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1 This PDF document is a sample chapter from the book... To order this book, complete and print out the order form on the last page of this document. Then mail, fax, or phone in your order. Copyright 2002 Society of Manufacturing Engineers

2 Process Planning 1 Process Planning Process planning the systematic determination of the methods by which a product is to be manufactured economically and competitively is an intermediate stage between design and manufacture. It synthesizes such factors as: functional requirements of the product; volume of output needed; operations; tools, materials, and equipment necessary; and estimated manufacturing costs. Process planning provides specifications for the proposed manufacturing line of action on process sheets that designate, in appropriate detail, the most efficient sequence of operations, facilities, and tools required to manufacture the product. No one method of process planning fits all plants, and many methods are discussed in various other reference books that deal specifically with this aspect of manufacturing. However, in bending and forming operations there are certain steps in process planning that merit discussion due to the specialized nature of the steps. MATERIALS SUITABLE FOR BENDING Bending machines today are widely used for: cold bending of extrusions; solid rod and bar; moldings and rolled shapes; and tubing and pipe. 1

3 Tube Forming Processes: A Comprehensive Guide Generally, most common metals can be cold bent, providing they have sufficient elongation to achieve the desired angle and radius before reaching their ultimate strength. Metals commonly formed without difficulty include low carbon and stainless steel, aluminum, brass, and copper. Simple forming operations also can be performed on magnesium, titanium, and certain copper/nickel alloys. Special tooling and bending techniques allow bending some of the so-called exotic and refractory metals. Steel Steel is the most common material formed on bending machines, and those types of steel with a carbon content of 0.35% or less are the most practical for production work. With a carbon content above 0.35%, work hardening occurs rapidly as a bend progresses. Scrap losses due to breakage can be considerable. As carbon content increases, bend radii should be enlarged, and the angle a piece is bent should be as small as possible. A second factor determining the suitability of steel for a particular application is hardness. Steels with a Rockwell rating of or less on the B scale are best for production. Harder materials, generally, do not have sufficient elongation to allow bending before fracture. The American Iron and Steel Institute (AISI) and the Society of Automotive Engineers (SAE) have established specifications covering the so-called standard steels, and carbon and alloy steels. The AISI designation system for standard carbon and alloy steels is: 1. The first two digits of the four-numeral series indicate the grade of steel. 2. The last two digits indicate (as far as is feasible) the approximate middle of the carbon range. It is necessary, however, to deviate from this and to interpolate numbers in the case of some carbon ranges and for variations in manganese, phosphorus, sulfur, chromium, and other alloying elements. An abridgement of the AISI standard carbon and alloy steels number code is shown in Table

4 Process Planning Series Designation 10xx 11xx 12xx Table 1-1. AISI standard carbon and alloy steels (Courtesy American Iron & Steel Institute) Types and Classes Basic and acid open-hearth and acid-bessemer carbon-steel grades, nonsulphurized and nonphosphorized Basic open-hearth and acid-bessemer carbon-steel grades, resulphurized and rephosphorized Basic, open-hearth carbon-steel grades, rephosphorized and resulphurized 13xx Manganese % 23xx Nickel 3.50% 25xx Nickel 5.00% 31xx Nickel 1.25%, chromium 0.60% 32xx Nickel 1.75%, chromium 1.00% 33xx Nickel 3.50%, chromium 1.50% 40xx Molybdenum 41xx Chromium-molybdenum 43xx Nickel-chromium-molybdenum 46xx Nickel 1.65%, molybdenum 0.25% 48xx Nickel 3.25%, molybdenum 0.25% 51xx Medium chromium 52xx Chromium, high-carbon 61xx Chromium-vanadium 86xx Nickel 0.55%, chromium 0.50%, molybdenum 0.20% 87xx Nickel 0.55%, chromium 0.50%, molybdenum 0.25% 92xx Manganese 0.80%, silicon 2.00% 93xx Nickel 3.25%, chromium 1.20%, molybdenum 0.12% 94xx Manganese %, nickel 0.45%, chromium 0.40%, molybdenum 0.12% 97xx Nickel 0.55%, chromium 0.17%, molybdenum 0.20% 98xx Nickel 1.00%, chromium 0.80%, molybdenum 0.25% The letters prefixed to the full series numbers of a given steel to designate the metallurgical process used are: A basic open-hearth alloy steel; B acid-bessemer carbon steel; C basic open-hearth carbon steels; and E electric-furnace steel. 3

5 Tube Forming Processes: A Comprehensive Guide Plain Carbon Steels On the basis of carbon content, plain carbon steels may be divided into three groups as follows: 1. low-carbon steels with a carbon content of between 0.05% and approximately 0.30%. 2. medium-carbon steels with a carbon content between 0.30% and approximately 0.70%. 3. high-carbon steels containing from 0.70% to approximately 1.30% carbon. Material in which carbon is not uniformly distributed, such as angles or reinforcing bars re-rolled from railroad rail, should be bent to the most generous radii possible, and to the smallest angle. There will be considerable scrap since high-carbon areas have insufficient elongation to allow bending. Such material should be avoided if at all possible. Alloy Steels Alloying elements are added to ordinary steels to modify their behavior during heat treatment, which, in turn, results in improvement of mechanical and physical properties. In selecting alloy steel for a particular application, the steel chosen should contain alloy content no greater than is necessary to meet operating conditions satisfactorily. Low-alloy steels. The low-alloy grades of steel may be divided into two distinct groups: 1. High-strength structural steels where the alloying elements serve principally to strengthen the ferrite. Such steels are used in the as-rolled condition without heat treatment other than normalizing or annealing. 2. AISI or SAE steels of higher quality than ordinary structural grades where alloying elements serve primarily to improve mechanical properties over equivalent carbon steel, and to enhance response of the steel to heat treatment. Structural grades. High-strength structural steels are used principally in the transportation and construction industries for 4

