Milling Machines

Introduction

A milling machine is a machine tool that cuts metal with a multiple-tooth cutting tool calleda milling cutter. The workpiece is fastened to the milling machine table and is fed againstthe 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:..

General Purpose machines - these are mainly the column and knee type (horizontal & vertical machines) High Production types with fixed beds- (horizontal types) 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 canbe 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 byby their heavy overarms which are integral with their columns. This provides rigid supportfor 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 minimumof operator involvement. On this type of machine the table is fixed permitting machine tablemovement in one horizontal direction only. the workpiece has to be fixed in one locationon 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 factorin horizontal milling. Conventional up milling requires that the workpiece is fedagainst the rotation of the cutter. In down milling the workpiece moves in the seam directionas 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 byproducing 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 cutterson roughing cuts. For finishing cuts the speeds would increase 20% to 50%. Forcermet (cemented carbide)tipped teeth significantly higher cutting speeds are possible see table below.

Surface Speeds Material 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

Feeds Feed per tooth (mm) for HSS and Different Milling Cutter Forms Material v Cutter 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.

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Material	Surface Speed (mm/m)	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 Gray	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

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