Saturday, August 16, 2008

Advanced Milling Operation

Indexing
Indexing is the process of accurately spacing holes, gear teeth or other machined areas on the perimeter or face of a workpiece. There are several variations in the indexing operation such as direct indexing, simple indexing, angular indexing, differential indexing and rotary table indexing.

Direct Indexing
Direct indexing is also known as quick indexing. It is a operation that is done with a plate that has 3 holes circles containing 24, 30 and 36 holes. Any integer that divides evenly into 24, 30 or 36 is one of the numbers of divisions that can be indexed directly. A regular procedure that is normally used for direct indexing is as shown below:
1) Engage the plunger pin in one hole, lock the spindle and machine one side of the square.
2) Unlock the spindle and move the index plate 6 holes (1/4 of 24). Do not count the holes in which the pin was located.
3) Lock the spindle and machine the second side.
4) Repeat the process until all the sides are machined.

Simple Indexing

It is also known as plain milling and has a much wider range of divisions that can be indexed. The indexing head that uses only one plate has the following hole circles available:

Side 1: 24, 25, 28, 30, 34, 37, 38, 39, 41, 42, 43
Side 2: 46, 47, 49, 51, 53, 54, 57, 58, 59, 62, 66

The plates have several dividing heads:

Plate 1: 15, 16, 19, 23, 31, 37, 41, 43, 47
Plate 2: 17, 18, 20, 21, 27, 29, 33, 39, 47

For an indexing head with 40:1 ratio between the crank and the spindle, the number of turns and/or fractional parts of a turn necessary to cut a particular number of divisions is computed by using the following rules:

Number of turns of index crank (T) = 40/√number of divisions (N)

Angular Indexing

Angular indexing is an operation used when certain milling operation needs to establish the exact angular relationship between holes, surfaces and other machined areas. The procedure for determining the indexing head setup for spacing these holes is as follows:

Number of turns=Number of degrees to be indexed/9

Differential Indexing

This particular procedure is used for indexing divisions that are beyond the range of simple or angular indexing.


Rotary Table Indexing

The rotary table is used in many milling operations. The lower part of the rotating table is graduated in degrees and the dial near the headcrank is divided into smaller increments. To compute the indexing for a particular number or figure of divisions, the following formula is used:

Number of turns = number of teeth on worm wheel/number of divisions

Once the rotary table is set up properly, the following procedures are taken for machining operation:

1) The rotary table is placed at the centre of the vertical axis of the spindle
2) Move the table of the milling machine an amount equal to the radius of the hole circle to be drilled. Lock all slides.
3) Drill the 1st hole, making sure that the left sector arm is against the index pin.
4) Unlock the rotary table lock and turn the crank clockwise one full turn and 21 holes farther. Lock the rotary table lock and move the sector arms clockwise until the left sector arm touches the pin.
5) Drill the next hole and repeat the processes until all holes are drilled.


Helical Milling

This kind of operation is necessary when making helical gears, cams, reamers, taps, cutters and other similar objects. A helix is a curve that moves around a cylindrical object and it advances at a uniform rate. Before the process for determining gears to be used can be started, the lead of the milling machine must be computed by using the following formula:

Lead of milling machine = lead of table feed × number of index crank turns to turn indexing the head spindle once

The angle on the other hand can be calculated by using the following formula:

Tan A = Circumference/Lead of milling machine

Thursday, August 14, 2008

Basic Milling Operations

Cutting Speed and Feed Rate
Determining the cutting speeds and feed rates are heavily dependent on the following 8 factors:
1) Type of material to be machined
2) Nature of the heat treatment
3) Rigidity of the setup
4) Physical strength of the cutter
5) Cutting tool material
6) Power available at the spindle
7) Type of finish desired
8) Cutting fluid to be used

Cutting Speeds
The cutting speed of a milling cutter is expressed in surface per minute (sfpm) or meters per minute (mpm) and is the distance that a point on the periphery of a cutter travels in 1 minute at a particular rotational speed. The formula of cutting speed is as shown below:
Cutting Speed, CS= (Diameter (D) ×∏×rpm) ÷12
Where the diameter is in inches
Feed Rates
Several conditions must be considered when choosing the proper feed rate for a milling operation. Two major factors are the number of teeth of the cutter and its strength. The formula to determine feed rate is as shown below:
Feed Rate=chip load × number of teeth × rpm

Types of Milling Operations

Plain Milling
Plain milling is the process of milling a surface that is parallel to the axis of the cutter and basically flat. It is done on plain or universal horizontal milling machines with cutters of varying widths that have teeth only on the periphery.

