Engineering manufacturing y2 - machining processes 4

What is milling

milling is a discontinuous cutting process using a multi-tooth tool, as teeth are not continuously engaged in the cutting process

What is peripheral milling

axis of rotation is parallel to the surface to be generated, where machining is performed by the cutting edges on the periphery of the tool

What is face milling

Axis of rotation is perpendicular to surface to be generated, where machining is performed on by teeth at the end and periphery of the tool

types face milling processes

end milling and face milling

face milling

a face mill is a large diameter cutter to produce flat surfaces, which can be more accurate than slab milling as it employs a generating action

end milling

small diameter tools with cutting edges at the end and periphery of tool. can be used to cut faces and shouldered features. ball-nose end mills have spherical end and are employed for contouring 3d surfaces

solid milling tools

tools comprised of a solid body of cutting tool material, into which the cutting edges have been ground. diameters in the range of <1-30mm. Typically made of high-speed steel, tungsten carbide and ceramic. Can be reground, where a thin layer of original cutting edge is being removed

Inserted milling tool

cutting inserts attached to a tool body. Tool body is made from high strength steel, whereas the inserts are typically made of tungsten carbide or ceramics. Only the inserts are involved in cutting. Hence, only they experience tool wear and, after some time, need to be replaced. Typical diameter range of 10-100 mm.

How do cutting tools bite into material

Each tool needs to exhibit a clearance angle behind cutting edge, otherwise it would just rub against material and not bite.

Cutting motion types

Up milling(or conventional)

Down milling

Up milling

cutting velocity and table traverse velocity are in opposite directions, chip thickness is 0 at tool entry and maximum at tool exit. May be bad bc vertical component of force might lift up component

Down milling

Cutting velocity and table traverse velocity are in same direction. Chip thickness is highest at tool entry and lowest at tool exit. Tool experiences abrupt large force on entry. cutting force directed into the table, allowing for thinner workpieces to be cut. low friction between teeth and workpiece, resulting in good surface finish

problems in drilling

cutting zone is located inside workpiece, which causes problems like: energy generated by cutting process remains confined to small region around tool, strong friction between tool margins and borehole wall, chip remains in contact with tool and workpiece, so heat in chip is given back to tool, very limited provision of cutting fluid

what can these problems cause in drilling

overheating , chips can bundle together due to high temp, chips clogging, resulting in premature tool fracture, chips can even weld together due to heat

what is peck drilling and why

hole drilled in short strokes and fully withdrawn after each peck. This is to increase chip evacuation

what is tapping

operation to produce a thread, requires a rotary motion and a linear motion along tool axis, these motions needing to be synchronised for as long as tool is engaged. to withdraw tap from hole, rotation must be reversed. Hole must be pre-drilled before tapping

tapping shape

-small and fragile cutting teeth, they exhibit small positive rake and flake angles, leafing to strong friciton

- exhibit a conical front section with cutting teeth that gradually increase in size, to allow the cutting forces to gradually increase.

helical tap for blind holes

short conical front section, positive helix conveys chips out of hole

straight fluted tap for through holes

longer conical front section, negative inclination of cutting edges pushes chips to be pushed forwards

What is turning

Turning is a machining process to produce axis-symmetric parts, and only uses single cutting-point tools. cutting motion is carried out by workpiece, feed motion by tool. This is a continuous machining process

turning tool main features

main features are cutting edge, rake and flank faces

if turning tool has negative inclination angle:

it is achieved by inclining cutting insert, results in high cutting forces due to high workpiece/chip shear strain

if turning tool has positive inclination angle:

both main and secondary clearance angles have to be built into cutting insert, weakening cutting edge.

lower cutting forces due to positive rake angle

chip breakers in cutting inserts

sintered into rake face, done to break chips when they flow over rake face. This is to stop any long continuous chips from forming and tangling up in the working process

cutting forces in turning graph (cutting force against cutting speed)

-initial increase in all 3 force components (cutting force, feed force, passive force due to increased BUE formation

-further increase in cutting speeds results in thermal softening of workpiece material at cutting point, therefore reducing the workpiece material's strength

cutting force against feed graph

increase in feed results in increase in force due to larger chip cross section. this increase is digressional because the increase in deed does not change the tool's cutting edge length engaged in cut.

cutting forces in turning, cutting force against depth of cut

proportional increase

cutting force components against tool cutting edge angle

as angle increases, feed force component increases and passive component decreases. Overall, slight decrease in cutting force due to reduction in cutting edge length engaged in the cut