Laps are usually made of soft cast iron, copper, brass, lead or ceramic. In general, soft close-grained iron is the best material to use for a higher degree of accuracy in the finished product. Whatever material is used, the lap should be softer than the work or the work will become charged with the abrasive and cut the lap. External laps are commonly made in the form of an outer ring or holder and an inner shell, which forms the lap proper. This inner shell is made of cast iron, copper, brass or lead. Ordinarily, the lap is split and screws are provided for adjustment. The length of an external lap should be equal to the diameter of the work or longer. Large ring laps usually have a handle for moving them across the work. Laps for producing plane surfaces are made of cast iron. In order to secure accurate results the lapping surface must be a true plane.
A flat lap that is used for roughing or blocking down will cut better if the surface is scored by narrow grooves. They are usually located about 1/2 inch apart and extend both lengthwise and crosswise thus forming a series of squares similar to those of a checkerboard. For fine work a lap having an unscored surface is used and the lap is charged with fine abrasive. After the lap is charged all the loose abrasive should be washed off with gasoline or solvent.
When lapping, the surface should be kept moist with a light oil. Loose abrasives should not be applied while lapping. If the lap is well charged with an abrasive in the beginning and is kept well moistened during moderate use, it will cut for a longer time. The pressure upon the work should be just enough to insure constant contact. The lap can be made to cut only so fast, and if excessive pressure is applied, it will become stripped in places. The cause of scratches can result from loose abrasive on the lap, poorly graded abrasive, or too much pressure on the work.
To produce a perfectly smooth surface, free from scratches, the lap should be charged with a very fine abrasive. When the entire surface of the lap is charged, clean and examine the lap for bright spots. If there are any visible bright spots, continue the charging until the entire surface has a gray appearance. If a lap is once charged is should be used without applying more abrasives until it ceases to cut. If a lap is overcharged and an excessive amount of abrasive is used, there is a rolling action between the work and the lap, which results in inaccuracy.
The surface of a flat lap is usually finished true prior to charging by scraping and testing with a standard surface plate or by a method of scraping where three plates are lapped together to secure a plane surface. In any case, the varying marks or spots must be uniform and close together. These spots can be blended, by covering the plates evenly with a fine abrasive and rubbing them together.
With the wet method of using a surface lap there is a surplus of oil and abrasive on the surface of the lap. As the specimen is moved there is shifting of the abrasive particles. With the dry method, the lap is first charged by rolling or rubbing the abrasive into the surface. All surplus oil and abrasive are then washed off, leaving a clean surface and one that has a uniform embedding of small particles of abrasive. It is like the surface of a very fine oilstone and will cut away the hardened steel that is rubbed over it. While this had been called the dry method, in practice the lap is kept moistened with gasoline, naptha or any other thin-filmed oil.
In lapping it is desirable to avoid unnecessary scratching. For a fine background even a minute scratch is evident and unsightly. In some cases it is detrimental to the general functioning of the part. To prevent scratching, several conditions must be observed and maintained. The room where lapping is being performed must be reasonably free from dust. The proper abrasive must be selected and a suitable vehicle or carrier used. The proper amount of abrasive must be applied and used at the right time.
Generally speaking, the most suitable abrasive is one that is readily embedded in the surface of the lap. Soft cast iron is used for making laps on mechanical machines with a fine abrasive. To insure straight work the lap surface must be perfectly flat. To prevent undue wear the lap width should be made to conform with the length of the work being lapped. The laps may be made flat with true plane surfaces by dressing together in sets of three as in making master surface plates. Dress plate No. 1 with plate No. 2, No. 1 with No. 3 and then No. 2 with No. 3. The surface condition of the laps will determine the quality of the finish. By the use of multi-motion and ever changing work movement relative to the work, a true lapping action is obtained, and uniform lap wear is assured.
The advice given to a machine operator should be similar to that given to the chef who makes the salad. "Be a miser with the vinegar and a spendthrift with the oil," but in this case it should be, "Be a miser with the abrasive and a spendthrift with the oil." This is the secret of good lapping. When there is a super abundance of abrasive rolling loosely on the laps and the work, it is almost impossible to obtain a good finish. The lap wears unduly and the accuracy of the product is sacrificed.
An old method of lapping consisted of lapping with a soft metal lap, into which was rolled an abrasive such as diamond dust, Silicon carbide or one of the natural abrasives. The metal laps serve much the same purpose as the bond in a grinding wheel. In the modern method, the abrasive is embodied in a suitable carrying base or vehicle and allowed to roll between the lap and the work.
Using this method, it is essential to select the lap material with care, considering the material of which the work is made. Strange as it may seem, in this process the greatest amount of stock is removed from the harder of the two parts. Therefore the lap must be softer. The reason for this is that in either very soft or very porous laps the abrasive grains are held temporarily stationary by being embedded in the lap. Since their position is fixed with respect to the lap but moving with respect to the work, they cut the work. Thus it is very easy to see that the lap material has a great influence on the speed, accuracy and finish of the work.
