Milling & Types of Milling Machines

Milling is the machining process that uses rotary cutters to take away the material from a work piece. This process takes place in a particular direction and at it makes an angle with the tool axis.

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It covers wide range of operations and machines, starting from individual components to large heavy duty gang millers and their corresponding operations. It is the most commonly used processes in industry as well as machine shops, which are used for manufacturing parts to précised sizes as well as shapes. The process of milling can be done with a wide range of machine tools. The part integrations of milling into turning and vice versa led to the live tool usage of lathes and occasional use of mills in the process of turning.

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Milling machine:

A milling machine can be found in many sizes and in variety of designs that enables the work piece movement relative to the tool.  Milling is a typical process that is used to produce the non-axial symmetric that have amenities such as holes, slots, pockets and contours of three dimensions. The fabricated components through milling are used in limited quantities and have custom designed brackets along with fasteners. It is also an ideal process used as a surface finish process to the work parts manufactured by various other processes. It offers an ideal precision service to the part. The basic components of milling tool are:

  • Base and Column: The base of the milling machine is just the platform on which the machine is fixed on the ground where it acts as a support to the machine. The column is attached to the base and this provides a connection to the other components.
  • Table: The work piece that is to be milled is mounted on the table platform. It has Tm shaped slots along the surface. A fixture called as Vise is used to secure work piece that in-turn is secured into the T-slots. The work piece is also clamped directly into these slots.
  • Saddle: The table provides a motion horizontally to the work piece in the X-direction on sliding along the platform that is present below known as saddle. It supports the table along with allowance to the longitudinal motion.
  • Knee: The work piece motion is provided horizontally in Y-direction by a sliding movement along the knee which is another platform provided, it supports saddle. Most of the milling machines are called as column or knee milling machines. Knee provides a vertical movement to the work piece while the cutter remains stationary. In a fixed bed machine the knee is fixed where the cutter moves vertically in order to cut the work piece.
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  • Table Cross Transverse Handle: This is used to adjust the table and saddle positions.
  • Cutter: Cutter is the tool used to cut the work piece i.e., mainly cutting the tooth of a gear.
  • Chuck: Chuck is a tool holder. It provides support and grip to the tool.
  • Head: Head is the place where this chuck is attached and this head supports tool holding devices.

Process:
It is an act or a process of grinding, cutting, pressing or crushing in a mill. A milling cutter is a rotary cutting tool that has multiple cutting points. The process of milling is opposite to drilling as the tool advances along the rotation axis. The cutter moves perpendicular to the axis of the cutting on the circumference of the cutter. The milling cutter enters the work piece and the cutting edges of the tool repeatedly cut into and get exited from the material of the work piece at each pass. This makes the process to be different from normal slicing. It removes materials from many separate small cuts. This can be easily accomplished by a cutter with many teeth that cuts at high speed corresponding to the tool advancements. Two major approaches in milling process are face milling and peripheral milling.

  • Face milling is used to cut flat surfaces of the work pieces or to cut the flat bottomed cavities. The cutting action takes place at the corners of the milling cutter.
  • Where as in peripheral milling the cutting action takes place along the cutter circumference. The cross section of the milled surface ends up with the shape received by the cutter. It is best suited method for deep slots, threads and gear teeth.

Types of milling machines:

  • Vertical mill:

Vertical mill is a spindle axis that is oriented vertically. The milling cutters are held in the spindle and are rotated on their axis. The spindle is extended that allows a plunge cuts and drilling. Bed mill and turret mill are the two sub categories in vertical milling machine.milling-1

  • Bed mill: In this type the table moves perpendicular to the spindle axis where the spindle moves parallel to the axis.
  • Turret mill: In turret mill it has a stationary spindle and the table is moved perpendicular or parallel in order to accomplish the cutting action. The milling cutter is raised and lowered similar to the drill press.
  • Horizontal mill:

The horizontal milling machine is a robust and sturdy machine with availability of variety of removed material that is generally held in strong machine vice. It is used it the cases where the vertical milling machines are less suitable. Bulk material is removed by the cutters with less accuracy with horizontal milling machine. The cutter changing can be done very easily. The arbor bracket is removed by loosening of the nuts and bolts that are held in firm position. The arbor can slit over the arm, and the spacers are removed as in the original cutter.  New cutters can be places in position and the spacers slide back on the arbor that is tightened back firmly in position.

