Manufacturing Processes – Turning

Turning: the removal of metal from the outer diameter of a rotating cylindrical workpiece. Turning is
 

used to reduce the diameter of the workpiece, usually to a specified dimension, and to produce a smooth finish on the metal. Often the workpiece will be turned so that adjacent sections have different diameters.

Turning operations:

ü  Turning.

·         Tapered turning 

a) from the compound slide b) from taper turning attachment c) using a hydraulic copy attachment d) using a C.N.C. lathe e) using a form tool f) by the offsetting of the tailstock - this method more suited for shallow tapers.

·         Spherical generation 

The proper expression for making or turning a shape is to generate as in to generate a form around a fixed axis of revolution. a) using hydraulic copy attachment b) C.N.C. (computerised numerically controlled) lathe c) using a form tool (a rough and ready method) d) using bed jig (need drawing to explain).

·         Hard turning 

Hard turning is a turning done on materials with a Rockwell C hardness greater than 45. It is typically performed after the workpiece is heat treated.

ü  Facing: Facing in the context of turning work involves moving the cutting tool at right angles to the axis of rotation of the rotating workpiece

ü  Parting: also called parting off or cutoff, is used to create deep grooves which will remove a completed or part-complete component from its parent stock.

ü  Grooving: Grooving is like parting, except that grooves are cut to a specific depth instead of severing a completed/part-complete component from the stock. Grooving can be performed on internal and external surfaces, as well as on the face of the part (face grooving or trepanning)

ü  Boring: Enlarging or smoothing an existing hole created by drilling, moulding 

ü  Drilling: is used to remove material from the inside of a workpiece

ü  Knurling: The cutting of a serrated pattern onto the surface of a part to use as a hand grip 

ü  Reaming: removes a small amount of metal from a hole already drilled

ü  Threading: standard and non-standard screw threads can be turned on a lathe

Explanation of the standard components of most lathes:

• Bed: Usually made of cast iron. Provides a heavy rigid frame on which all the main components are mounted.

• Ways: Inner and outer guide rails that are precision machined parallel to assure accuracy of movement.

• Headstock: mounted in a fixed position on the inner ways, usually at the left end. Using a chuck, it rotates the work.

• Gearbox: inside the headstock, providing multiple speeds with a geometric ratio by moving levers.

• Spindle: Hole through the headstock to which bar stock can be fed, which allows shafts that are up to 2 times the length between lathe centers to be worked on one end at a time.

• Chuck: 3-jaw (self centering) or 4-jaw (independent) to clamp part being machined.

• Chuck: allows the mounting of difficult workpieces that are not round, square or triangular. 

• Tailstock: Fits on the inner ways of the bed and can slide towards any position the headstock to fit the length of the work piece. An optional taper turning attachment would be mounted to it. 

• Tailstock Quill: Has a Morse taper to hold a lathe center, drill bit or other tool.

• Carriage: Moves on the outer ways. Used for mounting and moving most the cutting tools.

• Cross Slide: Mounted on the traverse slide of the carriage, and uses a handwheel to feed tools into the workpiece.

• Tool Post: To mount tool holders in which the cutting bits are clamped.

• Compound Rest: Mounted to the cross slide, it pivots around the tool post.

• Apron: Attached to the front of the carriage, it has the mechanism and controls for moving the carriage and cross slide.

• Feed Rod: Has a keyway, with two reversing pinion gears, either of which can be meshed with the mating bevel gear to forward or reverse the carriage using a clutch.

• Lead Screw: For cutting threads.

• Split Nut: When closed around the lead screw, the carriage is driven along by direct drive without using a clutch.

• Quick Change Gearbox: Controls the movement of the carriage using levers. 

• Steady Rest: Clamped to the lathe ways, it uses adjustable fingers to contact the workpiece and align it. Can be used in place of tailstock or in the middle to support long or unstable parts being machined.

• Follow Rest: Bolted to the lathe carriage, it uses adjustable fingers to bear against the workpiece opposite the cutting tool to prevent deflection.

• Classification of cutting tools

A] According to number of cutting edge.

1. Single point cutting tool

It is simplest from of cutting tool & it have only one cutting edge.

Examples – shear tools, lathe tools, planer tools, boring tolls etc.

2.      Multi point cutting tool

In this two or more single point cutting tools arranged together as a unit. The rate of machining is more & surface finish is also better in this case.

Example- milling cutter, drills, brooches, grinding wheels, abrasive sticks etc.

B] According to motion

1. Linear motion tools – lathe tools, brooches
2. Rotary motion tools – milling cutters, grinding wheels
3. Linear & rotary motion tools – drills, taps, etc.

TOOLS AND TOOL HOLDERS

While lathes use some of the same tools that mills use, including spot drills, drills, and taps, most turning is done using carbide inserts. Inserts are gripped in holders, which in turn are bolted to the lathe turret (see Figure). Figure 1 shows a typical insert-holder combination. This is a left-handed holder, because the tool cutting edge points to the left when viewing the holder from the top as shown in Figure 2.

Figure (1) Typical Lathe Tool Holder

Carbide inserts employ highly engineered composite structures, coatings, and geometry features to achieve great accuracy and high material removal rates. Some inserts can be indexed to use other edges when one becomes worn. Inserts are quickly and easily replaced at the machine.

Figure (2) Insert Terms

Chip Breaker

A chip breaker is a feature in the face of the insert that disrupts the flow of chips such that they break into short segments, rather than forming a long, stringy chip.

Figure (3) Chip Breaker

Relief Angle

Most inserts have drafted faces on the walls. This is called Relief Angle. Relief prevents the walls of the insert from rubbing against the part.

Figure (4) Relief Angle

Tool Cutting Angles

The edge of the tool in the cut direction forms an angle with a line perpendicular to the cut direction. This is called Side Cutting Angle. The angle formed by the trailing edge and parallel to the cut direction is called the End Cutting Angle.

The purpose of these angles is to provide proper clearance between the tool and work piece. For example, the 80 degree insert shown in Figure 5 is rigid and has enough side and end cutting angle for facing and rough turning operations. However, complex contours may require a 55 or 30 degree insert to provide tool side and end clearance for the tool and holder. Very steep or vertical walls may require a round or slot tool to carve.

Figure (5) Cutting Angles

Rake Angle

Rake angle is set by the tool holder. Rake angle helps control the direction of the chip and cutting pressure. Angle is measured from face of the insert to the Z-X plane of the machine.

•         Cutting Tool Characteristics:

1.      Maintaining hardness, strength, and wear resistance at elevated temperatures

2.      Toughness

3.      Thermal Shock resistance

4.      Wear resistance

5.      Chemical stability