|Bending - Course: Technique of Working Sheet Metals, Pipes and Sections. Methodical Guide for Instructors|
The following sections contain proposals on how to arrange the trainees' instruction, the demonstration of techniques as well as exercises and controls.
3.1. Introductory Instruction
The introductory instruction should be given, if possible, with the trainees in a classroom. Attention is to be paid during the instruction to the trainees' noting down necessary additions or replies to questions in the Trainees' Handbook of Lessons. An essential prerequisite for learning the bending techniques is the trainees' mastering the techniques in testing and hammering. This knowledge should be repeated, if necessary.
For your instruction you can use the Trainees' Handbook of Lessons with the following focal points:
Purpose of bending
Initially, it would be favourable to explain to the trainees the purpose of bending with the help of visual objects and illustrations. In this connection, it must be made clear that bending is, due to its versatility, used in many fields of manufacture and performed by various techniques. Predominantly, the techniques in folding, turning over, flanging, seaming, rounding and rolling should be discussed.
Selected tools, appliances and machines
Out of the variety of applicable working means it is recommendable to introduce or repeat the following:
- Welding torches
- Angle bending appliances, strip rolling devices, pipe benders
- Screw presses
- Folding bench, rounding device, crimping and flanging machine
- Vise, blacksmith's anvil
These working means may also be dealt with in a question-answer talk with the trainees, as far as the trainees have previous knowledge. Already here, particular characteristic features in their application can be stressed. If tools cannot be shown, the trainees should have a look at the illustrations in the Trainees' Handbook of Lessons.
Processes in the material
During this instruction section, the tensile and compressive stresses and their effects occurring during the bending process are to be discussed. The terms "stretching and upsetting" as well as "neutral axis" have to be explained.
In this connection, it is recommended to show the stretching and upsetting zone on a spot of rupture of a square section strongerly bent It is also favourable to make a blackboard drawing as to Fig. 14 of the Trainees' Handbook of Lessons to show the neutral axis. Subsequently, the influences of such material properties like "plasticity", elasticity", "strength" and "strain hardening" are to be explained.
The trainees should keep in mind the knowledge so acquired in the form of rules and key sentences. To deepen the knowledge, the trainees have now to answer the questions asked in the Handbook of Lessons following the respective section.
In supplement of the influence data depicted just now, me influences of the bending radius, particularly the minimum bending radius, and the influences of beat reaction for the quality of bending can be discussed.
It is recommendable to put side by side a hot bent and a cold bent square section of 16 × 16. The trainees are to state the differences in quality of bending and derive from it the findings for their own work.
Fundamentals of calculation
Comprehensively dealing with this focus is essential. The trainees are to be explained the necessity of the exact blank of the basic material. As for calculating the blank size, the straight length in dependence of the neutral axis position must be ascertained first.
The formulae required for it are distinctly introduced in the "Trainees' Handbook of Lessons".
With an example of application it is demonstrated that the blank length can be calculated out of a sum of different partial lengths. Here, the trainees should develop more examples for calculation to be solved by them without assistance, if possible.
The techniques depending on material thickness and form of bending edge, as mentioned in the beginning, are described in the Trainees' Handbook of Lessons, in principal.
Versions for each technique are shown, describing the manual making of bends with hammers or with mechanical devices as well as machines. It is not recommendable to describe the techniques all together during the lessons, what is more, it is favourable first to only explain the bending procedures which can be practised in the workshop with the help of existing working means.
That means that following the description of folding sheets practical exercises should be done, applying immediately in practice the knowledge just imparted.
Subsequently, the following techniques will be dealt with in the same way. It would be better, in case of slow learners or those having only a small previous knowledge, to impart the know-how in small, closely restricted sections and then support it with respective practical exercises.
Hints on Labour safety
Give the essential hints at the respective place, when bending procedures to be performed are described. These recommendations are contained in the text and as a compilation in the Trainees' Handbook of Lessons".
3.2. Practical Exercises
Basically, the necessary hints on labour safety have to be given prior to the practical exercises. Then, the trainees receive their working places and the technical equipment of the workshop is checked as to its serviceability. It is recommended to start any exercise with a demonstration in connection with the teacher's instruction related to the instruction example. Here, the trainees are to be motivated to perform the exercises in good quality. Difficulties to be expected have to be pointed out and evaluation key assessment should be mentioned at the same time.
It is necessary for the instructor to have previously performed the exercise himself.
Only in such a way does he know the difficulties that may arise during its execution.
The course of exercise may be in change of instructions on the special techniques and the appurtenant exercises. With the help of "Instruction Examples for Practical Vocational Training - Bending", 6 practical exercises can be done by applying different techniques.
For that purpose, the "Instruction Examples....." contain a material list (base material, tools, appliances, measuring and testing instruments as well as auxiliary means), the sequence of operations to carry out the practical exercises and an illustrative working drawing. The trainees receive all the required information in it to be able to perform the exercises purposefully.
To give a survey, to which practical pieces the knowledge previously imparted is to be applied, the following individual instruction examples are briefly described.
Instruction example 14.1.:
Sheet metals of 1 mm and 2 mm thick steel are to be folded in the vise. For that purpose, a sheet with a simple bending edge is to be bent manually.
A second sheet shall receive two bending edges by means of machinist's hammer and intermediate plate'
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Instruction example 14.2.:
A sheet steel strip is to be folded and rounded off in the vise with the help of a bending mandrel so that a serviceable half-round bracket is the result. Before bending, the straight length is to be calculated.
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Instruction example 14.3.:
During this exercise, a sheet steel is to be rolled on a rolling device as well as round sections to be folded and bent to a round object by means of bending mandrels, so that a serviceable hasp will be the result after assembly.
