Basic Operations Performed on CNC Lathes
The essence of turning is the formation of the surface of the part with a tool with a cutting edge, while, as a rule, the workpiece rotates and the cutter moves. The turning process is quite diverse in terms of the shape and materials of the processed parts, types of operations, processing conditions, requirements, cost price and many other factors. With the help of roughing and finishing operations performed on CNC lathes, parts of various configurations are obtained with a surface finish Ra of up to 1.25, and in some cases even higher. Surface accuracy depends on the rigidity of the machine-tool-part system, on the tool used and cutting conditions: the harder the cutting edge of the tool (hard alloys, cermets, CBN, cubic boron nitride, diamond, etc.), the higher the rotation speed of the workpiece, less feed and overhang of the cutter, the better the surface cleanliness and accuracy.
There are several basic types of turning operations, including:
- Processing of cylindrical surfaces.
- Processing of conical shaft-type parts.
- Design of complex surfaces of bodies of revolution, shaped turning, turning fillets and fillets.
- Trimming of blanks, processing of ledges.
- Turning grooves (external and internal).
- Drilling holes.
- Boring, reaming, countersinking of holes.
- Cutting of internal and external threads using cutters and tools: taps, thread-cutting heads.
- Cutting blanks.
Cylindrical Surface Treatment
It is one of the simplest operations for choosing the type of tool, calculating cutting data and programming machining.
Turning is a combination of two movements – rotating the workpiece and moving the tool. In the case of processing cylindrical surfaces, the tool is fed along the axis of the rotating workpiece, thus the metal allowance is removed, that is, the diameter of the workpiece is processed. A variety of external turning is the machining of stepped shafts using continuous thrust and scoring cutters.
On CNC machines, the optimization of the turning process occurs in the direction of increasing the speed and the possibility of machining with several tools in one setup, which allows both roughing and finishing in one cycle. It is also important to increase control over the turning process, which ultimately affects the quality of the processed parts and the reliability of the entire work.
When turning cylindrical surfaces on CNC machines, high turning accuracy is achieved due to the rigidity of the system, modern cutting tools and various processing control systems.
To ensure the rigidity of the machine-tool-part system, the following methods of fixing the workpiece are used:
1. when machining in a chuck – a decrease in the overhang of the workpiece (modern lathes have an enlarged hole in the spindle).
2. when machining long and heavy workpieces – fixation in the centers of the headstock and tailstock. As a rule, a rotating center is inserted into the quill and the workpiece is pressed against it. The drive faceplate transmits the torque from the lathe spindle to the product.
3. Clamping parts with a relatively short length in three- or four-jaw lathe chucks. Long workpieces can also be clamped in the spindle chuck, and their cantilever part is supported by a steady rest during cutting. The steady rest is installed on the bed guides or support.
4. A combined (1 and 2) fastening of the processed products is used.
5. Technological methods often include the ability to control the machine tool spindle at near-resonance frequencies (controlled oscillatory acceleration-deceleration of the spindle).
Shaft-type Conical Turning
With this type of processing, CNC lathes have an undeniable advantage. Accurate and productive turning of the conical surface of a part on universal machines is a laborious operation that requires not only the appropriate qualifications of a turner, but also additional devices (the use of simultaneous feed along two axes (if technically possible), a template, a copy ruler). While the CNC machine performs the simultaneous longitudinal and transverse feed of the tool. This allows linear movements in the X and Z axes to be specified in one block during machining programming. In this block of the NC program, the coordinates of the traversing end point, the cutting edge, are specified. This programming method is the most versatile, as it allows machining with any taper angle. Chamfering is often a standard CNC function that speeds up the programming process.
Design of Complex Surfaces of Bodies of Revolution, Shaped Turning
To obtain bodies of revolution with a curvilinear generatrix on universal machines, it is necessary to use through or shaped cutters using a copier or a hydrocopier support. Often, such operations require a high qualification of the turner, and profitability is achieved only in series production.
