Drilling is a mechanical process whose purpose is to create holes using rotating tools. Holes are usually divided into regular holes (10 cm or less in depth) and deep holes (10 cm or more in depth). In deep drilling, the depth of the hole can be 150 times its diameter. Normal drilling implies a depth of no more than 5 hole diameters.
A hole with a depth of ten or more diameters is machined using deep hole drilling technology with specialized equipment. This technology is used in various production areas: steel, nuclear power, oil and gas, aerospace, and so on. Always the most important parameters of the final result are the highest quality of machining, geometric accuracy and the specified dimensions of the hole.
Another important factor that determines the quality of hole machining is the formation of easily evacuated chips and their direct removal from the finished hole. To obtain quality results when drilling deep holes, it is important to ensure that the chips are crushed. In this way, chip buildup and the resulting damage to the machined surface can be avoided.
Deep hole drilling belongs to the category of continuous processes. It is therefore considered to be more productive and of higher quality than other processes.
Special tools are used to produce deep holes. It is customary to distinguish between three types of machining systems:
-Ejector (double rod).
-STS (single rod).
-Joule (tube and lobe) drills.
The ejector system uses two tubes, which are a one-in-one design. They are connected to the drill head. The cutting fluid (coolant) flows inside the drill, into the gap between the rods. The chips are flushed out through the inner cavity. The advantage of this method is that no high pressure is required for lubrication (compared to the single rod method). It is usually used for medium batches.
The second STS system consists of a single boom, and the fluid is fed through a special device that is placed on the end face of the workpiece. The coolant is fed at high pressure, flushing the chips, similar to an ejector system, through the boom. This ensures that the chips come out quickly without lingering, leaving no destructive marks on the walls of the hole. The single-bar system itself is considered more reliable.
STS drilling is used for low-carbon and stainless steels, that is, with materials that have strong chip formation. But since this method requires special equipment, it is most profitable and effective for large-scale production.
The use of gun drills (another name) or shot drills differs in the way the chips are washed out. Its exit is provided through an external V-shaped groove, which is equipped with a drill bit. Tubular drills are equipped with machining centers. The main condition for high quality machining is to provide the necessary coolant pressure. This method eliminates the need for countersinking and reaming.
There are different types of machines for which deep hole drilling is the primary process. Most of them are machines for producing holes in metal rotating cylinders. The tool itself (the drill bit) is simply moved at a certain feed rate. This operating principle is similar to a lathe. This equipment allows you to perform machining with a high level of accuracy, while at the same time it is characterized by the productivity of a medium level. However, this method is only suitable for holes centered on the axis of the workpiece.
However, for maximum holemaking accuracy and productivity, machines in which the drill bit and workpiece rotate simultaneously, but in the opposite direction to each other, can be used. If eccentric holes and heavy workpieces need to be machined, machines with rotating tools are used.
Deep Hole Drilling Machines come in different types: horizontal or vertical, rotary and multi-spindle. Some of them can perform additional manipulations, such as boring.
Since in deep hole drilling the supply of coolant under pressure at a certain flow rate is considered mandatory, the system must be equipped with pumping equipment: an oil pump or a pump for pumping viscous fluids. The flow and the required pressure of the coolant have a direct influence on the performance of the equipment.