 | New
Concepts in Milling Handbook A practical approach and illustrated guide to milling cutter selection and use <Table of Contents © 1973 Niagara Cutter Inc. |
| New
Concepts in Milling Handbook A practical approach and illustrated guide to milling cutter selection and use <Table of Contents © 1973 Niagara Cutter Inc. |
Selection of Speeds and Feeds and Cutting Tool Materials -
A definition of the formation of the chip, chemical composition of cutting tool materials and engineering charts for setting of speed and feed for a variety of materials
CHART 6 SELECTION OF CARBIDE GRADES
Chemical Composition and Properties they Impart
The carbide grades in each category vary in hardness, and transverse rupture strength primarily because of the cobalt content and grain structure. However, each group possesses specific properties to resist specific reactions from materials being machined. Each group has different properties due to the chemical composition, and carbide grades should be selected on this basis, rather than on the more general grade classification code.
SELECTION OF CARBIDE CUTTING TOOL MATERIALS
Chart 6 is a guide to the selection of carbide grades. Chemical composition is given and relative abrasion resistance, red hardness and toughness are indicated. The carbides are classified in five groupsGroup I - Straight Tungsten Carbide Grades; Abrasion Resistant.
The straight tungsten carbide grades in grade classifications C1 through C4 contain the highest resistance to abrasion (flank wear) of any carbide grades and have the greatest strength. The grain size and cobalt content affect the hardness, abrasion resistance and strength of the tool. Additions of other carbides reduce the strength and abrasion resistance.
The grades coded in the Group I category in Chart 6 should be used where the reaction of the material being machined on the cutting tool material is abrasive. The hardest grade in this group that doesn't chip or fracture should be used.
The straight tungsten carbides are generally not successful when machining steel. The continuous chip in steel machining passes across the face of the cutter under pressure. The cobalt binder and tungsten carbide particles are pulled out and a crater developed. This crater grows, weakening the tool until it breaks. The steel also has a tendency to adhere to the straight tungsten carbide forming a built-up edge. This built-up edge scores the workpieces, increases the cutting forces and is often responsible for premature fracturing of the carbide.Group II - Titanium Carbide Bearing Grades; Cratering, Seizing, and Galling Resistance.
Titanium carbide (code classification C-5 through C-8) adds resistance to higher cutting temperatures generated by the continous pressure of the steel chip passing across the face of the cutter. It gives "lubricity" to the carbide so that the chip slides across the face of the cutter with less heat and friction. Titanium carbide additives permit the carbide to maintain high hardness at elevated temperatures. However, the more titanium carbide added, the weaker the tool is. Where the material being machined tends to crater, bind, seize, or gall the workpiece, titanium carbide bearing grades should be used. The hardest grade in this group that doesn't chip or fracture should be selected.Group III - Titanium Carbide and Tantalum (or Columbium) Carbide Bearing Grades; Resist Cratering, Deformation under Pressure, Elevated Temperatures.
The Group III grades (code classification C-5 through C-8 ) contain titaniun and tantalum carbide additives. These grades resist cratering, seizing, and galling as the Group II grades do and are a later, improved development for machining steel. In addition, they resist deformation of the carbide under heavy load where very high temperatures are created. Although additions of tantalum carbide reduce the strength of the carbide, they do not reduce the strength as directly as titanium carbide additives do. Tantalum carbide maintains its hardness and strength at elevated temperatures better than titanium carbide or tungsten carbide. Where high temperatures are developed and crateriag, seizing, galling or deformation takes place, the hardest Group III grade that doesn't chip or fracture should be selected.Group IV - Tantalum Carbide Bearing Grades.
Special grades of carbide can be made to suit the application. The Group IV grades are high tantalum carbide bearing grades in the C-5 code classification. The 28 percent tantalum bearing grade with 17 percent cobalt has very high red hardness qualities and high transverse rupture strength and is used for removing flash from weld. The 18 percent tantalum bearing grade with 11 percent cobalt also has high red hardness and strength qualities and is used for machining magnesium. Other chemical compositions can be made for specific applications.Group V - High Titanium Bearing Grades.
The high titanium bearing grades vary from tungsten free titanium carbides with nickel as the binding agent to high titanium grades with other additives. These grades have high red hardness and good wear qualities. They are capable of machining steel in the very high speed ranges, providing good surface finishes and size control. They have low strength values and are recommended for light cuts only.
