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Milling Machines

Introduction

A milling machine is a machine tool that cuts metal with a multiple-tooth cutting tool called a milling cutter.   The workpiece is fastened to the milling machine table and is fed against the revolving milling cutter.   The milling cutters can have cutting teeth on the periphery or sidesor both.   The cutting teeth can be straight or spiral.

Milling machines can be classified under three main headings:..

  1. General Purpose machines - these are mainly the column and knee type (horizontal & vertical machines)
  2. High Production types with fixed beds- (horizontal types)
  3. Special Purpose machines such as duplicating, profiling, rise and fall , rotary table ,planetary and double end types

Milling attachments can also be fitted to other machine tools including lathes planing machines and drill bench presses can be used with milling cutters.


Additional Notes
Cutting Tool Materials... Cutting Tool Materials..
Cutting Fluids... Cutting Fluids..




Milling Cutters

There are a wide range of milling cutters as illustrated in BS 122-1.   these include

  • Plain cutters ( most types have straight or helical teeth)
  • End mills (taper or parallel shank, slotting or shell)
  • Cylindrical side and face slotting,screw slotting
  • Single,double and equal angle cutters:
  • T-slot, convex, single and double corner rounding cutters
  • Metal slitting saws:
  • Hollow Mills
Typical Milling cutters

Tee Slot Milling Cutter


Shell End Mill


High Helix Cylinderical Cutter


Slotting Cutter





Relevant Standards

BS 122-1:1953..Milling cutters. Milling cutters
BS 122-4:1980..Milling cutters. Screwed shank end mills and slot drills
BS 122-5:1986..Milling cutters. Specification for mounting dimensions and accessories
BS 122-6:1995, ISO 240:1994..Milling cutters. Specification for dimensions for interchangeability of cutter arbors and cutter mandrels




Column and Knee Machine -Vertical Milling Machine

Vertical milling machines have their cutting tool spindles vertical and are characterised by by their heavy overarms which are integral with their columns.  This provides rigid support for the spindle housing which is set at a fixed distance from from the column.   The milling head can be adjusted vertically

The diagram below shows the primary controlled motions available with this type of milling machine.



Column and Knee Machine - Horizontal Milling Machine

This is the basic milling machine configuration.  The workpiece can be fed in all three axes and is suitable for short production runs but it is not as rigid as a fixed bed machine and should not be used for heavy duty work.  Because of the large number of controls it is relatively slow to operate.



Fixed Bed Milling Machines

Fixed bed or production type machines are designed for rapid metal removal requiring the minimum of operator involvement.   On this type of machine the table is fixed permitting machine table movement in one horizontal direction only.  the workpiece has to be fixed in one location on the table and all vertical movements for feeds and lateral positioning are by moving the spindle in the spindle carrier mounted on the headstock.  A plain machine has just one spindle and a duplex machine has two spindles.  this type of machine can have one column or two columns providing more rigidity.




Horizontal Milling Feed Direction

The direction of feed in relation to the direction of cutter rotation is and important factor in horizontal milling.  Conventional up milling requires that the workpiece is fed against the rotation of the cutter.  In down milling the workpiece moves in the seam direction as the surface of the rotating cutter.  The tow alternatives are shown in the figure below

In up milling the cutter tooth starts cutting into clean metal with zero chip thickness and ends with the maximum chip width and rought metal.   The cutter stays sharp long and the motion of the feed compensates for wear or backlash in the feed system.  The cutter tends to lift up the workpiece and high clamping forces are required.

In down milling the cutter tooth starts cutting into the rough metal with a thick chip resulting and ends by producing a thin chip.  The finish is smoother but the forces tend to drag the workpiece along under the cutter . Any looseness in the feed system could result in vibration and chatter with more risk to the cutter.   This method is used to produce a good surface finish and for small workpieces that cannot be firmly clamped.


Cutting speed HSS cutting tools , Roughing

The table below provides some very approximate cutting speed for different materials using HSS cutters on roughing cuts. For finishing cuts the speeds would increase 20% to 50%.   For cermet (cemented carbide)tipped teeth significantly higher cutting speeds are possible see table below.