6 Process Planning applications where steel possessing moderately high strength is required and where weight reduction may prove beneficial. The carbon content is generally less than 0.15% although, in some of the higher-strength varieties, the steel may contain as much as 0.30% carbon. While these higher carbon grades have improved strength, they are less ductile and harder to form. Corrosion resistance, an important consideration in reduced weight-structures, is somewhat superior to that of equivalent carbon steels. This added corrosion resistance is attributed to phosphorus and copper. AISI or SAE grades. Low-alloy machinery steels are generally characterized by high tensile strength, good ductility, and excellent toughness when appropriately heat-treated. The AISI alloy steels are used particularly in the automotive and aircraft industries for highly stressed members and moving engine parts. Certain combinations of various alloying elements can, after appropriate heat treatment, impart to a particular steel unique and specialized characteristics for use in a specific application. For example, carbon-molybdenum and other molybdenum-bearing steels possess good creep characteristics and, therefore, find useful application for moderately high-temperature service where oxidation is not too severe. Typical applications are found in piping for steam and oil refineries. The nickel-chromium steels, as a group, exhibit excellent hardenability, high strength, good wear resistance, and toughness. The various nickel-chromium combinations, properly heat-treated, have tensile properties equivalent to the entire range available with alloy steels. The chromium-vanadium steels, after heat treatment, show remarkable toughness and good fatigue resistance. Ultra-high-strength alloy steels. Several steels have been specifically developed and applied at strength levels of 200, ,000 psi (1,379 2,068 MPa). Types 4140 and 4340 are examples. Modifications of these SAE grades containing higher silicon have been commercially used, generally in the specific strength range of 220, ,000 psi (1,517 1,724 MPa). Types 4130 and 4140 steel are suitable for bending operations when higher strengths are desirable. Both are high-carbon steels that should be used with the most generous radius possible. Since both work-harden rapidly as a bend is made, bend angles should be kept small to keep scrap to a minimum. Of the two grades, type 5

7 Tube Forming Processes: A Comprehensive Guide 4140 will prove the more difficult to work with and will require a larger radius than the same size piece made of type This is due to its higher carbon content. Stainless steels. This family of steels is described by composition as containing 4% or more chromium, generally more than 50% iron, and perhaps such alloys as nickel, molybdenum, columbium, titanium, manganese, sulfur, and selenium. These alloys confer specific characteristics to enhance corrosion and scaling resistance, mechanical properties, and formability at subzero, room, and elevated temperatures. Stainless steel, in most common grades, is routinely bent without problems. There are three broad metallurgical classifications that have a marked bearing on their utility the martensitic, ferritic, and austenitic steels. Austenitic grades in the 300 series are most commonly formed; examples are types 304 and 321. Austenitic stainless steels are relatively easy to fabricate and weld. They are tough but ductile. They cannot be hardened by heat treatment; cold working alone can increase their strength. The ferritic steels are readily cold formed. However, a low rate of work hardening and relatively high yield strength combine to encourage localized thinning or necking down under tensile stresses. Caution must, therefore, be exercised in cold-forming operations that involve stretching. Since stainless usually has greater elongation than mild steel, it is generally capable of being formed to greater angles and on smaller radii than comparable carbon-steel material. As with all metals considered for bending, hardness should be considered when specifying stainless work. Best results are achieved when workpieces are between a complete soft condition and 1/4 hard. When surface appearance of a bend is of prime importance, a small amount of hardness should be present to preclude the appearance of stretch marks (sometimes called an orange-peel condition) on the outside of bends. Evidence indicates that stabilized stainless steel has more uniform characteristics and thus will form with less possibility of scrap. Since stainless steel is often used where the highest quality bends are required, tooling is often used that gives maximum control of metal flow through close material confinement. Material for such work, especially for ultra-thin-wall aircraft tube bending, must 6

