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A Screw Machine is a metalworking machine used in the high volume manufacture of turned components. In operation, a Screw Machine is similar to a Lathe . Essentially a Screw Machine is an Automated Turret Lathe . Screw Machines have been replaced by CNC Lathes to some extent. However, for high volume production of turned components nothing is as cost efficient as a Screw Machine. In the hierarchy of manufacturing machines, the Screw Machine sits at the top when large volume of product is needed. An Engine Lathe sits at the bottom, taking the least amount of time to set-up but the most amount of skilled labor and time to actually produce a part. A turret lathe has traditionally been one step above a Lathe, needing greater set-up time but being able to produce a higher volume of product and usually requiring a lower skilled operator once the set-up process is complete. Screw Machines may require an extensive set-up but once running, a single operator can monitor the operation of several machines. The advent of the CNC Lathe or more properly: the CNC Turning Center, has blurred these distinct levels of production to some extent. The CNC Turning Center most appropriately fits in the mid-range of production, replacing the turret lathe. However, it is often possible to produce a single component with a CNC turning center more quickly than can be done with an Engine Lathe. To some extent too, the CNC turning center has stepped into the region traditionally occupied by the Screw Machine. The ease of set-up of CNC machinery makes it attractive alternative to the Screw Machine. However, in nearly all cases, a Screw machine, once set-up, can outperform any other turning machine in speed and cost of production. DESIGN A Screw Machine may have a single spindle but, in contrast to a lathe, a Screw Machines may have multiple spindles. Each spindle contains a bar of material that is being machined simultaneously. A common configuration is six spindles. The cage that holds these six bars of material indexes after each machining operation is complete. The indexing is very reminiscent of a Gatling Gun . Each station may have multiple tools that cut the material in sequence. The operation of these tools being very similar to that of a turret lathe. By way of example then: a bar of material is fed forward through the spindle. The face of the bar is machined (facing operation). The Outside of the bar is machined to shape ( Turning operation). The bar is drilled (boring operation) and finally, the part is cut off (parting operation). In a single spindle machine, these four operations would most likely be performed sequentially with four cross-slides each coming into position in turn to perform their operation. In a multiple spindle machine, each operation would be performed on each spindle simultaneously, with the material being positioned at each station in sequence. Screw Machines are mechanically driven, the position of the cutting tool is determined by the shape of a cam that rotates in step with the machine, but at a slower speed. For the machining of complex shapes, it is common to use a Form Tool . This contrasts with the cutting that is performed on an Engine Lathe where the cutting tool is usually a Single-Point Tool . A Form Tool has the form or contour of the final part but in reverse, so it cuts the material leaving the desired component shape. A Single-Point Tool is designed to cut on one point at a time and the shape of the component is dictated by the motion of the tool rather than its shape. The name "Screw Machine" is somewhat of a misnomer since Screw Machines spend most of their time making things that are not screws and are not even threaded. However, threading is frequently performed on a screw machine. Unlike a lathe, single point threading is rarely if ever performed, single point threading is too time consuming for the short cycle times that are typical of Screw machines. A Threading Die can cut rapidly but it requires the machine to reverse in order to be removed from the work. It is impractical to reverse the rotation of the spindle {Link without Title} of the machine so it is necessary to have a cutting tool that can cut in one direction and cut fast and be removed without interrupting the rotation of the machine. Threading is performed with a Die Head - a device that cuts the thread then opens and withdraws rapidly. |
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