Fig. 1585.
The bevel at a would also prevent any side motion to the apron should wear occur between it and the jaws. In addition to this bevel, however, there may be employed two vertical bevels c in the top view in [Fig. 1584]. In English practice, and especially upon large planing machines, the apron is sometimes made to embrace or fit the outsides of the tool box, as in [Fig. 1585], the object being to spread the bearings as wide apart as possible, and thus diminish the effect of any lost motion or wear of the pivoting pin, and to enable the tool post or holder to be set to the extreme edge of the tool box as shown in the figure.
It is desirable that the tool apron bed as firmly as possible back against its seat in the tool box, and this end is much more effectively secured when it is pivoted as far back as possible, as in [Fig. 1585], because in that case nearly all the weight of the apron, as well as that of the tool and its clamp, acts to seat the apron, whereas when the pivot is more in front, as m, in [Fig. 1573], it is the weight of the tool post and tool only that acts to keep the apron seated.
Fig. 1586.
In small planing machines it is a great advantage to provide an extra apron carrying two tool posts, as in [Fig. 1586], so that in planing a number of pieces, that are to be of the same dimension, one tool may be used for roughing and one for finishing the work. The tools should be wider apart than the width of the work, so that the finishing tool will not come into operation until after the roughing tool has carried its cut across.
When the roughing tool has become dulled it should, after being ground up, be set to the last roughing cut taken, so that it will leave the same amount of finishing cut as before.
The advantage of this system is that the finishing tool will last to finish a great many pieces without being disturbed, and as a result the trouble of setting its cut for each piece is avoided; on which account all the pieces are sure to be cut to the same dimension without any further measuring than is necessary for the first piece, whereas if one tool only is used it rapidly dulls from the roughing cut, and will not cut sufficiently smooth for the finishing one, and must therefore be more frequently ground up to resharpen it, while it must be accurately set for each finishing cut. A double tool apron of this kind is especially serviceable upon such work as planing large nuts, for it will save half the time and give more accurate work.