8 Process Planning be held to close dimensional tolerances. Tubes with walls of approximately in. (1.24 mm) or heavier can be drawn to size. Lighter wall tubes should be procured on-size, since sizing operations work-harden such material excessively and produce minute wrinkles, rather than changing dimensions through metal flow. Stainless-steel shapes should be produced on a roll-forming machine if there is more than one break in a piece. Roll forming produces more uniform shapes. Heat-resisting superalloys. Many heat-resisting superalloys, developed for application at temperatures from 1,000 2,000 F, (538 1,093 C), have high-temperature strength properties that are superior to those of low-alloy steels and stainless steels. The three most important basic requirements of an alloy for high-temperature service are strength, surface stability, and structural stability. Austenitic stainless steels were used as a basis for the development of high-temperature superalloys. The three basic metal systems from which useful alloys evolved are iron, nickel, and cobalt. Because of the outstanding high-temperature strength of the superalloys, they are inherently difficult to deform by hot working, and many of them are sensitive to cracking during hot working. They also cause considerable wear on die and roll materials. The alloys are likewise quite difficult to cold form. Type 19-9 superalloy is used in manufacturing aircraft components. It will bend in a manner similar to more common stainless grades, but with its higher tensile strength, it resists compression on the inside of a bend. Instead it tends to form wrinkles. Careful consideration should be given to using a bending machine with adequate power to form the bends using a tooling setup that provides maximum stretch over bend circumference. Tooling must be precisely fitted to the workpiece and exceptionally hard so it will avoid being marked by the workpiece should wrinkles develop. Aluminum and Aluminum Alloys Aluminum is another commonly formed metal. Unalloyed aluminum has many desirable characteristics, including its light weight, pleasing appearance, malleability, formability, and resistance to corrosive attack by industrial and marine atmospheres, 7

9 Tube Forming Processes: A Comprehensive Guide many chemicals, and food products. It has good electrical, thermal, and reflective characteristics, but has relatively low strength and hardness levels. To increase its strength and hardness, three methods are used: addition of other elements to form alloys; heat treatment of some types of alloys; and strain hardening by cold work. Alloy Designations Aluminum and its alloys are designated commercially in the United States by a series of numerals or by numerals and letters assigned by the producer to indicate composition. The 1xxx group is assigned to the 99% minimum aluminum category. The last two digits are the same as the two digits to the right of the decimal point in the minimum aluminum percentage when it expressed to the nearest 0.01%. The second digit indicates modifications to the impurity limits: 0 indicates no special control on the individual impurities; and 1 9 (assigned consecutively) indicates special control of one or more individual impurities. The 2xxx through 8xxx alloy groups are assigned to the major alloying elements: copper, manganese, silicon, magnesium, zinc, and other elements, respectively. In these groups, the last two digits are assigned arbitrarily to identify the different aluminum alloys in the group. The second digit indicates alloy modifications: 0 indicates the original alloy; and 1 9, assigned consecutively, indicates alloy modifications. Temper Designations The designations for temper of aluminum alloys are based on the sequence of basic mechanical and thermal treatments used to produce the temper, but only those operations recognized as significantly influencing the characteristics of products are indicated. Should some other variation of the same sequence of basic operations be applied to the same alloy and result in different characteristics, additional digits are added to the designation. The temper designation follows the alloy designation and is separated from it by a dash. 8

10 Process Planning Depending on the temper, all alloys extruded as shapes or tubing, or rolled and welded into tube, are suitable for bending. The temper of heat-treated alloys should be T6 or less; cold-worked temper should be H-18 or softer. As in all metals, the harder, less ductile alloys will require larger bend radii for successful forming. Alloys commonly bent include , H-12, H-14; 5052 in the 0 condition; and T-6. Alloy 6063-T832 is commonly bent, but on radii at least times the tube diameter, and in a wall thickness of in. (0.89 mm) or larger. Although it might appear that alloys in a very soft condition are best for bending due to their greater elongation, the bending tools more easily mark very soft metals. A comparatively long clamp die length is necessary to distribute the clamping force over a wide area and eliminate workpiece distortion and tool marks. COPPER AND COPPER ALLOYS Wrought coppers and copper-base alloys are available in various degrees of hardness or tempers, such as spring, hard, halfhard, and quarter-hard, developed by an appropriate amount of cold working after the last anneal. In the annealed or soft condition, tempers are based on the grain-size specification, and grain size is a determining factor in the success of forming. Tensile strength, yield strengths, and elongation vary somewhat with the shape of the section. For flat products, the section is taken at in. (1.02-mm) thick if possible. For rod, the section is taken at 1.00-in. (25.4-mm) diameter, or if that is not available, to the nearest diameter for which there is available data. Yield strength is the stress corresponding to an extension of 0.50%. Data under a soft condition are for in. (0.05-mm) grain size or, if that is not available, the nearest grain size or anneal available. Commercially pure copper is available in several grades, all of which have essentially the same mechanical properties. The three most commonly used (all of the same purity but varying in some respects) are: electrolytic tough-pitch copper, deoxidized copper, and oxygen-free copper. 9

