In this article we will discuss about:- 1. Meaning of Milling Machine 2. Classification of Milling Machines 3. Principal Parts 4. Work Performed 5. Methods 6. Setting Up the Machine 7. Parameters 8. Cutting Forces 9. Effect of Various Factors on Horsepower.
Contents:
- Meaning of Milling Machine
- Classification of Milling Machines
- Principal Parts of Milling Machines
- Work Performed by Milling Machines
- Methods of Milling
- Setting Up the Milling Machine
- Milling Parameters
- Cutting Forces in Milling
- Effect of Various Factors on Horsepower in Milling Operations
1. Meaning of Milling Machine:
Milling is the process of removing metal by feeding the work past a rotating multipoint cutter. In milling operation the rate of metal removal is rapid as the cutter rotates at a high speed and has many cutting edges. Thus the jobs are machined at a faster rate than with single point tools and the surface finish is also better due to multi-cutting edges.
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Milling machine is one of the most important machine tools in a tool room as nearly all the operations can be performed on it with high accuracy. The indexing head makes the machine suitable for so many purposes as exact rotation of job is possible by its use. Milling machine augments the work of a lathe and can produce the plain and curved surfaces and also helical grooves etc.
The milling machine may be so arranged that the several cutters are mounted on the arbor at the same time, thus increasing the metal removal rate and allowing several surfaces to be machined at the same time. The single set-up thus arranged also ensures accuracy. It is also possible to adopt the machine to two position works, so that one station is loaded while the other is being worked on, thus assuring continuous machining.
Further with the variety of milling cutters, the machine can produce a wide variety of flat and formed surfaces. It is possible to have relative motion between workpiece and cutter in any direction and thus mill surfaces having any orientation.
The action of a milling cutter is vastly different from that of a drill or lathe tool. In milling operation, the cutting edge of the cutter is kept continuously in contact with the material being cut. The cut picks up gradually only.
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The cycle of operation to remove the chip produced by each tooth is first a sliding action at the beginning, the cutter comes into contact with the metal and then crushing action takes places just after it leading finally to the cutting action. In some metals this peculiar action produces a hardening effect called ‘work- hardening’ which complicates the milling operation considerably, since it throws an increased strain on the teeth of the cutter.
Milling machines can be used for machining flat surfaces, contoured surfaces, complex and irregular areas, surfaces of revolution, slotting, external and internal threads, gear cutting, helical surfaces of various cross-sections etc. to close tolerances for both limited quantity and mass production. The versatility and accuracy of the milling process causes it to be widely used in modern manufacturing.
2. Classification of Milling Machines
:
There are many types of milling machines, from the simple hand mills to the complex tape-controlled machines. Each has a particular field, in which it performs best. Many are special, single purpose machines which can do only one job or may even be designed to do one operation on one workpiece.
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The usual classification is in accordance with the general design but in every classification there is some overlapping.
According to the design, the distinctive classification is as follows:
1. Column and Knee Milling Machines:
(a) Horizontal milling machine.
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(b) Vertical milling machine.
(c) Universal milling machine.
(d) Ram-type universal milling machine.
2. Bed-Type Milling Machine:
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(а) Simplex milling machine.
(b) Duplex milling machine.
(c) Triplex milling machine.
3. Piano-Type Milling Machine:
4. Special Purpose Milling Machine:
(a) Rotary table milling machine.
(b) Drum milling machine.
(c) Profile milling machine.
(d) Duplicating milling machine.
(e) Planetary milling machine.
3. Principal Parts
of Milling Machines:
The principal parts of the column and knee type milling machine are described below (Refer Fig. 16.1):
i. Base:
It is the foundation of the machine and is that part upon which all other parts are mounted. It gives the machine rigidity and strength. Sometimes it also serves as a reservoir for cutting fluid.
ii. Column:
It is the main supporting frame. The motor and other driving mechanisms are contained within it. The front is a machined surface called the column face. It supports and guides the knee in its vertical travel.
iii. Knee:
The knee projects from the column and slides up and down on its face. It supports the saddle and table and is partially supported by the elevating screw which adjusts its height.
iv. Saddle:
The saddle supports and carries the table and is adjustable transversely on ways on top of the knee. It is provided with graduations for exact movement and can be operated by hand or power.
v. Table:
The table rests on ways on the saddle and travels longitudinally in a horizontal plane. It supports the workpiece, fixture and all other equipment.
vi. Over-Arm:
The over-arm is mounted on and guided by the top of the column. It is adjusted in and out by hand to the position of maximum support for the arbor and then clamped.
vii. Spindle:
The spindle obtains its power from the motor through belts, gears and a clutch and transmits it to an arbor or sub arbor. Cutters are mounted directly in the spindle nose.
viii. Arbor:
The arbor is an accurately machined shaft for holding and driving the arbor type cutter. It is tapered at one end to fit the spindle nose and has two slots to fit the nose keys for locating and driving it.
