CNC Milling Machines

Introduction

Milling is the process of machining flat, curved, or irregular surfaces by feeding the work piece against a rotating cutter containing a number of cutting edges. The milling machine consists basically of a motor driven spindle, which mounts and revolves the milling cutter, and a reciprocating adjustable worktable, which mounts and feeds the work piece. Milling machines may be manually operated, mechanically automated, or digitally automated via Computer Numerical Control (CNC). The CNC milling machine is used to manufacture high precision quality components in large volume. CNC machines are also used for polishing welded area and for various fine tuning and finishing applications.

Classification and Advantages

Milling machines are broadly classified as vertical or horizontal according to the orientation of the spindle axis.

v  Vertical CNC Milling Machine and the Vertical Machining Centre (VMC)

They are probably the most common CNC machine tools found today and can be easily adapted to a great variety of work pieces. Vertical CNC milling machines are easy to load and their operators have good visibility of cutting zone. VMCs are usually considered as good, all purpose of machine tools but, they are not as heavy duty as HMCs. Removing chips is difficult, as they tend to fall back into the cutting zone. VMCs are typically made in a bed-type construction, whereas vertical CNC milling machine uses the knee and column construction.

v  Horizontal CNC Milling Machine and the Horizontal Machining Centre (HMC)

HMC is similar in construction to the conventional horizontal milling machine. However Conventional horizontal mills commonly use cutting tool that are mounted to an arbour. HMCs usually use spindle-mounted cutting tools in the same way as VMC. In fact, an HMC is basically a VMC that has been tipped over.

There are some advantages to HMC construction and configuration. First the spindle is mounted to a stationary and extremely rigid base. This rigid construction allows heavy cutting and high material removal rates. Next the tool and the work piece are mounted horizontally. When we remove lot of material, we will produce lot of chips. If they get stuck in the cutting zone can ruin the work piece and destroy the cutting tools. The horizontal configuration allows gravity to help remove the chips. The disadvantage of HMCs is the work piece is often mounted horizontally on a tombstone. It tends to be more difficult for the operator to hold and align heavy work pieces while fighting gravity and it will be difficult for the operator to see the tool and cutting zone.

Some Other Classifications

v  No of Axes (2,3,4 or more up to 9)

The ways of tool movements refers to axes. Multi axes machines offer several improvements over other CNC tools as follows; Amount of work is reduced, better surface finish can be obtained by moving the tool tangentially about the surface and more complex parts can be manufactured, particularly parts with curved holes.

v  Knee and Column Type

These tend to be fairly small and have a movable quill and used for general purpose milling operations.

v  Bed Type

Bed-type construction is suitable for larger machines. They most often use a solidly mounted spindle rather than a quill. These machines have high stiffness and are used for high production work.

v  Planer Machines

They are similar to bed type but are equipped with several cutters and heads to mill various surfaces.

v  Rotary Table Machines

These are similar to vertical mills and are equipped with one or more heads to do face milling operations.

v  Pallet Changing Type

The advantage of pallet system is that while the machine is machining one work piece, the operator can be setting up the next work piece on an identical fixture on the other pallet. This can reduce amount of down time, when the machine is not making chips.