Modern CNC lathes have wide technological capabilities. Shaped surfaces are very diverse; in many cases, they are obtained not by the geometry of the tool, but by the shaping movements of the working bodies of the machine according to the program. The use of shaped tools for working on CNC machines is extremely rare. Obtaining the whole variety of shapes of the surfaces of the part can be achieved through the competent design of the processing program. The precision of circular and linear interpolation allows for smooth transitions between frames.
This allows you to get by with a relatively narrow range of tools when processing various parts. The programmed cutting point is either the tip or the center of the corner at the tip.
On CNC lathes, the use of tools with multifaceted non-regrowth plates made of carbide and extra hard materials is especially effective. They provide stability of geometry, the ability to use the maximum power of the machine, increased tool life, simplify the adjustment of the machine with tool wear. When one of the cutting edges is worn, the plate is turned, introducing a new facet into operation. The error in the position of the new face usually does not exceed 0.05-0.1 mm and can be easily eliminated using the correctors of the CNC system.
Crosscutting of Workpieces, Processing of Shoulders
This type of machining is achievable by clamping the workpiece in the machine spindle chuck. The operation is performed with scoring or passing cutters. The best surface finish is obtained by machining “from the center to the periphery” or when moving to the center of the workpiece, a corresponding increase in the spindle rotation speed (constant cutting speed).
The grooves are cut on the cylindrical, conical and end surface of the part using grooved and slotted cutters in one or several passes (depending on the configuration and the required surface accuracy). When grooving relatively large dimensions, a combination of feed and groove cutters can be used. Canned cycles are provided for grooving and are programmed using conventional methods.
Cutting off a product or workpiece
produced by cutting tools, while the tool moves in the transverse direction to the center of the part. Depending on the size of the part, different methods of fixing the nearly cut or cut part are used. Tool breakage at the end of the cut is prevented by using supporting rests and reducing the cutter feed (by 45-55%) when approaching the center of the part by half the radius of the workpiece. Small parts fall into the chute, part catcher or are fixed in the turret fixture.
Drilling, Countersinking, Reaming Holes
Drilling is the main method of making holes. Drilling is the process of making cylindrical holes using a metal cutting tool. Drilling usually precedes operations such as boring or reaming. Machining can be performed both in the center of the part (when clamping it in a three-jaw chuck), and with an offset of the hole center. Displacement (eccentricity) is achieved by fixing the workpiece in a four-jaw lathe chuck or on the faceplate of the headstock. On a turning machining center, it is possible to use a driven tool and make holes both on the spindle axis and with an offset along the X axis.When using a radial drive unit, it is possible to machine holes located along the X axis.
In a universal machine, the processing tool: countersink, drill, reamer – is fixed in the tapered hole of the tailstock directly or through a chuck. in CNC machines – in the position of the tool holder using special tool blocks and mandrels.
With the development of tools for machining short holes, the sequence of the drilling process and preparation for it are undergoing significant changes. The modern tool allows you to drill into solid material and does not need to pre-center the holes. A high surface quality is achieved and, often, there is no need for subsequent finishing of the hole. The use of modern drills with replaceable inserts allows machining at high speeds and large volumes of generated chips, which in CNC machines are washed out of the hole by flows of coolant supplied under a certain pressure through the internal channels.
For the accuracy of turning, correct and uniform sharpening of the cutting edges of the drill, the perpendicularity of the end of the workpiece to the tool axis, the absence of burrs, and surface irregularities are necessary.
Using Renishaw’s control and tuning systems, the software in CNC machines allows you to set tool length and diameter offsets and perform breakage detection during machining. The tool is fed mechanically in the machine. The drill ensures the surface finish of the hole Ra 6.3 … 3.2, countersink – Ra 2.5, reamer – Ra 1.25 … 0.8.
Precise holes, large-diameter stepped holes and internal grooves can be produced with a boring operation. The product is clamped into the chuck of the headstock, supported by a steady rest (in the case of considerable length or mass). At the same time, access to the end face processed by the boring cutter remains free. Boring accuracy on a CNC lathe exceeds that of drilling, often provided by machining technology, cutting tools, the experience of a turner, cutting tool refined tuning systems, and equipment condition.