ME 3633 CH 22

Twist Drill

Most common cutting tools for hole‑making. Usually made of high speed steel

Tool Life vs. Cutting Speed

Natural log‑log plot. Slide 9

Dry Machining

No cutting fluid is used. Avoids problems of cutting fluid contamination, disposal, and filtration.

Milling Cutters principle types?

Plain milling cutter Face milling cutter End milling cutter Look at tool geometry on slides 39 and 40.

Ceramics

Primarily fine‑grained Al2O3, pressed and sintered at high pressures and temperatures into insert form with no binder

Taylor Tool Life Equation

Relationship credited to Frederick W. Taylor vT^n = C v = cutting speed T = tool life n = slope of the plot C = intercept on the speed axis at one minute tool life

Twist Drill Operation

Rotation and feeding of drill bit result in relative motion between cutting edges and work material to form the chips. - Cutting speed varies along cutting edges as a function of distance from axis of rotation. - Relative velocity at drill point is zero, so no cutting takes place. Instead, a large thrust force is required to drive the drill forward into the hole.

Synthetic Diamonds

Sintered polycrystalline diamond (SPD) - fabricated by sintering very fine‑grained diamond crystals under high temperatures and pressures into desired shape with little or no binder - Usually applied as coating (0.5 mm thick) on WC-Co insert

Holding and Presenting a Single-Point Tool

Solid shank tool, typical of HSS; brazed cemented carbide insert; and mechanically clamped insert, for carbides, ceramics, and other very hard tool materials

Alternative Drills?

Straight-flute drill Gun drill Spade drill

Cutting Fluid Classification

- Coolants - designed to reduce effects of heat in machining - Lubricants - designed to reduce tool‑chip and tool‑work friction

Preferred Mode is Gradual Wear. Why?

- Fracture and temperature failures are premature failures. - Gradual wear is preferred because it leads to the longest possible use of the tool. - Gradual wear occurs at two locations on a tool: Crater wear (occurs on top rake face) and Flank wear (occurs on flank - side of tool).

Cemented Carbides - General Properties?

- High compressive strength but low‑to‑moderate tensile strength - High hardness (90 to 95 HRA) - Good hot hardness - Good wear resistance - High thermal conductivity - High elastic modulus ‑ 600 x 103 MPa (90 x 106lb/in2) - Toughness lower than high speed steel

Cubic Boron Nitride

- Next to diamond, cubic boron nitride (cBN) is hardest material known - Fabrication into cutting tool inserts same as SPD: coatings on WC‑Co inserts - Applications: machining steel and nickel‑based alloys - SPD and cBN tools are expensive

Problems with dry machining?

- Overheating of tool - Operating at lower cutting speeds and production rates to prolong tool life - Absence of chip removal benefits of cutting fluids in grinding and milling

Cutting Fluid Filtration Advantages

- Prolong cutting fluid life between changes - Reduce fluid disposal cost - Cleaner fluids reduce health hazards - Lower machine tool maintenance - Longer tool life

Dealing with Cutting Fluid Contamination

- Replace cutting fluid at regular and frequent intervals - Use filtration system to continuously or periodically clean the fluid - Dry machining

Cutting Fluid Contamination

- Tramp oil (machine oil, hydraulic fluid, etc.) - Garbage (cigarette butts, food, etc.) - Small chips - Molds, fungi, and bacteria

High Speed Steel Composition Typical alloying ingredients? Typical composition (Grade T1)?

- Tungsten and/or Molybdenum - Chromium and Vanadium - Carbon, of course - Cobalt in some grades §18% W 4% Cr 1% V 0.9% C

Steel Cutting Carbide Grades

- Used for low carbon, stainless, and other alloy steels. - TiC and/or TaC are substituted for some of the WC - Composition increases crater wear resistance for steel cutting, but adversely affects flank wear resistance for non‑steel cutting applications

Lubricants

- Usually oil‑based fluids - Most effective at lower cutting speeds - Also reduce temperature in the operation

Cutting Fluids other functions and benefits?

- Wash away chips (e.g., grinding and milling) - Reduce temperature of work part for easier handling - Improve dimensional stability of work part

Coolants

- Water used as base in coolant‑type cutting fluids - Most effective at high cutting speeds where heat generation and high temperatures are problems - Most effective on tool materials that are most susceptible to temperature failures (e.g., HSS)

Typical hot hardness relationships for selected tool materials

-High speed steel is much better than plain C steel. -Cemented carbides and ceramics are significantly harder at elevated temperatures.

