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Aviation History
1928
1928 - 0666.PDF
uTO PLIGHT JILT 19, 1928 THE AIRCRAFT ENGINEER from 18 G. sheet steel. Each proof and its template must be reversible in the case of symmetrical sections, otherwise special markings are needed on the rolls as to which ends are to be threaded on to the rolling mill spindles : this should not be necessary. Fitting the proofs together gives the true contour and gap, and this must agree with the calculated shape. After the rolls or dies are made to the templates and the rolls are bored, broached, etc., it is usual to try them out before hardening, and it is usually necessary to touch them up here and there where the material shows signs of rolling in speed of production (except where the material is pulled through rolls) yet where an experimental section is required urgently, drawing may be resorted to in preference to rolling, as dies can be produced more quickly than rolls. For this reason also it is often more economical to make dies where only a small quantity of the section is likely to be required. If, for instance, a figure, as shown in Fig. 10, were required urgently in a special case, tools as shown in Fig. 11A and 11B could be made. 266, 136 K. 6c. being nipped : this is a matter for ordinary observation. It is always easier to form a symmetrical than an unsymmetrical section. Unsymmetrical sections are rarely required, but when they are, some ingenuity may be required from the operator because of the possibility of the strip being fed through the rolls at varying speeds across its width. Fig. 6, a, b and c, is a case of an unsymmetrical section. Rolls should be made from good case-hardening steel ; ordinary mild steel is often used, but a 0 • 1 per cent, nickel steel is better. Although a glass hard surface is not necessary, yet the harder the surface the better, since there is always a tendency for the edges of the strip to cut furrows in the sides of the rolls and in time such grooves may tend to throw the strip out of line. Owing to the large mass of metal that is sometimes necessary in a roll, it is not always easy to obtain a hard surface without some distortion after case hardening. The only operations after hardening should be polishing and grinding out the bore. Oil lubrication is, of course, an Apart from selecting appropriate stages for the forming process, the only other consideration of first importance lies in keeping the length of arc constant for each curve from stage to stage, as in Fig. 8A, 8B, and 8c, part A n = a., ; r., = a3 ; etc. Various artifices need to be introduced occasionally. For example, in members subjected to low values of stress intensity, arcs having large values of R/< may be used, and it is often better to have a surface composed of one large arc terminated by a small radius than a surface composed of a series of flats joined together by arcs of small radii. In the case of the arc of large radius, it will be found sometimes that the extremities of the calculated contour overlap, thus giving an impossible shape for a die. For example, jj parts of cross section, Fig. 12, were required, then the first operation of rolling could be performed with the rolls, Fig. 13 A and B, correct angles and radii for the spring-back give the shape shown in Fig. 14. In a case like this, a certain amount of 1-24 2>LAT PI 7A. •857' •I5s ,-657;| ROLL. 063* 075* ^, \ 1 . •614' 071? 7B.R0LL. 7c DIE FOR 28G. MATERIAL. 7D. DIE FOR 24G. MATERIAL. BA. 8 B ••206 R 093-9 094 s 6D. DIE FOR 8E.DIE FOR 28G. MATERIAL 24G. MATERIAL. advantage in rolling and a very liberal supply of oil is an absolute necessity in drawing. Die making is quite a different matter from roll making. Where only a small quantity of a section is required, ordinary cast-iron dies may be used, but the gap will be good only for a few hundred feet of strip. Indeed, for soft metals a simple wooden die can be made. For quantity production in steel it is usual to make the body of the die in cast iron, the lead-out portion being fitted with a hard steel face. Owing to the harsh nature of the drawing operation, this exit face should be glass hard, or rapid wear will take place and frequent renewal will be necessary. A sketch of a typical die is shown in Fig. 9. Special devices are frequently necessary for use with dies, such for instance as floating mandrils for forming complete curls, etc. There is usually no difficulty in fitting these to the dies. While drawing cannot compare with bending may be done along part of the arc, as shown in Fig. ] 5, portion x. Then allowing this to spring back, to bend a further portion (y) of the arc as shown in Fig. 16. Thus is the required shape obtained. The same process is necessary in working extremely thin strip. The above cases provide on their own account very good arguments for continuous heat-treatment. Sections of this contour, however, are not required in large quantities. On the drawings giving the stages of forming, the centres from which the various radii are struck should be specified in relation to two axes at right angles, as indicated in Fig. 1A. Such centres are readily found by calculation and otherwise, and these dimensions are of great service to the template makers, and should be used as a check on the lay-out of the contours. In new designs of members, close co-operation between 6146
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