Saturday, August 9, 2008
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Cutting Speed and Feed Rate.
Proper determination of cutting speed and feed rate can be done only when the following eight factors are known:
- Type of material to be machined
- Nature of heat treatment, if any
- Rigidity of the setup
- Cutting tool material
- Power available at the spindle
- Type of finish desired
- Cutting fluid to be used, if any
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.
- 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.
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.
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.
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.
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
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.
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, 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.
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.
Mounting the work
Wednesday, July 16, 2008
Plain milling machines, both vertical and horizontal, do not require table alignment in normal use because the table cannot be swiveled. Overarm-type vertical milling machines that do not have either a swiveling or fully universal head do not require head alignment because the head can move up and down only.
The table on universal milling machine must be checked for alignment whenever it is being returned to 0˚ position or when a job involving precise angular relationships is to be done. NEVER trust the graduations on the saddle or on any other parts of the machine when really accurate work must be done.
A rigid mounting for the dial indicator is necessary so that flexing and slippage do not alter the readings. The dial indicator must be mounted to the table of the machine because the motion of the table relative to the face of the column is being checked.
The head on vertical milling machines with semi universal or universal heads must be checked before doing jobs requiring accurate alignment between the head and table. It’s important to align the spindle perpendicular the table when drilling, boring, and flycutting operations are performed. The following procedure is suggested for alignment the head of vertical milling machines with semi universal or universal head.
- Clean the table thoroughly and place a flat and parallel plate on it if one is available.
- Attach the dial indicator to spindle.
- Feed the spindle down, with the dial pluger at operator’s right of left side until it registers about one-fourth of its operating range, and zero it.
- Carefully rotate the spindle one revolution.
(a)If the head and swiveling or semi universal type, the fore and aft readings in line with the cross feed will be identical; the right and left readings may vary if the head is not vertical.
(b)If the head is of the universal type, the reading may vary on both axes.
- Adjust a semi universal head by loosening the head and swiveling it so that the dial indicator reading is cut in half. Recheck the spindle and readjusting the head if necessary.
- Universal heads should be adjusted in one plane at a time.
- Securely tighten all head locking bolts and recheck.
Vise and Fixture Alignment
Vise alignment-the solid jaw of the vise is always the reference surface. The dial indicator is attached to the arbor of a horizontal milling machine or held in the collet of a vertical milling machine. The following procedure is suggested for aligning plates and other fixture with major flat surfaces
- Clean all parts thoroughly. Make sure that there areno burrs or nicks on mating surface.
- Lightly clam the vise or fixture in approximately the correct position.
- Bring the dial indicator in contact with one end of the part to be aligned. Move in about one-forth of the indicator’s operating range and zero the indicator.
- Move the table or cross slide the full length of the jaw or fixture.
- Note the variation in indicator reading, and move the vise or fixture in the appropriate direction, using a soft hammer.
- When the indicator shows no deviation, retighten all bolts and recheck.
High speed steel (HSS) cutters are the least-expensive and shortest-lived cutters. Cobalt steel is an improvement on HSS and generally can be run 10% faster. Carbide tools are more expensive than steel, but last longer, and can be run much faster, so prove more economical in the long run. HSS tools are perfectly adequate for many applications. The progression from HSS to cobalt steel to carbide could be viewed as very good, even better, and the best.
Larger tools can remove material faster than small ones, therefore the largest possible cutter that will fit in the job is usually chosen. When milling an internal contour, or concave external contours, the diameter is limited by the size of internal curves. The radius of the cutter must be less than or equal to the radius of the smallest arc.
Flutes: More flutes allows a higher feed rate, because there is less material removed per flute. But because the core diameter increases, there is less room for swarf, so a balance must be chosen.
Coatings, such as Titanium nitride, also increase initial cost but reduce wear and increase tool life.
Helix angle :
High helix angles are typically best for soft metals, and low helix angles for hard or tough metals.
