Draw-filing means grasping each end of the file and moving it back and forth sidewise along the work. For this purpose single-cut files are used. The smoothing is done with a very fine single-cut file, or if very particular, a float file is used. Then the polish is rubbed on with fine emery cloth and oil. The emery cloth is wrapped around the file and the same motion is continued. With some little practice a very creditable piece of work may be turned out. Such work is valuable because of the instruction. A good test of skill at blacksmithing is making an octagon punch that tapers true to the eye when finished. _Set-Screws._--It is customary to fasten a good many gear wheels, cranks and pulleys to machinery shafts by set-screws. There are two kinds of set-screws; one has a cone point, the other a cup end. Both screws are hardened to sink into the shaft. A cup is supposed to cut a ring and the point is supposed to sink into the shaft to make a small hole sufficient to keep the wheel from slipping. However, unless the cone-pointed screw is countersunk into the shaft, it will not hold much of a strain. The point is so small it will slip and cut a groove around the shaft. To prevent this, the set-screw may be countersunk by first marking the shaft with an indentation of the point of the screw. Then the wheel or crank or collar may be removed and a hole drilled into the shaft with a twist-drill the same size, or a sixty-fourth smaller, than the set-screw. Then by forcing the end of the set-screw into the drill hole, the wheel is held solid. [Illustration: Figure 94.--Machine-Bolt and Set-Screw. The bolt to the left is used to clamp cylinder heads in place. The set-screw to the right is the cup variety. The end is countersunk to form a cup with a sharp rim.] The principal objection to set-screws is that they are dangerous. The heads always project and are ready to catch a coat sleeve when the shaft is revolving. In all cases, set-screws should be as large as the hub will allow, and it is better to have them protected so it is impossible to catch anything to wind around the shaft. Cup set-screws are not satisfactory except for very light work. If necessary to use them, the ends may be firmly fixed by cutting a ring with a sharp, diamond-point cold chisel.