Tooling for Production

Unit-1Design of cutting tools

Content

• Design of single point turning tool• Tool signature• Design features for twist Drill• Milling cutters• Cutting tool materials

The Mechanics of Chip Formation

A wedge-shaped tool is made to move relative to the workpiece. As the too makes contact with the metal, it exerts a pressure on it resulting in the compression of the metal near the tool tip. This induces shear-type deformation within metal & it starts moving upward along the top face of the tool. As the tool advances, the material ahead of it is sheared continuously along a plane called shear plane.

Shear Plane

Classification of Cutting Tools

• Single Point Cutting Tool :-A single point Cutting tool has only one cutting point or edge. The tools are used for turning, boring, shaping, or planning operations.

•Multi Point Cutting tool:-A multi point cutting tool has two or more than two cutting points, for example, tools used on drilling machine, milling machine, broaching machines etc.

Single Point Cutting Tool

Shank

Major Flank

Major cutting edge

Cutting Part

Tool Axis

Base

Minor Flank

Corner

Minor Cutting Edge

Tool Angles

• Side Cutting Edge Angle:- Angle between the side cutting edge and the side of the tool shank.

• End Cutting Edge Angle:- Angle between the end cutting edge and a line normal to the tool shank.

• Side Relief Angle:- Angle between the portion of the side flank immediately below the side cutting edge & a line perpendicular to the base of the tool.

• End Relief Angle:- Angle between the portion of the end flank immediately below the end cutting edge & a line perpendicular to the base of the tool.

• Back Rake angle:- Angle between the face of the tool & a line parallel to the base of the tool.

• Side rake angle :- Angle between the tool face & a line parallel to the base of the tool.

Importance of cutting angles

• Side cutting edge angle :- It is the angle which prevents interference as the tool enters the work materials. The tip of the tool is protected at the start of the cut. Satisfactory values vary from 15 to 30 degrees.

• End cutting edge angle :- It provides a clearance or relief to the trailing end of the cutting edge to prevent rubbing or drag between the machined surface and the trailing part of the cutting tool. Satisfactory values vary from 8 to 15 degrees.

• Side relief angle :- These angles are provided so that the flank of the tool clears the workpiece surface and there is no rubbing action between the two. Varies from 5 to 15 degrees.

• Back & side rack angles :- Larger the cutting angle, lower the cutting force & power. However, since increasing the rake angle decreases the cutting angle, this leaves less metal at the point of the tool to support the cutting edge & conduct away heat.

Tool Signature

• The numerical code that describes all the key angles of a given cutting tool.

• It also describes how the tool is positioned in relation to the workpiece.

• For the most part, a tool signature specifies the effective angles of the tool normal to the cutting edge. This is always true as long as the tool shank is mounted at right angles to the workpiece axis.

Example of tool signature

Tool signature = 0 – 7 – 7 – 7 – 15 – 15 – 1/32

This implies• Back rake = 0°• Side rake = 7°• End relief = 7°• Side relief = 7°• End cutting edge angle = 15°• Side cutting edge angle = 15°• Nose radius = 1/32”

Twist Drill

• Drilling is the process of cutting or originating a round hole from a solid material. The tool & not the workpiece is revolved & is fed into the material along its axis. The most common type of drill is the flute twist drill. It is made from a round bar of tool material and has three principal parts:-1) The point2) The body3) The shank

Twist Drill geometry

• Axis: The imaginary straight line which forms the longitudinal centre line of the drill

• Back Taper: A slight decrease in diameter from front to back in the body of the drill.

• Body: The portion of the drill extending from the shank or neck to the outer corners of the cutting lips

• Body Diameter Clearance: That portion of the land that has been cut away so it will not rub against the walls of the hole

• Chipping: The breakdown of a cutting lip or margin by loss of fragments broken away during the cutting action

• Chisel Edge: The edge at the end of the web that connects the cutting lips

• Flutes: Helical or straight grooves cut or formed in the body of the drill to provide cutting lips, to permit removal of chips, and to allow cutting fluid to reach the cutting lips

• Land: The peripheral portion of the body between adjacent flutes• Margin: The cylindrical portion of the land which is not cut away to provide clearance• Drill Diameter: The diameter over the margins of the drill measured at the point• Point: The cutting end of a drill, made up of the ends of the lands and the web; in form it it

resembles a cone, but departs from a true cone to furnish clearance behind the cutting lips• Lips: The cutting edges of a two flute drill extending from the chisel edge to the periphery

• Shank: The part of the drill by which it is held and driven• Tang: The flattened end of a taper shank, intended to fit into

a driving slot in a socket• Lip Relief: The axial relief on the drill point• Lip Relief Angle: The axial relief angle at the outer corner of

the lip; it is measured by projection into a plane tangent to the periphery at the outer corner of the lip

• Chisel Edge Angle: The angle included between the chisel edge and the cutting lip, as viewed from the end of the drill

