January 5, 2022

What is a knee mill

A knee mill is a vertical milling machine that moves along the y axis. That is a direction in which the workpiece can move to the milling tool shaping it. The table is not stationary like standard milling machines that can only move along both x, horizontal, axes. The head drills and performs boring movements along the y-axis.

The head does not consist of the stationary top slide and spindle. What separates the knee mill from the rest of the milling machines is its head, which can also move along an x-axis. Due to their movement capabilities, they take on different workpiece sizes.

Milling machines work and like shape metal, unlike a wood or metal lathe, where the workpiece turns at high revolutions and by a stationary tool. The workpiece is kept stationary and clamped to the table while milling tools spin at high revolutions.

That is how the machine makes precise cuts. The difference between milling machines is in the position of the workpiece.
A knee mill performs a wide variety of cutting and forming processes. Some of these processes include surfacing, cutting, and slot machining. You achieve these capabilities by employing milling tools. That is similar to drill bits. These tools come in different shapes and sizes, for different-sized shapes when used.

Rickwashburn1, CC BY-SA 4.0 https://creativecommons.org/licenses/by-sa/4.0, via Wikimedia Commons

The advantage of using the knee mill comes in machining larger objects. Sometimes the moving spindle will not provide enough torque to drill vertical holes. The process is called boring. Knee mills have stationary spindles, and all y-axis movement through the table the workpiece is situated on.

That results in a significant torque increase and makes drilling holes in harder material. The knee mill featured a vertical movement of the table that accommodates larger objects. That is a result of an increase in height capacity.

What is a knee milling machine?

A knee milling machine is a type of vertical mill. It features a horizontal X-axis that moves along the vertical Y axis, too. The worktable is not fixed but moves along the machining axis. The spindle head of the knee mill performs all cutting operations on the Y-axis.
A knee mill machine is a flexible tool suitable for different operations and workpiece types. It comes with a vertically adjustable worktable, resting on a knee-supported saddle.

Tosaka, CC BY 3.0 https://creativecommons.org/licenses/by/3.0, via Wikimedia Commons

The knee is a supportive and massive casting of the machine. The knee ensures the stability of a knee milling machine by riding vertically on the column. It also secures the column rigidly and firmly in a fixed position. That makes the milling head and spindle vertically fixed during operations.

Modern vertical knee machines have the spindle installed vertically. Sometimes it is parallel to the column face. The design is for the head to have the ability to swivel on angular surfaces. The turret and swivel head assembly gives cuts a higher precision and mobility.
For angular cuts, the setting of the head allows better precision when done on the horizontal plane. The versatility and stability of knee milling machines have made it one of the most popular manufacturing machines.

The knee milling machine is one of the most widely used machine tools in machine shops due to the unenclosed nature of the knee machine. It allows for quick setups for simple jobs. It is easy to operate. Just put a workpiece into the mill, drill a few holes or tap something. Also, the tilt-able head, movable ram, and turret make it versatile.

Why is it called a knee mill?

The name comes from the way the machine works. Its design of the knee mill dates to 1936 when Rudolph Bannow’s company produced a knee-and-column vertical mill with a rotating turret and sliding-ram head.

Types of knee mills

Horizontal Milling Machine

Horizontal milling machines are also known as plain milling machines. These are more robust than hand millers and have a horizontal spindle, also called horizontal milling machines.

You are allowed to feed the table in vertical, cross, or horizontal directions. The feed includes a vertical, cross, and longitudinal. Feeding the table vertically, you are adjusting the table vertically. Cross feeding moves the table parallel to the spindle, and longitudinal rotates the table

Vertical Milling Machine

When using the vertical milling machine, the position of the spindle is perpendicular or vertical to the table. You are allowed to use this machine for slots, machining grooves, and flat surfaces. The spindle head is fixed to a vertical column, which rotates at an angle.
The milling cutter is fixed on the spindle to work with angular surfaces. Some vertical milling machines allow you to adjust the spindle up and down.

What Is a Vertical Knee Mill

A vertical knee mill is a versatile machine tool that allows an operator to perform a wide range of tasks such as facing, slotting of keyways, drilling, or boring. A vertical milling machine is the most used knee mill due to oriented cutting head or spindle. The spindle holds a rotating cutting tool against the workpiece and moves vertically, along the Y-axis.

Vertical mills are ideal for projects on a single side, such as sectioning operations or finishing operations like end millwork. Vertical mills are into two vertical milling machine types: bed and turret milling machines. Each mill functions differently from the other and offers unique advantages.

Features of a knee mill

Column and Base

The column comes with a base, and it is the casting that supports all other parts of the milling machine. The column features an oil reservoir and a pump that lubricates the spindle. It rests on the base, and the knee base has a coolant reservoir and a pump. These are required to cool off the milling machine during teaching operations.

Knee

The knee is a casting that supports the saddle and table. All gearing mechanisms are within the knee. It is secured to the column by dovetail ways, supported, and adjusted by a vertical positioning screw. That is the elevating screw. It adjusts the knee up and down, raising or lowering the lever. Use the help of a hand or power feed.

Table

It is a rectangular casting that comes on the top of the saddle. It holds the work or work holding devices. The table comes with multiple T-slots holding the work and work holding devices. You are allowed to control the table by hand or by power. To move the table by hand, engage and turn the longitudinal hand crank. To move it through power, engage the longitudinal direction feed control lever.

Spindle

It is the shaft used to hold and drive the cutting tools of the milling machine and mount the spindle on the bearings and supported by the column. Move the gear trains by the electric motor. The gear trains are inside the column.
The face of the spindle lies near to the table and comes with an internal taper machined on it. The internal taper at the front of the spindle allows only a tapered cutter holder. It has two keys at the front face that gives a positive drive for the cutter holder.

David D. Hughes
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