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.

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.

Design

Close up view of a Brown & Sharpe Single Spindle screw machine. Notice the six station turret, the front and rear slides, and the two vertical slides. Also notice the black shaft just over the turret for use of swing stop.

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.

View of Lead Cam and Seconds Gear Box for a Brown & Sharpe screw machine, Sq-base 542 Series.

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.

Exercise 4. Find English equivalents in the text:

1) металлообрабатывающий станок

2) обработанная на токарном станке деталь

3) автоматизированное производство мелких деталей.

4) первичное использование

5) компьютерное числовое программное управление

6) механическая автоматизация

7) револьверный токарный станок

8) полуавтоматический

9) одношпиндельный токарный автомат в сравнении с многошпиндель­ным токарным автоматом

10) токарный многоцелевой станок

11) крупносерийное производство

12) токарно-винторезный станок

13) квалифицированная рабочая сила

14) режущий инструмент

15) обработка сложных форм

Exercise 5. C omplete the sentences:

1) Each spindle contains ….

2) A common configuration is six ….

3) Each station may have multiple tools that cut ….

4) In a single spindle machine, these four operations would most likely be performed sequentially with ….

5) Screw machines are mechanically driven, the position of the cutting tool is determined by ….

6) A form tool has the form or contour of the ….

7) A threading die can cut rapidly but it requires ….

8) Threading is performed with ….

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