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TM 9-254
4-20. Abrasive Processes - Continued
a.
Forms of Abrasives. Abrasive products are used in three basic forms:
(1)
Bonded, to form a solid shape tool such as disks, cylinders, rings or sticks.
(2)
Coated, on backings made of paper or cloth in the form of sheets, strips or belts.
(3)
Loose, held in some liquid or solid carrier, (for lapping and polishing) or propelled by centrifugal force such as
air or water against the work surface (blast cleaning).
b.
Application for Abrasives. Application for abrasives are multiple and varied.
(1)
Cleaning, of surfaces. Coarse removal of excess material.
(2)
Shaping, such as tool sharpening.
(3)
Surface finish improvement, primarily as in lapping, honing, and polishing.
4-21. Lapping.
a.
General. Lapping is an operation for removing a small amount of metal with an abrasive compound either by
hand or machine. Lapping produces a smooth, but not necessarily polished, surface that is not usually obtainable
through ordinary machine and hand operations such as turning, thread cutting, filing. Lapping must be performed,
therefore, to properly finish such bearing surfaces as sleeves, worm gears, and eyepieces. Machine lapping is not
normally within the field of work for the instrument repairperson as it is used chiefly in production work.
b.
Lapping Compound. The abrasive mixture used in the lapping process is known as the lapping compound and
must be mixed by the instrument repairperson. It is composed of artificial abrasives such as silicon carbide,
manufactured boron carbide, or manufactured aluminum oxide mixed with oil, grease, or water.
c.
Lapping Procedures. In the lapping process, select a grade of compound that is best suited for the job. Select a
compound which will resist the tendency to embed itself in to the material being lapped. The following list will be
useful in making such a choice:
Metal
Compound (grade)
Steel on steel
Medium and/or fine
Steel on brass
Medium and/or fine
Brass on brass
Medium and/or fine
Aluminum on aluminum
Fine
(1)
Apply the compound sparingly to the parts to be lapped, then work the parts together until they are
perfectly mated. Frequent inspection should be performed to prevent the removal of excess material,
thus causing possible spoilage. After lapping has been accomplished, and before each inspection, the
parts must be washed off thoroughly. Cleaning solvent such as mineral spirits or paint thinner may be
used. The washing-off process must remove the lapping compound; otherwise the cutting action will
continue as the parts are worked together in use.
4-50 Change 6
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