Metal fabrication is a process of producing a metal component by modifying a raw piece of material in a machine shop. These materials are processed on different temperatures and depending on the range of temperatures; the process is classified as cold, warm and hot. Precision sheet metal fabrication describes various different processes that form sheet metal into finished products. Objects that are fabricated and used for machinery and other instruments are among the daily used objects like paper clips, computers, bolts, nails, automotive parts and many more.

There are various factors like rate of production, desired geometry, and other physical requirements that influence the fabrication process. The benefits of the metal fabrication process are far and wide because it is used by all industries. Every precision sheet metal fabrication process undergoes three primary processes that include forming, cutting and finishing. Forming is a process that alters the form of the flat metal sheet. Forming can be done using various different processes like annealing, bending, cold rolling, drawing, forging, mechanical working, press forming, roll forming and welding. After forming, a metal sheet is cut to alter the shape by removing some unwanted material. The cutting process includes stamping, shearing, sawing, drilling, blanking and punching. Finally, the last stage is the finishing process. In this process, the shaped metal sheet is given a finished and soft surface using sanding and vibration techniques. Aesthetic considerations like painting and visual design applications are also few of the method applied to give the finishing touch. Some of the latest techniques in metal fabrication involve laser cutting, electro discharge machining, water jet cutting and wire cut EDM. In some cases CNC machining is also used.

There are various products that are created during precision sheet metal fabrication. The most common objects produced are metal cabinets, enclosures, ventilation shafts, hoods, exhaust systems, tanks and prototypes. Metal fabrication is also used by various other industries for food dispensing, food storage, communications, automotive, computer, medical, electronics, aerospace, telecommunication, pharmaceutical, residential and construction.

Some of the types of Precision Sheet Metal Fabrication include processes like Annealing (heating), Bending (straining), Cold forming, Cold rolling (shaping sheet metal using rollers), Drawing (material is forced into a die with a punch to form a cup-like shape), Forging hammering or pressing), Rolling (reducing thickness of the material), Extrusion (producing cylindrical bars or hollow tubes by forcing round billets through one or more dies) and Spinning (forms axially symmetrical shapes).

George is a well-known author who writes on the topics related with Precision Sheet Metal Fabrication, metal fabrication for the site www.emachineshop.com.

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Question 5.

As we have seen, chips carry away the majority of the heat generated during machining. If chips did not have this capacity, what suggestions would you make in order to be able to carry out machining process without excessive heat. Explain?

Solutions:

When the chips are formed they have long grain structures, traverse to the direction of cutting. However, as the chip rubs the surface of the workpiece, the grain structure on the lower surface bends to form a "long tail" due to secondary deformation and the heat build up. This lower surface then recrystallizes due to the grain structure and intense heat. This recrystallized area is highly undesirable since it is more difficult to have smaller, broken chips.

We can improve the chip breaking by

• increasing the brittleness of the chip material by the low temperature application of the cryogenic coolant;
• reducing the secondary deformation that occurs at the bottom surface of the chip which rubs on the tool face;
• reducing the long tail in the grain structure of the chips, which is a major cause of the difficulty in breaking ductile chips;
• avoiding the welding or recrystallizing of the bottom layer of the chip (to avoid the strong bonding of chip structure which adds to the difficulty in chip breaking) by the coldness and reduction of the secondary deformation;
• bending and curling the chip for breaking by the use of both a mechanical chip breaker and the pressure from the cryogenic fluid

Explanation:

The build-up edge formation is automatically prevented by injecting the super cold cryogenic fluid to the cutting tip area. This reduces the adhesion of the chip material to the tool tip which lowers the temperature in the cutting zone and prevents the welding phenomenon. Also, this invention can clean and remove the possible build-up edge with the cryogenic jet. Because the build-up edge is the main cause of poor surface quality, the method of the present invention also indirectly improves the finish of the machined surface. Therefore, it is an object of the present invention to provide a cutting tool which injects liquid nitrogen or other liquified gas as close as possible to the highest heat affected area of the cutting tool, not the workpiece, through a nozzle, between the chip and the tool face. This provides a fluid cushion which reduces the friction of the chip rubbing on the tool face, and reduces the cutting force involved, as well as the abrasive wear.

