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CNC TURNING - Sabner

CNC TURNING

Turning and milling are the two main machining technologies. Turning is used to separate the layer of material from the processed with the use of a tool, usually the turning tool. In this technology, the processed part is turning, while the tool is fixed.

Due to such characteristics of the process, turning technology is used in the production of all types of parts formed as turned shapes. The main components which are produced on lathes include rollers, axles, bushings, discs, rings, collars, cylinders, cones, etc.

Our modern CNC turning centers allow the processing of all available materials, from plastics to heat-resistant super alloys or titanium alloys. However, the most popular material that is machined is steel. Depending on the chemical composition, they are present in various grades with different properties. Our company usually performs the turning of carbon steel, low-alloy steel and stainless steel. The turning of non-ferrous metals such as aluminum, brass or bronze is also popular.

In order to ensure the high performance of production in larger batches, we have equipped our turning centers with devices that allow automatic operations – the machine feeds the material for subsequent parts without the operator’s intervention. The modern control of our lathes allows even complex shapes to be quickly programmed, which would be impossible on conventional machines. The starting point on the lathes is a 3D model for the part. By working on such a model, our specialists can design the paths to be followed by the machine in order to obtain the final product. An additional benefit of the theoretical model is easy correction, when needed during the work on the product. After CNC turning, the part can be further machined on other equipment or heat-treated, heat and chemical treated, galvanized, etc., to complete the production process.

A well planned turning process on CNC turning centers requires many factors to be taken into consideration. The most important one is experience in turning technology and advanced programming methods. Knowing the latest achievements in the technology of cutting tools is also important.

Our experienced specialists can study all the crucial aspects in detail, so that we can offer unrivalled quality and efficiency of production to our customers.

The turning process itself can be basically split into several categories. Each of them is specific and requires knowledge and experience to work effectively.

EXTERNAL TURNING

The process involves the removal of the material from the external cylindrical surface of a part and includes the following types: straight turning, profiling and facing (surfacing). In each of those cases, particular attention should be paid to the selection of tools and the specifics of particular products. First, one must correctly fix the machined part in order ensure maximum stiffness. The ability to precisely turn the machine part is the essential requirement for achieving tight dimensional tolerances. For thin-wall elements, wide fixing clamps should be used, as they distribute the clamping forces on a large area so as to avoid deformation of the part with sufficient clamping force. Another specific case is parts which are long and slender. It is assumed that the ratio of length to diameter equal to 2:1 is usually acceptable for processing, with the use of clamping on one side only. In the case of a more unfavorable ratio of these two dimensions, it is necessary to support the processed part with a center or downholders. Correct alignment of the headstock and tailstock also helps the maximum stiffness and good contact of the cone of the support center point – these factors contribute to the high quality of the finished product.

INTERNAL TURNING

This category of turning is also referred to as boring. It is characterized by limitations in the selection of processing strategy due to the work field being reduced by the diameter of the bore in a processed element. Similarly, the depth of the bore that is crucial for the required tool overhang is also problematic. In accordance with the general rule, tools with the shortest overhang and the largest diameter should be selected. The selection of the correct tool for the performed task, proper use and accurate clamping has an impact on keeping tool deviation and vibration at a minimum. Another element that is crucial for the performance and safety of internal turning is the correct removal of chips. In this case, the centrifugal force pushes the chips to the outside and usually keeps them inside the bore. The tool may then press the chips against the machined surface and cause damage to the part and tool. In order to solve this problem efficiently, internal cooling can be introduced with cutting fluid in order to assist chip evacuation. The cutting liquid can be replaced with compressed air, while in through-holes, the chips can be blown out through the spindle. Additional solutions for this problem include boring with the reversed toolholder in order to keep the chips away from the cutting edge, reduction of the cutting speed or using a smaller cutting head in order to maximize the space for generated chips.

