Screw Machining: A Machining Process
Screw machining is an automated machining process very similar to that of CNC turning. A screw machine is configured in a very similar manner to a lathe in as much as the spindle is positioned horizontally and spins the part removing material in chips with a stationary tool to reduce the outside diameter.
While screw machines may require an extensive set-up; once running, a single operator can monitor the operation of several screw machines each producing different parts. It is hotly debated that screw machines can outperform any other turning machine in speed and cost of production in high volumes.
The argument for the screw machine’s speed and economic production is greatly aided with the addition of multiple spindles. One common configuration includes six spindles, each holding a bar of material. As the machining operation advances the machine’s turret indexes bringing up a new tool to perform the next machining task. These tasks may include facing, turning, threading, drilling and cutoff among others.
Unlike operations on a lathe where single point tools and CNC programming are primarily utilized to turn complex shapes, a screw machine may have form tool which has been ground to create a negative shape.
Traditional screw machines are “programmed” for operation by the use of cams and trip dogs. As cams are unique to each job, they have to be manufactured accordingly. Cams and trip dogs control the machines spindle speeds, stock feeds, infeeds, turret rotation and the length of the feed stroke. Set up for a beginner can be difficult.
Swiss Screw Machining
The term “Swiss screw machining” is derived from a type of screw machine that was first developed and manufactured in Switzerland. It was originally developed to make small turned watch parts.
Because the Swiss screw machine was developed to manufacture such small parts which required exacting tolerances, it is capable of holding +/- 0.0005 inches and 0.001 inches in both length and width on a part that is ¼” in diameter and up to 8” long. This is achieved because all turning is done while the material is fully supported.
Production speeds are greatly enhanced with the addition of an automatic bar feeder which is great considering this cam driven machine can take between four and eight hours to program.
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