metal cutting processes

Tool Wear

Adhesion wear Abrasion Diffusion wear

In each of these processes, the process parameters are:

Depth of cut: the amount of material removed per stroke (mm) Speed: the speed of the cutting (rpm or mm/min) Feed: the width removed per one revolution (mm/rev)

Types of Chip

Discontinuous chip. Continuous chip. Continuous chip with built-up edge. Serrated chips.

Tool Geometry:

Rake angle Cutting-edge angle Relief angle Nose radius

carbon tool steel

Hot hardness Temperature up to 200 Celsius

coated carbids

high red hardness 1100 c

High-speed steel (HSS)

up to 650 c

Diffusion wear:

At high temperatures, atoms from tool diffuse across to the chip; the rate of diffusion increases exponentially with temperature.

Advantages of machining:

Excellent dimensional tolerances, Produce both internal and external surfaces with same quality, Economical if small quantities

temperature is an important factor because

Excessive temperature lowers the strength, hardness, stiffness, and wear resistance of the cutting tool; tools also may soften and undergo plastic deformation; thus, tool shape is altered. Cutting Increased heat causes uneven dimensional changes in the part being machined, making it difficult to control its dimensional accuracy and tolerances. An excessive temperature rise can induce thermal damage and metallurgical changes in the machined surface, adversely affecting its properties.

During cutting, the tool suffers from:

Excessive wear, Fracture failure, High temperature

Adhesion wear:

Fragments of the work-piece get welded to the tool surface at high temperatures.

Abrasion:

Hard particles, microscopic variations on the bottom surface of the chips rub against the tool surface and break away a fraction of tool with them.

To be able to withstand these conditions, the tool should be:

High hot hardness, High toughness High wear resistance, Low friction Chemically stable.

how is it functioned turning ?

In turning, the workpiece is rotating with a speed (N) (cutting speed) whereas, the tool is moving radially (d) (depth of cut) and axially (f) (feed).

what are inserts made of ?

Inserts are usually made of carbides or diamond.

how can you install cutting inserts ?

Inserts can be clamped (the most common method) or brazed to the tool shank. Clamping is the preferred method of securing an insert because each insert has a number of cutting points and, after one edge is worn, it is indexed (rotated in its holder) to make another cutting point available.

Relief angle

It controls interference and rubbing at the tool-workpiece interface. Relief angles typically are 5 °.

rake angle

It is important in controlling both the direction of chip flow and the strength of the tool tip.

Machining or cutting might contain

Roughing (removing great part of the metal), semi- finishing, or finishing processes (removing a very small amount).

Serrated chips.

These chips are semi-continuous in the sense that they possess a saw-tooth appearance. This fourth type of chip is most closely associated with certain difficult-to-machine metals such as titanium alloys, nickel- base superalloys, and austenitic stainless steels when they are machined at higher cutting speeds.

Disadvantages of machining

They waste material (although the amount may be relatively small), The processes generally takes longer than other processes, They generally require more energy than do forming and shaping processes, They can have adverse effects on the surface quality and properties of

Common Processes

Turning Cutting-off Drilling Face milling (end) Plain Milling (slab) Shaping Grinding

Processes that can be performed on a lathe machine:

Turning: Facing: Face grooving Cutting with form tools: Boring: Drilling: Parting: Threading: Knurling:

Continuous chip.

When ductile work materials are cut at high speeds and relatively small feeds and depths, long continuous chips are formed. A good surface finish typically results when this chip type is formed. A sharp cutting edge on the tool and low tool-chip friction encourage the formation of continuous chips. Long, continuous chips (as in turning) can cause problems with regard to chip disposal and/or tangling about the tool. To solve these problems, turning tools are often equipped with chip breakers

Continuous chip with built-up edge.

When machining ductile materials at low-to medium cutting speeds, friction between tool and chip tends to cause portions of the work material to adhere to the rake face of the tool near the cutting edge. This formation is called a built-up edge. Built- up edge usually requires the tool to be re-sharpening otherwise the surface of the work will be rough.

Discontinuous chip.

When relatively brittle materials (e.g., cast irons) are machined at low cutting speeds, the chips often form into separate segments (sometimes the segments are loosely attached). This tends to impart an irregular texture to the machined surface. High tool-chip friction and large feed and depth of cut promote the formation of this chip type.

Cutting-edge angle

affects chip formation, tool strength, and cutting forces to various degrees. cutting-edge angle is around 15°.

Nose radius

affects surface finish and tool-tip strength. The smaller the nose radius (sharp tool), the rougher the surface finish of the workpiece

Tool materials

carbon tool steel high speed steel (HSS) cast-nonferrous alloys (stellite tools) cemented carbids (sintered carbids) coated carbids ceramics cubic boron nitride dimond

design consideration for turning operation

clamp workpiece don't ask for high accuracy avoid sharp corners start with the nearest dimensions use standard cutting tool minimize tool length support workpiece use machine tools with high stiffness stop the machine when its vibrating

Turning

turning is the process of reducing the diameter of a circular cross- section bar from D to d.

ceramics

high red hardness tempreture up to 1200 c

cubic boron nitride

hot hardbess up to 1500 c

diamond

hot hardness tempreture uo to 1500 c

The importance of the cutting inserts

importance is to reduce the possibility of tool change after certain times of resharpening.

Cutting

is a group of processes at which the final geometry and dimensions are obtained by removing metal from the workpiece surface in the form of chip.

turning process is usually done on what machine ?

on Lathe Machine

cemented carbids (sontered carbids)

red hardness tempreture of 900-1000 c

cast-nonferrous alloys (stellite tools)

red hardness twnpreture up to 925 c

Cutting tool inserts are

replaceable attachments for cutting tools that typically contain the actual cutting edge.

turning is considered a ...... point tool

single