Surface Speeds

Material being machined...... Rough
Cut
m/min
Low Carbon Steel 24-30
Med. Carbon Steel
Annealed
23-30
High Carbon Steel
Annealed
18-25
Tool Steel
Annealed
18-25
Stainless Steel
Annealed
18-25
Grey Cast Iron 18-25
Malleable Iron 25-30
Brass 60-90
Bronze 30-60
Aluminium/ Alloys 120-300

These speeds are changed as follows
1) For finishing process .. Increase speeds by 25-50%

Feeds

Feed per tooth (mm) for HSS and Different Milling Cutter Forms
Material vCutter Type
Plain
Heavy
Plain
light
Face Side End Form Slitting
Steel Low Carbon.
0,25 0,15 0,3 0,15 0,15 0,1 0,08
Steel-Low Carbon 0,2 0,13 0,25 0,13 0,13 0,08 0,08
Steel-Med Carbon 0,2 0,13 0,23 0,13 0,1 0,08 0,05
Steel-High Carbon
Annealed
0,1 0,08 0,15 0,08 0,05 0,05 0,05
Stainless steel
Free cutting
0,2 0,13 0,25 0,13 0,1 0,08 0,05
Stainless steel 0,1 0,08 0,15 0,1 0,05 0,05 0,05
Cast Iron-grey 0,3 0,2 0,36 0,2 0,2 0,1 0,1
Cast Iron-medium 0,25 0,15 0,3 0,15 0,15 0,1 0,08
Cast Iron - Malleable 0,25 0,15 0,3 0,15 0,15 0,1 0,08
Brass /Bronze 0,25 0,2 0,33 0,2 0,15 0,1 0,08
Aluminium/alloys 0,41 0,2 0,51 0,3 0,25 0,18 0,1


Cermet Surface Speed /Feed Rates

The table below provides some indicative values for the surface speeds and feeds (mm/tooth) that can be achieved using cermet type e.g cemented carbide tipped milling cutters.   

The faster speeds and lower feeds for finishing. Thelower speeds and higher feeds for roughing



Material  being  machined ... Surface Speed
(m/min)
Feed / tooth (mm)
Aluminum
low silicon ( < 8% )
 450-550   0,12-0,18 
Aluminum, Aluminum-bronze
high silicon ( > 8% )
 250-350   0,13-0,2 
Bronze  300-330   0,13-0,2 
Cast Iron,Malleable  120-220   0,08-0,15 
Cast Iron Grey  180-360   0,1-0,4 
Cast Iron Nodular (ductile)  180-260   0,1-0,2 
Copper  370   0,18 
Inconel  30   0,08 
Stainless steel-Ferr/Mart  500-800   0,05-0,15 
Stainless steel- cast  150-250   0,08-0,15 
Stainless steel-304  120   0,08 
Stainless steel-316L  90   0,08-0,1 
Steel, unalloyed  250-430   0,003-0,006 
Steel, low alloy
 180-360   0,08-0,15 
Steel, low alloy
hardened
 120-220   0,05-0,15 
Steel, high alloy
annealed
 120-220   0,076-0,15 
Steel, high alloy
hardened
 76   0,05 
Steel, cast
low alloy
 180-360   0,1-0,2 
Steel, cast
high alloy
 120-220   0,08-0,15 
Titanium  30-64   0,05-0,08 
Tungsten 120 0,05-0,1


Links Providing information on Milling
  1. Budget Machinery..Specifications of Machine Tools
  2. Milling ..Machine Tool Manufacturing Education- Very useful notes
  3. OTM tools -Milling ..Millng technical information notes - Cermet feeds and speeds
  4. Milling Cutters ..Manufacturing Talk milling cutter reference page
  5. Coromant Sandvik ..Supplier and useful reference source
  6. Calculating Feeds and Speeds ..Modern Machine Shop Online Article
  7. Practical Machinist ..Lots of forums and useful articles
  8. Library of Manufacturing;.. A complete document providing detailed info on all primary forming processes

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Last Updated 15/01/2013