Geometry of Plain Milling Cutter&

Geometry of Twist Drill

BY –BY –GIRISH SAPRA Department of Mechanical EngineeringDepartment of Mechanical Engineering

NITTTR , CHANDIGARHNITTTR , CHANDIGARHMarch, 2016March, 2016

Objectives

• Understand the characteristic features of the milling process

• Select the various types of milling cutters based on the application

• Understand the different types of milling operations that can be done in the milling machines

Milling

• In milling, the workpiece is fed into a rotating milling cutter, which is a multi-point tool

• Interrupted cutting• Small size of chips• Variation in chip thickness

Milling operation

Milling Machine

Milling Cutters

• Based on construction Solid Inserted tooth type• Based on mounting Arbour mounted Shank mounted

Milling Cutters

• Base on rotation Right hand rotation (Counter clockwise) Left hand rotation (Clockwise)• Based on helix Right hand helix Left hand helix

Milling Cutters

Geometry of Plane Milling Cutter

Geometry of Plane Milling Cutter

• Radial Rake Angle• Radial Relief Angle• Land• Flute• Heel• Fillet• Lip angle

Up Milling

• In up milling the cutting tool rotates in the opposite direction to the table movement.

• In the conventional or up milling, the chip starts as zero thickness and gradually increases to the maximum size.

• However, this process is inherently safe.

Down Milling (Climb Milling)

• In down milling the cutting tool rotates in the same direction as that of the table movement.

• In the climb or down milling, the chip starts as maximum thickness and goes to zero thickness gradually.

• The cutting force will act downwards and as such would keep the workpiece firmly in the work holding device.

• This is good for thin and frail workpieces.

Up and Down Milling

String Milling

• A number of the workpieces will be kept on the machine table in a line and hence called as 'string milling' or 'line milling'.

• By having a number of workpieces kept in line the approach distance will be only at the beginning and end of the line, thus considerably saving the machine time.

Gang Milling

• In gang milling a number of milling cutters are fastened to the arbor to suit the profile of the workpiece to be machined.

• For example, two side and face milling cutter with a slab milling cutter at the centre to mill an inverted U-shape.

• The advantage of gang milling is that several surfaces are machined at the same time.

Gang Milling

Geometry of Twist Drill

Twist Drill

• A rotating cutting tool, used for cutting holes in rigid materials, that consists of an essentially conical point, relieved and fluted to form cutting lips, and spiral flutes which direct the chips away from the lips and toward ejection from the hole.

Twist drill

Drill geometry

Drill point

Twist drill nomenclature

• AXIS - The imaginary straight line which forms the longitudinal center line of the drill.

• BODY - The portion of the drill extending from the shank or neck to the outer corners of the cutting lips.

• CHISEL EDGE ANGLE - The angle included between the chisel edge and the cutting lip, as viewed from the end of the drill.

• DRILL DIAMETER - The diameter over the margins of the drill measured at the point.

Contd...

• 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.

• FLUTE LENGTH - The length from the outer corners of the cutting lips to the extreme back end of the flutes.

• HELIX ANGLE - The angle made by the leading edge of the land with a plane containing the axis of the drill.

• LIPS - The cutting edges of a two flute drill extending from the chisel edqe to the periphery.

Contd..

• POINT - The cutting end of a drill, made up of the ends of the lands and the web. In form it resembles a cone, but departs from a true cone to furnish clearance behind the cutting lips.

• POINT ANGLE - The angle included between the lips projected upon a plane parallel to the drill axis and parallel to the cutting lips.

• 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.• TANG DRIVE - Two opposite parallel driving flats on the extreme

end of a straight shank.• WEB -The central portion of the body that joins the lands. The

extreme end of the web forms the chisel edge on a two-flute drill.

Twist Drill Geometry

• In order to provide strength to the drill the cutting edge is thickened gradually from the bottom. It is termed as web.

• The web will only compress the material and as a result, the thrust on the drill increases with an increase in the web thickness.

• Also it is likely that out of round and over size holes may result in because of the additional thrust.

• The best geometry to use depends upon the properties of the material being drilled. The following table lists geometries recommended for some commonly drilled materials.

• A more aggressive angle, such as 90 degrees, is suited for very soft plastics and other materials. Such a bit is generally self-starting and can cut very quickly.

• A shallower angle, such as 150 degrees, is suited for drilling steels and other tougher materials. This style of drill requires a starter hole, but does not bind or suffer premature wear so long as a suitable feed rate is used.

• Drill bits with no point angle are used in situations where a blind, flat-bottomed hole is required.

Drilling Force Estimation

• The torque acting on a twist drill is M = C d1.9f0.8N-mm d is the diameter of the drill in mm f is the feed rate of the drill in mm/rev

C is a constant• The thrust force is T = K d f0.7 Newtons• The values of K are given as• Steel = 84.7, Cast iron = 60.5

References.

• Bhattacharya A., “Metal Cutting” Central Publishers, Kolkata,2000 140-145

• Donaldson, “Tool Design ”Tata- McGraw Hill ,3rd Edition,2000,PP.5OO-515

• Rajput, R. K. A textbook of Manufacturing technology.1. New Delhi: Luxmi Publishers (P) Ltd,2007,pp. 80-92.