Robert Gibson Griswold
Those who have attempted to drill or bore an accurate hole on a drill press know just what a difficult operation it is. This is due to the fact that, no matter how carefully a hole may have been laid out, to make a twist drill follow a hole true to the mark is next to impossible, owing to the fact that one lip of the drill may cut a little faster than the other, causing it to run. For ordinary work this will do very well when the difference does not exceed a few thousandths of an inch, ■ but if the work is being done it is very essential that the hole be exactly right.
Quite frequently one may see a machine " draw a drill " while starting the hole. When the hole is laid off a smaller concentric circle is drawn inside the first. When the points of the drill enters the material to ascertain depth it is an easy matter to see whether the cone-cup is exactly central with the smaller circle. If it is, the drill is following true, but if not a small groove is cut down the side of the cone on the side farthest from the line, or on the side toward which it is desired to bring the drill to make it coincide with the true center, or the circle denoting the periphery of the hole.
Then when the drill is again started the resistance on this side of the hole is lessened and the drill slips over slightly, cutting more on this side and bringing the hole more nearly in line with the intended center. But the eye is unable to accurately judge when a coincidence between the scribed circle and the edge of the cup occurs, at least within a few thousandths of an inch, and if great accuracy is desired this method cannot be used.
Let as suppose that it is required to drill three holes of different sizes in a die block, as shown in the figure. After finishing the sides of the block until they are truly parallel, coat one (the upper) side with copper the lower edges of which will lightly press on the upper edges of the sides of the drawers. These strips should be 15 in. long and about 3 in. wide and fastened with 1 in. wood screws and glue, putting on the glue"after locating the position, with two screws, one at each end.
The reader may at first thought reach the conclusion that the work here desribed is of considerable amount and rather difficult, but a careful reading of the directions and study of the illustrations will enable anyone of ordinary skill to make a very serviceable and presentable cabinet, and those having use for one will find the making of it will affect a very considerable saving.
upon which the fine marks will show very clearly. This coppering is done in the following manner:
After being sure that the surface is perfectly clean of grease, which may be effected by rubbing it with a rag and chalk, rub it with a rag wet with a solution of copper sulphate made by dissolving several large crystals of copper sulphate (blue-stone) in a bottle of water to which is added several drops of sulphuric acid. This solution will deposit on the surface of the metal a firm coating of copper upon which the finest lines will appear distinctly.
The holes are then carefully laid off and marked with a very sharp center-pencil. The piece is laid on the fac9 plate and the clamps adjusted, one of the holes being as nearly central as possible, which may be determined by measuring with a pair of hermaphrodite calipers from the periphery of the face-plate. Before the block is adjusted on the face-plate a sheet of writing paper should be placed beneath it which will give greater friction between the block and plate. The clamps are set up just firm enough to hold the block in place and the face-plate screwed onto the spindle.
Then the center indication mentioned in chapter V, of the January, '05, issue is placed in position with the point of the small or short end in the prick punch mark, and the long end point quite near the tail center. The face-plate is now slowly revolved to quarter turn position, which will indicate in which direction the block must be shifted to bring the center exactly in line with the center line of the lathe. A gentle tap with a hammer will move the block very slightly one way or the other until, when the spindle is rapidly revolved, the long end of the indicator shows no movement but remains stationary. The clamps are now set down firmly and the plate revolved to see that the screwing down of the clamps has not shifted the block slightly.
A centering tool, Fig. 24, page 24, November, '04, issue, is now placed in the tool post and a center carefully bored to start the drill. Then a very carefully sharpened drill of the size required, if a very small hole, is put in position with the point in the center just bored, and the pointed rear end placed in a cup center in the tail-stock. If the drill has a center hole drilled in the rear end, so much the better, as it can then be rested against the tail center. The drill is then gripped firmly with a dog or clamp and the work revolved, feeding the drill meanwhile with the tail-stock feed screws.
If the drill has been properly ground, that is, with both lips exactly even and of the same length, the will bore a true hole exactly where desired. This method, however, may be improved on in holes above 3/8 in., as a small boring tool can then be used. A hole slightly under size is first drilled and finished to exact size with the boring tool, which method is the most positive of any for securing a true hole exactly where designed.
The plate is then shifted to a new position and centered and the next hole bored, and so on. With very careful setting these holes may be bored without a greater variation than half-a-thousandth of an inch or less. If the holes are to be reamed they should be left about .01 in. smaller than finished size and then reamed with the reamer resting against the tail center.