Abrasive part machining is one of three processes that remove material from a part in order to shape and mold it for its subsequent application. It differs from other machining processes because the material removal is relatively small, so it is more of a part finishing process and the tool used for it is a grinding wheel, made up of grains of quartz, silicon carbide, carborundum or corundum and a binder.
In our article on the lapping machine we mentioned that the machining of parts by abrasion includes, in turn, various techniques, one of which is grinding. Basically, grinding is an operation performed on parts that require exacting measurements and tolerances, whether dimensional (diametric, longitudinal or angular), geometric (concentricity, parallelism, perpendicularity, etc.) or surface finish (roughness, scratch direction, etc.).
The machines used in grinding are known as grinding machines and since there are several different types of grinding, the industry also has a corresponding multiplicity of grinding machines. In this article we will mention only the main types.
1) Flat or surface grinders
These machines are the ones that present the simplest handling, since they consist only of a longitudinal slide that gives the translation movement to the piece and the grinding wheel, which prints the rotation movement. Two subtypes are distinguished according to the position of the tooth:
to) Front grinding machines: the grinding wheel rotates on a vertical spindle, works flat against the workpiece and moves in a rectilinear motion. They are generally used for fast stock removal, although some machines can achieve high accuracy.
b) tangential grinding machines: the grinding wheel rotates on a horizontal spindle, works edgewise on the piece and moves with a circular and pendulum movement. They are used for high precision work on simple flat surfaces, flared or sloped surfaces, slots, flat surfaces close to shoulders, recessed surfaces and profiles.
As their name implies, these grinders are used for flat parts, usually machined on other machines in the shop, such as lathes, mills, and filing machines. The parts are fixed to the table by a magnetic clamping plate and can be moved manually or mechanically under the grinding wheel. The machine is provided with an internal pump and a network of pipes for the application and automatic recirculation of a cooling liquid for the part and the grinding wheel. The figure below outlines the main parts of a tangential grinding machine.
The parts machined with this type of grinding machine are, for example, bearings, dies, guides, plates, piston rings or segments, moulds, pins and profiles for tools. The dimensions of the pieces can vary between 40 cm and 6 meters long, and between 70-80 cm and 1 meter long, which gives an idea of the dimensions and solidity of the machines.
2) Cylindrical grinding machines
The cylindrical grinding machine it can work in a variety of ways, however, the part must have a central axis of rotation. This includes but is not limited to shapes such as a cylinder, a cone, an ellipse, a cam, or a crankshaft. Several subtypes are also distinguished:
to) External cylindrical grinders: grinding is done on the external surface of a part between centers, which allow the rotation of the same. In turn, the grinding wheel also rotates in the same direction when it comes into contact with the piece, as shown in the following figure.
Where: a) represents the rotational movement of the grinding wheel; b) represents the rotational movement of the piece; c) represents the advance in depth that determines the thickness of the chip and d) represents the lateral advance, either of the piece (diagram 1) or of the grinding wheel (diagram 2).
b) Internal cylindrical grinders: grinding is done inside a piece. The grinding wheel is always less than the width of the part. A metal ring supports the piece, giving it movement, as shown in the following figure.
Where: a) workpiece spindle head; b) metal retaining ring; c) piece; d) tooth; e) grinding spindle; and f) head of the grinding spindle.
c) Centerless grinding machines: This type of machine grinds small cylindrical parts, such as bushings, bolts or pins. The mechanism consists of two grinding wheels that rotate in the same direction, between which the piece is placed, without clamping (that is why the mechanism is called “centerless”) that rotates in the opposite direction to that of the grinding wheels, driven by the movement of the drag wheel, which is inclined at a certain angle between 1 and 5 degrees, depending on the hardness of the material to be ground and the diameter of the piece. The figure below shows a schematic of the process.
Where: a) guide; b) tooth; c) drag wheel and the cylindrical piece between both wheels is observed.
3) Universal grinders
It treats of the machines that offer greater capacity of work, since they mechanize bodies of revolution. With these machines of great robustness and scope, it is possible to grind both the exterior and interior of camshafts, crankshafts, cylinder interiors, cones, liners and many other parts.
The longitudinal carriage of the machine provides the translational movement to the rotating parts through its automatic advance and return provided by a hydraulic mechanism, while the grinding wheels receive the rotational movement, opposite to that of the part. The hardness or the characteristics of the pieces define the speed of rotation of the shaft by means of speed regulators. The figure below outlines the main parts of a universal grinding machine.
4) Special grinding machines
For the grinding of parts with a certain mechanical geometry or that contain holes and that cannot rotate on themselves, special machines are used, whose size is generally small and large, which are called special grinding machines. They have a vertical spindle (sometimes called planetary spindle) whose grinding wheel, in addition to turning, performs a circular translation. These machines are used, for example, to grind a myriad of parts, such as, among others, gear teeth, profiles, sprockets, threads, rolling cylinders, bed guides, brake pads, splines, hob, bearings , spokes, turbine blades and aircraft landing gear.
As far as the main tool of the grinding machines, the tooth, there is a variety of wheels of different shapes and grain sizes, all normalized by international standards, whose characteristics we will see in future articles.
5) Grinders with CNC control system
Today, technological progress has brought to the market the CNC controlled grinding machines for all the types seen above, which have a series of advantages over conventional ones, among which we can mention:
- unification of movements for grinding (upper, front and lower) in a single machine
- full automation, with minimal operator intervention
- larger dimensions of the machine, which makes it possible to rectify large parts
- magnetic workpiece clamping systems
- availability of various forms of benches or work tables
- improvement of grinding times and precision
- incorporation of servomotors for each axis, which allows a more precise positioning of the piece
automatic grinding wheel condition control
- possibility of programming cartesian coordinates and setting the exact grinding distance
- operation in a closed environment, without external projection of chips, dust or debris