Cutting Tool Technology

basic types of HSS

1. Tungsten 2. Molybdenum

2 classifications of cutting fluids

1. coolants 2. lubricants

gradual wear occurs at two locations

1. crater wear 2. flank wear

basic types of milling cutters

1. plain 2. face 3. end

two categories of tool geometry

1. single point tools 2. multiple cutting edge tools

Two principle aspects of cutting tool technology

1. tool material 2. tool geometry

Tool Life Criteria in Production

1.Complete failure of cutting edge 2.Visual inspection of wear by the 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

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

cutting fluids

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

coated carbides

Cemented carbide insert coated with one or more layers of TiC, TiN, and/or Al2O3 or other materials

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 worseRubbing 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.

plain milling cutter

Cutter with teeth located around the circumference.

ceramics

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

fracture failure

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

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)

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

end milling cutter

Looks like a drill bit but designed for primary cutting with its peripheral teeth

twist drill

Most common cutting tools for hole‑making Usually made of 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

dry machining

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

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

twist drill operation

Rotation and feeding of drill bit result in relative motion between cutting edges and work material to form the chips

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

face milling cutter

Tool geometry elements of a four‑tooth face milling cutter

cutting fluid contamination

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

multiple cutting edge tools

Used for drilling, reaming, tapping, milling, broaching, and sawing

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

non-steel cutting carbide grades

Used for nonferrous metals and gray cast iron Properties determined by grain size and cobalt content

single point tools

Used for turning, boring, shaping, and planing

other benefits of cutting fluids

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

temperature failure

cutting temperature is too high for the tool material

coolants

designed to reduce effects of heat in machining

lubricants

designed to reduce tool‑chip and tool‑work friction

three modes of tool failure

fracture failure, temperature failure, gradual wear

flank wear

gradual tool wear on the flanks of a tool below the cutting edge

gradual wear

gradual wearing of the cutting tool

as grain size increases

hardness and hot hardness decrease, but toughness increases

crater wear

occurs on the rake face of the tool

as cobalt content increases

toughness improves at the expense of hardness and wear resistance

tool material properties

toughness, hot hardness, wear resistance

straight flute drill

using indexable cemented carbide inserts for higher cutting speeds