Open grained cast iron will work faster than dense cast iron or steel. For certain jobs laps are made of the softer metals or even wood. The speed depends upon several variables such as the material to be lapped, the material of the lap, the type of vehicle, the type and size of abrasive, the pressure on the lap and the personal skill and even the psychology of the operator.
THE DEFINITIONS OF LAPPING:
LAPPING is the balancing of abrasive grit size and proper hardness or kind of abrasive against lapping time and the pressure of the part on the plate or lap.
The proper balance is achieved when all abrasive particles break down completely into inert sizes, while removing the desired amount of metal and wearing out the abrasive power of the particles.
There are no hard, fast rules to follow and each operator must experiment to find the proper compound and proper lap. He then combines them with his own skill and experience to solve his lapping problem.
Lapping has four main purposes:
Good lapping plates are made of soft, close-grained cast iron or meehanite. A scleroscope hardness of 27 to 32 has proven best for cast iron surfaces. Harder laps will often cause glazing and scratching while softer laps cause a loss of flatness and parallelism and will produce grayer finishes.
The plate should be heavy enough and properly designed so it will not distort in use. Its surface may be plain or grooved. Plain plates are best suited for lapping cylindrical work and for extreme accuracy. Grooved plates, which have a working surface serrated with vee-grooves V.040 deep and 1/2" apart, are preferred for lapping flat work only. A rotating plate may have a spiral groove leading from the center to the outside edge.
LAPPING MACHINES & SPECIFICATION questionnaire (hand lapping with a rotating plate):
Vertical-spindle lapping machines, with single horizontal rotating plates, are made in sizes from 10" to 56" in diameter. These machines can be equipped with any desired lap material, bonded abrasive disc or lapping wheel. As the lap rotates the work, held by hand, is moved at random across the lapping wheel. If the size or weight of the part does not provide proper pressure, the operator must apply whatever is necessary. Light pressure gives greatest accuracy, heavy pressure removes stock more rapidly.
This method includes the addition of a mechanical means of applying pressure and motion. The work or parts are carried in a holder of the planetary type that revolves or rotates about the center of the lap. Mechanical lapping can also be performed with non-rotating laps for certain types of flat lapping: for gage blocks, etc. Machines of this type have a stationary lower and a removable non-rotating upper lap. The weight of the upper lap can be adjusted to meet lapping requirements. Such lapping machines may also use bonded abrasive laps.
Another method of lapping is a refinement of cente-less grinding to produce straight cylindrical objects. Such a machine has two rotating rollers. One roller is twice the size of the other. Both revolve slowly at the same speed and in the same direction. The work is held in the throat or V between them and is caused to rotate at the speed of the smaller roller. Because of its increased surface speed over that of the work, the large roller causes a rapid lapping action. The lapping pressure is applied manually through a notched fiber stick held against the work and moved evenly over the entire surface. It is desired to correct taper or other error, the stick is held longer on the portion requiring more stock removal.
Cylindrical parts to be lapped by this method must be ground within 0.005" of the finished size and well within the tolerance for the roundness desired. Usually tow machines are available, or two sets of laps-one set for roughing and the other for finishing. Diamond compounds must be used for tungsten carbide parts. Several of the alumina compounds can be used to impart color or luster to the part and in most cases this finishing is done with a slight addition of naptha or gasoline to the rotation rollers.
These laps are usually in the form of a ring which is split and adjustable by means of a screw arrangement. Some external laps have an inner-ring of copper, brass, lead, tin, etc. External lap length should be at least equal to, or longer than, the diameter of the part being lapped.
In wet lapping, the abrasive is mixed into a carrier or vehicle. This even film of abrasive covers the total lap area. The carrier acts as a lubricant and absorbs the heat generated by the abrasive against the metal. It allows the abrasive to cut the material by resisting the rolling of the abrasive granules.
This method is used for the final finishing of a part. Laps are charged with the proper abrasive and the surface is barely moistened with a very thin oil such as spindle oil, gasoline, naptha, mineral seal oil, etc.
Caution: Kerosene may cause scratches.
Just enough pressure to insure contact is used. If too much pressure is used, or if the lapping operation is too fast in motion, the lap may strip. Foreign matter, such as dirt and lint from the operators clothing and various sources, can also cause scratches in the work. A wash stand for frequent use by the operator is recommended for all lapping departments.
NOTE: Lapping often precedes polishing, and the quality of a polished surface is largely dependent on the lapping process. The selection of lapping abrasives, vehicles, additives, pressures, speed, plates and many other factors play an important role in this complex, controlled stock removal process. When the quality of a polished surface is critical, we strongly recommend selecting the finest uniform-graded abrasive compound available.
LAPPING RULES OF THUMB:
(knowledge gained through practical experience)