How to Calculate the Development of a Bend

When making a piece from a technical drawing it is very important to know how to calculate the development of a bend, especially to make sure that all the measurements in the drawing, before and after our bends, are perfect.

The development of the bend changes according to the material we use, the thickness and hardness of the tube or profile we are bending, the die and the counterbending die mounted on the machine and lastly the tube bending machine we are using. For this reason it is very important to know how to calculate the development of a bend, to know where and how to take the measures, so that we can realize our technical drawing step by step.

How to calculate the development of a curve starting from a technical drawing

Let’s start with our technical drawing, the drawing that shows us in section and with all the measurements, the piece we need to make.

As you can see from the drawing below, the pipe we are using has a total length of 1852 mm and to avoid cutting it again, we need to make two bends after a straight line of 539 mm and an outer distance between them of 714 mm. 

Even if the drawing gives us approximate indications about the length of the bend, we cannot totally rely on it, because the variables of these measures are really too many, and therefore it is better to know how to calculate the development of a bend, to respect to follow all the other measures of our technical drawing. But let’s see how to do it.

The first thing to do is to take a piece of the tube we are using, a scrap piece of the right size to make a single bend with our machine. This piece will be thrown away, we only need it to take the first measures and to calculate the development of the bend, bends that we will make later on our “good tubes”.

Let’s make a mark at a distance of about 20 cm from the head of the tube and put it inside our bending machine. This mark must be placed in the center of the die, corresponding to the notch placed on it to indicate the ZERO of the bend. To us, this mark will indicate the beginning of our bend.

Another important thing to keep in mind is the closing of the vice of the counter bending die on the tube, in order to have all bends equal, it must always have the same value, this is why Mackma machines have a contactor at the height of the handwheel used to close the vice.

This value of the tachometer, which must always be the same, is very important.

Once the 90 degree bend has been made, taking into account the springback of the tube and of the tools (if you do not know how to calculate the springback you can read this article) we take the tube out of the machine, take a pen and paper and start measuring.

Helping ourselves with a stop placed on the long side of the pipe, we take the exact measurement that goes from our mark made earlier on the pipe indicating the beginning of the bend, to the stop we placed.

In our example, the exact measurement is 115 mm and if we look again at our technical drawing, we understand that we have to make two bends, so the first operation to do is 115 x 2 = 230 mm. 

Another important measurement to keep under control on our drawing is the external distance that must be between our bends, which in our example is 714 mm. So the next operation to do is: 714-230= 484 mm. This value is the exact distance from the beginning of the right bend to the beginning of the left bend. But we have to divide it by two because we have to position ourselves in the center of the tube so 484:2=242 mm. 

Ok let’s recap for a moment here is the operation we did:

115×2=230     714-230=484     484:2=242  this is it!

We are almost there, we take our “good tube” which I remember is 1852 mm, we measure the center of the tube (so 926mm) and make a mark. From that mark we move 242mm to the right and make a mark and to the left and make another mark. There you have it, we have marked the exact points on which to start our bends.

Now we take the tube inside our bending machine, remember to center the mark we made with the sign of the matrix that indicates ZERO, we tighten the counterbending machine being careful to respect the value of the vice closing turns counter and we start our bend. Once finished, we turn the tube and repeat the same thing with the other sign to make the second bend and if we have been good and have respected all the rules, our piece is ready. We have learned how to calculate the development of a bend, starting from a technical drawing.

Watch the video we made and learn how to calculate the development of a bend starting from a technical drawing.

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