Machine Lapping

Machine Lapping

Machine Lapping - Hand lapping as a means of improving the surface finish of work such as gauges, has been practised for many years. Unlike hand lapping, machine lapping is preferred for finishing work in larger quantities.

Both cylindrical and flat lapping operations can be performed on lapping machines.

Machine lapping of outer cylindrical surfaces is done between

(a) Two opposed cast iron laps using loose abrasives,

(b) Two bonded abrasive circular plates or laps.

Machine lapping of flat surfaces is done by using

(i) Two bonded abrasive laps, if both ides of the workpiece are to be lapped at the same time,

(ii) Single rotating cast iron or bondedabrasive lap (plate) if one side of the workpiece is to be lapped only or at one time. Mechanical lapping machines are of vertical construction.

They have two cast iron laps

The lower lap supports the work held in a holder or work retainer disc. The lower lap rotates at relatively slow speeds (about 60 RPM). The upper cast iron lap is stationary and it rests and floats on the work pieces and supplies pressure for the abrading action. Lapping compound (abrasives) is applied with a brush or fed through a pump.

Cylindrical work is loosely held and guided in a plate type work guide or holder so that it follows a random, complicated path between the two laps.

Lapping of External Cylindrical Work

A similar holder is used in the lapping of flat surfaces, where the holder propels the work. Either it is provided with drive pins which impart to it a rotary and gyratory motion, or it is given a planetary motion. Either way, the work is brought in contact with, the entire surface 01 the lap in an ever changing path.

Commercial accuracy can be held to 0'00060 mm, and to even closer limits if needed.

Products commonly finished by this process include:

Gauges

Piston pins

Valves

Gears

Roller bearings

Thrust washers and optical parts. 

Lapping of Flat Components

Very fine finish of the order of 0.02 µm and tolerance or 0.5 µm are feasible when cast iron laps are used.

Various external and infernal surfaces both cylindrical and flat ones, can be machined on the lapping machine using two bonded abrasive circular laps lapping is done on crank shaft journals, cams of camshafts, gauge blocks, gauge plugs, gears etc.

Parts of Vertical Lapping Machines	Retainer Disc with Components for Lapping
1. Bed	2. Machine mounted in column
3. Lap	4. Disc 5. Lap
A - α - Axis B - Disc	C - Retainer disc with components for lapping D - Relative path of the workpiece in lapping

The drive of lap 5 is housed within bed 1; lap 3, connected to the spindle of the machine mounted in column 2, receives simultaneously rotation and straight vertical movement.

When bonded abrasive laps are used, both laps are rotated. The laps are rigidly supported on spindles and separately driven at speeds higher than that used for cast iron laps. Lapping is carried out with laps 3 and 5, between which work retainer disc 4 is placed.

The work pieces are freely located in openings of the retainer, which is positioned either coaxially or eccentrically with respect to the lap axes. In the first instance, the retainer is freely mounted on a shaft and is rotated in the same direction as lap 5. In the second, the retainer receives an oscillating reciprocation from a separate drive.

Retainer disc 4 is displaced transversely to change the eccentricity α of its axis to the rotation axes of the laps: this is necessary to ensure uniform wear of the laps. Fig shows the approximate relative path of the workpiece in lapping.

Owing to the eccentric position of the upper lap in reference to the lower one, cylindrical work pieces in the slots of the work holder, not only roll between the laps, but have an axial motion as well.

Kerosene may be used as a coolant as well as to flush the chips.

Lapping takes place at a faster rate and consequently, the machine does not produce the extreme accuracy possible with machines using cast iron laps. Since bonded abrasive laps must be dressed with diamond tools, it is not possible to make them as flat as cast iron laps.