11 Tube Forming Processes: A Comprehensive Guide Copper tubes as extruded or extruded-and-drawn are bent by many fabricators. When considering copper for its formability, hardness is an important factor. Pieces in the range between fully annealed and half-hard are commonly used for small-radius bending for example, radii of approximately 1.5 times workpiece diameter and larger. Harder material will require bend radii two to three times the diameter or larger. Skin hardness imparted by a single light draw or sizing after the final anneal is considered most suitable because of the risk of possible tool marking in tempers 1/4 hard or less. When making critical small-radius bends in thinwall material, such as those used in U-shaped condenser tubes, grain size is important. Copper-base Alloys Binary alloys of copper and zinc are known as brasses, and alloys of copper and tin are bronzes. Some true brasses, solely because their color is similar to that of the copper-tin alloys, are called bronzes. Likewise, the term bronze is also used in modern metallurgy to refer to copper exhibiting a characteristic bronze color, to which elements other than tin are the principal alloying materials. Figure 1-1 plots the percent elongation and tensile strength of various chemical compositions of brass. Brass is widely used in bending, especially to manufacture plumbing waste traps and elbows. Fully annealed material is best for bending light-wall brass tubing to centerline radii that are one-totwo times the diameter. Often it is necessary to anneal only that material actually bent, leaving a length of hard tubing for clamping against the bending die. Larger radius bends in all grades of brass generally are made without annealing, and with no difficulty. MAGNESIUM AND MAGNESIUM ALLOYS The principal property of magnesium is lightness. While magnesium can be cold bent to some simple shapes with large radii, formability is so greatly improved at elevated temperatures that most working of magnesium is done hot. Small radius bends have been accomplished by heating the work to slightly elevated temperatures before forming. For example, tubing made of alloy AZ31B has been bent at room temperatures on a radius of four times the 10

12 Process Planning Figure 1-1. Influence of composition on certain mechanical properties of annealed wrought brasses. diameter. Working the same material in dies heated to 200 F (93 C) has made possible bends on a diameter of three times the radius. Heat ranges of F ( C) are commonly used. TITANIUM AND TITANIUM ALLOYS The strength-weight ratio for titanium exceeds that for most other engineering metals. As a result, titanium is finding increasing use in the aerospace industries where this ratio is a critical design factor. To conveniently differentiate between the various titanium and titanium-alloy compositions, available commercial grades can be classified as commercially pure titanium, all alpha (single-phase) weldable alloys, alpha-beta (two-phase) weldable alloys, and alpha-beta non-weldable alloys. A fifth group, the allbeta alloys, is available in sheet form. 11

13 Tube Forming Processes: A Comprehensive Guide Commercially pure titanium is an unalloyed composition containing over 99% titanium. The remaining percentage consists of carbon, oxygen, nitrogen, hydrogen, and iron. The amount of oxygen and nitrogen determine strength levels. Various grades are listed in the Appendix of this book. All grades are available in billets, bars, wire, sheet, strip, tubing, and some in extruded forms. There is one all-alpha weldable alloy in the commercial alloy group. This 5% Al, 2.5% Sn alloy is available as sheet, bar, and wire. Alpha-beta weldable alloys comprise the majority of titanium alloys. They are heat-treatable; all are available in bars and billets, and nearly all in sheets. Titanium alloys containing 6% Al and 4% V were developed for forging and are available in wrought mill shapes. Alpha-beta, non-weldable alloys are non-weldable by fusion welding; flash or spot welding may be practical for some. They are available in bar, wire, extrusions, sheet, and forgings. Although only a limited amount of titanium has been formed on bending machines, experience indicates that certain grades of titanium tubing can be bent. For best results, titanium for bending should be fully annealed, commercially pure alloy A-40. The annealing process is very critical and may vary between tubing suppliers, individual workpieces, and even between sections of the same tube. In diameters over 3-in. (76.2-mm) outside diameter, best results have been obtained by bending titanium at elevated temperatures of F ( C). This is accomplished on the bending machine itself by electrically heating the pressure die and mandrel body. A pressure die booster is applied in many instances. By exercising close material quality and temperature control, fabricators presently have formed thousands of bends. These include bends such as: a 1.50-in. (38.1-mm) diameter; in. (1.25-mm) wall on a 2-in. (50.8-mm) centerline radius to 90 ; and a 1.25-in. (31.8-mm) diameter; in. (0.89-mm) wall on a 1.50-in. (38.1-mm) centerline radius to 110. NICKEL AND HIGH-NICKEL ALLOYS All nickel and high-nickel alloys have nickel as the major element, except Incoloy, an iron-nickel-chromium alloy, and Ni-O- Nel, a nickel-iron-chromium alloy. The high-nickel alloys are 12

14 Process Planning designed for specific service applications involving high corrosion and/or oxidation resistance in a broad range of temperatures. In addition, moderate anti-galling characteristics are designed into several of the cast alloys. Nickel alloys are divided into five main groups having the following typical characteristics and applications. Group 1 is a commercially pure nickel for chemical equipment, electrical uses, high temperatures, and corrosion resistance. It is also produced in cast form. Group 2, Monel, is a nickel-copper alloy for general applications requiring corrosion resistance in addition to toughness and high strength. Monel is quite suitable for bending and is usually approached in a manner similar to common grades of stainless steel. Group 3, Inconel, is a nickel-chromium, heat-and corrosionresisting alloy able to withstand temperatures up to 2,200 F (1,204 C). It has a high hot strength, is resistant to progressive oxidation and fatigue, and is non-magnetic. This alloy is also produced in cast form. Group 4, Incoloy 901, is an age-hardenable nickel-iron-chromium alloy used for aircraft and industrial components requiring low creep and high rupture properties in the temperature range of 1,000 1,400 F ( C). Group 5, Incoloy, is an iron-nickel-chromium, oxidation- and heat-resistant alloy that also resists moderately sulphid-izing atmospheres, green rot, molten cyanide salts, and fused neutral salts at high temperatures. The average room-temperature mechanical properties of the wrought high-nickel alloys normally used for subzero-, room-, and elevated-temperature service are given in the Appendix of this book. The cold work-hardening characteristics of nickel versus other metals are shown in Figure 1-2. COLD-BENDING SUITABILITY In considering any material for its cold-bending suitability, a general rule is to use the following equation as a guide to determining the elongation necessary in a metal to make a given bend. 13

