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Posted on Jul 23, 2020
Numerically controlled (CNC) turning machine centers cut and shape a range of precise products from car parts to general machine parts. Operating in horizontal or vertical positions, CNC machines include machining tools such as lathes, multi-axis spindles, and milling and boring machines; functions previously performed by people are performed by a computer control module. CNC machines are loaded manually or automatically. Most CNC machines are partially or completely enclosed in metal housings equipped with thermoplastic vision panels, most often made of polycarbonate.
Surface roughness is often considered the main goal in the modern numerically controlled computer processing (CNC) industry. Most of the existing optimization tests for CNC turning have been carried out in specific production circumstances or as a result of numerous hardware operations.
Turning is a form of machining, a material removal process that is used to create rotating parts by cutting off unwanted material. The turning process requires a lathe or lathe, workpiece, chuck and cutting tool. The workpiece is a piece of pre-shaped material that is attached to the chuck, which is itself attached to the lathe and can rotate at high speed. The cutter is usually a one-point cutting tool that is also secured in the machine, although some operations use multi-point tools. The cutting tool is inserted into the rotating workpiece and cut off the material in the form of small chips to achieve the desired shape.
Turning is used to produce rotary, usually axially symmetrical parts that have many features, such as holes, grooves, threads, cones, steps of different diameters, and even contouring surfaces. Parts that are made entirely by turning often contain components that are used in limited quantities, perhaps for prototypes such as custom shafts and fasteners. Turning is also commonly used as an additional process of adding or refining parts elements that have been produced using a different process. Due to the high tolerances and surface finish that turning can offer, it is ideally suited for adding precision rotary elements to a part whose basic shape has already been formed.
The time required to produce a certain number of parts includes the initial configuration time and cycle time for each part. The set-up time consists of the time to set up the lathe, plan tool movements (done manually or by machine) and install the clamping device in the lathe. The cycle time can be divided into four types:
● Loading / unloading time - the time required to load the workpiece into the lathe and attach it to the device, as well as the time to unload the finished part. The loading time may depend on the size, weight and complexity of the workpiece, as well as the type of clamping.
● Cutting time - the time required by the cutting tool to make all the necessary cuts in the workpiece for each operation. The cutting time for any operation is calculated by dividing the total cutting length for this operation by the feed rate, which is the tool speed relative to the workpiece.
● Time of inactivity - also referred to as non-productive time, is the time required for all tasks that occur during the process cycle that do not involve the workpiece, and therefore remove material. This idle time includes approaching and retracting the tool from the workpiece, tool movement between functions, adjusting machine settings and changing tools.
● Tool change time - the time it takes to change a tool that has exceeded its service life and must therefore be used to cut effectively. This time is usually not performed in every cycle, but rather only after the service life of the tool has expired. When determining the cycle time, the tool change time is adapted to the production of a single part by multiplying by the tool change frequency, which is the cutting time divided by the tool life.
No post-processing required after the turning cycle. However, if necessary, secondary processes can be used to improve the surface finish of the part. Scrap in the form of fine chips cut from the workpiece is moved away from the workpiece by the movement of the cutting tool and the spraying of grease. Therefore, no process cycle step is required to remove scrap that can be collected and discarded after production.
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When turning, the speed and movement of the cutting tool are determined by several parameters. These parameters are selected for each operation based on the workpiece material, tool material, tool size and more.
● Cutting feed - the distance the cutting tool or workpiece moves during one spindle revolution, measured in inches per revolution (IPR). In some operations, the tool moves to the workpiece, while in others the workpiece moves to the tool. For a multipoint tool, the cutting feed is also equal to the feed per tooth, measured in inches per tooth (IPT), multiplied by the number of teeth per cutting tool.
● Cutting speed - the surface speed of the workpiece relative to the edge of the cutting tool during cutting, measured in feet of surface per minute (SFM).
● Spindle speed - rotational speed of the spindle and workpiece in revolutions per minute (RPM). The spindle speed is equal to the cutting speed divided by the circumference of the workpiece at which the cutting is performed. To maintain a constant cutting speed, the spindle speed must change depending on the cutting diameter. If the spindle speed is kept constant, the cutting speed will change.
● Feed speed - the speed of movement of the cutting tool relative to the workpiece when the tool is cutting. The feed rate is measured in inches per minute (IPM) and is the product of the cutting feed (IPR) and spindle speed (RPM).
● Axial cutting depth - Depth of the tool along the workpiece axis when cutting, as in the case of face machining. A large axial depth of cut will require a low feed rate, otherwise it will put a heavy load on the tool and shorten tool life. Therefore, the feature is usually machined in several passes when the tool moves to a specific axial depth of cut for each pass.
● Radial depth of cut - The depth of the tool along the radius of the workpiece when cutting, as in turning or boring. A large radial depth of cut will require a low feed rate, otherwise it will put a heavy load on the tool and shorten tool life. Therefore, the function is often machined in several stages when the tool moves at a radial depth of cut.
During the process cycle, you can process the workpiece with various operations to get the desired shape of the part. These operations can be classified as external or internal. External operations modify the external diameter of the workpiece, while internal operations modify the internal diameter. The following operations are determined by the type of cutting tool used and the path of that tool to remove material from the workpiece. Here are the steps:
● Facing - a single-point turning tool moves radially along the end of the workpiece, removing a thin layer of material to provide a smooth flat surface. The face depth, usually very small, can be machined in one pass or can be achieved by machining at a smaller axial depth of cut and making multiple passes.
● Grooving - a single-point turning tool moves radially to the side of the workpiece, cutting a groove equal to the width of the cutting tool. Multiple cuts can be made to create grooves larger than the width of the tool, and special shaping tools can be used to create grooves with different geometries.
● Parting off (parting off) - Like grooving, the single point cut off tool moves radially to the side of the workpiece and continues to work until the center or inside diameter of the workpiece is reached, dividing or cutting off the workpiece section.
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