Screw machine
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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Screw machines have been replaced by CNC lathes to some extent.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.
As CNC components and controllers have matured, CNC has moved into the world of screw machines as well. Multispindle CNC screw machines have opened up possibilities for producing a number of parts per hour that was not obtainable before. CNC screw machines and rotary transfers are a recent, and highly expensive manufacturing option. The number of axes which must be controlled can number over a hundred, and as a result, programming is highly complex. The costs in initial purchase and tooling plus ongoing programming costs relegate this flexible, high-end piece of equipment to only the largest manufacturing operations.
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[edit] 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.
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[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.
[edit] 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.
[edit] 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.