4. Work Performed
by Milling Machines:
i. Next to the lathe, milling machine performs a versatile role in the production of variety of components. Here are some of the most common operations which can be performed on milling machine.
ii. All kinds of grooves; straight, spiral, vertical and formed.
iii. Splines and key-ways on shafts.
iv. Slots for inserting teeth in milling cutters.
v. Flats surfaces of all kinds at any angle.
vi. Contours of infinite variety with straight and spiral elements.
vii. Concave and convex surfaces.
viii. Facing operations of all kinds.
ix. Plate and barrel cams.
x. Cavities for plastic, glass or die casting moulds.
xi. Forging and punch press dies.
xii. Templates.
xiii. Jet and steam-turbine buckets, root and bucket surfaces.
xiv. Indexing operations of all kinds; gear teeth, slots, flutes in twist drills and holes etc.
5. Methods of Milling
:
Following variations of milling methods are possible depending upon the setup of job and tool:
(i) Single Piece Milling:
This is the simplest method of milling in which a single workpiece is milled in a single machine cycle.
(ii) String Milling:
In this case, two or more parts are fixed on the table and milled one after the other.
(iii) Abreast Milling:
In this case, two or more parts are fixed on table and are milled simultaneously.
(iv) Gang Milling:
In this case a number of cutters are used in combination to produce the desired shape on the workpiece.
(v) Progressive Milling:
In this method, two or more similar or different operations are performed either simultaneously or one after the other on separate workpiece on the same machine. Workpieces are progressively moved from one fixture station to next to complete all operations.
(vi) Reciprocal Milling:
In this case, both ends of the tables are utilised by providing fixtures at both ends. The workpiece held in one fixture is milled and at the same time other fixture is kept ready. This way setting up, loading and unloading times are minimised.
(vii) Index Milling:
In this type of milling, identical multiple operations are done on one or more pieces by indexing each time to present a new position in each cycle, e.g., cutting gear teeth.
(viii) Copy Milling:
It is performed on special milling machines (like tracer controlled). The path of the cutter is guided by a master or contour template. It is used for manufacture of complicated contours which are normally difficult and time consuming.
6. Setting Up the Milling Machine:
The selection of proper cutter for doing a job is very important aspect in milling operation. Depending on the cutter and workpiece, suitable feeds and speed are calculated and set on the machine.
Depth of cut is dependent on the amount of material to be removed. Usually two cuts, one roughing and one finishing are taken to achieve better surface finish and higher dimensional accuracy. Depth of roughing cut in limited by horsepower of machine or rigidity of set up and is usually 2.5 to 5 mm. Finishing cut is usually 0.4 to 0.8 mm.
For setting up the machine, the knee locking clamp and the cross slide lock are loosened. The spindle is turned on and its rotation checked. The table is positioned so that the workpiece is under the cutter. The knee is raised slowly by turning the vertical hand feed crank until the cutter just touches the workpiece.
The micrometer dial on the feed screw is then set to zero. The table is then lowered slightly down (by half revolution of hand feed crank to clear the work slightly from the cutter) and the table is moved longitudinally until the cutter is clear of the, workpiece. The knee is again raised to zero mark. The knee lock and the cross slide lock are tightened.
The machine is then ready to cut. The coolant is turned on and the table is slowly moved into the revolving cutter until the full depth of cut is obtained before engaging the power feed. When cut is completed, the power feed is disengaged, spindle rotation stopped and coolant turned off.
7. Milling Parameters:
Cutting speed in peripheral or slab milling operation:
8. Cutting Forces in Milling:
The cutting forces in milling are of pulsating type since the cutting edges of a milling cutter engage with the workpiece only in a part of its rotary path.
The chip thickness varies along the cut and as such average value of chip thickness (fc) is considered for calculation of cutting force.
fc = (57.3 / θ3) ft sin A (cos θ1 – cos θ2)
where ft = feed per tooth in mm
and A = approach angle in degrees
θ1, θ2 and θ3 are the angles subtended at entry and exit, and angle of contact with the work piece in degrees respectively as shown in Fig. 16.34.
Aggregate tangential force Ft = Zs Ks fc w.
where Zs = no. of teeth in simultaneous engagement with workpiece
= Z/360 x θ3 (Z = no. of teeth in cutter)
w = chip width (mm) = d/sin A (d = depth of cut in mm)
Ks = specific cutting force, (which depends on the material and chip thickness)
Power at spindle (kW) =ftʋ/61.20 (V= cutting speed in m/min)
Torque = Ft D/2 kgf mm (D = cutter diameter in mm)
9. Effect of Various Factors on Horsepower in Milling Operations:
The following useful information will serve as a guide in planning a milling operation to obtain economical results:
(i) Keeping depth and width of cut constant, and doubling the feed rate increases power consumption by 50 per cent, whereas power increases by 90 per cent if depth is doubled and other factors kept constant. Power requirement is doubled if the width of cut is doubled keeping feed and depth constant. Thus more stock can be removed with less power in face milling than in peripheral milling.
(ii) If feed is halved and speed doubled then 30% extra power is required; which suggests increasing of feed is more economical to remove more stock as the power required is only in ratio of 3 : 2 on doubling of feed whereas power requirement will be doubled on doubling speed.
As feed is always per tooth, so number of teeth also has same effect as speed. For this reason the number of teeth in sintered carbide cutters should be low for same H.P. as these are operated at high speed.
(iii) Soft materials having high ductility offer greater resistance to the formation of chip, produce a large built up edge, produce poor surface finish, require more power than harder but less ductile materials.
(iv) Power consumption is decreased by increasing positive rake angle because larger rake angle favours the flow of the deformed metals in the form of chip. However, tool strength should not be lost sight of.