Basics of Bending Stainless Steel Tubing
Terms of stainless steel pipe and tube
- Generally, the tube is measured by the outside diameter while the stainless steel pipe is measured by the inside diameter, so the pipe may appear thicker than stainless steel tubing of the same measurement. For example, a piece of one-inch stainless steel pipe will seem to have a greater diameter than a piece of one-inch stainless steel tubing.
- When bending stainless steel tubes or pipes, use the tool designed for the right item and the right size. Do not use a tube bender on stainless steel pipe or vice versa.
- The center line radius, or CLR, is the radius down the center of the stainless steel tube and is the standard way to measure a bend. The smaller the CLR, the sharper the bend; the larger the CLR, the more gradual the bend.
- The bend radius measures the inside curvature and indicates the minimum radius one can bend a stainless steel pipe or tube without kinking or otherwise damaging it. The greater the flexibility of the material, the smaller the bend radius.
- The die of a pipe bender is the curved form around which the stainless steel pipe bends.
- Springback is the tendency of a stainless steel pipe to revert to its original shape after bending. You may need to bend slightly further than your target to compensate for spring back, based on the kind of material and the thickness of the stainless steel tubing wall.
Why need bending stainless steel tubing?
Designs using bent stainless steel tubes can be preferable to mechanical or welded connections for a number of reasons:
- Welded joints, for example between straight tubes and elbows, require the prevention or removal of heat tint. Shielding gas may be difficult to apply, especially on-site. The chemical removal of heat tint employs acid-containing products, usually involving environmental and safety precautions. Mechanical removal is only possible for the outside of the tube. A one-piece design in bending stainless steel tubing avoids these fabrication steps.
- In mechanical joints with couplers or flanges, crevices are inevitable. Depending on the conditions, these may be undesirable because they can trap corrosive substances. The risk of crevice corrosion must also be taken into account. A bending stainless steel tubing ensures a continuous, even surface.
Bending stainless steel tubing can therefore be the easiest and most efficient solution to a design task. Indeed, tube bending is one of the most frequently used fabrication techniques for stainless steel.
Four methods for bending stainless steel tubing
Both welded and seamless stainless steel pipe or tubing can be curved, when the desired radius is very tight, seamless usually bends better.
There are 4 methods of bending to meet the requirements of stainless steel pipe and tubing of different sizes and materials(such as 204, 304, and 316). A variety of methods can be used for bending stainless steel tubing. If the bend has a large radius, the tubing can be curved with three-roll benders (also called section benders, profile benders, or angle rolls.) If the bend has a tight radius, the tubing can be bent on a rotary-draw bender through a process called mandrel tube bending.
- Methods 1# Compression Bending: Compression bending involves holding one end of a stainless steel tube in place and bending it around a die or other kind of former. The simplest applications involve bending by hand, such as bending a stainless steel tube around your knee, as well as using simple manual pipe benders and tube benders.
- Methods 2# Draw Bending: Draw bending tends to be more effective on tubing with thicker walls or made of a stiffer material such as stainless steel.
- Methods 3# Roll Bending: Roll bending often uses three rolls in a pyramid shape that roll the stainless steel pipe through the machine as the top roller pushes down to bend the pipe. Tube rollers are often used for large bends in a stainless steel pipe or tubing workpieces in construction.
- Methods 4# Hot Bending: Induction bending involves placing a heat induction coil around a stainless steel tube at the bend point. The user heats the coil and applies pressure to bend the softened tube to the desired angle. This is only a viable process, in fact, we have not heard of an induction bending process for bending stainless steel pipe because usually there is no very large diameter stainless steel pipe or tubing.
Commonly used bending methods of stainless steel pipes
There are 2 main types of CNC bending machines: the CNC profile bending machine and the CNC pipe bending machine, in both types, the rollers are classed as tools since they must be changed over to be specific for different tube diameters.
- Profile bending machine: The profile bending machine often referred to simply as a roll bender or section bender, just has three rolls, with one or more of them rotationally power-driven to pull the tube in and between them. Often the tube is run backward and forwards between the rolls a number of times to achieve the desired bend radius. This type can be semi-manual, NC, or CNC.
- CNC pipe bending machine (Draw bending): The second type is where the rolls are free running rotationally and the tube is pushed through them (hence push bending). This type is normally CNC controlled and the bend is performed in a single pass, which we called a CNC pipe bending machine.
Draw bending vs roll bending?
Actually, they both have their place because they do quite different things. Draw bending is for fairly tight curves, typical of most engineering applications, while roll bending is for large radius curves often found in furniture or architectural work.
