Understanding that drawing a tube requires more than just pointing and drawing is a good start to a successful draw. Procuring the raw material, pointing, drawing, straightening, and finishing are the five steps fabricators need to keep in mind.
Figure 1: Rod drawing uses a die to determine the tube’s OD and a rod to determine the tube’s ID.
Drawing a tube from one size to another sounds simple. The process has two main steps: crushing one end (also known as pointing the tube), then drawing it through a die that has the correct ID. When the process is finished, the tube’s OD matches the die’s ID.
In reality, it’s much more complicated than that. A successful draw is a product of five distinct steps:
- Procuring the raw material
- Preparing it for drawing
- Finishing and final inspection
Tubing for redrawing can be either welded or seamless. The redrawing process for each is essentially the same; therefore, processes described in this article apply to both.
Welded tubing is produced from strip that has been rolled, slit, and coiled. After the coil is delivered to the tube production facility, it is uncoiled and fed into a mill that forms it into a tubular shape and the resultant seam is welded. Carbon and low-alloy steels usually are electric resistance welded (ERW), whereas stainless steels are gas tungsten arc welded (GTAW).
Seamless tubing may originate from pierced tubing (carbon or low-alloy steel) or extrusions (stainless, high-alloy steels, and nickel-based alloys). They may be further processed by pilgering or reducing. Another raw material is a drilled bar, which usually is used for special alloys or tolerances.
While the equipment and procedures discussed here may be applicable to most alloys, they are aimed primarily at carbon and low-alloy steel, stainless steel, and nickel-based alloys. Copper and aluminum usually are produced by high-volume processes, while titanium and zirconium alloys are better suited to low-volume, specialized processes such as pilgering and tube rolling.
Drawing begins with procuring the raw material. The purchase order should specify the material’s chemistry and dimensions including tolerances—size, wall thickness, concentricity, and straightness. In most cases, the annealed properties are specified for maximum softness. These requirements may be included in a proprietary specification or an ASTM, AMS, or MIL code or specification.
The next step is pointing, which is the process of decreasing the diameter of several inches of material at the tube’s end so it can enter the drawing die. The three most common methods for pointing are push pointing, rotary swaging, and squeeze pointing. In some cases, phosphate coating or soap film is applied before drawing.
Draw benches are usually mechanical and have three components: a back bench, die head, and front section. Jaws on a trolley grip the tube and a hook on the back of the trolley engages a moving chain, pulling the tube through a die. Dies are most commonly sintered tungsten carbide inserts with a cobalt binder that have been shrunk-fit into a steel casing.
Tubes are drawn to a finished size using one or more of the following operations:
Figure 5: Straighteners use bending forces and a rolling motion to straighten the tube. Common configurations use either six or 10 rolls.
- Rod or mandrel drawing
- Plug drawing, including fixed, floating, and semifloating (tethered)
Rod Drawing. During rod drawing, a hardened steel mandrel is inserted into the bore of the tube that has been pointed. After the tube has been introduced into the die (see Figure 1), lubricating oil is pumped onto the surface of the tube, the trolley jaws grip the tube or rod tip, the trolley hook engages the chain, and the tube is drawn through the die. The die diameter determines the OD; the rod diameter determines the ID size. Proper die selection minimizes wall thickness changes before the tube contacts the mandrel.
In general, heavy-wall tubes tend to thin before contacting the rod; light walls thicken. High-angle dies tend to thin the wall and low-angle dies tend to thicken the wall. It is critical to remember that the optimum die angle varies with the diameter-to-thickness (D/t) ratio.
After the tube is drawn, it must be expanded for rod removal. A common method is to apply pressure by rotating the tube while passing it through cross rolls. This process generates radial stresses and expands the tube. The process is repeated until the tube is at finished size.
Advantages of rod drawing are that drawing speeds are relatively high and high area reductions (approximately 45 percent for stainless steel) are possible. Disadvantages are that it is a two-person operation and it requires an additional drawing operation, such as a plug draw or sinking, to remove the spiral pattern.
Plug Drawing. Two varieties of plug drawing are fixed and floating. Fixed plug drawing uses a hollow rod anchored at the back of the bench. A lubricant is pumped through the rod to a small hole near the front, allowing lubricant to enter the ID of the tube. A slightly tapered tungsten carbide plug is threaded or brazed onto the end of the rod; the tube is loaded over the rod, lubricant pumped onto the OD surface, and the tube is drawn.
