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Covington instructions, vibrating laps
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Basic instructions for vibrating laps, (model 392, 394, and 396), compliments Covington Engineering. The following is a general guide. If you have lost your instructions, a complimentary copy can be e-mailed to you by ordering a copy on this web page. The vibrating lap is designed to abrade and polish automatically.  The inertia of the heavy stone slab holds it relatively still while the lap pan vibrates underneath.  This action replaces the circular rotation used by conventional lapping machines.  Place  your automatic vibrating unit on a sturdy work bench, table or flat surface and secure the lap base to prevent it from vibrating from your work area.  Use a level to ensure the bottom of the pan is level.  Preparation of material: bevel grind rough edges and remove the saw cut nub from the slab to help reduce chipping.  The smaller or thinner the piece to be ground, the more important weight becomes.  Weight can be applied to the slab by the use of double back carpet tape, dop wax, or Fiber-glass tape on a dry surface.  The weight should be evenly distributed over the bottom surface of the stone to be lapped.  Weight in the center of the stone slab will cause an irregular lapped surface to be ground.  It is necessary to lap three or more pieces so each will move the others around.  If a work piece stops in one place, it will cause excessive wear to the pan at that location.  If the work pieces are fragile, surround each piece with plastic or rubber bands.  To start, apply two or three tablespoons of grit over the surface of the metal pan.  Apply 1/3 cup of water slowly to the pan to form slurry.  Water acts as a lubricant.  Have the grit slurry as dry as possible while still retaining movement of the pieces.  Should grit splash take place, stop and dry out excess water using newspaper or sponge.  Clean machine before restarting operation.  Run stone slab in 80 grit until surface has been ground completely across the face of the slab.  Clean slab.  Be sure any holes have been purged of grit.  Remove pan and bumper ring and thoroughly clean.  Use a brush.  Check corners and grit recesses.  (IMPORTANT TO REMOVE ALL GRIT RESIDUE.)  As a general rule, 80 grit abrades twice as fast as 220 grit, which abrades twice as fast as 400 grit. Run stone slab in 220 grit.  To check finish and flatness, remove and wash slab.  Using a soft lead pencil, make square marks across the face of the slab.  Run stone again for two minutes.  If all marks are not removed, continue grind and repeat test.  When rock slab has passed the test, clean slab, pan and bumper thoroughly and repeat grind using 400 grit.  Suggested sequence of grinds depends on the hardness of the material.   The harder the material, the more steps of the grind must be made to obtain a smooth semi-gloss surface. A grind of 600 grit may be in order.  A good polish can only be had when all marks of the coarser grind have been removed.  POLISH: Important that the rock slab, pan and bumper ring be completely clean.  Insert polish pad into pan.  Slowly add water until pad is saturated.  Then add polish, (usually cerium oxide) and water until a light surface of polish is on the pad.  Polish should be the consistency of cream.  If polishing two or more pieces, protect the slab edges.  Remember water acts as a lubricant.  Too much will lengthen the polishing time. LAPPING TIME: The hardness of glass is 5 to 5 1/2.  Based on a slab of onyx (hardness 7) under ideal conditions: grit - 80, grind time - 2 to 3 hours; grit -  220, grind time, 1 to 2 hours; grit - 400, grind time 1 to 2 hours; polish - 1 to 2 hours.  Harder material will take longer for each lapping operation.  Softer material, less time.