PARTING AND GROOVING

This is a very wide category of CNC turning covering both cutting material and external/internal grooving. As for cutting, the basic factors that should be considered at the planning stage include material saving and the reduction of the cutting speed. This determines the selection of a tool with the smallest width, with geometry adjusted to form narrower chips than in grooving. Thus, cutting offers good control of the chips and proper surface finishing. Due to the fact that during cutting the inserts are often cut very deeply into the metal, the risk of vibrations and tool bending increases. In order to reduce the scale of this problem, one should select a parting blade or toolholder with the smallest overhang possible, the largest toolholder grip, adjust the height of the blade equal or larger than the length of its cutting depth, select plates with geometry for light machining and plates with sharp edges. Equally important for cutting is the correct setup of the toolholder at an angle of 90 degrees to the axis of the processed product and the setup of the blade height within 0.1 mm in relation to the product axis.

As for grooving, this process covers a series of specific operations, such as general grooving, circlip grooving, face grooving, profiling and undercutting. The most cost-effective and efficient method in all cases is cutting with a single cut. Unfortunately, it is not always possible due to the geometry of a machined part. Other methods include: angled ramping, the multi-cut method or deep cut turning. All these methods are designed for rough machining and must be followed by finishing operations. Face grooving is the unusual type of grooving. In such processing, one should pay particular attention to the selection of the correct toolholder which must have the correct bending radius of the supporting part adjusted to the bending radius of the groove. Due to the fact that many products machined on CNC units require further processing, such as grinding, it is often required to make an undercut to allow the running of the grinder disc. A similar case is threading to collars, where the undercut ensures the correct mating of the threaded part. For this type of work, toolholders with an overhang provided with a round insert are usually used.

THREADING

Machined elements frequently have helical screw ridges cut on the surface of a machined part, internally or externally, referred to as threads. They are cut to form the mechanical fixing of two threaded parts or allow the transmission of movement by translating the rotary movement into linear movement, or vice-versa. Thread turning is often performed with CNC machines. This method can be used safely and effectively thanks to the employment of modern tools equipped with V-shaped inserts, or inserts with a full profile or multipoint plates. In thread turning, the most important factor is machine feed; it should be equal to the thread pitch, i.e. the distance between two turns of the helix at corresponding points. The adjustment of the pitch to feed per turn is performed with the use of procedures stored in the memory of CNC machines. Due to the specifics of thread geometry, the selection of the correct tool ensuring clearance between the cutting edges and thread sides is a very important factor. To this end, the cutting insert should be inclined to ensure the required flank/radial clearance. Depending on the thread geometry, an experienced process engineer selects one of the many available processing strategies, such as radial infeed, flank infeed or incremental infeed.

In CNC machining, the experience of process engineers responsible for the design of processing is very important. Prior to starting a new task, it is necessary to deeply analyze each individual case in order to plan the technology in an optimum way.

First, the dimensions and requirements for the machined part should be analyzed:
  • the type of operation (internal or external turning, facing, straight turning, profiling, etc.) The type of operation affects the selection of tools.
  • the type of machining (roughing, finishing)
  • the features of the product (large and stable or small, long and slender, thin-walled)
  • the finishing of undercut corners and the required corner radii
  • the required quality (surface finishing and tolerances)
After the analysis of geometry, the processing method should reviewed:
  • batch size (this factor influences the correct adjustment of tools to improve productivity)
  • material (what is the machinability and what are the good properties related to chip breaking
  • can the part be securely mounted in a standard lathe chuck or is a special fixture needed
  • is chip control important
Finally, some important aspects related to the cutting machine should be taken into account:
  • stability, required power, torque. It is particularly for larger parts.
  • is counter spindle or tailstock necessary
  • cooling fluid feeding
  • is feeding the cutting liquid under high pressure necessary (e.g. for chip breaking in materials generating long chips)
  • the number of tool changes and number of tools in the turret
  • the spindle rpm
  • the bar feeder