Robert Gibson Griswold,
The finishing of machined surfaces by the process of lapping is in reality a fine art. No other method presents such possibilities in the way of finished surfaces aud accurate fits as does this very excellent method of grinding.
This process is used very largely for truing up holes in cylinders in which a very fine running fit is desired, such as the spindle of a boring mill or vertical drill. Guides of delicate precision machinery are frequently finished by this method, and in the construction of fine measuring instruments it is invariably used.
To illustrate with what degree of accuracy work finished by this method may attain, the writer begs permission to cite the personal experience of an expert tool maker. The question of accurate methods in the construction of sizing blocks was under discussion when the tool-maker took from his tool-chest a neat, flat case which, upon being opened, displayed neatly arranged each in its own velvet-lined pocket, twenty-four sizing blocks ranging from 1-32 in. to 1 in. in thickness. The workmanship was faultless and each bore a polished surface so bright that even the smallest exterior detail was perfectly reflected as if from the surface of mercury. These blocks were made of hardened tool steel, finished by hand exactly to size.
Accompanying this set was a signed certificate from one of the foremost tool-making concerns in the country, stating that this set of blocks had been tested by micrometers and found to vary less than the ten-thousandth part of an inch from size, and also that the angles were perfect right angles. So perfect were these blocks that if any two of them were wiped perfectly dry and placed together by sliding over one another, they would adhere very tenaciously, proving that this fit was so perfect that even a layer of air could not possibly be imprisoned between them. Not only would two of them hang together but almost the entire lot could be supported one from the other in a string when properly placed in contact. This beautiful finish and accuracy in size was obtained by hand lapping on a true cast iron surface plate, without lubricant or abrasive. Unless one has seen such work they can scarcely realize what patience and skill are required to accomplish this result. The simplest piece of lapping which, by the way, is really nothing more than fine polishing, is the finishing of a cylindrical, turned piece in a lathe. This is shown in Fig. 1. Two pieces of hard wood are shaped so that they may be easily manipulated and a semicircular recess cut in each to fit the work. The two pieces are then fastened together with a strip of leather to act as a hinge.
The clamps are now charged with a thin coating of rather fine emery and flour and oil and closed over the revolving piece in the lathe. This coating of emery 60on cuts away the imperfections on the surface of the work when another charge of a finer grade of emery and oil should be applied. Plenty of oil must be used in order that the inside surfaces of the blocks do not become dry. When the grinding has brought the surface to a very smooth finish, clean out the blocks and charge with a thin coat of washed Turkish emery and oil, or crocus and oil. This will give the work a beautiful, smooth polish and the work will be found remarkably true and round.
These clamps are held in one hand and only sufficient pressure exerted to cause the charge to cut. '1 he blocks are constantly moved to and fro along the piece.
so that the minute scratches cross each other, thus removing the appearance of a series of circular scores in the surface. With a little practice this method will produce beautiful surfaces, especially in hardened work. Soft materials are not so easily polished by by this method as the pieces or grains of emery are easily -embedded in the wood, and they often cut very deeply.
The lapping of a hole is not so readily accomplished. In the first place the laps are made of either soft cast iron, copper or lead, the latter being most frequently used. This lap is shown in Fig. 2. A mandrel is first turned up with roughened surface, to which the lead "will stick, and around this mandrel is cast a layer or cylinder of lead slightly larger than the finished hole. It is then placed in a lathe and turned to such a size that it will just push into the hole. Several small grooves are then cut spirally around the circumference, with a small, round-nose chisel.
The lap is then charged with flour of emery and oil until the grooves are filled and the lap is pushed into the hole. It is then twisted backwards and forwards either by a machine or a wrench applied to a square end on the mandrel. As the grains of emery roll out of the slots between the inside surface and the lead, they imbed themselves into the lead, leaving numberless small teeth exposed which cut the surface of the steel as it passes over them. When worn down so that little cutting is accomplished a fresh charge is applied and the grinding continued. A final charge of crocus or washed emery finishes the hole to a beautiful polish and almost true to size.
Of course the most accurate way to finish a hole is by grinding, but there are many times when a grinder cannot be used and the above method forms a cheap and effective substitute. The same precautions are necessary in this case - keep the lap constantly moving to and fro in an axial direction so that the grains cannot follow their previous cuts, and always turn in one direction.
The lap for finishing a tapered hole is shown in Fig. 3. This lap is made in exactly the same manuer as above described, but the grooves should all be cut in one direction. Then the lap should be turned in such a direction that the tendency of these grooves is to throw the lap out of the hole. If turned in the opposite direction the tendency is to screw itself into the taper so that it is impossible to twist it, especially where the taper is slight. It is more difficult to tap a hole of this character than a straight one, owing to the lack of the axial motion to prevent cutting.
The lapping of a square piece is undoubtedly the hardest job. A rather different scheme is used in this oase. The block is first filed or ground nearly to size within one or two thousandths) and one side is rubbed on a smooth cast iron plate with only a very small quantity of crocus and a light oil. This rubbing is a difficult operation to prevent the piece rocking slightly while being moved, which would undoubtedly cause the edges to wear faster than the central portions.
While not entirely overcoming this tendency, placing the tip of the middle finger on the center of the piece and thus imparting motion thereto will afford a fairly even pressure over the entire surface.
When this surface has been ground perfectly true, the next one is finished until it is square with the first. Only patience and labor will bring successful results. When the four sides are ground true with one another, the ends, if the piece is oblong, may be lapped by holding against the blade of a small square to prevent rocking, as shown in Fig. 4. The finishing operation is done by rubbing the surface on a clean cast iron plate, free from oil or any abrasive. This puts on a beautiful polish.
The process of lapping, while only resorted to in cases where grinding cannot be done, is one that should be understood by all mechanics, as there are many cases where it is absolutely necessary to use this method. The finish produced on flat surfaces by lapping is beyond criticism and cannot be equalled by any other method, a buff finish being the nearest approach thereto.