If the threads are cut so that the reverse is true then they are "left-hand threads ".
Eye Bars. These are used in pin connected trusses and structures to take care of tensile strains. The heads at each end are formed by upsetting machines and the pin holes afterward bored out. See page 331 of Cambria Handbook for dimensions, etc., of standard eye-bar heads.
Batten Plates. In Fig. 225 a batten plate is placed at each end of the strut on the top and bottom of the flanges. It is used merely to tie together the two parts of the strut. Batten plates (also called tie plates) are used generally wherever lacing is used in order to tie the parts of a member together at each end of the lacing. The Pencoyd Iron Works specifications for railroad bridges gives the following in regard to tie plates: "All segments of compression members, connected by latticing only, shall have tie plates placed as near the ends as practicable. They shall have a, length of not less than the greatest depth or width of the member, and a thickness not less than one-fiftieth of the distance between the rivets connecting them to the compression members ".
Chas. Evan Fowler, in his Specifications for Roofs and Iron Buildings, refers to tie plates as follows: "Laced compression members shall be stayed at the ends by batten plates having a. length equal to the depth of the members ".
The rules given in various specifications are somewhat different as regards the length and thickness, being determined by each authority merely on his own judgment of what will prove satisfac tory. There is no method of proportioning batten plates except in accordance with such specifications as may be furnished in rela tion to the particular job of work in hand.
Lacing. Single lacing is used on the girder shown in Fig. 225, but if two systems of lace bars are used crossing each other and riveted at their intersections, it is called double lacing_ This is only used on very heavy members. Single lacing is usually placed at an angle of about GO degrees with the axis of the member, while double lacing is placed at about 45 degrees to the axis.
The size of lace btrs to use is somewhat a matter of judgment, but certain rules have been established by common practice and experience which it is well to observe when practicable. Chas. Evan Fowler's specifications give the following: The sizes of lacing bars shall not be less than that given in the fallowing table. When the distance between gauge lines is They shall generally be inclined at 45 degrees to the axis of the member, hut shall not be spaced so as to reduce the strength of the member as a whole. Where laced members are subjected to bend ing, the size of the lacing bars shall be calculated, or a solid web plate used.
Shop Drawings. In making shop drawings, the outlines of the member (in other words, the ``picture" of it) should be done in fairly heavy lines, so as to show up clearly on the blue prints, and the dimension lines should be very light so that they will not be confused with the outlines of the members. All distances should he given from center to center, wherever possible. Dimensions from the edge of an angle, beam, or plate, should never be given unless there is a special reason for so doing; because all rolled shapes vary in the width of the flanges, and Z-bars also vary in height. The reason for this variation is that different sizes are rolled by the same set of rolls and the difference is made in the spacing of the rolls. See Figs. 25, 26, 27 of Part I. Also, angles of a thickness of one half inch or more vary somewhat in the length of legs unless they are given what is called a finishing pass or rolling which is not always done.
Make all drawings on the dull side of tracing cloth with a No. H H or a No. II H H pencil. After the drawing is completed the pencil marks are easily removed with a piece of sponge rubber.
Do not draw out your work on paper first and then trace it. You will find that this is a waste of valuablq,time. Learn to draw directly on the tracing cloth, as you will be expected to do when you begin work in an office. You will need the following outfit in the way of drafting instruments and equipment: A shop drawing is a drawing which gives all the information necessary to lay out, cut, punch, and rivet the piece shown. It is the medium by which instructions are conveyed from the engineer's office to the shop. It must convey full, accurate and explicit instruc tions for every operation. It must be so clear and explicit that no further explanations are needed to enable the shop to correctly interpret it, and the information must be given in such form that only one interpretation is possible. The draftsman making a shop drawing must constantly bear in mind that the man at the shop will work entirely from this drawing; that he does not have access to the sources of information which are consulted by the draftsman in making the drawing, and that what might be clear in connection with these other drawings will be blind or uncertain to the shop man not familiar with them. The draftsman should further understand that it is distinctly the duty of the shop man not to read into the drawing anything not there, and that consequently the responsibility is entirely upon the draftsman to make his drawing so complete that such action will be unnecessary and impossible. Neatness in exe cution of a shop drawing is desirable, but accuracy and clearness are absolutely essential.