ROBERT GIBSON GRISWOLD

For truing up flat surfaces, the piece to be operated upon is strapped to the face plate either directly in contact therewith or, if it has one finished surface, it is laid on two parallel strips which insure this surface being exactly parallel with the surface of the face plate.

But before going further it is well to know whether the face plate is true; that is, whether it is a true plane, and whether the tool travels across the carriage in a line exactly at right angles with the center line of the lathe. If it does not it will be an extremely difficult operation to true up the face plate, or to make a good job of the work strapped to it. But assuming that the face plate is reasonably true, we will consider the steps in facing a bass on a rough, flat casting, like that shown in Fig. 1, and boring a hole through it having a shoulder about half way through, as shown in section in Fig. 2.

The operation of strapping the the pieces to the face plate is more readily performed if the plate is removed from the spindle and laid on the bench. Strapping work to the plate while in the lathe is a laborious operation excepting when the piece can be held against it by the tail center.

After running a file over the back to make the surface reasonably flat and free from small projections, it is placed against the face plate with a piece of paper between. The object of this paper is to prevent the shifting of the piece when the tool takes a cut. It affords greater friction between the work and the plate. Three or four clamps are now set up against the piece, these clamps being simple straps of say 1/2 x l 1/4 in. flat iron with a hole through them for the bolt, as shewn in Fig. 3. These bolts are first set up so as to clamp the piece firmly against the plate, but not so that it will not move when struck on the edge with a hammer. The piece is now ready to be centered.

When placing it on the plate it is centered as nearly as possible by eye, but the final adjustments are made in the lathe where the piece can be rapidly revolved. After screwing the face plate home the work is set in motion either by putting the belt on, running by power at a slow speed, and a piece of chalk is held in the hand and up against the edge of the work. The" hand or arm rests against some solid part of the lathe, generally the tool part or carriage, and the chalk is just allowed to touch the high spots. When the work is stopped it will be found to bear chalk marks on the "high side " or on the side furthest from the center. It is quite obvious, therefore, that the piece should be moved slightly to the opposite side, and this is done by gently tapping it with a hammer. The chalking process is again repeated and the work moved until it is truly centered. Then the clamps are set down hard, care being exercised to«ee that the work is not moved during this operation.

The question as to whether the tool should be fed in towards or out from the center of the work is hard to decide, as this question, too, has its opponents. It is frequently decided by the character of the work itself, as the position of a shoulder on the face will determine in which direction the tool must be fed.

But in this case we will feed the tool in toward the center. A diamond point tool is the proper one to use first, setting it on an angle, as shown, and grinding the point so that it has a left-hand top rake. A point to be remembered in all work of this kind where castings are to be machined, is that the surface of a casting is very hard and that this "skin" or scale is very apt to take the edge off a tool even at a low cutting speed. Sometimes it seems almost impossible to make the tool penetrate below this skin, in which event it is well to score the edge about to be cut with a cold chisel. This breaks the surface and allows the tool to get under the scale.

When the roughing cuts have reduced the piece to within a few hundredths of an inch of finished dimensions, the side tool is put in and the finishing cuts are made with a sharp corner. A right-hand offset cornering tool must be used, as shown in Fig. 5. If a round fillet is required, a round-nose tool is used, as in Fig. 6.

Supposing the piece has now been finished in size, boring the hole next commands our attention. A centering tool is now placed in the post and a center bored in the face of the piece just deep enough to start the drill. A drill slightly smaller than the finished size of the hole is then set in the cup just bored, and the small center hole in the opposite tapered or stock end of the drill is placed on the drill to prevent its turning, the tail of the dog resting against the tool carriage.

When the hole has been drilled completely through (and right here is one reason for placing the work on parallel strips. If the hole to be bored is smaller than the topmost hole in the spindle, the drill and tool will not touch it, but if larger, the work must be mounted on strips that will allow the points of the drill and the tool passing completely through without touching the face plate) the drill is removed and the boring tool placed in the post. It is fed into the work with a small cut and fine feed, the tool being gradually fed out until the hole is finished exactly to size. The boring tool always cuts on the front edge and should be so ground as to present only a small cutting surface; the tool, owing to its shape, is not very stiff and if a broad cutting edge is used considerable chatteiing will surely occur.

But this hole is shown provided with a shoulder about half-way in. The first hole bored is the size of the smaller one. The tool is then fed out so as to enlarge the hole, and this cut is run in until until say 1-32 in. of finished depth and a few hundredths of an inch of finished diameter. Then the point of the tool, or better still, another tool kept especially for such work, is ground with a square corner and the finishing taken with this. Make the bottom cut by feeding the tool towards you rather than towards the face-plate, as the latter method presents a broad cutting surface, while the former takes only a small cut on the advancing edge of the tool.

This finishes this particular piece. In calipering the diameter of the hole, do not allow the calipers to drop below center, but measure the hole with the calipers standing as nearly perpendicular to the surface of the work as possible, as shown in full lines, Fig. 9, and not as shown in dotted lines.