Materials & Processes in Manufacturing

Transcription

1 Materials & Processes in Manufacturing ME 151 Chapter 19 Cold Working Processes 1 Introduction Forming materials their recrystallization temperature Attractive features of cold working: Strain hardening increases materials strength Lower cost starting materials can be used Possible elimination of product heat treatment Good surface finish and dimensional precision Lower total energy cost as compared to hot-working 4 Basic categories of cold working processes Squeezing Operations Bending Operations Shearing Operations Drawing Operations 2 Bending Shearing Drawing Squeezing Operations Material is deformed (cold) to a desired shape by rolling or forcing into or through a die Process Characteristics Have identical hot-working counterparts Primary reason for cold deforming is better dimensional accuracy and surface finish Equipment must be more powerful due to material starting strength and effects of strain hardening Types of Squeezing Processes Cold Rolling Sizing Peening Swaging Riveting Burnishing Cold Forging Staking Hubbing Extrusion Coining Thread Rolling 3

2 4 Cold Rolling Cold Rolling Process using rolls to cold deform metal to obtain a reduced cross section Products: Sheet Stock Strip Stock Bar Stock Rod Stock Dead Soft Skin-Rolled (1/2 to 1% Reduction) Quarter Hard Half Hard (Up to 50% Reduction) Full Hard Alternative to machining or extrusion 20% additional strength possible Smooth surfaces and good precision Requires large volumes to justify expense Cold Rolling Swaging Swaging Process that, tapers, or points round bars or tubes by external hammering Both external and internal shaping (using a shaped mandrel) is possible Figures 19-1 Page 409, 19-2 Page 409, and 19-3 Page 409 Process Characteristics Quick process => High production rates Excellent surface finish Good dimensional accuracy 5 Cold Rolling Swaging Cold Forging Cold Forging Process where metal is cavity of the desired shape Figure 19-4 & 5 Page 410 (Basic Cold Forging Steps) 2 Types of cold forging processes: Cold Heading - Create enlarged sections on the end of rod or wire stock Upsetting - Create enlarged sections at locations other than the ends of rod or wire stock Process Characteristics Reduced waste High machine cost High production capability Good dimensional accuracy and excellent surface finish 6

3 7 Machining vs. Cold Forging Figure 19-6 Page 410 Cold Rolling Swaging Cold Forging Extrusion Extrusion Process where metal is to obtain desired cross sectional shape Types of cold extrusion processes: Impact or Cold Extrusion (Figure 19-8 Page 411) Forward Backward Hydrostatic Extrusion - Workpiece extruded through a die using high pressure fluid force (Figure Page 413) Continuous Extrusion - Process utilizing continuous feedstock (Figure Page 414) Roll Extrusion - Used to make thin walled cylinders (Figure Page 415) 8 Steps in Forming a Bolt Figure 19-9 Page 412 9

4 10 Steps in Cold Forming a Part Figure Page 412 Other Squeezing Processes Sizing - Squeezing areas of forgings, ductile castings, or P/M parts to achieve a desired thickness or precision Riveting - Head is formed on the end of a fastener for joining sheets or plates (Figure & 15 Page 415) - One part protrudes through another and is deformed using a punch to lock the parts together (Figure Page 416) Coining - Squeezing of metal while all surfaces are confined within a set of dies (Figure Page 416) Hubbing - Process used to form cavities in female dies (Figure Page 416) Thread Rolling - Threads formed by rolling a thread blank between hardened dies Processes used to improve or alter metal surfaces: Peening - Mechanical working of surfaces by blasting with steel shot or hammering with a round nose tool - Rubbing a smooth hard object over a surface to remove small surface protrusions (Figure Page 417) 11 Squeezing > Bending Shearing Drawing Bending Operations Plastic deformation about a linear axis with little or no change in surface area (Figure Page 418) Bending Terms Forming - Multiple bends made simultaneously with a single die Neutral Axis - Material is neither compressed or stretched - Materials tendency to unbend or restore Bending Design Factors Directional properties - Part orientation should be considered Minimum bend radius - Smallest radius formable without cracking Flat blank length - Produce the desired dimensions and precision Minimum leg length -Necessary to minimize distortion Realistic tolerances - Not less than 1/32 (0.030) 12