15 Tube Forming Processes: A Comprehensive Guide Figure 1-2. Increase in hardness of various metals and alloys with cold working. 0.50D E = R where: (1-1) E = necessary elongation, % D = outside diameter of the material, in. (mm) R = radius of the bend to the centerline, in. (mm) Then compare the calculated elongation factor with the published elongation factor for that metal, either in the Appendix of this book, or similar tables found in other reference books or handbooks. It is quite common to make quality bends where the calculated elongation exceeds the published figure. However, it is unwise to exceed 14

16 Process Planning the published figure too far, such as attempting a bend requiring approximately a 50% elongation in a metal having only 10%. MATERIAL SHAPES AND FINISHES Apart from material specifications, the shape of the workpiece should be considered. In tubing, welded tube is often preferred over seamless mechanical material because closer tolerances are maintained between the outside diameter and inside diameter of the tubing. This is particularly important in lighter-wall tubing where a mandrel must be used inside the tube to support the walls during the bend. Shaped sections formed by hot rolling are preferred over coldrolled materials because hot working leaves a greater elongation percentage and thus allows bending to smaller radii and greater angles without excessive breakage. Bending machines are capable of producing a wide variety of material shapes. Tubing Tubing is the most commonly bent material shape. For quality bends and long tool life, round-welded tubing in either steel or aluminum should be procured as close to the specified diameter and as round as is possible with modern tube mill processes. Holding such quality control standards will result in consistent accuracy, mar-free bends, and lower scrap rates. Weld flash must be considered if an internal mandrel is used to support the tube during bending. For critical bends, either flashremoved tubing should be used or the mandrel must be grooved to accommodate the flash. Flash-in tubing is most commonly used and the mandrel is made undersize to accommodate the flash. Flattening of the bend equal to the mandrel clearance can be expected. In addition, tube lengths that have a heavy burr or dimple left from the cutting operation may require that the ends be deburred or de-dimpled, depending on the mandrel clearance and amount of burr. 15

17 Tube Forming Processes: A Comprehensive Guide Tubes should be free from abrasive dust, such as that left by an abrasive wheel cutoff. This is particularly true of abrasives left inside a tube to be bent over a mandrel because such dust will wear this tool excessively or cause pickup and breakage. Excessive rust or dirt inside steel tubing can cause this same problem. Because of its physical properties, aluminum tube may have a coat of oxide both inside and out. Tubing with only minimum oxide should be used since the oxide is extremely abrasive and will shorten tool life considerably. Square or Rectangular Welded Tubing Square or rectangular welded tubing, in either steel or aluminum, should receive much the same consideration as round tubing, but with additional emphasis. Good material is held to uniform dimensions. It is almost mandatory that a mandrel be used in this work and that it fit the inside dimensions of the tube with only a few thousandths of an inch (micrometer) clearance overall. Thus, the corner radii of the tube must be held uniform. If any weld flash is present, the mandrel must be grooved to accept the flash. Since the mandrel cannot be rotated out of plane, the tube must be procured with the weld flash running consistently along one point of the tube, preferably in the center of one side. Seamless Tubing Produced in steel, aluminum, copper, and brass, seamless tubing should be selected for bending based on the criteria of material uniformity and freedom from scale or surface oxide. In seamless steel tubing, the wall thickness often varies considerably, resulting in varied inside-diameter dimensions. This condition makes it difficult to obtain maximum effectiveness from use of an internal mandrel if such is required. In addition, concentricity of the inside and outside diameters of seamless steel is usually not consistent, which can lead to sporadic appearance of wrinkles, excessive flattening, or inconsistent bend-angle accuracy. Finally, 16

18 Process Planning seamless steel tubing sometimes varies in hardness, which results in breakage or inconsistent springback of the bends. Seamless aluminum tubing is produced by an extrusion process and should be checked for the same variance in wall thickness and/or inside and outside diameter concentricity as steel. Usually aluminum, as extruded, is of uniform hardness. Drawing after extrusion produces seamless aluminum. This eliminates inaccuracies and produces uniform tubing that presents no special bending problems. Of course, such tubing should be kept free of tube-end dimples and burrs, and have minimum surface oxide. Copper and brass are most commonly bent as tubing that has been brought to its final form by drawing. It presents few problems as to shape. Best tool life is obtained if tubing is used with a minimum of surface oxide. Frequently, brass tubing must be annealed, either overall or in the specific bend area, to make it suitable for bending. If this annealing is done in a gas furnace or salt bath, the resulting film of oxide should be removed by pickling. This minimizes friction as the material is drawn over a mandrel or through other stationary dies. Lock-seam Tubing Lock-seam tubing requires closer control than seamless or welded tubing, but is readily formed on bending machines. Attention to two factors in the manufacture or purchase specification of lockseam tubing greatly facilitates bending. First, the seam should be rolled to as tight a lock as possible. To check for seam tightness, grasp an approximately 3-ft (0.9-m) length of the material at either end and twist the tube. If the seam lock is loose, it will produce a squeak or cracking noise or be felt to shift. Such tubing flattens considerably more and produces a higher scrap rate than quality material rolled to a tight lock. Second, variation in the outside diameter of the tube increases bending problems because oversize or undersize material does not fit the bending tools closely enough to permit dies to control metal flow into a quality bend. This factor is usually controlled at the point of slitting the steel stock before it is rolled into tubing. 17

