After all pieces are roughed out the screw and dowel holes are drilled. The dowel holes, however, are drilled, and then tapped with a fine-pitch thread; if a 3/8-inch dowel is used, the holes can be tapped, say, ½ - 32 pitch. The object of tapping the dowel holes is to permit screwing in tight-fitting soft-steel plugs after the pieces have been hardened, following which the plugs are dressed off flush with the top and bottom of the piece. Then after all pieces are ground to size and securely fastened in proper place by means of the screws, the dowel holes are drilled and reamed through the soft-steel bushings or plugs and into the die shoe. This is better practice than to drill and ream the dowel holes in the pieces before hardening, for, after hardening the pieces, the holes are slightly distorted; but, granting that the holes remained true, it becomes necessary to transfer the holes to the die shoe, and in order to do this a drill is used, using the dowel holes in the hardened pieces as a jig. The drill used must of necessity be somewhat smaller than the hole in the hardened piece, possibly not more than one or two thousandths, but whatever the difference between the drill and the hole is, that difference can cause an error in the alignment of holes in the hardened piece and in the die shoe, as the drill can bear against one side of the guide hole, drilling the hole off center. Using the hardened pieces for a guide prohibits the use of a reamer, for, in order to have the reamer size the hole in the die shoe and to bring the hole in the die shoe absolutely in line with the hole in the piece, the reamer must fit the hole in the hardened piece, which of course would ruin the reamer. The greatest objection to using the holes in the hardened piece for a guide for the drill and reamer is that the holes are invariably distorted during the hardening process.
Having drilled and deeply counterbored all screw holes, and drilled and tapped all dowel holes, the pieces are hardened, but, as the die is to cut iron having scale, the die is left harder than for ordinary sheet steel.
The first grinding operation is to take a chip across a temporary bed, or the grinder bed itself, or the face of a magnetic chuck to insure the surface being parallel with the travel of the cross-slide and the travel of the bed. The pieces are examined on the bottom side to make sure no burrs are protruding or scale adhering to the pieces that would tilt them, for there is only an allowance of .010 inch to remove, and a slight tilt might be sufficient to prevent the pieces cleaning up all over. All pieces are then placed on the newly finished bed and waxed to the bed, unless a magnetic chuck is employed. After all pieces are ground on the top side, the wax is thoroughly removed from the bed, the pieces cleaned, and the surfaces just ground are waxed to the bed, and the bottoms then ground. All pieces now are of uniform thickness and parallel. An angle iron, or, what is better suited for this class of work, a hollow square, absolutely square in every position, Fig. 30, is now waxed to the surface of the newly machined bed. In either case the angle or hollow square must be absolutely square and must be waxed to the bed so that the vertical face of the angle is absolutely parallel to the line of travel of the bed. This is accomplished by clamping an indicator to the side of the wheel. Do not trust a square against the machined surface of the uprights of any machine, if the surface of the work must be parallel with the line of bed travel, for by so doing we are trusting to the accuracy employed by the machinist who built the machine: eliminate all chance.
Fig. 30. Hollow Square Used for Holding Pieces for Grinding.
Assuming that the hollow square is correctly located and waxed to the bed, the first piece to be ground can be a, Fig. 29. By clamping the piece to the hollow square and using the indicator attached to a surface gage, the piece can be positioned parallel with the surface of the bed by testing each end of the piece for height, using the indicator. The piece need project above the hollow square only a slight distance, say 1/16 hich. As the top and bottom of piece a are perfectly parallel and the hollow square is perfectly square - at right angles - it is obvious that when the upper face of piece a is ground, the face will be at right angles with the top or bottom.
Piece b, Fig. 29, is clamped to the hollow square, as in Fig. 31, and the end of the piece is lined up with the bed by placing a square on the bed. The use of a square will be found accurate enough for the grinding of the first edge, but, after one edge is ground, if the remaining edges are trued up at right angles with each other by holding the base of a square against the vertical edge of the work, and using an indicator clamped to the emery wheel and with pointer sliding along the blade of the square, as in Fig. 32, the work can be made more accurate than by holding the blade against the work while the base of the square is on the bed of the machine. After the first edge has been ground, the piece should be tested, as in Fig. 33, to prove that the side of the hollow square is at right angles with the bed, as grit can get under the hollow square unless extreme care is exercised. The piece 6 now being ground on two sides, its succeeding sides or edges may be ground by a similar procedure. Each piece must be ground on the edges as above described, always using the indicator to square up the work, for the use of parallels is not safe. A slight nick on a parallel or a piece of grit or even a slight taper of the parallels would of course be transferred to the work.
Fig. 31. Piece Clomped to Hollow Square on Machine Bed.
Fig. 32. Emery Wheel and Indicator Mounted for Grinding.
Fig. 33. Proving Accuracy of Work after First Edge Is Ground.