Milling is perhaps the most versatile metal machining operation. With today’s multi-axis CNC milling machines, you can fabricate nearly any shape imaginable.
Do you know one important difference between a Lathe Machine and Milling Machine? Any guess?
Unlike in a Lathe where the tool is fed against the revolving work-piece, in a Milling Machine, the work-piece is fed against a revolving multi-teeth tool (Milling Cutter).
As the milling cutter rotates, each cutter-tooth removes a small amount of material from the advancing work-piece in every cutter revolution.
What is a Milling Machine?
Your milling machine is the second most important machine tool after the lathe machine in terms of versatility. Your milling machine can machine flat, curved, or irregular surfaces and also do drilling, reaming, boring, and many other operations.
Your simple milling machine will have a motor-driven spindle, which mounts and rotates the milling cutter, and a movable table that mounts and feeds the workpiece.
Your milling machine can be classified as Vertical or Horizontal, based on the orientation of the spindle. Further, your milling machine can be knee and column-type, fixed bed-type, ram-type, planer-type, turret-type, etc. Your milling machine has a spindle with an independent motor, pressurized cooling system, variable spindle speeds, and feeds, manual and power-operated table feeds (including rapid movement). Some of your milling machines have more than one milling head, either vertical or horizontal, or a combination of both.
Your milling machine has movements in the X, Y, and Z-axis, depending on the design of your milling machine these movements are given to the table or milling head; also, the movements may be distributed between the table and the milling head. Many of your milling machines have spindle movement through a quill unit.
Parts of a Milling Machine
Most of the milling machines you use are of conventional Knee and Column type and others are fixed-bed-type, where a fixed bed takes the place of the knee. Structural parts of your milling machine are made from cast-iron since cast-iron has good shock absorption quality.
So, major parts of your Knee and Column Type Milling Machine are:
- Base supporting the complete machine structure.
- The Vertical hollow column is mounted on the base and houses the drive system for the spindle, table, knee etc.
- The knee is mounted on the column guide-ways and supports the saddle and table; knee has up and down movement.
- The saddle is mounted on the guide-ways of the knee and has transverse movement.
- The machine table is mounted on the guide-ways of the saddle and has longitudinal to and fro movement.
- And other parts like overarm and cooling system.
Quill-unit: In your milling machine, the positioning of the work-piece and milling is accomplished by using movement in X, Y and Z axis. With the arrangement of the Quill-unit, you can move the machine spindle for giving depth of cut. Your Milling Machine may have auto power movement for the quill. . You are able engage / disengage the quill movement or lock the spindle at a particular position.
2 Types of Milling
Depending on the direction of the cutter rotation and direction of work-piece feed, you can classify the milling process as Up (conventional) milling and Down (climb) milling.
- Up (conventional) milling: Here the direction of your work-piece feed is opposite to the direction of the milling cutter.
- Down (climb) milling: Here the directions of your work-piece feed and the direction of the milling cutter are the same.
So, what is the difference between up and down milling?
Climb Milling vs. Conventional Milling
When you do the up-milling or conventional milling, the cutting forces tend to lift the work-piece and the table on which your work-piece is mounted; up-milling is favorable to the cutter since the starting load on the cutter teeth is at the minimum, however, it results in bad surface finish. You generally use the up-milling process for rough milling and on machines that are sturdy and can withstand the cutting forces.
In climb milling (down-milling), the cutting forces push the work-piece downwards and this helps in milling thin work-pieces and also results in good surface finish. However, Sudden and maximum load on your milling cutter at the start of down-milling is not favorable to the cutter.
The cutting process in your Milling Machine is an interrupted cutting operation, where there is a very small time gap between the exit of one tooth from the work-piece and entry of another tooth into the work-piece.
You have to use a combination of up and down milling in real life to achieve optimum metal removal and surface finish. Properly designed tool geometry, cutting fluid and coolant helps you to achieve this.
Milling Flat Surfaces
First, let’s see the different types of milling operations that produce flat work surfaces.
In peripheral milling, the axis of your milling cutter rotation is parallel to the surface being machined and cutting is performed by the cutting edges on the outside periphery of the cutter. You can achieve the cutting process by feeding the work-piece against the rotating cutter.
Types of peripheral milling
In Slab Milling the width of your milling cutter is far more than the width of the work-piece.
In slotting or slot milling the width of the milling cutter called ‘slotter’ is less than the work-piece width; the slotter has cutting edges on the periphery and also on both the faces.
You can do side milling of your work-piece by engaging only one ‘side face teeth’ of the slotter.
Straddle milling is similar to side milling and cutting takes place simultaneously on two parallel sides of the work-piece; you can do straddle milling by mounting two slotters on the spindle arbor separated by spacers of required width. When you do straddle milling, the two slotters mounted on the arbor work together.
You can use a thin slotter called ‘slitting cutter’ or ‘slitter’ for slitting operation; the slitter is thinner than the slotter and has cutting edges on the periphery only. You can use a slitter to mill thin slots or to cut the work-piece into two halves.
When you do peripheral milling, the milling cutters are held firmly on the arbor and this gives you a good surface finish at minimum power.
In face milling the machined surface is perpendicular to the axis of cutter rotation and you can do face-milling in a Horizontal or Vertical Milling machine. Your face milling cutter has cutting edges on the face as well as on the periphery.
You generally use a face milling cutter for milling bigger work-pieces; you use a bigger face milling cutter for high and fast removal of material.
However, the surface finish achieved is inferior to your peripheral milling; also your machine should be rigid enough to withstand the cutting forces and this method is generally adopted for rough milling.
You can do end milling using an end mill cutter for machining closed or open key slots, pockets, etc.; the machined surface is perpendicular to the axis of cutter rotation.
In form milling, the cutting edges of your peripheral cutter or end mill have a special profile and called a form milling cutter. You can transfer the special profile onto the work-piece by milling. You can use the available standard form milling cutters or get one custom made for your requirement.
With a manual milling machine usually, you will have the Z-axis locked and the workpiece is moved in the X and Y-axis. This created a 2D profile on your work. When you want complex 3D profiles machined on the surface of the workpiece you may need to move all the 3 axes simultaneously. The CNC milling machines makes this process easy.
Today, the multi-axis CNC milling machines are used in a wide variety of industries including die making, mould making, metal component production etc.
How Do You Specify a Milling Machine?
When you are planning to purchase a milling machine, you must know how to specify it. The general specification of your Milling machine should include
- Type of Milling Machine- Vertical or horizontal.
- Work-piece clamping area (length x width)
- Maximum safe weight on the table
- Power operated table traverses-longitudinal, cross and vertical (in mm).
- Number of speeds and feeds, and their range.
- Main motor and feed motor power.
- Maximum and minimum size of the job you can load on the table (L x W x H).
- Overall size of the machine and area required for its installation.
- Net weight of the machine.
Your Milling Machine is the second most important machine (after Lathe) in terms of versatility. You cannot imagine a workshop without a Milling Machine.