19 Tube Forming Processes: A Comprehensive Guide Stainless-clad and Butt-seam or Open-seam Tubing Stainless-clad and butt or open seams are two relatively uncommon types of tubing formed on bending machines. Stainlessclad tubing is usually made by roll forming a sheet of stainless steel over an open-seam, mild-steel tube. The stainless steel is held in place by a lock seam rolled into the open area of the mildsteel tube. This material is sometimes used where high-volume production offsets the additional difficulty of bending stainlessclad tubing. When considered for use, best results are obtained with material having a stainless layer in. (0.51 mm) in thickness or heavier, and where the two layers are rolled together as solidly as possible. Though not a direct consideration in material selection, it should be kept in mind that bending stainless-clad tubing usually requires use of a wiper die of aluminum-bronze, a mandrel, and unusually high tooling pressures. These factors, when combined, may result in high tooling costs. In addition, the locked seam in the tube must be located on the direct inside or outside of the bend, thus limiting application of this material. Butt-seam material is formed from strip stock rolled into a tubular shape, but without welding or otherwise fusing the seam. To obtain best results, such material should be free from scale and rolled to within close tolerances on the diameter. A mandrel is almost always required and the open seam must be located either on the direct inside or outside of the bend, limiting the planes in which bends can be made. Often the additional difficulty in bending this material more than offsets any savings in material cost over comparable welded-steel tubing. Decorative Finishes Certain finishes are often applied to material (most commonly aluminum, welded steel, or lock-seam tubing) prior to bending. Most prepainted or other precoated tubing can be bent without marring the finish. Nearly all of the paint or coating material used has sufficient elasticity to resist cracking or chipping as the metal stretches or compresses in the bend. Pre-anodized aluminum tubing (plus extrusions or shapes) is also commonly bent without marring or disfiguring the finish, or adding any special tooling or handling problems. 18

20 Process Planning Although not usually considered a decorative-type finish, pregalvanized material is also readily bent without scratching the zinc coating. No great degree of success has ever been achieved, however, with preplated material because the plating is usually marked, cracked, or chipped in the bend area. Pipe Common pipe in all weight schedules and sizes is one of the most frequently and easily bent materials formed on bending machines. For longest tool life, pipe should be obtained with as little scale or dirt, both inside and out, as possible. This is especially important when a mandrel is required. Excessive scale can bind against a mandrel to the point where the bending machine will stall or break the pipe. Rod and Bar Almost all metals suitable for bending present few forming problems, provided they have sufficient elongation to bend to the required radius and angle without fracturing. In ferrous metals, hot-rolled bar usually has better elongation and thus will withstand more severe bends. Such material, however, may have excessive scale, a condition that should be avoided if possible to prolong machine and tool life. Cold-rolled bar has less surface oxide but, because of the stresses remaining after cold rolling, may creep or distort after bending. In addition, cold-rolled bar is usually somewhat work-hardened and therefore will not withstand as severe a bend as comparable hot-worked material. Sections Shapes and sections in many configurations are commonly bent. Examples include T-shaped rolled sections used as sink rim water seals, garage door tracks, and standard steel channel used in truck frames. To obtain the best results, shapes and sections should be clean, free from excess scale or other surface oxide, and uniform in their manufacture. Uniformity between various pieces and material lots is essential since workpieces must fit the bending 19

21 Tube Forming Processes: A Comprehensive Guide dies within close tolerances to obtain smooth, wrinkle-free bends. Because of their greater uniformity, shapes formed on roll-forming machines are usually bent with less difficulty than brakeformed shapes. Extrusions The majority of extruded shapes are suitable for bending, provided the major segments of the shape are approximately equal in their length and thickness. Both stretch forming and draw bending can be used to bend extruded sections. Large, irregular shapes are usually stretch-formed, while draw bending handles the slightly smaller, more symmetrical extrusions. DESIGNING BENDS The proper design of tubular parts incorporating bends can contribute greatly to production efficiency and low unit costs. Select a Reasonable Radius Usually a reasonable radius means a bend centerline radius that is an even multiple of the outside diameter of the tube. Radii would be selected as 3 D, 2 D, or 1 D, where D is the outside diameter of the tube. For example, in a 2-in. (50.8-mm) diameter tube, a 2D bend would be made on a 4-in. (101.6-mm) centerline radius. Occasionally a slight deviation from this rule of thumb will help keep bend radii simple, as in the case of making bends in 5/16-in. (7.9-mm) outside diameter tubing on a 3D radius. It is only common sense to specify a 1-in. (25.4-mm) centerline radius instead of 15/16-in. (23.8-mm) centerline radius, thereby keeping tooling, production, and record-keeping operations as simple as possible. Selecting a radius that is an even multiple of the tube diameter reduces the amount of money invested in tooling by avoiding the possibility of having a number of sets of dies for the same tube diameter, each made to produce a random radius. In addition, there is less chance for error in manufacturing the tooling. Keeping design consistent will reduce the lead time required to make or buy the proper tooling. 20