Side Milling
A cutter that has teeth on the periphery and on one or both sides is used for side milling operation. There are several types of side milling. Straddle Milling and Gang Milling are some example of side milling operations.

Straddle Milling
Two-sided milling cutters are used. The cutters are half-side or plain-side milling cutters and have straight or helical teeth. Stagger-tooth side milling cutters can also be used. The cutters cut on the inner sides only or on the inner sides and the periphery.


Gang Milling
Three or more cutters are mounted on the arbour and several horizontal, vertical and angular surfaces are machined in one pass.

Face Milling
This operation can be done on both vertical and horizontal milling machine. The purpose of face milling is that it produces a flat surface that is perpendicular to the spindle on which the cutters are mounted. The cutters are of varying diameters and complexity.

Contour Milling
Contour milling operations are milling operations done on workpieces with an irregular shape such as connecting rods and pistons.

End Milling
End milling is probably the most versatile milling operations. Many types of end mills can be used on both the vertical and the horizontal milling machines. End mills are available in sizes ranging from 1/32 to 6 inches and are in almost any shape needed.

Wednesday, August 13, 2008

Alignment and Setup

Machine Alignment
1) Clean the table and column face, making sure there are no nicks that rise above the surface
2) Attach the dial indicator
3) Bring the dial indicator in contact with one edge of the column face
4) Move in one-fourth of the indicators operating range, and zero the dial by turning the bezel
5) Move the table manually and note any changes in the dial indicator reading. The gibs on the knee, saddle and the table must be in good condition and properly adjusted to eliminate lost motion. The table cannot be adjusted accurately if there are any loose mating surfaces.
6) If any error is noted, loosen the saddle clamps and move the table one-half of the error in the proper direction and retighten. If the dial reads 0 at the right-hand side of the column and 0.008 inches at the left-hand side, shift the table so that the reading becomes 0.004 inches.
7) Repeat the process until the dial indicator reading does not change as it traverses the column face.

Head Alignment
1) Clean the table thoroughly and place a flat and parallel plate on it if one is available.
2) Attach the dial indicator to the spindle.
3) Feed the spindle down with the dial plunger at the operator’s right or left-hand side until it registers about one-fourth of its operating range and zero it.
4) Carefully rotate the spindle one revolution. If the head is of the swivelling or semi-universal type, the fore and aft reading in line with the cross feed will be identical; the right and left readings may vary if the head is not vertical. If the head is of the universal type, the reading may vary on both the axes.
5) Adjust a semi-universal head by loosening the head and swivelling it so that the dial indicator reading is cut in half. For example, if the highest dial indicator reading is 0.010 inches, stop rotation of the spindle at the point. The head should be moved so that the reading is reduced to 0.005 inches. Recheck by rotating the spindle and readjusting the head if necessary.
6) Universal heads should be adjusted in one plane at a time. For example, adjust the head to correct fore and aft tilt until both the fore and aft readings are zero. Then adjust the head so that the right and left readings are zero.
7) Securely tighten all head locking bolts and recheck.

Vise and Fixture Alignment
Vise Alignment
1) Clean all parts thoroughly. Make sure that there are no burrs or nicks on mating surfaces.
2) Lightly clamp the vise and fixtures in approximately the correct position.
3) Bring the dial indicator in contact with one end of the part to be aligned. Move in about one-fourth of the indicator’s operating range and zero indicators.
4) Move the table or cross slide the full length of the jaw or fixtures.
5) Note the variation in indicator reading and move appropriate direction using a soft hammer.
6) When the indicator shows no deviation, retighten all bolts and recheck.