5 13 Squeezing > Bending Shearing Drawing Types of Bending Processes Angle Bending Bar Folder (Figure Page 418) Press Brake (Figure Page 419) Roll Bending - Forming to desired curvature using forming rolls (Figure Page 422) Draw / Compression Bending (Figure Page 422) Cold Roll Forming - Forming complex sections from flat strip (Figure Page 423) Seaming / Flanging Sheet metal forming operation for joining edges (Figure Page 424) Straightening - Forming process to flatten or straighten material Roll Straightening Stretcher Leveling Squeezing Bending > Shearing Drawing Shearing Operations Mechanical cutting of materials without the formation of chips or melting the materials 2 Primary Stages of Shearing deformation Figure Page Types of Shearing Processes Straight Cutting Blade (Simple Shearing) Square Shearing - Sheets of metal are sheared along a straight line Slitting - Process for cutting strip stock into narrower widths Fineblanking - Process that produces a smooth and square edge part Fineblanked Part Characteristics Usually less than 1/4 in thickness Complex shaped perimeters Close tolerances achievable Curved or Closed Cutting Blade (Punch & Die) Types of Curved Blade Shearing Piercing - Piece being punched out is scrap Blanking - Piece being punched out is part Figure Page 426 Figures & 42 Page

6 16 Variations of Piercing & Blanking Notching - Edge of strip forms part of the piece punched out Shaving - Edge finishing operation to remove a small amount of metal (Improve dimensional accuracy or edge refinement) Trimming - Process to remove unwanted material Lancing - Piercing operation to allow easier follow-on operations Perforating - Piercing a large number of closely spaced holes Nibbling - Variation of notching used to cut contours Cutoff - Operation to separate a part from a continuous strip Dinking - Modified shearing operation for low strength materials Techniques for Reducing the Cutting Force Cutting forces can be reduced dramatically by adding a to the face of the punch Figure Page Piercing and Blanking Dies Basic components of a die set Die Set Punch Holder Die Shoe Figure Page 428 Bolster Plate Figure Page

7 19 Types of Dies Subpress Dies - Standardized self-contained die sets (Figure Page 430) Steel Rule Dies - Low cost dies for cutting thin nonferrous and nonmetal materials Dies - Multiple sets of punches and dies (stations) contained within the same die set (Figure Page 430) Compound Dies - Cutting from both sides of the part in a single station (Figure Page 431) Squeezing Bending Shearing > Drawing Drawing Operations Cold drawing refers to 2 types of processes depending on the starting material Wire, Rod, or Tubing Stock - Reduction of the cross section by pulling through a die Sheet Metal Stock - Forming of parts where plastic flow occurs over a curved axis or form Cold Drawing presents unique problems due to: Higher deformation forces Thinner metal Limited ductility Closer dimensional tolerances 20 Types of Drawing Processes Wire, Rod, & Tubing Stock Rod and Bar Drawing - (Figure Page 433) Tube Drawing - (Figure & 56 Page 433) Wire Drawing - (Figure & 58 Page 434) Sheet Metal Stock Spinning - (Figure & 61 Page 435) Stretch Forming (Figure Page 436) Rubber Tooling or Fluid Pressure Forming Guerin Process - Utilizes rubber tooling (Figure Page 439) Hydroforming - Utilizes hydraulic pressure (Figure & 76 Page 441) Sheet Metal Drawing - (Figures & 69 Pages 437 & 438) Shallow Drawing - Part depth is less than diameter Deep Drawing - Part depth is greater than diameter 21

8 22 Press Classification Things to consider when selecting a punch press: Capacity (Tonnage) Type of Power (Mechanical or Hydraulic) Type of Drive Mechanical Drive - Faster motion and better displacement control Hydraulic Drive - Accurate control of forces and more flexibility Type of Frame Arch-Frame Gap-Frame or C-Frame (Figure Page 448) Straight Sided Frame (Figure Page 449) Chapter 19 Cold Working Processes Benefits of cold-working processes Basic knowledge of the four major categories of cold-working operations and general knowledge of the types of processes within the categories General understanding of presses and the different types Review Questions: 23