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Parameters of milling cutter:

  • Cutting feed: The distance covered by the cutting tool or the work piece advancement in the duration of one revolution of the spindle and tool is termed as cutting feed, which is measured in Inches per Tooth. Depending on the type of operation the tool feeds into the work piece and the work piece feeds into the tool. In the case of a multi-point tool the feed is equal to feed per tooth which is measured in inches per tooth (IPT) and it is multiplied by the teeth present on the cutting tool.
  • Cutting speed: It is the speed of the work piece surface in relative to the cutting tool edge and is measured in surface feet per minute (SFM).
  • Spindle speed: The spindle rotation relative to the tool rotation is measured in revolutions per minute (RPM). The circumference of the tool divides the spindle speed into equal cutting speeds.
  • Feed rate: Feed rate is the speed of cutting tool and its movement in comparative to the work pieces that makes the action of tools cut. It is a product of cutting feed and spindle speed.
  • Axial depth of cut: The depth of the tool along the axis of the work piece to make a cut is known as axial depth of cut. Low feed rates require larger axial depths. This may result in high load on to the tool and also reduces tool life. This action is performed in accordance to the phases.
  • Radial depth of cut: The depth of the tool cut along the work piece radius in order to make a cut and is generally less than tool radius. It offers machining in several steps over distance that makes a radial cut.

Types of milling cutter:

       Milling cutters are the cutting tools that are typically used in milling machines to perform the milling operations. The cutter removes the material by the movement within a machine or directly from the cutter’s shape.

  • Milling cutter: These are used for a variety of operations from normal styles of facing as well as cutting angles and shapes along with their grooves and slots.

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Various types of mill cutters are:

  • Key seat cutter
  • Narrow width key seat
  • T-Slot Cutter
  • Dove Tail Cutter
  • Bevel Cutter
  • V-Slot Cutter
  • Staggered toother Milling Cutter
  • Straight tooth milling cutter
  • Large diameter and High speed Steel Face Milling Cutter
  • Square tool bit flat
  • Fly cutter

Slitting cutter:

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It is used to remove narrow slits into the material. These cutters do not have any side tooth that makes them as a good choice in slotting as well as in thin slits. They are concave ground that provides free cutting process. Usage of more teeth in a cut is preferred for thin and hard materials.

  • Gear cutter:

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Gear cutter is used to cut the provided stock to make a gear. The cutter number is determined on the basis of the number of teeth that are to be provided in the gear.

  • End mill:

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End mills are the tools that have cutting teeth on one end as well as on the sides. These types of end mill cutters are used in various operations as edge cutting, slots or channel cutting. Types of end mill cutters are:

  • Roughing end mill
  • Finishing end mill
  • Square end mill
  • Ball end mill
  • Rounded edge end mill
  • Tapered end mill
  • Drilling end mill
  • Chamfer end mill
  • Corner end mill
  • Convex radius end mill

Cutter Operations:

In a process cycle several operations are performed to the work piece in order to get a desired shape. The cutter path is defined with various operations, where the material is removed from the work piece.

  • End milling:

It is an end mill that performs either slot cuts or peripheral cuts. It is determined by a step over distance across a work piece that is worked in the machine to get an identified feature such as profile, pockets, slots and also complex surface contours. The final cutting can be achieved by a single pass or multiple passes.

  • Chamfer milling:

It provides a peripheral cut along the edge of the work piece that creates an angular surface known as chamfer. The angle may be generally upto 45 degrees.  It can be done on a straight path or else curved path that is machined on either exterior portion or interior portion.

  • Face milling:

A face mill is used to machine a flat surface that provides smooth surface finish. The cut depth is generally small and it is reached by the process of machining at a very smaller axial depth of cut in a process of multiple passes.

  • Drilling:

A hole is cut with the diameter of the tool when the drill is axially made to enter into the work piece. The drilling operation produces a blind hole that extends upto some depth in the work piece.

  • Boring:

Boring is a single point cutting tool that is set to cut a desired diameter with the adjustable head of the bore. It enters the work piece axially and cuts the internal surfaces in order to form different features. It is generally performed after drilling operation just to enlarge the diameter and obtain précised dimensions.

  • Counter boring:

Counter boring is also performed after drilling and it provides a head of fastener such as bolt or slit below the part surface. It penetrates in to the work piece axially and enlarges the existing hole. The counter boring has a pilot at its end to guide it straight into the existing hole.

  • Counter sinking:

The counter sink head fasteners are screw, slit flush with the surface of work piece that is mostly performed after drilling. The most common included angles in counter sink operation are 60, 82, 90,100,118 and 120 degrees.

  • Reaming:

Remer removes the minimal material from the work piece that is performed after the drilling operation to obtain accurate dimensions of diameter and a smoother internal surface.it enters axially to enlarge the existing hole.

  • Tapping:

Tapping is a process of cutting internal threads. It penetrates into the work piece axially and cuts the existing hole. The hole that exists is drilled by the selection of required tap drill dimensions in order to accommodate the size of the desired tap. The threads can be cut upto desired depth in the hole or can be done throughout to reach the depth of the hole.

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