In setting out plater's work for boilers or other similar class of work, a high degree of accuracy is required if joints are to be properly constructed, and the various parts made to fit together as they ought to do. The settings out for the inside and outside plates of a cylindrical shell are shown in Fig. 292. The thickness of metal is purposely drawn out of proportion to the diameter, so as to better exhibit the construction lines. The lengths of the plates can be obtained, as previously stated, by measuring the lengths of the centre lines of each ring in section and setting out for inside and outside plates respectively. A much better plan, however, and one that will give more accurate results, is to calculate the lengths of the plates. Thus, suppose the inside diameter of inner tier of plates is 12 in. and the plates 1 in. thick, then the girth of outside plates will be: -
And inside plates
Difference in lengths
It should be observed that the difference in length between the inner and outer plates is 2 x 3.1416, and this gives us a rule by which we can always determine the difference between their lengths: -
Difference = twice thickness of plate x 3.1416
Or thickness of plate x 6.2832
If 3 1/7 be used instead of 3.1416, then this difference will always be -
Thickness of plate x 6 2/7
For an accurate-fitting joint, the calculation of this difference is really of more importance than the exact girths. It should be borne in mind that before proceeding to calculate, the thickness of the plate should be carefully gauged. A plate may be called a certain thickness; but as plates are usually rolled to a given weight per square foot, the thickness may be a little more or less than that stated. Consequently, if the calculations are based on a given thickness, and the plate happens to be a shade thinner, the joint will be slack, and if the plate is thicker than that allowed for, the joint will be too tight.
The pitch of the rivet-holes in the two plates can be measured directly from the centre line circles on the section of the two rings. Thus the length along the arc from A to B will be the pitch of the holes on inner plate, and the length measured along the curve from C to D will equal the pitch of holes in outer plate.
Whilst the above method is accurate enough for rough work, or for jobs bringing in only a small part of a circle, it is not of much use where very particular work is wanted. The pitch can be determined by arithmetic from the following rule -
Pitch of holes = diameter of neutral circle x 3.1416 / number of holes in circle
Thus in the present case: -
Pitch of holes in outer plate = 15 x 3.146 / 12 = 3.927 in.
Pitch of holes in inner plate = 13 x 3.1416 / 12 = 3.4034 in.
When the distances between the hole centres run out to such awkward figures as those above, we are confronted with a fresh difficulty in not being able to set the compasses, with exactness, to this length. So that, in practice, it is a good plan to mark the two end holes and then carefully subdivide the distance; the calculations above giving considerable aid. Usually, the centre of end holes would come on the end lines of net template; but in the present case no lap has been allowed for so as to simplify the problem. As the holes are arranged in Fig. 292, it will be observed that the distance from edge of plate to first hole will be equal to half the pitch on each plate.
The way to calculate the required pitch for any given thickness of plate, and the proper formation of the various riveted joints, will be dealt with later.
In most of the better-class boiler work the plates are rolled and the joints tacked together before drilling, the bulk of the holes being drilled in position. In this way holes with irregular walls are obviated, the joint left stronger than with punched holes, and no stresses set up in the joint. The calculations for lengths of plates and pitch of rivets will, of course, have to be carried out whether the plates have punched or drilled holes, or holes drilled after the plates are shaped and fixed.