21-07-2012, 04:48 PM
Cutting Tool Materials
Cutting_tool_materials.ppt (Size: 673 KB / Downloads: 67)
Characteristics And Applications Of The Primary Cutting Tool Materials
Basic composition of HSS is 18% W, 4% Cr, 1% V, 0.7% C and rest Fe.
Such HSS tool could machine (turn) mild steel jobs at speed only up to 20 ~ 30 m/min
However, HSS is still used as cutting tool material where;
The tool geometry and mechanics of chip formation are complex, such as helical twist drills, reamers, gear shaping cutters, hobs, form tools, broaches etc.
Brittle tools like carbides, ceramics etc. are not suitable under shock loading
The small scale industries cannot afford costlier tools
The old or low powered small machine tools cannot accept high speed and feed.
Effectiveness and efficiency of HSS tools and their application range were gradually enhanced by improving its properties and surface condition through -
Refinement of microstructure
Addition of large amount of cobalt and Vanadium to increase hot hardness and wear resistance respectively
Manufacture by powder metallurgical process
Surface coating with heat and wear resistive materials like TiC, TiN, etc by Chemical Vapour Deposition (CVD) or Physical Vapour Deposition (PVD)
Stellite
Cast alloy of Co (40 to 50%), Cr (27 to 32%), W (14 to 19%) and C (2%).
Stellite is quite tough and more heat and wear resistive than the basic HSS (18 – 4 – 1)
Stellite as cutting tool material became obsolete for its poor grindability and specially after the arrival of cemented carbides.
Composite carbides
The single carbide is not suitable for machining steels because of rapid growth of wear, particularly crater wear, by diffusion of Co and carbon from the tool to the chip under the high stress and temperature bulk (plastic) contact between the continuous chip and the tool surfaces.
For machining steels successfully, another type called composite carbide have been developed by adding (8 to 20%) a gamma phase to WC and Co mix. The gamma phase is a mix of TiC, TiN, TaC, NiC etc. which are more diffusion resistant than WC due to their more stability and less wettability by steel.
Mixed carbides
Titanium carbide (TiC) is not only more stable but also much harder than WC.
For machining ferritic steels causing intensive diffusion and adhesion wear a large quantity (5 to 25%) of TiC is added with WC and Co to produce another grade called Mixed carbide.
But increase in TiC content reduces the toughness of the tools
For finishing with light cut but high speed, the harder grades containing up to 25% TiC are used
For heavy roughing work at lower speeds lesser amount (5 to 10%) of TiC is suitable.
Coated carbides
Reduction of cutting forces and power consumption
Increase in tool life (by 200 to 500%) for same vc or increase in vc (by 50 to150%) for same tool life
Improvement in product quality
Effective and efficient machining of wide range of work materials
Pollution control by less or no use of cutting fluid through reduction of abrasion, adhesion and diffusion wear
Reduction of friction and BUE formation
Heat resistance and reduction of thermal cracking and plastic deformation
The cutting velocity range in machining mild steel could be enhanced from 120 ~ 150 m/min to 300 ~ 350 m/min by properly coating the suitable carbide inserts.
About 50% of the carbide tools being used at present are coated carbides which are obviously to some extent costlier than the uncoated tools.
Different varieties of coated tools are available and appropriate one is selected depending upon the type of the cutting tool, work material, desired productivity and product quality.
The properties and performances of coated inserts and tools are getting further improved by;
Refining the microstructure of the coating
Multi layering (already up to 13 layers within 12 ~ 16 μm)
Direct coating by TiN instead of TiC, if feasible
Using better coating materials.