In the article on pattern blocks we mentioned that two of the parallel faces of these pieces, called measurement facesThey have a very fine finish that gives them the properties they must have to fulfill their function.
Now, how is this perfect finish achieved? For this purpose, a series of operations are used that are part of the machining of parts by abrasion.
At least four different techniques are highlighted in the abrasion machining:
The one used for finishing the measuring faces of gauge blocks, as well as for many other applications, is the technique of lapped (also known as stoned or stoning).
Although it may seem recent, lapping is a process that has been used for centuries. The first lapped materials were stones that were carved in ancient towns for religious and cultural purposes. Later, the art of jewelry and precious stones gave a great commercial impetus to this technique.
Basically, lapping is a surface finishing through a abrasive powder of extreme fineness, for example, diamond powder, aluminum oxide, cerium oxide, boron carbide, silicon carbide and others. This abrasive is impregnated in a block of soft material (cloth, copper, cast iron, steel, brass, wood or other softer material than the one to be stoned) that receives the name of disk or lapping stone and that it only serves as a support for the abrasive.
The abrasive is transported between the lapping disc and the part in a vehicle which can be water, fat or oil. By rubbing the disc impregnated with the abrasive on the surface of a piece under a certain pressure P, the abrasive granules remove small amounts of chips from said surface, as shown in the following figure.
As the abrasive is impregnated on the surface of the moving disc, it does not wear away because it remains fixed. This characteristic is always present in any system in which two materials of different hardness are rubbed together in the presence of an abrasive: the softer one constitutes the polisher and the harder one is the one that is polished.
The amount of material removal in the lapping process generally ranges from 0.003 to 0.03 mm, although 0.08 to 0.1 mm is sometimes achieved. It is an extremely slow process that can be done both by hand and on special machines.
By the way, the lapping machines they’re not the kind of tools you’d find in a DIY shop or hobbyist’s garage. They are very expensive devices used at an industrial level in a wide range of applications, from the aforementioned pattern blocks to mechanical, aerospace, electronic, ceramic, optical and other parts.
How does a lapping machine work?
Based on the lapping process described, the machines that perform this function incorporate a generic mechanism such as the one shown in the following figure.
In the flat surface lapping machines, the pieces are introduced loosely in a workpiece ring and stand against lapping disc through floating heads. Workpiece carriers, which move at low speed, move the pieces following an irregular path. When you have to produce two parallel surfaces, two disks can be used, one that rotates under the pieces and the other on top, which is the example shown in the figure. to stone round surfaces There are various types of machines. For cylindrical parts small, such as plunger pins and ball bearing races, a special spotless lapping machine is used.
Parts of a lapping machine
The figure below shows the schematic of a lapping machine for flat surfaces. Its main parts and the function they perform are as follows:
Work table: It is on which the rotating components of the machine sit, which are kept separate for greater safety and ease of cleaning.
lapping disc: in this device, generally made of cast iron, the mixture of abrasive + vehicle is impregnated. The disk rotates on an axis, so that the abrasive film comes into contact with the workpiece.
Workpiece rings: They are also made of cast iron and it is where the piece to be stoned is placed. Lapping machines typically contain 3 or 4 of these rings.
mobile arms: easily move the workpiece rings.
Deposits: there are usually two: one is the feed one and contains the mixture of abrasive + carrier vehicle; the other is the emptying one and receives this mixture already used during the process.
Metering pump and valve: the pump ensures the homogeneous composition of the abrasive + vehicle mixture, keeping it in constant agitation. The metering valve distributes the mixture on the lapping disc, so as to impregnate it before starting the process.
Control Panel: depending on the lapping machine model, it can be analog or equipped with a programmable logic controller (PLC). In addition to starting and stopping the machine, the analog control regulates the speed, the duration of the dosage, the interval between dosages and the duration of the cycle. The PLC with TFT screen not only regulates these functions, but also has a database to archive production histories and set parameters and conditions capable of guaranteeing the repeatability of working conditions.
The advantages of lapping compared to the usual mechanical procedures are:
- Absence of tension and deformation of the work piece, since it is not subject to clamping claws.
- Absence of thermal distortion.
- A single phase is sufficient to guarantee an optimal finish.
- Slow part roughing, without sudden removal of material.
- You can work with any compatible material, regardless of its fragility. Only materials that are too soft are not suitable for lapping because the abrasive tends to embed in them.
- Great versatility and versatility in the abrasive + vehicle mixture compared to different tools and composites for each type of material.
- Metallic, non-metallic, magnetic and non-magnetic materials can be lapped.
Finally, what elements can we stone in a lapping machine? Countless of them: automatic transmission, hydraulic and pneumatic components; gear cutters-notchers; valve seats and seals; engine parts, for air brakes, cast aluminium, for fuel burners, gasometers and compressors; Medical instruments; printed circuit boards; contact lenses; quartz crystals and many more, covering a wide variety of applications.