Basic Setup Procedures
Mounting the Work
1) When the work is held in a vise, it should be placed and supported so that the loads imposed by the cutter are directed at the solid jaw of the vise.
2) Parallel bars should be used under any work that is too thin to protrude above the jaws of the vise.
3) When castings are held in a vise, the part that contacts the fixed jaws should be ground as smooth as possible. If this is not possible, place a sheet of soft aluminium or copper between the vise and the casting.
4) Place a mild steel or aluminium between the movable jaw and the casting. This protects the rear jaw and applies pressure more evenly to the work piece. The round bar should be place in line with the screw of the vise is possible.
5) A combination of straps, clamps, T bolts, step blocks and many other work-holding devices can be use to hold a work piece on the milling machine.

Monday, August 11, 2008

Milling Cutters

There is a wide variety of milling cutters available today. They are classified under:
1) Plain Milling Cutter
2) Side Milling Cutter
3) Inserted Tooth Cutters
4) End Mills
5) Special Cutters




Plain Milling Cutters
Plain milling cutters could only cut periphery and are normally used to machine flat surfaces and T-slots. The surface produced is always parallel to the spindle of the machine. Plain milling cutters comes in many different sizes ranging from 0.0032 inches to about 8 inches. There are 3 types of plain milling cutters. They are the saws, Light-duty Plain Milling Cutters and Heavy-duty Plain Milling Cutters.
Saws
This type of plain milling cutters are narrow cutters. And they vary in diameters form 2.5 inches to about 8 inches. This type of cutters has relatively small teeth and is almost always ground with side clearance to reduce friction when making deep cuts.
Light-duty Plain Milling Cutters
This type of cutters has smaller teeth and are used with fairly low chip loads and feed rates. This type of cutters also ranges in size from 2.5 to 4 inches in diameter and usually has 14 to 20 teeth. The helical teeth reduces the possibility of chatter because of tooth engages the work gradually and cuts with a mild shearing action.
Heavy-duty Plain Milling Cutters
This type of cutters comes in diameters ranging from 2 to 6 inches and has 8 to 10 teeth. The space between the teeth is large and heavy which allows higher feed rates without clogging the cutter with chips. The helix angle is about 45 degrees and the teeth have 10 degrees positive rake. The arbor hole size ranges from 1 inch to 2.5 inches in diameter cutters to 2 inches for the 4.5 inch diameter cutters.





Side Milling Cutters
Side milling cutters are cutters that have teeth on the periphery of the cutter and on one or both sides. There are 2 types of side milling cutters which are the Straight-Toothed Side Milling Cutters and the Staggered-Toothed Side Milling Cutters.
Straight-Toothed Side Milling Cutters
This cutter is normally used for slotting operations, gang milling and straddle milling. They normally come in diameters ranging from 3/16 inches to 1 inch. The teeth have 10 degrees positive rake angle. When this cutter is sharpened on both sides, they will no longer be suitable for cutting in one pass slots that are the nominal width of the cutter.
Staggered-Toothed Side Milling Cutters
These type of cutters are normally used for deep slotting and heavy-side milling operations. The teeth on the periphery are helical with alternate right and left helix angles. The side thrust is eliminated and heavy cuts can be taken.
Inserted Tooth Cutters
Cutters of this kind are normally used for rapid metal removal on powerful milling machines. They are generally used in facing operations. There are 2 types of this kind of cutters which are the Face Mills and the Brazed-in Inserts.
Face Mills
The Face Mills vary in Diameter ranging from 4 to 6 inches or in some cases more. It is made up of alloy steel and it is machined to accept various insertions. The angles at which the cutter inserts determines whether the cutter will have negative, neutral or positive rake.
Brazed-in Inserts
Some milling cutters are made up of carbide inserts nickel-silver-brazed-in place. The shell end mill is typical of this kind of cutters. This types of cutters ranges from 1.25 to 6 inches and mount on a standard shell end mill adapter.
End Mills
This type of cutter can be used on both vertical and horizontal milling machines for a variety of facing, slotting and profiling operations. There are 3 types of the end mill cutter which are the Solid End Mills, Shell End Mills and the Special End Mills.
Solid End Mills
This type of cutters comes in two, three, four or more flutes and cutting edges on the end and on the periphery. The 2-flute end mill cutter can be fed directly along the X-axis into the solid because the cutting faces on the end meet. The 3 or 4-fluted end mill cutter with one end cutting edge that extends past the centre of the cutter can also be fed directly onto the solid material. The solid end mills are normally double or single ended with straight or tapered shanks.
Shell End Mills
This type of cutters are normally made up of high-speed steel and mounted on adapters that fit into the milling machine spindle. They’re sizes ranges from 1.25 to 6 inches and are available in right and left hand helix cut. It is normally used for roughing operations.
Special End Mills
Ball end mills are available in diameters ranging from 1/32 to 2.5 inches in single or double ended types. Single-purpose end mills are used in both vertical and horizontal milling machine. They are made of high-speed steel and may have straight or tapered shanks.
Special Cutters
This type of milling cutters is used to produce certain forms and shapes on the finished parts. There are 2 types of this cutter which are the Form Cutters and Gear Cutters.

