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