Turning

Turning

زهرة شوبينج اونلاين    April 16, 2009   

Turning: Engine Lathe

Turning is another of the basic machining processes. Information in this section is organized according to the subcategory links in the menu bar to the left.

Turning produces solids of revolution which can be tightly toleranced because of the specialized nature of the operation. Turning is performed on a machine called a lathe in which the tool is stationary and the part is rotated. The figure below illustrates an engine lathe. Lathes are designed solely for turning operations, so that precise control of the cutting results in tight tolerances. The work piece is mounted on the chuck, which rotates relative to the stationary tool.

Turning

Turning refers to cutting as shown below.

The term "facing" is used to describe removal of material from the flat end of a cylindrical part, as shown below. Facing is often used to improve the finish of surfaces that have been parted.

Turning: Engine Lathe Detail

Engine Lathe Carriage

The figure below illustrates the carriage of an engine lathe. The carriage allows cross-feed and diagonal movements in addition to axial movement.

Turning: Chucks

The chuck is integral to a lathe's functioning because it fixtures the part to the spindle axis of the machine. Below is shown a three-jaw chuck with jaws that are all driven by the same chuck key. This arrangement provides convenience in that parts can be mounted and dismounted quickly.

Turning: Engine Lathe Tool Post

Since the tool is stationary on a lathe, there is great flexibility for mounting the tool to best advantage. The tool post and carriage of a lathe provide several ways of positioning and feeding the tool. Below is shown the work area of an engine lathe.

The cutting tool is fixtured on the tool post, which sits atop the carriage assembly. The carriage can move the tool post along the axis of part rotation, perpendicular to the axial direction, and on a diagonal.

The tool post is shown below. The tool post can pivot the tool about a vertical axis and the cutting tool can be moved in and out along its long axis. The cutting tool is held in by the vertical screws, the heads of which can be seen above the cutting tool groove.

Engine Lathe Tail Stock

The tail stock of an engine lathe is used to provide a fixture at the end of the part opposite from the chuck. The tail stock can be used to support a long, thin part so that more radial cutting force can be applied and higher rotational speeds can be attained without a "whipping" instability effect. Below is illustrated another use for the tail stock. Drill bits can be fixtured in the tail stock to cut axial holes in a turned part. These central holes are more accurate than a drill press or mill could provide since the lathe is dedicated to operations in which an axially-symmetric part is rotated about its central axis. The fixturing is more accurate since all fixturing is based upon surfaces of revolution about the central axis, and the machining is more rigidly supported for the same reason.

Boring

Boring can be accomplished on a mill or even a drill press, but is most accurate on a lathe. The boring tool is fixtured in the tail stock. Again, since all fixturing is relative to the central spindle axis, boring on a lathe is more accurate than most other boring methods, an exception being jig boring on a dedicated jig boring machine. The length of the boring bar is of critical importance because of its tendency to bend. The figure below illustrates a boring tool which is double-ended so that it is less prone to the cantilever "diving board" effect.

For design guidelines for bored holes in parts, please check the design for boring section.

Below are illustrated some of the many types of machining that can be accomplished on a lathe.

Turning: Standard Tool Post Tool
The tool inserted in the tool holder is shown below:

CUTTING TOOLS FOR LATHES

Tool Geometry. For cutting tools, geometry depends mainly on the properties of the tool material and the work material. The standard terminology is shown in the following figure. For single point tools, the most important angles are the rake angles and the end and side relief angles.

The back rake angle affects the ability of the tool to shear the work material and form the chip. It can be positive or negative. Positive rake angles reduce the cutting forces resulting in smaller deflections of the workpiece, tool holder, and machine. If the back rake angle is too large, the strength of the tool is reduced as well as its capacity to conduct heat. In machining hard work materials, the back rake angle must be small, even negative for carbide and diamond tools. The higher the hardness, the smaller the back rake angle. For high-speed steels, back rake angle is normally chosen in the positive range. There are two basic requirements for thread cutting. An accurately shaped and properly mounted tool is needed because thread cutting is a form-cutting operation. The resulting thread profile is determined by the shape of the tool and its position relative to the workpiece. The second by requirement is that the tool must move longitudinally in a specific relationship to the rotation of the workpiece, because this determines the lead of the thread. This requirement is met through the use of the lead screw and the split unit, which provide positive motion of the carriage relative to the rotation of the spindle.

Tool Geometry. For cutting tools, geometry depends mainly on the properties of the tool material and the work material. The standard terminology is shown in the following figure. For single point tools, the most important angles are the rake angles and the end and side relief angles.

The back rake angle affects the ability of the tool to shear the work material and form the chip. It can be positive or negative. Positive rake angles reduce the cutting forces resulting in smaller deflections of the workpiece, tool holder, and machine. If the back rake angle is too large, the strength of the tool is reduced as well as its capacity to conduct heat. In machining hard work materials, the back rake angle must be small, even negative for carbide and diamond tools. The higher the hardness, the smaller the back rake angle. For high-speed steels, back rake angle is normally chosen in the positive range.

Most lathe operations are done with relatively simple, single-point cutting tools. On right-hand and left-hand turning and facing tools, the cutting takes place on the side of the tool; therefore the side rake angle is of primary importance and deep cuts can be made. On the round-nose turning tools, cutoff tools, finishing tools, and some threading tools, cutting takes place on or near the end of the tool, and the back rake is therefore of importance. Such tools are used with relatively light depths of cut. Because tool materials are expensive, it is desirable to use as little as possible. It is essential, at the same, that the cutting tool be supported in a strong, rigid manner to minimize deflection and possible vibration. Consequently, lathe tools are supported in various types of heavy, forged steel tool holders, as shown in the figure.

The tool bit should be clamped in the tool holder with minimum overhang. Otherwise, tool chatter and a poor surface finish may result. In the use of carbide, ceramic, or coated carbides for mass production work, throwaway inserts are used; these can be purchased in great variety of shapes, geometrics (nose radius, tool angle, and groove geometry), and sizes.

Single-Point Cutting Tool Variety

There are many types of cutting tools for different operations. Below is shown a few of the variety, here shown with a tool holder adapter that fits into a larger tool post fixture.

Below is shown how single-point lathe tools can be used.

Parting Tool

The illustration below shows how a parting tool is fixtured and used. Parting is important at the end of a turning process in order to separate the part from the raw material. Parting must be carried out slowly and carefully since the tool is quite long and is prone to chattering. Parting is not very accurate, and a finishing cut must often be undertaken after parting if the parted surface is to be accurate.

Knurling
Knurling is an operation used to produce a texture on a turned machine part. Handles are often knurled in order to provide a gripping surface. The two wheel inserts shown on the tool below contact the work piece, and with pressure, cold-form a pattern into the surface of the part.

Screw Machines

Screw machines are automated lathes which can quickly mass-produce turned parts. A screw machine uses cutting methods similar to that of a lathe but is highly automated. Screw machines are typically used for high-volume, low-cost turned parts. Feed stock for a screw machine is a long cylindrical rod of material. The screw machine automatically turns/faces the part, parts it off, and advances the rod for the next part. A screw machine is illustrated below.

Cross Slide Simultaneous Operation

Below is illustrated an on-axis view of how cross slides in a screw machine sequentially cut the work piece. Simultaneous action increases throughput. The view is down the axis of the spindle and shows how tools on cross slides can cut the work piece, some simultaneously with others.

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