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
Cutting Speeds and Feeds
Machinability Ratings of Materials
Selecting Speed in (SFM) Surface Feet Per Minute
Selecting Feed in Chip Load per Tooth
Selection of High Speed Steels
Selection of Carbide Grades
Cutting Tool Geometry
Cutting Tool Failures
CUTTING TOOL GEOMETRY
In the practical day-to-day application of cutting tools, the tool engineer is not confronted with laboratory conditions. Production requirements dictate the time allowed for corrective action. Rather than correct a machine problem that exists, the tool engineer may find it necessary to change the geometry of the milling catter, the cutting tool material or adjust speeds and feeds to suit the situation.
Confusion often exists when referring to the cutting tool nomenclature. The single point tool is as basic to all cutting tools as the simple engine lathe is to all machines. The effects the functional angles have on the single point tool are also basic to the milling cutter, end mill, drill, tap and other cutting tools.
The illustrations in Figures 72, 73 and 74 list the nomenclature of a single point tool, a staggered tooth side milling cutter and a face mill. An understanding of tool geometry and the function of the angles is part of this set of tools.
Using the proper geometry for a specific application will provide many quick solutions to poor tool life and breakage problems. A value analysis approach will aid in determining what action must be taken to continue producing parts.
FUNCTION OF THE ANGLES
Radial Rake Angle (Milling) - Side Rake Angle (Single Point Tool).
This angle has a major effect on power efficiency and tool life.
Axial Rake Angle (Milling) - Back Rake Angle (Single Point Tool).
This angle controls the chip flow, and thrust force (into spindle or away from) of the cut and the strength of the cutting edges.
Corner (Chamfer) Angle - Side Cutting Edge Angle (Single Point Tool).
This angle reduces the thickness of the chip. Shock is absorbed behind the cutting point, adding strength that influences tool life.
True Rake Angle (Milling - True Rake Angle (Single Point Tool).
The combination of radial rake, axial rake, and corner angle determines the chip formation shear angle, the power requirements, tool force and temperature. True rake angle is the most significant angle in the metal removal process. The higher the positive true rake angle, the lower the force, the power requirements and the heat generated. The cutting tool material, machine rigidity and other variables determine the positive or negative values that can be used.
Inclination Angle (Same in Milling and Turning).
This has a significant effect on the direction of the chip. Positive inclination directs the chip outward and negative inclination directs the chip toward the center of the cutter. The inclination angle is perpendicular to the direction of tool travel. Any change in axial rake angle, radial rake angle or chamfer angle can change the inclination and therefore the direction of the chip flow:
Dish Angle (Milling) - End Cutting Edge Angle (Single Point Tool).
This provides clearance between the cutter and the finished surface of the work which blends into the radius or chamfer of the tool. All angle close to zero adds strength but causes rubbing and generates heat. Too large an angle Weakens the tool. Flats parallel to the finished surface are often ground on the end cutting edge or dish angle to produce good surface finishes.
Nose Radius (Milling and Single Point)
This strengthens the culting edge, improves finish and influences tool life. Too large a radius increases radial forces and induces chalter. Too small a nose radius may result. in premature chipping or prevent proper distribution of the heat and break down the properties of the cutting tool material.
Clearance Angles (Milling and Single Point).
Primary clearance is directly below the cutting edge and is selected for the material being machined. It prevents the cutter or tool from rubbing on the workpiece. It also affects the strength of the tool. The secondary clearance is on the tooth form of the cutter or the shank of the single point tool. It must be large enough to clear the workpiece and permit chips to escape but not so large that it weakens the cutter or tool. The correct geometry for the milling application is vital to profitable metal removal. The correct geometry with the wrong cutting tool material can result ill successful machining but the wrong geometry with the correct cutting tool material may result in prohibitive tool costs or a loss in production. The tool engineer must know the function of the tool angles to correct tool failure due to changes in machine set-up, or part configuration.
One of the most important variables when machining is "rigidity" - rigidity of set-up, machine, cutting tool and workpiece. Where a lack of rigidity exists in any one of these, a change in cutter geometry, cutting tool material, speed and feed or a combination of these variables may be necessary.
Large arbors increase rigidity significantly and can eliminate chatter, improve tool life and surface finish. The spindle bearings, feed screws and arbor are all subject to deflection from the cutting and reaction forces and must be kept in good condition. Figure 45, page 15 illustrates the importance of arbor size when machining.