ME37 - Chap 21

Difference between + and - Side Rake Angles (RA): (Or true rake angle????)

(+) Increasing the back or side rake angles reduces the compression, the forces, and the friction, yielding a thinner, less deformed, and cooler chip. Unfortunately, it is difficult to take much advantage of the desirable effects of larger positive rake angles because they are offset by the reduced strength of the cutting tool, due to the reduced tool section, and by its greatly reduced capacity to conduct heat away from the cutting edge. (-) A small rake angle causes high compression, tool forces, and friction, resulting in a thick, highly deformed, hot chip. To provide greater strength at the cutting edge and better heat conductivity, zero or negative rake angles are commonly employed on carbide, ceramic, poly diamond, and PCBN cutting tools. While the negative rake angle increases tool forces, it keeps the tool in compression and provides added support to the cutting edge.

Difference between + and - Back Rake Angles (BRA):

(+) Positive rake angles reduce the cutting forces, resulting in smaller deflections of the workpiece, tool holder, and machine. For high-speed steels, back rake angle is normally chosen in the positive range, depending on the type of tool (turning, planing, end milling, face milling, drilling, etc.) and the work material. (-) In machining hard work materials, the BRA must be small, even negative for the carbide and diamond tools. Generally speaking, the higher the hardness of the workpiece, the smaller the BRA.

HSS main strengths are as follows:

.Great toughness (Superior transverse rupture strength); .Easily fabricated; .Best for sever applications where complex tool geometry is needed (Gear cutters, taps, drills, reamers, dies).

The selection of the cutting-tool material and geometry followed by the selection of cutting conditions for a given application depends upon many variables like:

.Work material (hardness); .Type of cut (roughing vs finishing); .Part geometry and size (rigidity); .Lot size (small batch vs mass production); .Machinability data (tool life/specific HP); .Quality/Capability needed; .Past experience of decision maker; .Availability (of materials, their compositions, sizes, shapes, geometry, costs, delivery dates, etc...)

The most important properties of tool steels are:

1. Hardness 2. Toughness 3. Wear resistance

Carbide cutting tool inserts are traditionally divided into two primary groups:

1. Straight tungsten grades, which are used for machining cast irons, austenitic stainless steel, and nonferrous and nonmetallic materials. 2. Grades containing major amounts of titanium, tantalum, and or columbium carbides, which are used for machining ferritic workpieces. There are also the titanium carbides grades, which are used for finishing and semi-finishing ferrous alloys.

What are three main sources of heat?

1. The shear front itself, where plastic deformation results in the major heat source. Most of this heat stays in the chip. 2. The tool/chip interface contact region, where additional plastic deformation takes place in the chip and considerable heat is generated due to sliding friction. This heat goes into the chip and the tool. 3. The flank of the tool, where the freshly produced workpiece surface rubs the tool.

Cause of failure in Built-up Edge?

A portion of the workpiece material adheres to the insert cutting edge (Chemical).

Define BRA

Back rake angle.

what should the application of a coolant be copious and of some velocity?

Because the extent to which the flow of a cutting fluid washes the very hot chips away from the cutting area is an important factor in heat removal.

Why are carbides, which are non ferrous alloys, also called sintered (or cemented) carbides?

Because they are manufactured by powder metallurgy techniques.

Cause of failure in Crater Wear? (Important)

Carbide particles are removed due to degradation of tool performances and the chemical reactions at high temperature (Chemical).

What is hot hardness?

Cutting tool materials that do not lose hardness at the high temperatures associated with high speeds are said to have hot hardness.

Cause of tool failure in Flank Wear? (Important)

Due to the abrasive effect of hard grains contained in the work material (Physical).

Cause of tool failure in Partial Fracture?

Due to the mechanical impact when an excessive force is applied to the cutting edge (Physical).

Cause of tool failure in Groove?

Due to wear at the DCL or outer edge of the cut (Physical).

Define ECEA

End cutting-edge angle.

Define ERA

End relief angle.

Cause of tool failure in Chipping?

Fine chips caused by high-pressure cutting, chatter, vibration, etc. (Physical).

Where is HSS used today?

HSS is still widely used for drills and many types of general-purpose milling cutters and in single-point tools used in general machining. (PS: For high-production machining, it has been replaces about completely by carbides, coated carbides, and coated HSS.)