One of the benefits of fixed plug drawing (see Figure 2) is that it produces a smooth ID. Another advantage is that the taper makes it possible to adjust the ID to meet a tight tolerance. While it requires only one operator, the drawing speed is quite slow, and maximum area reductions are low—about 25 percent for stainless steel.
Floating plug drawing (see Figure 3) is well-suited to producing long-length coils economically. This method was used for drawing copper and aluminum for many years. After the lubricant is pumped into the ID of the tube, a tapered plug is inserted, the tube is crimped to hold the plug in place, and the tube is pointed. During drawing the plug is held in position by a combination of forces between the tube ID and the plug. The tooling design is critical to the success of this process. Die angles are generally between 28 and 32 degrees, with plug angles between 20 and 24 degrees. The bearing length should be about 10 to 15 percent of die diameter. Be aware that a plug that is too long can cause scratches on the ID; a plug that is too short will not seat.
Semifloating drawing and tethered plug drawing are floating plug processes adapted for drawing straight lengths. The plug is attached loosely to a back rod and the tube is loaded over the rod and plug for drawing (see Figure 4).
Sinking. Sinking is the term for drawing a tube with no internal support. It is usually performed as a sizing pass after a rod draw. The proper die angle depends on theD/t ratio; a properly chosen die angle minimizes the change in wall thickness. If the wall thickens too much, the ID surface finish will deteriorate.
The bearing length is longer than with other operations, up to 50 percent of the die’s diameter, to ensure the roundness of the finished tube.
Plug drawing and sinking can be used to draw a tube to a finished size.
When designing a drawing schedule, the ratio of wall reduction to diameter reduction is an important quality consideration. Wall reductions tend to iron, or smooth, the ID surface; diameter reductions tend to roughen the surface. A convenient expression for the ratio is the Q value, which is equal to the perce nt wall reduction divided by the percent ID reduction. A Q value of 2 or higher tends to smooth the ID surface. When the schedule does not lend itself to a series of high-Q-value draws, it is better to use a high-Q-value rod draw followed by a hard sink rather than a series of low-Q-value drawing operations. High Q values also result in low residual stress levels for cold-worked tubes. In a recent project, a Q value of 0.91 yielded a residual stress of more than 52,000 pounds per square inch (PSI) as measured by the Sachs and Espy procedure described in ASTM E1928. A draw with a Q value of 2.2 had a residual stress level of only 5,200 PSI. High Q values would result in negative, or compressive, values.
Lubrication. Lubrication is another important consideration, along with tooling and drawing schedule. Most tube mills use chlorinated oils for lubricating stainless steels and nickel alloys. The correct viscosity can be as low as 8,000 SUS (Saybolt Universal Seconds) or more than 100,000 SUS depending on the alloy, tube size, and type of reduction.
Straightening is usually performed using a six- or 10-roll rotary straightener (see Figure 5) with a combination of flex and pressure. While flex has little effect on properties, pressure tends to increase yield strength and raise the residual stress level. Exerting the minimal pressure is the best practice.
Finishing operations may include polishing, pickling, or sandblasting to improve the surface appearance and remove minor imperfections. Final inspection techniques are determined by the customers’ order requirements.
The editors of TPJ-The Tube & Pipe Journal® thank the Tube & Pipe Association, International®’s Extrusion, Drawing & Tube Reducing Technology Council for its efforts in arranging the publication of this article.
Tube bending is the umbrella term for metal forming processes used to permanently form pipes or tubing. One must differentiate between form-bound and freeform-bending procedures, as well as between heat supported and cold forming procedures.
Form bound bending procedures like “press bending” or “rotary draw bending” are used to form the work piece into the shape of a die. Straight tube stock can be formed using a bending machine to create a variety of single or multiple bends and to shape the piece into the desired form. These processes can be used to form complex shapes out of different types of ductile metal tubing. Freeform-bending processes, like three-roll-pushbending, shape the workpiece kinematically, thus the bending contour is not dependent on the tool geometry.
Generally, round stock is used in tube bending. However, square and rectangular tubes and pipes may also be bent to meet job specifications. Other factors involved in the bending process are the wall thickness, tooling and lubricants needed by the pipe and tube bender to best shape the material, and the different ways the tube may be used (tube, pipe wires).
A tube can be bent in multiple directions and angles. Common simple bends consist of forming elbows, which are bends that range from 2 to 90°, and U-bends, which are 180° bends. More complex geometries include multiple two-dimensional (2D) bends and three-dimensional (3D) bends. A 2D tube has the openings on the same plane; a 3D has openings on different planes.