• Clearance: The space provided to eliminate undesirable contact between the drill and the workpiece

• Clearance Diameter: The diameter over the the cut away portion of the drill lands

• Heel: The trainling edge of the land• Helix Angle: The angle made by the leading edge of the land with a plane

containing the axis of the drill• Point Angle: THe angle included between the cutting lips projected upon a plane

parallel to the drill axis and parallel to the two cutting lips• Web Thickness: The thickness of the web at the point, unless another specific

locationis indicated• Neck: The section of reduced diameter between the body and the shank of a

drill Oil Grooves: Longitudinal straight or helical grooves in the shank, or grooves in the lands of a drill to carry cutting fluid to the cutting lips Oil Holes or Tubes: Holes through the lands or web of a drill for passage of cutting fluid to the cutting lips

Design Features• Materials:- 1) Carbon tool steels:- Low cost but should be used occasionally & at

slow speeds.2) HSS:- Most popular & have good strength.3) Drills tipped with carbide tip:- Economical of high production but

are expensive & must be handled carefully

• Drill Sizes:-Standard drills are available in four size series, the size indicating the diameter of the drill body

1) Fractional size:- Range 1/64” to 1/4“ with increments 1/64”2) Millimeter size:- Range 0.5 to 10 mm with increments 0.1 mm.3) Numbered size:- Range 0.0135” to 0.228” with slight increments.4) Lettered size:- Range 0.234” to 0.413” with slight increments.

• Lip relief angle:-The heel of the drill point is backed off when ground to give

relief behind the cutting lips. This will allow the cutting edges to cut without interference. It is kept 12° to 15°.

• Point angle:- The point angle is selected to suit the hardness & brittleness of

the material being drilled. It is 116° to 118° for medium hard steel & cast iron.

• Helix angle:- This angle is equivalent to back rake angle of a single point

cutting tool. It is 24° to 30° for most drill.• Web thickness:- If the web is too thin, the drill will not be rigid enough to

withstand a high drilling torque. But, with a thinner web, the axial thrust is reduced & the drilling is easier since the chiesel edge becomes shorter.

Milling cutters

• Milling cutters are multi-point cylindrical cutting tools with the cutting teeth spaced around the periphery.

• Conventional Milling:- Table and cutter rotates in the same direction.

• Climb Milling:- table and cutter rotates in the opposite direction.

Milling Cutter Nomenclature

• Body:- Part of the solid or tipped cutter exclusive of the teeth or shank

• Arbor:- It is the shaft on which arbor type cutters are mounted or driven.

• Shank:- It is a cylindrical or tapered extension along the axis of the cutter employed for holding & driving shank type cutters.

• Cutting Edge:- It is the edge formed by the intersection of the face and the circular land.

• Land:- The narrow surface of a cutter tooth immediately behind the cutting edge.

• Flute:- The chip & coolant space between the back of one tooth & the face of the following tooth.

• Face:- It is the portion of the flute adjacent to the cutting edge on which the cut chips impinge.

• Fillet:- It is the bottom surface of the flute.• Helix angle:- The cutting edge angle which a helical cutting

edge makes with a plane containing the axis of a cylindrical cutter.

• Diameter:- Cutter diameter or the outside diameter of the cutter is the diameter of the circle passing through the peripheral cutting edge.

• Relief:- The result of the removal of the tool material behind or adjacent to the cutting edge to provide clearance & prevent rubbing.

• Primary Relief:- Relief immediately behind the cutting edge• Secondary relief:- The additional space provided behind the

relieved land of a cutter to eliminate undesirable contact between the cutter & the workpiece.

• Relief angle:- The angle formed between a relieved surface & a given plane tangent to a cutting edge or a point on a cutting edge.

• Radial rake angle:- It is the angle in a plane perpendicular to the axis of the cutter, between the face of the tooth and a radial line passing through the cutting edge.

Cutting tool Material

Basic requirements of a cutting tool is that the tool should be harder than the metal to be cut.

The first metal cutting occurred during the bronze age. However, the first real evidence of progress in metal cutting was found

after the discovery of iron.Then after there was significant development in the area of

development of new materials for making cutting tools.

Characteristics of Primary cutting tool Materials

• High Speed Steel (HSS):-The 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 upto 20 ~ 30 m/min (which was quite substantial those days) 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. the tool is to be used number of times by resharpening.

• 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. So 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. Therefore, for finishing with light cut but high speed, the harder grades containing upto 25% TiC are used and for heavy roughing work at lower speeds lesser amount (5 to 10%) of TiC is suitable.

• Plain ceramics:-Inherently high compressive strength, chemical stability and hot hardness of the ceramics led to powder metallurgical production of indexable ceramic tool inserts.. Alumina (Al2O3) is preferred to silicon nitride (Si3N4) for higher hardness and chemical stability. Si3N4 is tougher but again more difficult to process. The plain ceramic tools are brittle in nature and hence had limited applications.