How To Bending Stainless Steel Pipe and Tubing
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.
Both welded and seamless tubing can be curved. When the desired radius is very tight, seamless usually bends better.
- 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 hands, such as bending a stainless steel tube around your knee, as well as using simple manual pipe benders and tube benders.
- Draw bending tends to be more effective on tubing with thicker walls or made of a stiffer material such as stainless steel.
- 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.
- 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 tubes 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.
Is the draw bending better than 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 the 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.
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.
Standard pipe or tubing size
Choosing a “standard” tube diameter has a number of advantages. Firstly the material is more readily available and prices will tend to be lower, especially if you only require small to moderate quantities, as otherwise, you will pay minimum batch order charges. Secondly, your tube manipulation subcontractor is much more likely to already have suitable tooling, avoiding your tooling costs which could range from £1500 to £3000 even for relatively small tube diameters, meaning that you can get a rapid response and/or prototypes produced quickly.
When we talk about bend radius it refers to the radius measured to the center line of the tube.
The bend radius is a fundamental parameter for the bend feasibility analysis of a tube: it is essential because it is the value that defines the coefficient which will indicate the degree of difficulty of cold bending. Bending metal tubes is a process that is done using special tube bending machines with particular tools.
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-itch 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.
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 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 bend (or the arc of the bend) is L = 0.001745ur, in which u = the angle of bend in degrees and r = the radius of the bend in inches.