LAB 2 MILLING MACHINE

INTRODUCTION
The milling machine, invented by Eli Whitney in 1818, carries out cutting
operation on a workpiece with a revolving cutter as the workpiece is fed against it. A
milling cutter has a series of cutting edge on its circumference. Each acts as an individual
cutter during the cycle of rotation.



AUTOMATIC TABLE FEED OPERATION
Feed speed conversion chart
The speed corresponding to the lever positions are as follows:



Cautions when converting speeds
a. Only move the select levers after the feed motor is stopped
b. Move the variable speed dial with the motor running.

Difference in table movement directions
a. Right-left and front-back movement is as written on the variable speed dial.
b. Up-down movement is 1/3 of the numbers on the variable speed dial.


Main spindle rpm
There are 12 main spindle speeds, as shown in the table.



Types of milling machines
There are two main types of milling machines: a general purpose milling machine
and a production milling machine.
a.General purpose milling machine – designed for multi-type, small lot milling
and knee type
b.Production milling machine – designed for heavy cutting and is bed type.
There are also milling machines with the spindle vertical, or horizontal.


MILLING CONDITIONS
The milling conditions are the cutting speed, the feed rate and the cutting depth. These
vary with the type of cutting type of cutting head, cutting head material and workpiece
material.

Cutting speed
The relation between cutting speed and main spindle speed is as follow:
Where
v = cutting speed (m/min)
d = cutter diameter (mm)
N = main spindle rpm.

Feed rate
The feed value is expressed in terms of one blade, but the feed rate for milling is in terms of the table speed.
Where
T = table speed (mm/min)
f = speed in mm/blade
N = main spindle rpm
Z = number of blades.

The calculated cutting conditions for a face milling cutter (150 dia., 10 cutting
chips) and an end mill (12 dia., 2 blades), where v = 100 ~ 120 and 20 ~30
m/min., and f = 0.15 mm./ blade and 0.075 mm./blade are as follow:


Cut depth
For workpieces with mil scale, the rough cut should be 2 mm. deep and the finish
cut 0.3 ~ 0.5 mm.

The maximum cut depth with an end mill should be the diameter of the end mill.

MILLING OPERATIONS
Milling includes a number of highly versatile machining operations capable of
producing a variety of configurations with use of a milling cutter, a multitooth
tool that produces a number of chips in one revolution. Parts can be machined
efficiently with various types of milling cutters.

Some of the basic types of milling cutters and milling operation

Slab Milling
In a slab milling, also called peripheral milling, the axis of cutter rotation is
parallel to the workpiece surface to be machined. The cutter, generally made of
high-speed steel, has a number of teeth along its circumference, each tooth acting
like a single-point cutting tool called a plain mill.


Conventional Milling and Climb Milling
In conventional milling, also called up milling, the maximum chip thickness is at
the end of the cut. In climb milling, also called down milling, cutting starts at the surface of the workpiece, where the chip is at its thickest.

(a) Schematic illustration of conventional milling and climb milling.
(b) Slab milling operation, showing depth of cut,d, feed per tooth,f, chip depth
of cut,tc,and workpiece,v.
(c) Schematic illustration of cutter travel distance lc to reach full depth of
cut.


Face Milling
In face milling, the cutter is mounted on a spindle having an axis of rotation
perpendicular to the workpiece surface. When the cutter rotates, the operation is
climb milling; when it rotates in the opposite direction the operation is
conventional milling. The cutting tools are mounted on the cutter body.


Face milling operation showing
(a) action of an insert face milling
(b) climb milling
(c) conventional milling
(d) dimension in face milling


The relationship of cutter diameter and insert angles and their position relative to the surface to be milled is important in that it will determine the angle at which an insert enters and exits the workpiece. For climb milling that if the insert has zero axial and radial rake angle, the rake face of the insert engages the workpiece directly.


Terminology for a face-milling cutter.


(a) Relative position of the cutter and insert as it first engages the workpiece
in face milling
(b) insert positions toward the end of cut, and
(c) example of exit angles of insert, showing desirable (positive or negative
angle) and undesireble (zero angle) position.


End Milling
Flat surface as well as various profiles can be produced by end milling. The cutter in end milling (end mill); it has either straight or tapered shanks for smaller and larger cuter sizes, respectively. The cutter usually rotates on an axis perpendicular to the workpiece, although it can be tilted to machine-tapered surfaces.


Cutters for (a) straddle milling
(b) form milling
(c) slotting and
(d) slitting with a milling cutter.


High-Speed Milling
One of the more common applications is high-speed milling using an end mill,
which observe the same general provisions regarding the stiffness of machines,
workholding device, etc. A typical application is the milling of aluminium-alloy
aerospace components and honeycomb structures, with spindle speeds on the
order of 20,000 rpm. Chip collection and disposal can be a significant problem in
these operations.


EQUIPMENTS / APPARATUS
1. Mild Steel Block (18 mm x 28 mm x 150 mm)
2. Vernier Calliper
3. Inside Micrometer
4. Fine flat file
5. Copper hammer
6. Levelling stand
7. Rough brush


PROCEDURES
1. Remove workpiece burrs with the file
2. Check the workpiece measurements with the calipers.
3. Attach the protective plate to a vise jaw
4. Place the workpiece and levelling stands in the vise jaws so that the workpiece
is level.
5. Determine the main spindle rpm and set the rpm to the nearest value.
6. Determine the table feed rate.
7. Move the workpiece to the center of the cutter
8. Set the knee scale to 0, and then drop the workpiece by a ¼ turn of the scale.
9. Move the table to the left and set the cut depth to 1 mm.
10.Using manual feed, machine the workpiece. When finished, drop the knee and
return the workpiece to the original position.
11.Stop the main spindle and check that the machined surface looks normal.
12.Apply (5) to (9) for the next process.
13.Consult with your engineers if you facing difficulties.

Note: Before start your project, make sure to follow all the safety procedure




NOTE :
1. ALL DIMENSIONS ARE IN MILIMETER (mm)
2. TOLERANCE ±0.2mm
3. DO NOT SCALE THE DRAWING
4. REMOVE ALL SHARP EDGES

13 comments:

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