One slight downside to roll-bending is that there is a degree of trial and error involved in getting the right curvature, certainly the first time the job is bent, since every different tube diameter, wall thickness, and material behaves slightly differently under force and because there is no fixed form that the tube is being bent around these factors come into play more significantly. Consequently, there is a longer development time and more tube wastage, all of which ultimately has to be paid for, than for roll forming. Obviously, this is not such an issue for larger volumes where the cost can be spread. Fundamentally draw bending is often considered more accurate than roll bending.
Bending square and stainless steel rectangular tubing
While the procedures are the same for bending round, rectangular and square material, square and rectangular tube requires special consideration.
Hard-way versus easy-way bending. When a rectangular tube is bent, the material often has less distortion if it is bent the hard way. The heavier the wall thickness, the tighter it can be formed without excessive distortion.
Tips for Bending Stainless Steel Pipe and Tubing
Since the application of bent stainless tubes often involves products for the pharmaceutical and food industries, care must be taken to avoid carbon contamination of the steel. The machinery and tooling should be cleaned and prepared to avoid such contamination. Furthermore, care should be taken to avoid having metal strapping in contact with the stainless parts. Cardboard, wood, or plastic can be used to protect the tubing during transit.
After bending stainless steel tubing, a nut, and sleeve can be slid over the ends which then can be flared. Process piping is often supplied this way. For example, 316 seamless stainless steel tubing 1.5 OD x 0.065 walls (16 ga) was bent with two 90-degree bends into a flat-back U. The ends had a 37-degree JIC flare. JIC (Joint Industry Council) fittings are widely used in fuel delivery and fluid power applications, especially where extremely high pressure is involved. Another tubing of the same size and grade was bent with an offset.
How to Make a Reference for a 90-Degree Bend
When bending a tube or pipe, the interior wall of the bend becomes more compressed and thicker, while the exterior wall becomes stretched and thinner. To determine how long a piece of tubing you need and to make sure it achieves the correct shape, use this method to create a 90-degree reference bend.
- Begin by estimating the needed length of the tube or pipe.
- Mark the beginning and end of the desired bend, along with a longitudinal line on the side of the pipe opposite to the chosen bend direction (i.e., the exterior side).
- Bend a test pipe 90 degrees to use as a reference.
- Check the angle of the pipe by laying it against a carpenter’s square with the outer bend facing the corner.
- Find and mark the distorted places where the bend begins and ends with a permanent marker. They should be the same distance from the inner corner of the square.
- Measure the distance between the beginning and end of the pipe by adding the distances on the x and y-axis. For instance, if they’re both six inches from the end of the square, the length of the bent section of the pipe will be 12 inches.
- Place the 90-degree reference tube back in the bender with the matching die. Note the places on the die where the bend begins and ends on the tube and mark them. These will be the reference points for future 90-degree bends.
- If the pipe springs back after the initial bend, bend a second time using the reference point.
Tip: The formula for the length of the bend (or the arc of the bend) is L = 0.001745ur, in which u = the angle of the bend in degrees and r = the radius of the bend in inches.
General parameters used in stainless steel tube bending
In round stainless steel hollow sections, a rule of thumb for the tightest bend radius is the diameter multiplied by three. There is no corresponding rule for rectangular or square profiles.
A common objective in tube bending is to form a smooth, round bend. When a metallic tube is bent, two things happen. The outside wall reduces in thickness, due to the stretching of the material, and the inside wall becomes thicker. The material that forms the outside of the bend has further to travel and is therefore stretched, while the inside of the bend is compressed. This is simple when a tube has a heavy wall thickness and is bent to a large radius. To determine if a tube has a thin or heavy wall, its wall thickness is compared to its outside diameter. This ratio is called the wall factor.
Wall factor = (Tube outside diameter) / (Tube wall thickness)
When the wall factor is greater than 30, the tube is classed as a thin-wall tube. Wall thickness is a meaningless measurement if not related to the tube diameter.
The same comparison is made to determine if a bend radius is tight or large (degree of bend).
Degree of bend = (Bend centerline radius) / (Tube outside diameter)
Two factors, wall factor, and bend radius are used to determine the severity of a bend. little or no support is needed inside the tube when the tube diameter is small and the wall is thick. As the tube diameter increases, the tube becomes weaker. If the wall thickness of the tube decreases this too makes it weaker. The forces acting on the tube also become greater as the bend centerline radius becomes smaller.
PDF: How Does Bending The Stainless Steel Tubing
Works Cited: Bending stainless steel tube Bending Stainless Steel Tubing: Effective Processes and Top Considerations