Form Cutters
This type of cutter is used for making angular grooves. The points of the teeth are sharp or rounded as necessary. Single-angle cutters are plain or side cutting. Plain cutting edges only on the conical surface. The cutter is classified by the angle between the face perpendicular to the arbour and the conical cutting surface. Double angle cutters, on the other hand, is classifies by the included angle between the 2 cutting faces.
Gear Cutters
Cutters for involute gears are made in sets of 8 for each diametrical pitch. Since the gear tooth cutters are a type of form cutters, the face is radial with neither positive nor negative rake. This type of cutter is form-relieved and sharpened by grinding the face only.


Saturday, August 9, 2008

Milling Machine Attachments and Accessories

There are many accessories that have been developed for milling vertically and horizontally. These attachments help smoothen the milling process.
Special Heads
Several types of special heads have been created to give the milling machine versatility. The three main types’ of special heads are the:
 Vertical Heads
 Rack-Milling Attachments
 The Slotting Attachments

Vertical Heads
These special heads are normally attached to the face of the column or to the over arm of the horizontal milling machine. There are two types of vertical heads.
1) Semi universal heads-pivots only on the axis parallel to the centre line of spindle
2) Fully Universal heads-can be set to cut compound angle
Both of the two types of vertical heads are powered by the spindle of the milling machine and accept standard arbors and collets.
Rack-Milling Attachments
The rack-milling attachment bolts to the spindle housing of the milling machine. Its spindle is at right angles to the main spindle. Spur and helical racks can be milled with these attachments and can also be used to mill worms.
Slotting Attachments
This particular attachment is bolted to the column of a horizontal milling machine and can be swivelled 90 degrees in either direction from vertical position. It is used in toolmaking and prototype work for cutting keyways, internal splines and square or rectangular cavities.
Vises and Fixtures
All milling operations require fixtures, vises or other clamping devices to hold the work piece. There are 5 types of vises and fixtures. They are
1) Plain Vises
2) Universal Vises
3) Angle Plates
4) Indexing Heads
5) Rotary Table
Plain Vise
This type of vise is actuated by an Acme threaded screw, and the movable jaw moves on either dovetail or rectangular slide. The vises are actually made of cast of high-grade gray cast iron or ductile iron that can be heat treated. Steel keys are attached in slots machined into the bottom of the vise parallel with and perpendicular to the fixed jaws to allow accurate placement on the milling table. The jaw inserts are usually heat-treated alloy steel and are attached by cap screws. This vise is classified by the jaw width and maximum opening.. Cam-operated plain milling vises are mostly used in production work because of the savings in time and effort and the uniform clamping pressures that can be achieved.
Universal Vise
This type of vise is commonly used in the making of tools, dies and prototype work. This type of vise is the least rigid amongst all the milling machine vises. The base of the vise is graduated in degrees and held by T bolts. The intermediate part of the vise has a horizontal pivot upon which the vise itself can rotate up to 90 degrees.
Angle Plates
Several types of angle plates can be used to hold work pieces or work holding devices. Plain milling plates are available in T-slotted or blank form made of strong iron casting. Adjustable angle plates may tilt in one direction only or have a swivel base. These plates are very useful for holding irregular milling work pieces.
Indexing Heads
This attachment can be used in both horizontal and vertical milling machines to space the cuts for such operations as making splines, gears, worm wheels and many other curating divisions. This attachment can also be fitted to the machine for helical milling operations. There two types of indexing heads. The plain and universal type. The plain heads cannot be tilted but the universal one can. The indexing heads could also be used to hold work-holding devices. The spindle of the indexing head can also be fitted with chucks and collets. Most indexing heads have a worm and wheel reduction ratio by 40 to 1, requiring 40 turns of the hand cranks to make the spindle revolve once.
Rotary Tables
The rotary table comes in many different types of sizes by which it fits both the vertical and horizontal millimg machines. Most can be clamped with a 90 degrees angle to the face. The faces of this table have 4 T-slots and are accurately bored hole in the centre which is concentric. The base of this table houses worm drive mechanism, graduated in degrees and the handwheels can be graduated with and increment as small as 5 inches. Rotary table can also be geared to the table screw which can make cam plates to generate irregular shapes. The axis of the tilting rotary table can also be positioned between 0 degrees to 90 degrees. This particular feature is useful when the rotary table is used on a plain or semiuniversal milling machine.
Arbors and Collets
Arbors
There are three types of arbors which are style A, style B and style C.
1) Style A-consists of tapered portions that fits the spindle , the shaft on which the cutter or
cutter fitters, the spacers and the nuts
2) Style B-used for heavy milling machine operations especially where it is necessary to
provide support close to milling cuttersuch as in straddle milling operations.
3) Style C-used to hold and drive shells end mills and some types of milling machine taper,
shaft size and length from shoulder to nut.