22 Process Planning Standardized tooling is also important from the standpoint of the machine operator. Proficiency increases as the operator becomes accustomed to the performance of certain tools. The purchase of a different design of tool would require another period of trial and error with regard to tool alignment, effect on material, etc. Many firms in a variety of industries, including aircraft, have carried this standardization of radius a step further and realized extensive savings of tool costs. These firms have standardized on a 2D-centerline radius. With today s precision bending machines, production bends often are made on radii of 1D in many materials. However, a 2D radius represents a reasonable balance between production speed, tooling cost, and the assembly space required by a bend. With this minimum practical radius, there is less chance for product redesign outmoding existing tooling and necessitating new tooling at additional expense. Bends on larger radii, such as 3D, 4D, 5D, or larger, may be preferable from the standpoint of design or production. Larger radii require less elongation and take slightly less material. For example, as shown in Figure 1-3, a bend radius of 4D to centerline in 2-in. (50.8-mm) tubing requires 1.71 in. (43.4 mm) less material than a 2D bend. These savings are often more than offset by the additional space required for the larger radius bend shown in Figure 1-3. Whatever multiples are chosen for radii, it is more economical to design to standards at every opportunity. Figure 1-3. Larger bend radii use less material. 21

23 Tube Forming Processes: A Comprehensive Guide Specify Bend Radii to Tube Centerline It is general practice to indicate bend radii on prints or drawings to the center of round tubing or pipe, or round solid stock. Radii for square or rectangular tubing or solid bar should be shown to the inside of the bend, or to a major face line of an extrusion, molding, or other shaped section. Following this procedure can help avoid error when ordering tooling from suppliers. Make All Bends in One Workpiece to the Same Radius If it is possible, making all bends in one workpiece to the same radius usually allows the machine operator to produce a completed part with a quick progression of bends. Setup time and handling are minimized, production is increased, and unit costs tend to be lower with this method. Often, a single large-radius bend can be replaced with two bends on the smaller common radius with savings more than offsetting the cost of the additional machine cycle. Allow Sufficient Clamp Length Between Bends When at all possible, avoid compound bends. A compound bend is one designed so close to adjacent bends that it does not allow sufficient straight material for clamping between bends. Consequently, material slips in the clamp during the second bend. This almost always results in wrinkles. The amount of clamp length required to distribute pressure over sufficient area to prevent distortion or collapse of the tube depends on material type and grade, diameter, wall thickness, surface condition, and radius of the required bend. Other factors that help determine clamp length include: marking of the work due to high clamping pressures distributed over a minimum area; amount and rate of tool wear; type of mandrel and use; number of balls used on multi-ball mandrels; and scrap loss due to extreme clamp lengths. 22

24 Process Planning In certain cases, special provisions may be necessary to keep the tube from slipping. Such provisions include use of a serrated or knurled clamp die (and clamping insert of the bending die). Other techniques involve inserting resin, abrasive dust, or abrasive-impregnated cloth into the clamping area. These aids increase the coefficient of friction between the tube and clamping dies with minimal marking of the work. In some extreme cases, even the longest practical clamp length and insertion of extra friction-producing elements into the clamp area does not prevent slippage. This is often the case in bending thin-wall stainless-steel tubing, such as 6-in. (152.4-mm) outside diameter in. (0.50-mm) wall tubing to 120 on a 12-in. (304.8-mm) centerline radius. Here, cleats are used in both the clamping portion of the bending die and the clamp die. Hard, knifelike cleats penetrate the tube as the clamp die closes, eliminating slippage. This method of minimizing clamp length and/or stopping slippage is used only where the cleat-marked clamp length is later cut off. Obviously, so many variables and special conditions govern the amount of straight material needed for clamping that it is not practical to publish a complete or inviolable chart of clamp lengths. Table 1-2 is a starting point for determining clamp length in tubing. The diameter multiples presented in this table are not absolute, but represent a conservative clamp-die length that yields a quality bend. Specific clamp-die lengths for a given tube outside diameter, wall thickness, and centerline radius can be found in Table 1-3. If bend tangents must be so close together that a straight clamp of sufficient length cannot be used, then the part can be fabricated in one piece by using compound tools. A clamp die incorporating a groove curved to fit the previous bend (radius, angle, and plane), plus a bending die with a similar curve in the clamping insert, must be produced. These tools are considerably more difficult and expensive to manufacture than comparable tools with straight clamp sections. They can usually be used only on the job for which they were specifically designed, and it is necessary to handle the tube separately for each compound bend, thus increasing production costs. 23