What are High Speed Steels (HSS)?

High-alloy steel is superior to steel in that it retains its cutting ability at temperatures up to 1100 degrees Fahrenheit. Compared with tool steel, it can operate at about double or triple cutting speeds to about 100 sfpm with equal life, resulting in its name.

What importance does the nose radius have on surface finish?

IT has a major influence on surface finish as increasing the nose radius usually decreases tool wear and improves surface finish.

What does the Back Rake Angle (BRA) affect?

It affects the ability of the tool to shear the work material and form the chip.

What does the chip groove do?

It can cause the chips to curl tightly and break into small, easily disposed lengths. (Also with a positive rake angle at the tool tip may be used to reduce cutting forces without reducing the overall strength of the insert significantly.)

Importance of a very thin final coat of TiN coating?

It can effectively reduce crater formation on the tool face by one to two orders of magnitude relative to uncoated tools.

What is the main advantage of TiN-coated HSS tooling?

It is the reduced tool wear (Less tool wear results in less stock removal during tool regrinding, thus allowing individual tools to be reground more times.)

Define Wear resistance

Resistance to abrasion and erosion.

Define Toughness

Resistance to breakage and chipping.

Define Hardness

Resistance to deforming and flattening.

What do triple-coated carbide tools provide?

Resistance to wear and plastic deformation in machining of steel, Resistance to abrasive wear in cast iron, Resistance to Built-up edge formation.

Define SCEA

Side cutting-edge angle.

Define RA

Side rake angle.

Define SRA

Side relief angle.

What are some restrictions of coating?

The coatings must be thick enough to prolong tool life but thin enough to prevent brittleness.

Cause of failure in Deformation?

The cutting edge is deformed due to its softening at high temperature (Chemical).

What are the two primary roles of the cutting fluid?

The cutting fluids acts primarily as a coolant, and secondly as a lubricant, reducing the friction effects at the tool/chip interface and the work flank regions.

What do Chemical Tool Failures include?

The mainly include wear on the rake face of the tool (crater wear), are rapid, usually unpredictable, and often catastrophic failures resulting from abrupt, premature death of the tool.

Importance of multiple coatings:

The purpose of multiple coatings is to tailor the coating thickness for prolonged tool life. Multiple coatings allow a stronger metallurgical bond between the coating and the substrate and provide a variety of protection processes for machining different work materials, thus offering a more general-purpose tool material grade.

What is the effective rake angle, or true rake angle, or resultant rake angle of the tool?

The side rake angle and the back rake angle combine to form the effective rake angle.

Cause of failure in Thermal Crack?

Thermal fatigues in the heating and cooling cycle with interrupted cutting (Chemical).

What is the importance of TiN-coated HSS tools?

These tools provide significant improvements in cutting speeds, with increases of 10 to 20% being typical. (PS: they have demonstrated their ability to more than pay for the extra cost of the coating process.)

What do the relief angles mainly affect?

They affect the tool life and the surface quality of the workpiece.For high-speed steel, relief angles in the range of 5 to 10 degrees are normal, with smaller values being for the harder work materials.

What are Polycrystalline diamond tools?

They are carbides with diamond inserts and are restricted to simple geometries.

What do the side and end cutting-edge angles do?

They define the nose angle and characterize the tool design.

What do Physical Tool Failures include?

They mainly include gradual tool wear on the flank(s) of the tool below the cutting edfe (called flank wear) or wear on the rake face of the tool (called crater wear) or both.

As the tool wears, its geometry changes. What does this geometry change influence?

This geometry change will influence the cutting forces, the power being consumed, the surface finish obtained, the dimensional accuracy, and even dynamic stability of the process.

What do tool life test data for various coated drills show?

TiN-coated HSS drills outperform uncoated drills. Life based on the number of holes drilled before drill failure.

What is Chemical vapor deposition used for?

To apply layers (TiC, TiN, etc.) to carbide cutting tools.

What have improvements in cutting tool material lead to?

increased cutting speed and productivity.

What does the reduction in temperature thanks to cutting fluids aid in?

it greatly aids in retaining the hardness of the tool, thereby extending the tool life or permitting increased cutting speed with equal tool life.