blanking process. Journal of Materials Processing Technology, Vol.80-81, Page no [3] R. Hambli, S. Richir, P. Crubleau, and B. Taravel, (2003), Prediction of optimum clearance in sheet metal blanking processes. International Journal of Advanced Manufacturing Technology, Vol. 22, page no [4] Emad Al-Momani, Ibrahim Rawabdeh, (Mar. 2008), An Application of Finite Element Method and Design of Experiments in the Optimization of Sheet Metal Blanking Process Jordan Journal of Mechanical and Industrial Engineering. Volume 2, Number 1, Pages [5] S. Maiti, A. Ambekar, U. Singh, P. Date, and K. Narasimhan, (2001), Assessment of influence of some process parameters on sheet metal blanking.journal of Materials Processing Technology, Vol. 102, page no

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