A two plane bend or compound bend is defined as a compound bend that has a bend in the plan view and a bend in the elevation. When calculating a two plane bend, one must know the bend angle and rotation (dihedral angle).
One side effect of bending the workpiece is the wall thickness changes; the wall along the inner radius of the tube becomes thicker and the outer wall becomes thinner. To reduce this the tube may be supported internally and or externally to preserve the cross section. Depending on the bend angle, wall thickness, and bending process the inside of the wall may wrinkle.
Tube bending as a process starts with loading a tube into a tube or pipe bender and clamping it into place between two dies, the clamping block and the forming die. The tube is also loosely held by two other dies, the wiper die and the pressure die.
The process of tube bending involves using mechanical force to push stock material pipe or tubing against a die, forcing the pipe or tube to conform to the shape of the die. Often, stock tubing is held firmly in place while the end is rotated and rolled around the die. Other forms of processing including pushing stock through rollers that bend it into a simple curve. For some tube bending processing, a mandrel is placed inside the tube to prevent collapsing. The tube is held in tension by a wiper die to prevent any creasing during stress. A wiper die is usually made of a softer alloy such as aluminum or brass to avoid scratching or damaging the material being bent.
Much of the tooling is made of hardened steel or tool steel to maintain and prolong the tool's life. However, when there is a concern of scratching or gouging the work piece, a softer material such as aluminum or bronze is utilized. For example, the clamping block, rotating form block and pressure die are often formed from hardened steel because the tubing is not moving past these parts of the machine. The pressure die and the wiping die are formed from aluminum or bronze to maintain the shape and surface of the work piece as it slides by.
Pipe bending machines are typically human powered, pneumatic powered, hydraulic assisted, hydraulic driven, or electric servomotor.
Press bending is probably the first bending process used on cold pipes and tubing.[clarification needed] In this process a die in the shape of the bend is pressed against the pipe forcing the pipe to fit the shape of the bend. Because the pipe is not supported internally there is some deformation of the shape of the pipe, resulting in an oval cross section. This process is used where a consistent cross section of the pipe is not required. Although a single die can produce various shapes, it only works for one size tube and radius.
Rotary draw bending
Rotary draw bending (RDB) is a precise technology, since it bends using tooling or "die sets" which have a constant center line radius (CLR), alternatively indicated as mean bending radius (Rm). Rotary draw benders can be programmable to store multiple bend jobs with varying degrees of bending. Often a positioning index table (IDX) is attached to the bender allowing the operator to reproduce complex bends which can have multiple bends and differing planes.
Rotary draw benders are the most popular machines for use in bending tube, pipe and solids for applications like: handrails, frames, motor vehicle roll cages, handles, lines and much more. Rotary draw benders create aesthetically pleasing bends when the right tooling is matched to the application. CNC rotary draw bending machines can be very complex and use sophisticated tooling to produce severe bends with high quality requirements.
The complete tooling is required only for high-precision bending of difficult-to-bend tubes with relatively large OD/t (diameter/thickness) ratio and relatively small ratio between the mean bending radius Rm and OD. The use of axial boosting either on the tube free end or on the pressure die is useful to prevent excessive thinning and collapse of the extrados of the tube. The mandrel, with or without ball with spherical links, is mostly used to prevent wrinkles and ovalization. For relatively easy bending processes (that is, as the difficulty factor BF decreases), the tooling can be progressively simplified, eliminating the need for the axial assist, the mandrel, and the wiper die (which mostly prevents wrinkling). Furthermore, in some particular cases, the standard tooling must be modified in order to meet specific requirements of the products.
Main article: Roll bending
During the roll bending process the pipe, extrusion, or solid is passed through a series of rollers (typically three) that apply pressure to the pipe gradually changing the bend radius in the pipe. The pyramid style roll benders have one moving roll, usually the top roll. Double pinch type roll benders have two adjustable rolls, usually the bottom rolls, and a fixed top roll. This method of bending causes very little deformation in the cross section of the pipe. This process is suited to producing coils of pipe as well as long gentle bends like those used in truss systems.
