Saturday, August 9, 2008

Basic Milling Operations

Before any milling job—no matter how simple—is attempted, the machinist has to make several decisions. In addition to selecting the best means of holding the works and the most appropriate cutters, the machinist must make an initial estimate of the cutting speed and feed rate that will provide good balance between rapid metal removal and long cutter life.

Cutting Speed and Feed Rate.
Proper determination of cutting speed and feed rate can be done only when the following eight factors are known:
  1. Type of material to be machined
  2. Nature of heat treatment, if any
  3. Rigidity of the setup
  4. Cutting tool material
  5. Power available at the spindle
  6. Type of finish desired
  7. Cutting fluid to be used, if any

Cutting Speeds
The speed of milling is the distance in FPM at which the circumference of the cutter passes over the work. The spindle RPM necessary to give a desired peripheral speed depends on the size of the milling cutter. The best speed is determined by the kind of material being cut and the size and type of cutter used, width and depth of cut finish required, type of cutting fluid and method of application, and power and speed available are factors relating to cutter speed.

There are no hard and fast rules governing the speed of milling cutters; experience has shown that the following factors must be considered in regulating speed:
  • A metal slitting saw milling cutter can be rotated faster than a plain milling cutter having a broad face.
  • Cutters having undercut teeth (positive rake) cut more freely than those having radial teeth (without rake); hence, they may run at higher speeds.
  • Angle cutters must be run at slower speeds than plain or side cutters.
  • Cutters with inserted teeth generally will stand as much speed as a solid cutter.
  • A sharp cutter may be operated at greater speeds than a dull one.
  • A plentiful supply of cutting oil will permit the cutter to run at higher speeds than without cutting oil.
The formula for calculating spindle cutting speed in revolutions per minute is as follows:





Where RPM = Spindle speed (in revolutions per minute).
CS = cutting speed of milling cutter (in SFPM)
D = diameter of milling cutter (in inches)

For example, the spindle speeds for machining a piece of steel at a speed of 35 SFPM with a cutter 2 inches in diameter is calculated as follows:





Therefore, the milling machine spindle would be set for as near 70 RPM as possible.


Feed Rate
The feed rate, or the speed at which the work piece passes the cutter, determines the time required for cutting a job. In selecting the feed. There are several factors which should be considered.

Forces are exerted against the work piece, the cutter, and their holding devices during the cutting process. The force exerted varies directly with the amount of feed and depth of cut, and in turn are dependent upon the rigidity and power of the machine. Milling machines are limited by the power they can develop to turn the cutter and the amount of vibration they can resist when using coarse feeds and deep cuts. The feed and depth of the cut also depend upon the type of milling cutter being used. For example, deep cuts or coarse feeds should not be attempted when using a small diameter end milling cutter.

Coarse cutters with strong cutting teeth can be fed at a faster rate because the chips may be washed out more easily by the cutting oil.
Coarse feeds and deep cuts should not be used on a frail work piece if the piece is mounted in such a way that its holding device is not able to prevent springing or bending.

Over speeding may be detected by the occurrence of a squeaking, shaping sound. If vibration (referred to as chattering) occurs in the milling machine during the cutting process, the speed should be reduced and the feed increased. Too much cutter clearance, a poorly supported work piece, or a badly worn machine gear is common causes of chattering.

Example: the formula used to find the work feed in inches per minute

IPM = CPT x N x RPM

IPM = Feed rate in inches per minute.

CPT = Chip per t

N = Number of teeth per minute of the milling cutter.

The first step is to calculate the spindle speed before the feed rate can be calculated.





The second step is to calculate the feed rate.
IPM = CPT x N x RPM

= 0.005 x 2 x2400

= 24

Therefore, the RPM for an l/2-inch-diameter end mill machining aluminum revolves at 2,400 RPM and the feed rate should be 24 inches per minute.



Direction of Feed
It is usually regarded as standard practice to feed the workpicce against the milling cutter. When the workpiece is fed against the milling cutter, the teeth cut under any scale on the workpiece surface and any backlash in the feed screw is taken up by the force of the cut. See Figure 8-26.

As an exception to this recommendation, it is advisable to feed with the milling cutter when cutting off stock or when milling comparatively deep or long slots.

The direction of cutter rotation is related to the manner in which the work piece is held. The cutter should rotate so that the piece springs away from the cutter; then there will be no tendency for the force of the cut to loosen the piece. No milling cutter should ever be rotated backward; this will break the teeth. If it is necessary to stop the machine during a finishing cut, the power feed should never be thrown out, nor should the work piece be fed back under the cutter unless the cutter is stopped or the work piece lowered. Never change feeds while the cutter is rotating.

PLAIN MILLING

General

Plain milling, also called surface milling or slab milling is milling flat surfaces with the milling cutter axis parallel to the surface being milled. Generally, plain milling is done with the work piece surface mounted parallel to the surface of the milling machine table and the milling cutter mounted on a standard milling machine arbor. The arbor is well supported in a horizontal plane between the milling machine spindle and one or more arbor supports.

Mounting the Work piece

The work piece is generally clamped directly to the table or supported in a vise for plain milling. The milling machine table should be checked for alignment before starting to cut. If the work piece surface to be milled is at an angle to the base plane of the piece, the work piece should be mounted in a universal vise or on an adjustable angle plate. The holding device should be adjusted so that the work piece surface is parallel to the table of the milling machine.

Selecting the Cutter

A careful study of the drawing must be made to determine what cutter is best suited for the job. Flat surfaces may be milled with a plain milling cutter mounted on an arbor. Deeper cuts may generally be taken when using narrow cutters than with wide cutters. The choice of milling cutters should be based on the size and shape of the work piece. If a wide area is to be milled, fewer traverses will be required using a wide cutter. If large quantities of metal are to be removed, a coarse tooth cutter should be used for roughing and a finer tooth cutter should be used for finishing. A relatively slow cutting speed and fast table feed should be used for roughing, and a relatively fast cutting speed and slow table feed used for finishing. The surface should be checked for accuracy after each completed cut.

Setup

A typical setup for plain milling is illustrated in Figure 8-27. Note that the milling cutter is positioned on the arbor with sleeves so that it is as close as practical to the milling machine spindle while maintaining sufficient clearance between the vise and the milling machine column. This practice reduces torque in the arbor and permits more rigid support for the cutter.

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