Home Shop Machining/Equipment/Mills

Milling machines spin a cutter to remove material from the work. There are a lot of different kinds of mills and many, many different kinds of cutters.



Vertical Mill
The vertical mill spins the cutter on a primarily vertical axis. The spindle holding the cutter is mounted in the 'head'. This head may be fixed in a single position on the body of the milling machine or be mounted in a way that allows it to be moved to many angles but the term 'vertical' is still used to differentiate the overall machine from an horizontal style. The head may also be on an arm that allows additional movements to extend the work area.

The spindle may be fixed in the head or be able to move along its axis under control of a lever or wheel. This moving part is called a 'quill' and allows it to be used for drilling as well as adjusting depth of cut. Although these extra movements make the machine very versatile they also make it less rigid, more expensive, and need to be set back to true vertical sometimes which is time consuming.

Cutters
Cutters are long and thin a bit like short drills. The most common have 4 spiral flutes (early ones had straight flutes for simplicity of manufacture) and 4 cutting teeth at the end but leave an area in the middle of the end that has no teeth. This is for ease of manufacture but means they cannot be plunged down into the work.

They are intended to cut by being moved across the work so it is the sides of the flutes that cut, not the end teeth, and they must be moved along the edge of the work so only about 1/3 of the diameter of the cutter is against the work. They will cut full width eg to make a groove, but are unhappy doing so.

Versions do exist with an end tooth right to the center which can therefore plunge cut but are more expensive and difficult to sharpen.

The next most common type is just 2 flutes. This is also called a 'slot drill' and is specifically designed to make slots as it will cut full width quite happily. It also has an end tooth extending to the center to make is suitable to plunge into the work then move sideways to make a slot.

3 flute cutters are available that compromise between the properties of 2 and 4 flute at extra cost. The advantages can be weighed against the cost in industrial applications.



Horizontal Mill
There are two main types of horizontal mill, plain and universal. In both the cutter is a disc of variable thickness that has teeth around its edge, looking a little like a gear wheel, which is mounted on an horizontal axle called an arbor (not arbour). In the plain mill the arbor is at a fixed height and the work-piece is fitted to a table with two directions of movement (X and Y) at right angles. The table is supported on a bracket called the 'knee' to move it up to the cutter.

In the universal mill the table has an additional movement. The top section is able to rotate about a vertical axis while still being able to move on its longitudinal axis. This results in the X and Y directions no longer being at right angles which has a particular advantage for certain operations (gear cutting). Universal mills are less common than plain mills.

(Note: this description has been included to point out that an Universal Mill is not one that is able to work both in vertical and horizontal mode; that is a Combination Mill. Obviously you can have a Universal Combination Mill such as the Elliott Omnimill)

Cutters
The cutter must be fed into the work along a tangent. This limits the type of shape that can be made but it is ideal for slots and grooves, and if the cutter has a profile, eg curved or angled then the groove is formed with that profile easily. It is also possible to fit several cutters simultaneously to an arbor with spacers so that several grooves can be cut together both to speed production and to achieve consistent accurate spacing.

Cutters can be very thin or extend to several inches across known as 'slab mills' which are typically used to flatten off the top of a casting or reduce the thickness of a piece of stock material quickly. Diameters of cutters vary from about 2inch to 6inches and may have fine or course teeth. Larger diameters have to turn more slowly to keep the speed of the cutting edge down to a reasonable value and this can often be the limiting factor. Bigger is not always better.

Teeth can take many forms and a common version is to have a cutting edge extending down the side or diameter of the disc. This is known as a 'side and face' cutter which gives a cleaner edge but is more expensive and more difficult to sharpen.