The home master is well aware of the advantages of pipe metal. From the basics of strength of materials, we know that the moment of resistance of a tubular section is much greater than that of a solid one with the same area. Therefore, pipes have increased values of allowable bending stresses. But this also presents certain difficulties when trying to bend a product at home: without the appropriate tool, you will most likely receive a part with a deformed section and with unaesthetic folds. Renting (and, even more so, purchasing) the necessary equipment is in most cases unprofitable. However, you can bend the pipe without a pipe bender, and in several ways.
Why do metal pipes bend badly
They bend well, the problem lies in the unsatisfactory appearance of the product, which is accompanied by a distortion of the perimeter. The latter circumstance is especially critical for profile pipes of rectangular, square or oval sections.
In the process of plastic bending, different sections of the pipe are under the influence of stresses of the opposite sign. The outer sections at the fold are stretched, and the inner ones are compressed. If compressive stresses are safe for most metals, then tensile stresses contribute to the formation of cracks and tears. This is especially true for profiles that are made from materials of medium plasticity, for example, medium and high carbon steels, nickel-free stainless steels, some brands of bronze, etc. It is very difficult to determine the exact grade of the metal from which the product is made without the appropriate tools and devices difficult.
Bending, as a process of plastic deformation, is generally quite capricious to the conditions of its implementation. To achieve a positive result, you will need:
- Take into account the phenomenon of springback of the bent profile, in which the final value of the bending angle is always less than the calculated one.
- Ensure accurate fixation of the bend area to prevent displacement of the workpiece.
- Determine the minimum required value of the radius of curvature of the deforming tool.
Therefore, before starting work, they are determined with the minimum possible bending shoulder, bending method (open or closed), as well as the sequence of operations. In particular, successful bending is guaranteed if the ratio r/s > 5 (here r is the assigned bending radius, s is the wall thickness).
In the future, options for self-bending pipes are considered, which can be implemented at home.
Option one — bend the pipe with heating
In this case, no special devices are required at all, it is enough for the master to have an oxygen-acetylene torch or a conventional blowtorch. In the first case, the mouthpiece should be located at some distance from the place of the intended bend. The torch should not be made concentrated, since in this case the temperature difference between the zones will be too large, and the plastic characteristics of the material will be uneven (this is especially true for carbon steels containing an increased percentage of manganese and silicon). ).
After heating to the required temperature (it is easily calculated from the tint colors), the part to be bent must be clamped in a bench vise, and the desired section must be free-bent.
Heating with a burner is more uniform, but is associated with the risk of reducing the strength of the workpiece material, especially if it is intended for use in pressure pipelines. It is better to use a blowtorch in order to avoid unnecessary vacation. The following sequence of operations is recommended:
- The section to be deformed is clamped in a bench vise, for which special rubber pads are used that do not damage the metal.
- Light a blowtorch, bring it to the required area, and, evenly turning the flame source around the outer diameter of the profile, warm it up. Note that it is impossible to overheat the pipe with the heat emanating from the blowtorch torch, but you can smoke it.
- Turn off the blowtorch, move the heated area to a distance equal to the allowable bending arm. Further, using a tubular wrench (sometimes even manually), bend the product, sometimes using a template.
Bending with heating, for all its simplicity, is characterized by the following limitations:
- Degrades the appearance of the material.
- If the heating temperature is insufficient, the formation of folds along the inner radius of the bend is likely (it is impossible to correct such a disadvantage).
- The process requires as quick bending as possible, because thin-walled pipes cool quickly.
- The technology is not recommended for small bending radii.
- Increased safety requirements must be met because open flames are involved.
Most often, a pipe made of stainless steel or non-ferrous metals is bent in this way, which, when heated, are not prone to oxidation.
Option Two — Using Back Pressure
Failures in conventional pipe bending are often associated with the presence of an unbalanced stress-strain state of the workpiece, when the external pressure from the working tool is not balanced by internal counterpressure (the resistance of the metal itself is not enough in this case, since it is obviously below the plastic limit).
Such backpressure can be achieved by injecting a gas, liquid, or other incompressible working medium, which is characterized by high complexity. Gas and liquid are excluded, because at home it is very difficult to perform a stable sealing of the open ends of the tubular blank. But sand or fine slag is always available — they are homogeneous, non-toxic and easy to handle.
Consider the sequence of bending a round tubular profile with the application of counterpressure:
- First you need to install a plug on one of the ends of the workpiece, for which you can use a regular bolt (see Fig. 5). The bolt shaft must fit snugly into the inner hole.
- We prepare the aggregate, it must be uniform and small so as not to create internal voids. For periodic sand/slag compaction, any pin of suitable diameter is used as a piston (see Fig. 6). The entire tubular blank must be completely filled with filler (see Fig. 7).
- The workpiece is placed in a vise and bent. Since the tensile strength of sand or slag is always lower than that of the material of the part, rupture or cracking will not occur. After bending, it is necessary to carefully clean the inner lumen of the pipe from the remnants of the filler.
Option three — the use of tension springs
To implement the method, a special spring is required, the outer diameter of which should be as close as possible to the inner diameter of the workpiece. The range of tension springs is given in GOST 18794–80; their characteristic feature is the presence of an annular grip on one of the ends. This capture can be closed, as shown in Fig. 8, or in the form of a curly hook. The latter is considered to be preferable, since it provides a more convenient extraction of the spring from the workpiece after the bending process is completed.
The selected spring is inserted inside and securely fixed at the opposite end of the pipe. Since the outer surface of the spring fits tightly against the wall, it provides the same back pressure as the intermediate medium in the previous embodiment. The advantage of the method is that such bending springs can be used repeatedly.
The last option — why even come up with some kind of bending technology
In some cases — for example, when the r / s ratio is much higher than the minimum recommended value — successful bending of the pipe in a cold state with your own hands is possible, provided that one of the ends is firmly fixed, followed by bending according to a pre-prepared template.
The template is made of multilayer plywood, you can even use the outer surface of any durable object that is suitable in diameter — a barrel of water, or a wooden flowerpot. One of the ends is fixed in a clamp / vice, and the opposite one is bent along the required radius (see Fig. 10). In this way, it is quite possible to obtain, for example, blanks for the steel frame of a greenhouse or greenhouse. After deformation, it is not worth immediately releasing the part from the clamps, since any metal has inertial properties.
In all cases of bending without a pipe bender, you should prepare:
- Stock up on levers (the bending moment, depending on the required parameters and pipe material, can be quite large).
- Prepare a template that must be accurately dimensioned for the characteristics of the bending area.
- Thoroughly clean the inside of the bent pipe (particularly if the product is to be used for pumping liquid media).
Before starting work, it is worth remembering the simple safety rules. Before bending by hand, it is advisable to wrap the workpiece with coarse leather, burlap or other dense fabric — this will protect your hands and create additional friction that will prevent slippage during bending.