Tool Life Criteria in Production

1. Complete failure of cutting edge 2. Visual inspection of wear by machine operator 3. Fingernail test across cutting edge 4. Changes in sound emitted from operation 5. Chips become stringy and difficult to dispose 6. Degradation of surface finish 7. Increased power 8. Workpiece count 9. Cumulative cutting time

Three Modes of Tool Failure

1. Fracture failure 2. Temperature failure 3. Gradual wear

Two main problems addressed by cutting fluids?

1. Heat generation at shear and friction zones 2. Friction at tool‑chip and tool‑work interfaces

Two basic types of Cemented Carbides?

1. Non‑steel cutting grades - only WC‑Co 2. Steel cutting grades - TiC and TaC added to WC‑Co

Common Insert Shapes

1. Round 2. square 3. rhombus 80° point angles 4. hexagon 80° point angles 5. triangle 6. rhombus 55° point angles 7. rhombus 35° point angles From 1 -> 7: versatility and accessibility increases while strength, power requirements, and vibration tendency decreases.

Tool Geometry two categories?

1. Single point tools - Used for turning, boring, shaping, and planing. 2. Multiple cutting edge tools - Used for drilling, reaming, tapping, milling, broaching, and sawing.

Two principal aspects:

1. Tool material 2. Tool geometry

Two basic types of HSS (AISI)

1. Tungsten‑type, designated T‑ grades 2. Molybdenum‑type, designated M‑grades

Cutting Fluids

Any liquid or gas applied directly to the machining operation to improve cutting performance.

Seven elements of single‑point tool geometry

Back rake angle (alpha b) Side rake angle (alpha s) End relief angle (ERA) Side relief angle (SRA) End cutting edge angle (ECEA) Side cutting edge angle (SCEA) Nose Radius (NR)

Coated Carbides

Cemented carbide insert coated with one or more layers of TiC, TiN, and/or Al2O3 or other materials. - Coating thickness = 2.5 ‑ 13 mm (0.0001 - 0.0005 in) - Coating applied by chemical vapor deposition or physical vapor deposition

Twist Drill Operation - Problems

Chip removal - Flutes must provide sufficient clearance to allow chips to move from bottom of hole during cutting - Friction makes matters worse. Rubbing between outside diameter of drill bit and newly formed hole. Delivery of cutting fluid to drill point to reduce friction and heat is difficult because chips are moving in opposite direction

Cemented Carbides

Class of hard tool material based on tungsten carbide (WC) using powder metallurgy techniques with cobalt (Co) as the binder.

Cermets

Combinations of TiC, TiN, and titanium carbonitride(TiCN), with nickel and/or molybdenum as binders.

Fracture failure

Cutting force becomes excessive and/or dynamic, leading to brittle fracture

Temperature failure

Cutting temperature is too high for the tool material

Gun drill

For deep holes, it has a carbide cutting edge, a single straight flute, and a coolant hole running its entire length

Spade drill

For large diameter holes - up to 152 mm (6 in)

Gradual wear

Gradual wearing of the cutting tool

End miller cutting

Looks like a drill bit but designed for primary cutting with its peripheral teeth Applications: Face milling, Profile milling and pocketing, Cutting slots, Engraving, Surface contouring, and Die sinking.

Tool failure modes identify the important properties that a tool material should possess:

Toughness ‑ to avoid fracture failure Hot hardness ‑ ability to retain hardness at high temperatures Wear resistance ‑ hardness is most important property to resist abrasive wear

Non‑steel Cutting Carbide Grades

Used for nonferrous metals and gray cast iron.

Coated Carbides applications?

cast irons and steels in turning and milling operations. - best used at high speeds where dynamic force and thermal shock are minimal

Non‑steel Cutting Carbide Grades: Properties are determined by?

grain size and cobalt content - As grain size increases, hardness and hot hardness decrease, but toughness increases - As cobalt content increases, toughness improves at the expense of hardness and wear resistance

Cermets applications?

high speed finishing and semi-finishing of steels, stainless steels, and cast irons - Higher speeds and lower feeds than steel‑cutting cemented carbide grades - Better finish achieved, often eliminating need for grinding

Synthetic Diamonds applications?

high speed machining of nonferrous metals and abrasive nonmetals such as fiberglass reinforced polymer, graphite, and wood - not for steel cutting

Ceramics applications?

high speed turning of cast iron and steel - not recommended for heavy interrupted cuts (e.g. rough milling) due to low toughness - Al2O3 also widely used as an abrasive in grinding

High Speed Steel (HSS)

high-alloyed tool steel capable of maintaining hardness at elevated temps better than high carbon and low allow steels. Especially suited to applications involving complicated tool shapes: drills, taps, milling cutters, and broaches

Tool Wear vs. Time

slide 7

Effect of Cutting Speed

slide 8

Straight-flute drill

using indexable cemented carbide inserts for higher cutting speeds