GMT has a wide range of dimensional metrology products, from simple ones such as basic Surface Plates, to complex structures for special applications. Used widely in the metalworking and manufacturing industry in India, the secret of the success of these products, is GMT’s expertise in the process of lapping, which has been developed over several decades.
Lapping is a process used to obtain precision flatness, surface finish, and/or parallelism of the work piece, during which workpiece slides on the surface of a “lap plate” with a lapping compound between the contact surfaces. The “lap plate” is driven and the workpiece rotates relative to the plate, in a sliding-rolling motion. This process is a “super finishing” process, where the surface is further machined after finishing processes such as grinding. Grinding uses bonded abrasives, whereas lapping generally employs a loose slurry of abrasives, though today lap plates with bonded grits are available for large volume production. The grits essentially break during the process, forming new cutting edges.At the end of the process, the “slurry” has to be discarded, as the grit will no longer be fit f or use.
Either one side of the workpiece can be lapped to get flatness and surface finish, or both sides to obtain both flatness and parallelism of component surfaces.
Lapping can be done manually or with machines.
A very large variety of materials can be lapped, both by hand and by machine.
The workpiece is placed on a Lap Plate, and surface material is removed as a result of the rolling and sliding action of abrasive grains mixed with a fluid, called a “medium” or “vehicle” to form a “slurry”. Usually aluminium oxide, silicon carbide, boron carbide are used. As granite is not corrosive, water can be used. This “slurry” of fluid and and abrasive grit is introduced between the Lap Plate and the workpiece. As the workpieces and lap plate/s rotate relative to each other, the direction of rolling and sliding changes continuously, at very low speeds, typically less than a metre/second. Water, white kerosene or diesel can be as the fluid used to lap granite. In manual lapping, the lapping grit is moved in figures of eight over the surface of the granite, rather than as a rolling movement.
In the case of single-sided lapping, the medium is pumped on to a spinning plate that carries it beneath a work piece that is held in place by a hold-down pad. In double-sided lapping, the work pieces are placed within a carrier (a special gear with holes cut into it to accommodate the work piece) that is sandwiched in-between an upper and lower lapping plate. The medium is fed in the lapping plates through a series of holes drilled into the upper plate. The lap plate also wears with use and has to be replaced when it wears beyond usable limits.
There are other types of flat lapping machines too, such as planetary lapping machines. It is possible to produce flatness of 0.025 micrometre, parallelism tolerances of 0.25 micrometre and surface finishes to 1 Ra, with lapping. The finished surface is dull rather than reflective. Lapping is not a polishing operation.
Challenges of Lapping Granite
The appropriate abrasive grit size has to be selected according to the required accuracy and material removal rate.
During the process, different abrasive grit sizes are used at different stages and also vary for different jobs. Therefore, at every completed stage the grit should be completely washed out. Even a single particle of remnant large grits can scratch the surface and will affect the finish. This is a big job. Lap plate grooves. Grain turns at groove. This is one of the major aspects of lapping any material. There is no specific challenge to lapping granite as against other materials.
The lapping master has to be continuously calibrated and, if required, has to be corrected.
GMT makes custom-built granite products for many applications. These involve not only lapping, but also cutting (for T-Slots or recesses), drilling, tapping and bonding of the material. Please see www.gmt.co.in/...
19 авг 2024
