When it is undesirable that the slots run out to the edge of the work they may terminate in a recess, as at a in [Fig. 400], which affords ingress of the bolt head to the slot; or the bolt head may be formed as in [Fig. 401], the width a b of the bolt head passing easily through the top a b of the slot, and the bolt head after its insertion being turned in the direction of the arrow, which it is enabled to do by reason of the rounded corners c d. In this case, also, there may be a square under the head to prevent the bolt head from locking in the slot, but the corners of the square must also be rounded as in [Fig. 402].
Fig. 403.
The underneath or gripping surface of a bolt head should be hollow, as at a in [Fig. 403], rather than rounding as at b, because, if rounding, the bolt will rotate with the nut when the latter grips the work surface. It should also be true with the axial line of the bolt so as to bear fairly upon the work without bending. The same remarks apply to the bedding surface of the nut, because to whatever amount the face is out of true it will bend the threaded end of the bolt, and this may be sufficient to cause the bolt to break.
Fig. 404.
In [Fig. 404], for example, is shown a bolt and nut, neither of which bed fair, being open at a and b respectively, and it is obvious that the strain will tend to bend or break the bolt across the respective dotted lines c, d. In the case of the nut there is sufficient elasticity in the thread to allow of the nut forcing itself to a bed on the work, the bolt bending; but in the case of the bolt head the bending is very apt to break off the bolt short in the neck under the head. In a tap bolt where the wrench is applied to the bolt head, the rotation, under severe strain, of the head will usually cause it to break off in all cases where the bolt is rigidly held, so that it cannot cant over and allow the head to bed fair.
A plain tap bolt should be turned up along its body, because if out of true the hole it passes through must be made large enough to suit the eccentricity of the bolt, or else a portion of the wrench pressure will be expended in rotating the bolt in the hole instead of being expended solely in screwing the bolt farther into the work.