Three-roll push bending
Three-roll push bending (TRPB) is the most commonly used freeform-bending process to manufacture bending geometries consisting of several plane bending curves. Nevertheless, 3D-shaping is possible. The profile is guided between bending-roll and supporting-roll(s), while being pushed through the tools. The position of the forming-roll defines the bending radius. The bending point is the tangent-point between tube and bending-roll. To change the bending plane, the pusher rotates the tube around its longitudinal axis. Generally, a TRPB tool kit can be applied on a conventional rotary draw bending machine. The process is very flexible since with a unique tool set, several bending radii values Rm can be obtained, although the geometrical precision of the process is not comparable to rotary draw bending. Bending contours defined as spline- or polynomial-functions can be manufactured.
Simple three-roll bending
Three roll bending of tubes and open profiles can also be performed with simpler machines, often semi-automatic and non CNC controlled, able to feed the tube into the bending zone by friction. These machines have often a vertical layout, i.e. the three rolls lie on a vertical plane.
An induction coil is placed around a small section of the pipe at the bend point. It is then heated to between 800 and 2,200 degrees Fahrenheit (430 and 1,200 °C). While the pipe is hot, pressure is placed on the pipe to bend it. The pipe is then quenched with either air or water spray. Heat-induction bending is used on large pipes such as freeway signs, power plants, and petroleum pipe lines.
The pipe is filled with a water solution, frozen, and bent while cold. The solute (soap can be used) makes the ice flexible. This technique is used to make trombones.
A similar techniques using pitch was formerly used, but discontinued because the pitch was hard to clean out without excessive heat.
In the sand packing process the pipe is filled with fine sand and the ends are capped. The filled pipe is heated in a furnace to 1,600 °F (870 °C) or higher. Then it is placed on a slab with pins set in it, and bent around the pins using a winch, crane, or some other mechanical force. The sand in the pipe minimizes distortion in the pipe cross section.
A mandrel is a steel rod or linked ball inserted into the tube while it is being bent to give the tube extra support to reduce wrinkling and breaking the tube during this process. The different types of mandrels are as follows.
- Plug mandrel: a solid rod used on normal bends
- Form mandrel: a solid rod with curved end used on bend when more support is needed
- Ball mandrel without cable: unlinked steel ball bearings inserted into tube, used on critical and precise bends
- Ball mandrel with cable: linked ball bearings inserted into tube, used on critical bend and precise bends
- Sand: sand packed into tube
In production of a product where the bend is not critical a plug mandrel can be used. A form type tapers the end of the mandrel to provide more support in the bend of the tube. When precise bending is needed a ball mandrel (or ball mandrel with steel cable) should be used. The conjoined ball-like disks are inserted into the tubing to allow for bending while maintaining the same diameter throughout. Other styles include using sand, cerrobend, or frozen water. These allow for a somewhat constant diameter while providing an inexpensive alternative to the aforementioned styles.
Performance automotive or motorcycle exhaust pipe is a common application for a mandrel.
These are strong but flexible springs inserted into a pipe to support the pipe walls during manual bending. They have diameters only slightly less than the internal diameter of the pipe to be bent. They are only suitable for bending 15-and-22 mm (0.6-and-0.9 in) soft copper pipe (typically used in household plumbing) or PVC pipe.
The spring is pushed into the pipe until its center is roughly where the bend is to be. A length of flexible wire can be attached to the end of the spring to facilitate its removal. The pipe is generally held against the flexed knee, and the ends of the pipe are pulled up to create the bend. To make it easier to retrieve the spring from the pipe, it is a good idea to bend the pipe slightly more than required, and then slacken it off a little. Springs are less cumbersome than rotary benders, but are not suitable for bending short lengths of piping when it is difficult to get the required leverage on the pipe ends.
Bending springs for smaller diameter pipes (10 mm copper pipe) slide over the pipe instead of inside.
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- ^Mentella, A.; Strano, M. (10 October 2011). "Rotary draw bending of small diameter copper tubes: predicting the quality of the cross-section". Proceedings of the Institution of Mechanical Engineers, Part B: Journal of Engineering Manufacture. 226 (2): 267–278. doi:10.1177/0954405411416306.
- ^Strano, Matteo; B.M. Colosimo; E. Del Castillo (2011). "Improved design of a three roll tube bending process under geometrical uncertainties". Esaform. AIP Conf. Proc. 1353: 35–40. doi:10.1063/1.3589488.
- ^Engel, B.; Kersten, S.; Anders, D. (2011), "Spline-Interpolation and Calculation of Machine Parameters for the Three-Roll-Pushbending of Spline-Contours", Steel Research International, 82 (10).
- ^ abhttp://www.thefabricator.com/article/shopmanagement/brass-instrument-manufacturing-how-metal-makes-music