End mills (middle row in image) are those tools which have cutting teeth at one end, as well as on the sides. The words end mill are generally used to refer to flat bottomed cutters, but also include rounded cutters (referred to as ball nosed) and radiused cutters (referred to as bull nose, or torus). They are usually made from high speed steel(HSS) or carbide, and have one or more flutes. They are the most common tool used in a vertical mill.
Slot drills (top row in image) are generally two (occasionally three or four) fluted cutters that are designed to drill straight down into the material. This is possible because there is at least one tooth at the centre of the end face. They are so named for their use in cutting keyway slots. The words slot drill are usually assumed to mean a two fluted, flat bottomed end mill if no other information is given. Two fluted end mills are usually slot drills, three fluted sometimes aren't, and four fluted usually aren't.
Roughing end mill
Roughing end mills quickly remove large amounts of material. This kind of end mill utilizes a wavy tooth form cut on the periphery. These wavy teeth form many successive cutting edges producing many small chips, resulting in a relatively rough surface finish. During cutting, multiple teeth are in contact with the workpiece reducing chatter and vibration. Rapid stock removal with heavy milling cuts is sometimes called hogging. Roughing end mills are also sometimes known as ripping cutters.
Ball nose cutter
Ball nose cutters (lower row in image) are similar to slot drills, but the end of the cutters are hemispherical. They are ideal for machining 3-dimensional contoured shapes in machining centres, for example in molds and dies. They are sometimes called ball mills in shop-floor slang, despite the fact that that term also has another meaning. They are also used to add a radius between perpendicular faces to reduce stress concentrations.
Slab mills are used either by themselves or in gang milling operations on manual horizontal or universal milling machines to machine large broad surfaces quickly. They have been superseded by the use of Carbide tipped face mills that are then used in vertical mills or machining centres.
The side-and-face cutter is designed with cutting teeth on its side as well as its circumference. They are made in varying diameters and widths depending on the application. The teeth on the side allow the cutter to make unbalanced cuts (cutting on one side only) without deflecting the cutter as would happen with a slitting saw or slot cutter (no side teeth).
Involute gear cutter
The image shows a Number 4 cutter from an involute gear cutting set. There are 7 cutters (excluding the rare half sizes) that will cut gears from 12 teeth through to a rack (infinite diameter). The cutter shown has markings that show it is a
- 10 DP (diametrical pitch) cutter
- That it is No. 4 in the set
- that it cuts gears from 26 through to 34 teeth
- It has a 14.5 degree pressure angle
These cutters are a type of form tool and are used in hobbing machines to generate gears. A cross section of the cutters tooth will generate the required shape on the workpiece, once set to the appropriate conditions (blank size). A hobbing machine is a specialised milling machine.
Face mill (indexable carbide insert)
A face mill consists of a cutter body (with the appropriate machine taper) that is designed to hold multiple disposable carbide or ceramic tips or inserts, often golden in color. The tips are not designed to be resharpened and are selected from a range of types that may be determined by various criteria, some of which may be: tip shape, cutting action required, material being cut. When the tips are blunt, they may be removed, rotated (indexed) and replaced to present a fresh, sharp face to the workpiece, this increases the life of the tip and thus their economical cutting life.
A fly cutter is composed of a body into which one or two tool bits are inserted. As the entire unit rotates, the tool bits take broad, shallow facing cuts. Fly cutters are analogous to face mills in that their purpose is face milling and their individual cutters are replaceable. Face mills are more ideal in various respects (e.g., rigidity, indexability of inserts without disturbing effective cutter diameter or tool length offset, depth-of-cut capability), but tend to be expensive, whereas fly cutters are very inexpensive.
Woodruff cutters make the seat for woodruff keys. These keys retain pulleys on shafts and are shaped as shown in the image.
Hollow milling cutters, more often called simply hollow mills, are essentially "inside-out endmills". They are shaped like a piece of pipe (but with thicker walls), with their cutting edges on the inside surface. They are used on turret lathes and screw machines as an alternative to turning with a box tool, or on milling machines or drill presses to finish a cylindrical boss (such as a trunnion).