25 Tube Forming Processes: A Comprehensive Guide Table 1-2. Guide to clamp length Centerline Wall Thickness Radius of Bend of Tube, in. (mm) Clamp Length 1D Up to (0.89) 4 to 5 diameter ( ) 3 to 4 diameter Over (1.65) 2 to 3 diameter 2D Up to (0.89) 3 to 4 diameter ( ) 2 to 3 diameter Over (1.65) 1-1/2 to 2-1/2 diameter 3D Up to (1.65) 2 to 3 diameter Over (1.65) 1 to 2 diameter Find the Minimum Centerline Radius of the Bend The minimum radius to which a tube can be bent is a function of the elongation of the material. If the outside of the bend is stretched beyond maximum elongation, it breaks. The formula that follows should be used only as a guide to the minimum bend radius possible. It does not take into consideration friction between tube and tools. The elongation percentage used is derived from a test, which is not quite the same as stretching the outer periphery of the tube during bending. The equation to determine the minimum radius of a bend is: 0.50D R = (1-2) E where: R = minimum centerline radius of bend, in. (mm) D = outside diameter of the tube, in. (mm) E = elongation in 2 in. (50.8 mm), % For example: 2-in. (50.8-mm) outside diameter type 321 stainless-steel tube, 40% elongation R = =

26 25 Table 1-3. Clamp die lengths* Outside Diameter and Wall Thickness, in. 1/2 to 5/8 3/4 to 7/8 Centerline Radius, in /2 2-1/2 2-1/ /2 1 3/4 5/8 2-1/ /4 3/ /2 1 3/4 3/ /2 2-1/ / /2 3/4 3/4 5/ /2 2-1/ /2 1-1/2 1 3/4 5/8 1/ /2 2-1/ /8 1-1/2 1-1/2 1 5/8 5/8 1/ /2 2-1/ / /8 5/ / / / /4 5/8 1/2 2-1/2 2-1/ / / /4 1/2 2-1/2 2-1/ / /4 1/2 2-1/2 2-1/ / /2 3/4 1/2 2-1/ / / / / / /2 1-1/ /2 1-1/ /2 1-1/ /2 1-1/ / Process Planning

27 26 Table 1-3. (continued) Outside Diameter and Wall Thickness, in. 1 to 1-1/8 1-1/4 to 1-3/8 Centerline Radius, in /2 5/8 3/4 7/ / / / / / / / / / / / / / / /2 5-1/ / /2 2-1/ / / / / Tube Forming Processes: A Comprehensive Guide

28 27 Table 1-3. (continued) Outside Diameter and Wall Thickness, in. Centerline 1-1/2 to 1-5/8 1-3/4 to 2 Radius, in /2 5/8 3/4 7/ /8 1-1/4 1-1/ / CP CP CP P CP CP CP / CP CP P CP CP / CP CP CP P CP P P-8 P Process Planning

29 28 Table 1-3. (continued) Outside Diameter and Wall Thickness, in. 2-1/4 to 2-1/2 2-3/4 to 3 Centerline Radius, in /2 5/8 3/4 7/ /8 1-1/4 1-1/2 1-3/ /2 CP CP CP P CP CP P-10 P CP CP CP CP P /2 CP CP P-10 P CP CP CP P-10 P CP CP P-8 P CP CP CP P-10 P CP P CP CP P-10 P CP P CP CP P CP P CP CP P P-10 P CP P-10 P P-10 P CP P P-8 P CP P P-8 P P-10 P P P * For all materials except soft aluminum and dead soft copper. Key:C = cleated clamp, P = clamping plug, CP = both cleat and plug Note: 1 in. = 25.4 mm Tube Forming Processes: A Comprehensive Guide

30 Process Planning This example indicates that 2.50 in. (63.5 mm) to centerline is the minimum radius bend that can be achieved. However, actual experience has proved that bends can be made very successfully on a 1D-centerline radius (a radius equal to the tube diameter). Many bending machines are now forming bends on a production basis on radii smaller than indicated by the published elongation factor, such as making 1D bends in stainless and mild steel, copper, and brass. Despite the inexactness of Equation 1-2, it does provide a practical guide. It is not recommended that the radius attempted for production bending be much smaller than that calculated. Find the Tube Wall Reduction After Bending (Thinning) The amount that the outside wall of a tube is reduced or thinned in the bend area is dependent upon the ratio between the centerline radius of the bend and tube diameter. In practice, this relationship is directly influenced by friction and the amount of flattening allowed in the bend. Friction is introduced as the tube is pulled over a mandrel or wiper die. The force exerted by the pressure die, the method of mounting this die, the fit of the tools to the tube, the type and surface finish of the tube, and the use of lubricant on the tools or tube directly affect the amount of friction. Friction is always present in various amounts in any bending machine setup and tends to increase the amount of thinning experienced. Flattening (the tendency for a tube to assume a somewhat oval shape) is also experienced to some small degree in all bends, and its presence tends to offset the friction factor. Flattening is dependent on the material type and grade, wall thickness, bend angle and radius, and the introduction of various types of internal mandrel support. Thus, it cannot be mathematically calculated in advance. Disregarding friction and flattening and using the measurable factors of starting wall thickness, tube diameter, and bend radius Table 1-4 can be used as a general guide to the amount of wall thinning that can be expected in a given bend. Because it does not consider friction or flattening, Table 1-4 should be used only as a starting point. The friction-producing elements in the tooling setup plus the factors that reduce or offset friction should 29