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Saturday, October 31, 2020

Calculating the shaping process for drawn tubes

For the manufacturing of drawn tubes round tubes are pulled through one or more drawing rings depending on the complexity of the final shape. The drawing rings are approximately between 40 and 100 mm thick. The incoming side needs to be large enough to cover the round tube. The outgoing side has the final shape of the tube or a defined pre-shape. The faces between the incoming side and the outgoing side build a curve. Of course it would be easier if the two contours (round tube and final shape) would be connected with straight lines. But in this case the friction between tube and drawing ring gets too high and causes other problems.

Especially the design of the outgoing shape of the drawing ring is not trivial because the straight legs of the final shape needs to be converted into arcs. It has been nearly impossible to do a true design keeping the circumference of the tube only with the help of a drawing board. Design as well as manufacturing of drawing rings has been quite difficult up to now.

The design of drawing rings has been characterized by following difficulties

  • The layout of the drawing ring was mostly made free hand
  • The length of the individual lines was very hard to estimate

The manufacturing of drawing rings has been making difficulties as well

  • The shape of the incoming tube, for example, was made with a machining tool which had mostly the same contour
  • The manufacturing process itself was very long and winded
  • A negative had to be made from the outgoing contour and the shape of the
    machining tool
  • The negative had to be casted to get an original part
  • This part was manufactured with a copy mill. It also had to be reworked in order to remove intersections with the incoming tube
  • Therefore a CAD/CAM system for the design and manufacture of drawing rings should allow an easy design of the drawing ring shape, a high accuracy material flow, an easy machining of the drawing ring. It should avoid additional intermediate steps like the need to machine a negative and it should provide the possibility to design and manufacture additional tools required, like cores etc.

The design process is done in the following steps:

1) design of the final section
In the first step the final section needs to be designed. This can be done with the features provided by COPRA® RF or by using the basic CAD-system. In this case the outside contour of the section will be designed with CAD-functions and then its data will be converted for the use with COPRA® RF.

2) design of individual shaping steps
This is the most important - and also most difficult - step in the process chain. The shape of the drawing ring is determined by a number of sections through the ring. How many shapes will be used to create the drawing ring geometry can be user-defined. Beyond that a compression factor can be defined for each individual section of the drawing ring. With this option it is possible to get the necessary curvature.

With these information the computer program will calculate the individual sections as selected. As COPRA® RF does know the individual position of any arc at any time it can make sure that the material flow of the shaping process will be an optimum. It may not happen that the length of an arc is too short or too long as it used to happen during a design on the drawing board.
The result supplied by the program are closed shapes (poly lines) which can be processed by a real 3D-CAD-system .

Forming steps calculated by COPRA
Forming steps calculated by COPRA® RF