Instruction example 14.4.:
Double pipe knee
A steel pipe is to be rounded off twice with a bending device as well as with a machinist's hammer in the vise. In this case, sand filler and local heating are to be applied to. The heating length has to be calculated before.
Instruction example 14.5.:
A thin sheet steel is to be folded with the folding press, turned over and rounded off. Subsequently, a jacket seam joint is fabricated.
Instruction example 14.6.:
The previously fabricated beaker jacket is further worked on. After flanging, a bottom piece is connected with the beaker jacket by a bottom seam joint.
All trainees can do the exercises at the same time so far as material and equipment are available in sufficient quantity and number.
In this case, the trainees are able to perform any single exercise individually - each trainee should have as much time as needed.
If there are not enough working means, the trainees must be split into groups. Here it is favourable to divide them into groups according to the application of different tools and appliances.
As versions for the exercises to be done, the bendings can be made with various working means as well. In such a way, foldings should not only be made on the vise, but also on the folding press and folding bench. Even so should roundings not only be exercised manually, but also on the rounding-off device, if available.
Should the proposed instruction examples not be used for exercises, it would be possible to select other practising pieces. In this case, attention is to be paid to the fact that all the techniques previously discussed can be practised on these pieces as well.
Focal points for practical execution
It is recommended for the to fix certain criteria for the assessment of the exercises.
These could be the following:
- Do the trainees prepare their working places carefully?
- Is the straight length of workpiece exactly calculated?
- Is the basic material exactly cut to size?
- Are the appropriate working means selected and properly operated?
- Are the workpieces tightly clamped?
- Do the workpieces receive the required form and accuracy to size?
- Are workpieces damaged by improper work?
- Are the trainees able to assess the quality of their work themselves correctly?
- Do the trainees adhere to the labour safety regulations?
3.3. Questions for Recapitulation
To strengthen and check the knowledge and abilities so acquired, the following tasks have been made out in this section and are also contained in the Trainees' Handbook of Lessons:
1) What is the purpose of bending?
(To remodel workpieces that must have angular or rounded shapes for a specified purpose of application.)
2) Which techniques are used for bending of sheet metals?
(Folding, turning over, flanging, seaming, crimping, rounding, rolling.)
3) What does the term "neutral axis" mean?
(Transition zone in the bent workpiece where no tensile and compressive stresses occur.)
4) Which material properties must parts to be bent not have?
(They must not be brittle or spring-tempered.)
5) What does the term "spring-back" (elastic recovery) mean?
(Elastic materials spring back after an action of force by a certain measure.)
6) What will happen when a sheet metal is being bent around a bending edge that is in accordance with the streak flow of rolling direction?
(Cracks may occur on the bending's outer edge.)
7) What does the term "strain hardening" mean?
(When tensile and compressive stresses change for several times, the internal stresses grow and result in hardening at the bending point.)
8) Which influence has the bending radius on developing cracks in the workpiece?
(The larger the bending radius, the smaller the risk of cracks.)
9) Which influence has workpiece thickness on bending radius?
(The thicker the workpiece, the larger the bending radius must be.)
10) Which influence has the material heating on the bending procedure in case of thick workpieces?
(With growing heat, the internal resistance within the material decreases, the bending procedure becomes easier and is without risk of crack formation.)
11) What does the term "stretched length" mean?
(Length of workpiece before being bent: is calculated from the neutral axis length.)
12) How does the position of the neutral axis change when workpieces are bent around a radius that is smaller than five times the workpiece thickness?
(It shifts to the internal side of the bending.)
13) How is the stretched length of a workpiece calculated, if several different bendings have to be effected?
(Each bending is calculated individually as a partial length, then the partial lengths are summarized.)
14) To which side have you to hammer when a sheet metal is to be folded on the vise?
(Towards the side of fixed jaw.}
15) Which side of the workpiece has to be scribed with the steel scriber?
(On the bending's internal side.)
16) From which sheet thickness onwards should workpieces be hot-bent?
(From 8 mm sheet thickness onwards.)
17) What is the significance of hardwood or metal intermediate plates for bending on the vise?
(They serve to compensate distances when workpieces with several bending edges are to be folded. They also absorb the impact when you strike with a machinist's hammer.)
18) How are sheets with long bending edges clamped in the vise?
(With sheet clamp or angular sections, both being additionally fixed with a ferrule.)
19) What is to be done before sections are folded?
(Prior to bending, the upsetting zone is to be separated.)
20) How are sheet metals to be turned over?
(They are folded by 90° and turned over with presses or hammers to a bending angle of up to 180°.)
21) How is a sheet metal cylinder flanged manually?
(Slightly flange with the hammer pane (striking face of a hammer) on the bordering tool and finish flanging with the hammer face on a plane striking plate.)
22) By which working steps is seaming marked?
(Turning over with shims, hooking-in, pressing together.)
23) What does crimping mean?
(Producing curl-like recesses in sheets to stiffen the metal)
24) How can sheet metals be rounded off in a vise?
(Clamp the sheet with the bending form and turn it overstep by step with the hammer.)
25) With which appliances can sections be rounded off?
(With bending devices, bending dies, section bending machines, swage block.)
26) What is to be noted when pipes of over ½" in diameter are rounded off?
(Pipe must be filled with sand prior to being bent.)
28) What is to be noted when welded pipes are rounded off?
(Welding seam must be lateral to bending radius in neutral axis.)
29) Which length has to be exactly calculated prior to hot-bending?
(The bend length to be heated.)
30) How is the bend length scribed?
(From the dimension length, the dimension leg is scribed to one side on the unbent pipe and the bending leg to the other side.)
31) How may sheet rims be rolled?
(With hammers on round sections, with devices or presses.)
32) How may springs be wound?
(Mechanically with lathes or manually with winding mandrels and wood clamp in the vise.)