Vc generally less than 5 m/min when machining steels.
High Speed Steels
Possible alloys: C, Si, Mn, Cr, W, Mo, V, Co, Ti, Ni, Bo
Most common types: Mo base or W base
Co between 2.5 and 24%: greater hot hardness
Classic: W = 18% Cr = 4% V = 1% known as 18 4 1
Stellites
Ternary alloys of Cr, Co and W They are obtained by casting Only machinable
by abrasion
Disadvantages: more fragile than high speed steel. They must work at high Vc and
small progress They are insensitive to heat treatments
Melting point close to that of 1280 o C steels) and identical coefficient of expansion,
They allow oxyacetylene recharging on steel support bars
Being a cast product, bars of this material may present
porosity
Sintered Carbides (Hard Metal or Widia)
Powder metallurgical compounds of carbides of W, or of Ta and W, including carbides of Ti and Nb
Commonly agglomerated with Co
Manufacturing grinding, mixing, pressing, sintering between 1400 and 1600 ºC
Vc much superior to high speed steels
They do not admit any heat treatment
Less tough than high speed steels
Increasing application replacing high speed steel drills, small cutters, taps, etc. in which the entire volume is carbide (solid carbide tools)
Other gun drill bits, disposable drill heads, etc.
Disposable inserts or plates, with or without coating
Hard Metal Grades
QUALITIES: are designated with numbers
Within each TYPE they are designated by numbers from 0 to 50
They have antagonistic behavior: wear resistance or toughness.
Wear resistance increases with increasing No. and toughness decreases.
The quality is chosen according to the requirements of the operation.
Toughness is sought for interrupted cutting or for low or medium Vc and feed.
Wear resistance is sought for continuous cutting or high VC and feed
Types of inserts and tool holders
Ceramics
Powder sintering at 1700 o C, of Al oxides or Si nitride between 90 and 99 with additions of other oxides such as Zirconium, Cr, Mg, Fe, etc.
The hardness of the final compound exceeds that of its individual components
Al oxide or alumina (Al 2 O 3 also used as an abrasive, has greater hardness than CW, but is more fragile and sensitive to shocks and vibrations. It is used only in continuous machining with very rigid, stable, and powerful machines.
Si nitride (Si 3 N 4 ceramic superior to Al 2 O 3 in toughness and resistance to thermal shocks. Its toughness is comparable to carbide
Tool holders should be as rigid and robust as possible
The cutting laws that apply to other materials are not applicable in the case of ceramics, namely
-There is no formation of built-up edge
-No crater-shaped tool wear
-The minimum thickness of chip that can be removed
Ceramics must work with to negative so that the effort is net compression, avoiding bending that could give rise to traction components
They are supplied as disposable inserts. The Vc are higher, up to 1000 m/min.
Cermets
It is the name assigned to hard metals with hard particles based on TiC, titanium carbonitride (and/or TiN instead of WC
Name derived from CERamic METal ceramic particles in metal binder
Powder metallurgy products are pressed and sintered to obtain inserts. High melting point metallic elements such as Mo Cr and V and non-metallic elements such as SiC BoC and silicates are added.
Among the cermets with the best cutting characteristics are those composed of Al 2 O 3 Mo 2 C, and VC. The percentage of metallic carbides can vary between 5 and 40%.
Despite their relative fragility, they have acceptable toughness and are not only used for finishing, but also for milling and turning stainless steels.
Outstanding features: high resistance to wear in incidence and crater, high chemical stability and heat resistance, low tendency to built-up edge and oxidation wear.
Cubic boron nitride (CBN)
Second in hardness after diamond, high hardness at high temperatures (2000 o C), great wear resistance, chemical stability during machining More tenacious than ceramics despite their greater hardness, but they have lower thermal and chemical resistance
An important application for turning hard parts avoiding grinding
Other typical applications Forged steels, hardened steels and castings, powder metallurgical metals with Co and Fe, rolling mill rolls, high heat resistance alloys
CBN is produced at high pressure and temperature to bond cubic boron crystals with a ceramic or metallic binder. The loosely oriented particles form a very dense polycrystalline structure. The actual CBN crystal is similar to that of synthetic diamond.
The properties of CBN can be varied by altering the crystal size, content and type of binder in order to produce a variety of qualities.
Parts that are too soft cause more wear than parts made of hard materials.
Cutting forces are high because negative geometry must be used, due to the material to be cut and the high friction. High machine stability and power, high tool rigidity and generous tip radius are crucial. Chamfered or lapped edge provides greater strength and durability than carbide and ceramics.
They are excellent for precision finishing, for Ra= 0 3 and tolerances of +- 0.01 mm.
Very abundant and uninterrupted cutting fluid, or work dry
There are integral CBN inserts or CBN segments bonded on vertices of carbide inserts.
Polycrystalline Diamond (PCD)
Hardness very close to that of natural monocrystalline diamond. High wear resistance, and is widely used as an abrasive for grinding wheels.
They are fine diamond crystals joined by sintering at high pressure and temperature. Their orientation is disordered, which eliminates directions that cause fractures.
Small PCD plates are welded on a corner onto carbide inserts fixed in tool holders. Cutting edge life up to 100 times longer than carbide.
Apparently ideal cutting material but it has critical points: The temperature in the cutting zone must not exceed 600 o C, it cannot be used for ferrous metals due to their affinity, nor for tough materials with high tensile strength. This excludes PCD from most non-abrasive machining applications.
Used for its correct application, it is excellent: for non-ferrous or non-metallic abrasive materials. When high precision and high finish quality are required. For turning and milling abrasive Si and Al alloys. In fact, uncoated fine-grained carbide and PCD are the two main materials for machining aluminium.
It is essential to have very sharp edges and a positive angle of attack.
Other materials that can be machined with PCD are composites, resins, plastics, carbon, ceramics and presintered hard metals, as well as Cu, bronze, Mg alloys, Zn alloys, Pb and brass.
Due to its high chemical stability, friction with the piece does not affect the edge. PCD does not leave burrs and the tool life is many times longer. Its high fragility demands very stable conditions, very rigid tools and machines working at high speeds. Fluid can be used for cooling.
Typical operations are finishing and semi-finishing in turning and boring. For milling, flush inserts in special seats are used. Depths and feeds should be small and interrupted cuts should be avoided.
Diamond
The highest hardness attainable in cutting tools: long cutting edge life.
Main disadvantage: fragility, which makes it incapable of withstanding vibrations.
For continuous, low-depth machining, responding to tolerances of +- 0.002mm, with a surface finish superior to grinding.
For machining plastic materials, some bronzes, Al, Cu, Brass, Rubber, Asbestos, Ebonite, Cardboard, etc. alloys.
To reduce the risk of brittle fracture, the tool tip is rounded with a large radius curve, either continuously or following a faceted polygonal shape.
They can be used in the form of inserts on special tool holders, or they can be welded onto a corner of a triangular or rhomboidal carbide insert. Carbide inserts with a surface diamond deposit have also appeared.
They are only used for finishing. The Vc is only limited to the appearance of the first vibrations in the machine or in the piece. Small feed, between 0.03 and 0.05 mm/turn, and depth of 0.1 to 0.5 mm.