Collets
Certain types of milling machine have bored holes that fit the collets. The collets are secured by a drawbar that is screwed into a tapped hole in the back of the collets and tightened from the top of the spindle.

Special-Purpose Milling Machines.

Special-purpose milling machines are built to accommodate larger work, duplicating parts of work pieces, surfaces the work piece precisely or to do many other unusual milling operations which regular milling machines cannot do. There are three types of special-purpose milling machines.
Planner-type Milling Machines
The tables of these machines carries the work piece across the rotating cutter heads which are dividually powered and the speed of which can be changed. As many as four cutter heads can be used with two mounted on the cross rail and two on the vertical pillars. These type of machines are normally used to machine parts like the bed ways of large machine tools and other long work pieces.
Profile Milling Machine
Two-dimensional profiling can be done by using a template or by controlling the vertical milling machine vertically. Certain types of profilers have several spindles and a number of duplicated parts can be produced for every cycle. Hydraulics-type profilers have a stylus that touches the template when starting the operations. The operator moves the stylus along the template which causes the thet table to move past the cutters thus duplicating the template. Diesinking and other processes that involve machining cavities can be done on 3D profilers.
Jig Borers
Jig borer machines are somewhat similar to vertical milling machines but they are more accurately or more precisely constructed. These kinds of machines are used for boring operations. Some jig borer machines have a digital readout system which is more accurate.

Production-type Milling Machines

Production-type milling machine are less versatile than the regular knee and column, horizontal or vertical milling machines. This machine has much greater rigidity, strength and power and is best when it comes to heavy milling operations. This machine is very easy to handle and requires less skills when compared to other types milling machine. The work pieces are held in fixtures and sometimes can be fitted with more than one work piece at a time. There are two types of production-type milling machines. There’s the fixed-bed machine and the rotating-table machine.
Fixed-bed Machine
On simplex machines, the table moves longitudinally. The work piece is fed by using hydraulics or run by a screw and nut arrangement. Normally, there is no provision for hand feeding the table. The position of the spindle can be adjusted vertically while the machine is being set up, after which the position of it is locked. In some machines, the spindle can be raised or dropped as the table moves. These kind of machines are used for simple profiling operations.
In a duplex machine, the table can only be moved longitudinally between the heads and spindles. The height of each head can be raised or lowered and can be fitted with a wide variety of mill cutters. These type of machine is best for facing operations.
There are also more complex triplex machines that are used to machine three surfaces at once on the part that is being moved past the cutters by the table.
Rotating-table Machine
These types of machines are usually used in factories and have one or more spindles that are of the vertical type. The work pieces are held by a lot of fixtures. The movement of the table is continuous and the main purpose of the operator is to place unmachined parts and remove machined parts. Tooth face milling types of cutters are used on these types of machines.