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A screw machine is a metalworking machine tool used in the high-volume manufacture of turned components. Screw machines are fundamentally a type of lathe that is specialized for the automated production of small parts. The name screw machine is somewhat of a misnomer, because screw machines spend much of their time making things that are not screws and that in many cases are not even threaded. However, the archetypal use for which screw machines were named was screw-making.
All screw machines are fully automated, whether mechanically (via cams) or by CNC (computerized control), which means that once they are set up and started running, they continue running and producing parts with very little human intervention. This has been true since the 1870s. Mechanical automation came first, beginning in the 1870s; computerized control (via first NC and then CNC) came later, beginning in the 1950s.
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Nomenclature
The definition of the term screw machine has changed with changing technology. Any use of the term prior to the 1840s, if it occurred, would have referred ad hoc to any machine tool used to produce screws, that is, there would have been no established differentiation from the term screw-cutting lathe. When turret lathes were developed in the 1840s, the term screw machine was applied to them in overlapping usage with the term turret lathe. In 1860, when some of the movements, such as turret indexing, were mechanically automated, the term automatic screw machine was applied, and the term hand screw machine or manual screw machine was retronymously applied to the earlier machines. Within 15 years, the entire part-cutting cycle had been mechanically automated, and machines of the 1860 type were retronymously called semi-automatic. From that time on, machines with fully automated cycles were called automatic screw machines, and eventually the term screw machine no longer was used to refer to manual or semi-automatic turret lathes, having become reserved for one class of machine, the fully mechanically automated type. This narrow meaning of screw machine remained stable from about the 1890s until the 1950s. Within this class called screw machines there were variations, such as single-spindle versus multispindle, horizontal-turret versus vertical-turret, etc.
With the advent of NC, screw machines diverged into two classes, mechanical and NC. This distinction continues today with mechanical screw machines and CNC screw machines. However, in shop-floor jargon, the term screw machine by itself is still often understood in context to imply a mechanical screw machine, so the retronym mechanical screw machine is not consistently used.
Choice of machines
Mechanical screw machines have been replaced to some extent by CNC lathes (turning centers) and CNC screw machines. However, they are still commonly in operation, and for high-volume production of turned components it is still often true that nothing is as cost-efficient as a mechanical 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 an engine 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 they are running, a single operator can monitor the operation of several machines.
The advent of the CNC lathe (or more properly, 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 (mechanical) screw machine. CNC screw machines do this to an even greater degree, but they are expensive. In some cases they are vital, yet in others a mechanical machine can match or beat overall performance and profitability. There are many variables involved in answering the question of which is best for a particular part at a particular company.
Design
A screw machine may have a single spindle but, in contrast to a lathe, a screw machine may also 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: 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 Tools. 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.
Unlike on 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[s] 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.
Automatic chucking machine
An automatic chucking machine is very similar to an automatic screw machine, except it is not bar-fed, but rather is fed by a magazine full of blanks (pieces of stock), each of which gets a turn at being chucked. (This is analogous to the way that each round of ammunition in the magazine of a semi-automatic pistol gets its turn at being chambered.) While a screw machine is limited to around 3.5 inches in practice, automatic chuckers are available that can handle up to 12" chucks arranged in the same way that a screw machine would arrange multiple spindles. The chucks are air-operated.
Rotary transfer machine
Rotary transfers can be relatively small to very large, CNC multi-station milling and turning centers. It utilizes between 6 and 24 and more turret stations, with each turret face holding a chuck or collet. The parts are loaded by a parts bin into the holding fixture, and the part is then taken through whatever number of stations for machining. Each station can be fitted with various CNC turning, milling,slotting and grinding fixtures, and the holding stations are fully indexable. Rotary transfer machines can forgo turning operations entirely, and perform pure milling operations on bar stock. These are the singlemost expensive machine tools available today, and the highest throughput capability. One manufacturer has reported that a 12-station cnc rotary transfer machine has taken the production load off six CNC screw machines, and several CNC lathes and mills used in secondary operations. There is a Swiss manufacturer of Transfer Machines who claims production output of up to 450 parts per minute performing up to 8 machining operations on each part. These machines are often multiple tooled i.e. several parts are machined at the same time. Rotary Transfer Machines, also called, Dial Index Machines are used wherever metal components require multiple machining operations and large volume production output. Modern Transfer Machines can hold very close tolerances and achieve astounding surface machining finishes.
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