The first step in designing for bending is the choice of material and its physical size and shape, for all other considerations will be directly related. In many cases, the type of material is governed by the end use of the part, such as structural high temperature and pressure applications, corrosion resistance, weight or for food and dairy applications. Perhaps an economic factor is alone the criterion, but in any event the material selection should be decided first.

Ductility or elongation of the material will determine the minimum centre-line radius to which a section of a given size can be bent. For example, high carbon steel as rolled, magnesium alloys, and certain aluminium alloys have only a 10 per elongation value while low-carbon steel, chrome- vanadium steel, and some aluminium alloys have values of 25 to 30 per cent.

The 18-8 stainless steels, phosphor and silicon bronzes and copper lie in the ductility range of 50 to 70 pelf cent. All the metals mentioned are in the annealed condition except as noted, and careful consideration should be given to the heat-treatment annealing or temper of the material. The uniformity of structure of the material will enhance duplication in bending and reduce scrap to minimum.

1. Welded Pipe or Tube:

When welded pipe or tubing is bent on rotary machine, some attention may be necessary in placing the material in the machine so that the welded seam lies on the top in approximately the location of the neutral axis.

2. Selection of Bend Radii:


Although the ductility of the material determines the minimum centre-line radius for a given section, it is most sensible and economical to design the bend to the largest practical radius. The more difficult the bend, the more costly the tooling initially and in maintenance.

If there is more than a very little bending to be done definite economic advantages are offered when the same size material is bent to the same shape or where a part has several bends and the same radius is used as often as possible.

As a guide in the designing for bending tubular sections, tool-selection graph (Fig. 6.34) is helpful. The ratios of centre-line radius to the outside diameter and wall thickness to outside diameter are used to determine the required tools. The areas between the curves limit the classification of tooling.

Where conditions of size and centre ­line radius place the tooling in a borderline region, i.e., near or on a curve; it is always better to start with the minimum amount of tools and, after test bends are made, add others if necessary since the graph is based on ratios where several conditions of outside diameter, wall thickness, and centre-­line radius can result in the same relationships, the graph is more accurate for tube sizes than for relatively thick-wall pipe sections. Because of the surface roughness of common pipe, ball mandrels or shoes are rarely used.

3. Tooling Considerations:


A machine is no better than the tools provided for it. The finest bending machine will turn out poor work if the tools are designed incorrectly or badly built. Figures 6.35 (a), illustrate basic bending tools. Relationship of the tools with reference to the bending machine can be readily understood from the illustrations shown.

Bending Tools for Rotary-Type Machines

4. Forms:

The most basic part of bending tool is the form.

A form must be built to satisfy two general requirements:


(a) Provision for attachment to the machine on which it is to be used and

(b) Design for proper guiding of the material as it is being bent.

The form must be exactly centered on the machine so that an arc of constant radius will be circumscribed as the form rotates. This is a rule which must be adhered to whenever circular bends are made. Forms with changing radii or special contours, such as elliptical shapes, can be used on rotary-type machines, provided a compensating pressure-die unit is used in conjunction with the pressure- die arrangement.  

That part of the form which comes in contact with the bent material should fit the shape of the material as closely as possible. Figure 6.35 (a) shows the way in which forms may be made to do this for various sections. The purpose of this close fitting is to prevent as far as possible, distortion in the shape of the material being bent.


The form straight, or that part of the form where the material is clamped, is important, for the imperative rule in bending on rotary machines is that the material must not slip when bending. Adequate length of the straight portion of the form must be provided to meet this requirement.

Clamp lengths may be approximated:

2D for 3 to 5 times (D) CLR

3D for 2 to 3 times (D) CLR


4D for 1 to 2 times (D) CLR

(D is tube diameter and CLR is centre-line radius.) In special cases of very thin-wall sections, a plug is used inside the tube to prevent crushing the material in clamping.

Tooling for Rotary-Type Bending Machine and Types of Mandrels

A form designed for production of a circular bend should press die in a roller which may be grooved to fit the material and which rotates around a shaft fixed to the frame of the machine so that rolling action rather than sliding is caused to not have quite the same radius as the final bend desired.

All metals are elastic to some extent and when the bent material is released, it tends to spring back a small amount toward its original shape. Roughly, the shorter the bend radius, the less the spring back. Several factors, such as size, wall thickness, center-line radius, and hardness, influence the amount of spring.

The following empirical formula has been found nearly accurate for annealed- seamless and welded-steel-tubing and steel-pipe sizes:

where S = spring allowance for centre-line radius

C = size factor

R = centre-line radius

D = outside diameter

Z = section modulus

t = wall thickness.

5. Clamp Dies:

The clamping die operates in such close conjunction with the form that it may be considered part of the same tool. The clamping die should also have the same contour as the material, as nearly as possible, so that it will grip the greatest possible area of material with the least chance of slipping.

6. Pressure Dies:

Stationary pressure dies must be built to withstand the wear and abrasion caused by the sliding material. In most instances these dies are made exactly the same shape as the clamp die, as shown in Fig. 6.35 (a), but must have a hard, smooth surface which contacts the material. Where more difficult bends are being made, a follower type solid-pressure die travelling on a series of rollers offers better guidance and more uniform restraining of the material.

7. Shoes:

The shoe is made to fit into the form groove in such a manner that the tip or front edge comes as close as possible to where the radius of the form starts. The shoe shown in Fig. 6.35 (a) has been moved back, away from the form to show the general construction.

8. Mandrels:

Mandrels are used to support hollow sections internally, preventing collapse of the wall during bending. Several common types of mandrels are shown in Fig. 6.35 (b).

9. Making Drawings of Bent Parts:

In many ways, the drawing of bent parts is as important as the other design considerations, for it is through drawings and sketches that the man in the shop interprets the thoughts of the designer. This is especially true where the part is to be made by an outside source.

Fig. 6.36 and 6.37, an isometric sketch and a three view drawing, illustrate a preferred and advantageous procedure of dimensioning drawings for bending. A considerable number of shop hours may be saved and costly errors avoided by establishing such standards for dimensioning drawings. In both examples the identical methods are used.

Control dimensions used mainly for layout and inspection are designated by capital letters. A sequence of operations results from the subscript of each tangent and angular dimension. Further the angular dimensions are the actual required degree of bend rather than the complement or supplement of the angle. Where rotation of planes between bends is other than 90°, the given rotation is so labeled.

Isometric Sketch of a Bent Part

Preferred Method for Dimensioning

Care should be taken wherever possible to make the tangents sufficient in length to avoid the use of special tooling, such as offset clamping, and make sure that ample machine clearance is maintained during bending.

Actual length of material required to make a bend (theoretical minus the stretch) may be found from test or calculated from displacement of the neutral axis, which varies from 5 to 25 per cent of the width or diameter of the material. From this data the developed length may be determined and length stop gauges for the sequence of bending operations set.