Question 4. Why does the temperature in cutting depends on the cutting speed, feed and depth of cut? Explain in term of the relevant process variable.

Temperature in cutting refers to the temperature at the tool/chip interface. It consists of the temperature rise in the body of the chip due to shear at the interfaces as the hot chip rubs on the tool faces, and the workpiece temperature. Those rises varies with cutting speed, in general, decreasing with an increasing in cutting speed.

Most of the mechanical energy expanded in metalcutting is transformed into heat. About 75%-80% of this heat is carried away by the chip; the remainder is divided between the workpiece and the tool.

An increase in speed or feed decreases the proportion of the heat transferred to the tool and workpiece. The relationship of cutting temperature to cutting speed is shown below:

where,
θf = tool/chip interface temperature
V = cutting speed
C = constant (dependent on tool/workpiece combination and cutting variables other than cutting speed)
n = exponent

An empirical equation of similar from relates cutting temperature to feed. It is seen that the increase in feed is less hazardous as far as temperature is concerned than a corresponding increase in speed. The depth-of-cut influences the cutting temperature in a limited manner.

If the depth-of-cut is more than twice the nose radius, a further increase in depth will have little effect on the cutting temperature. Basically, a depth of cut of 10 times of the feed rate creates a little temperature effect.

For all metal-cutting processes, "speeds and feeds" are important parameters. The colloquial term "speeds and feeds" refers to the speed, feed, and depth of cut of a metal-cutting process. To describe these parameters, we will be using the turning process. The figure below shows the important geometry. The speed (Labeled on the figure as V.) is the cutting speed, which is a measure of the part cut surface speed relative to the (here, stationary) tool.

Speed is a velocity unit, which is typically listed in terms of feet/min, inches/min, meters/second, or meters/min.

Feed (Labeled on the figure as f_r), is the amount of material removed for each revolution or per pass of the tool over the workpiece. Feed is measured in units of length/revolution, length/pass, length/tooth, length/time, or other appropriate unit for the particular process.

The depth of cut, DOC (Labeled on the figure as d.), represents the third parameter for metal cutting. For turning, DOC is the depth that the tool is plunged into the surface. The DOC is half of the difference in the diameters D_a and D_b,

the initial and final diameters, respectively.





Below is a summary list of the terms used.






Speeds and feeds are important since they are critical input parameters which determine the output of a machine tool. Other inputs include the material of the part, the shape of the raw stock and final part, and the geometry of the machine tool used. Outputs include the surface finish, the time to machine each part, the temperature of the cutting tool and cut part, the residual stresses left in the part, and warping of the part.

Speeds and feeds typically take much experience and experimenation to determine accurately, but a good place to start is a table of recommended speeds and feeds. The MRR, or the Metal Removal Rate, is listed at the end of the above relation list. For turning, MRR values range from 0.1 to 600 in³ per minute. Most processes have MRR's that can be expressed as the volume of metal removed divided by the time needed to remove it:
MRR = (volume of cut)/(cutting time).
MRR can be used to estimate the power required to sustain the cutting operation. With turning, the cutting time can be expressed as the following:



Where the symbols have the same meanings as defined earlier on this page and the allowance is an estimation factor which is added to the L term to allow for the tool to enter and exit the cut.


A single-point tool is a cutting tool that cuts only at a single edge or area of the cutting tool. Turning and shaping are examples of single-point cutting. Multiple-point cutting processes are milling and drilling.

High Carbon Steel

Contains 1 - 1.4% carbon with some addition of chromium and tungsten to improve wear resistance. The steel begins to lose its hardness at about 250° C, and is not favoured for modern machining operations where high speeds and heavy cuts are usually employed.

High Speed Steel (H.S.S.)

Steel, which has a hot hardness value of about 600° C, possesses good strength and shock resistant properties. It is commonly used for single point lathe cutting tools and multi point cutting tools such as drills, reamers and milling cutters.

Cemented Carbides

An extremely hard material made from tungsten powder. Carbide tools are usually used in the form of brazed or clamped tips. High cutting speeds may be used and materials difficult to cut with HSS may be readily machined using carbide tipped tool.

For efficient cutting a tool must have the following properties:

Hot Hardness

This means the ability to retain its hardness at high temperatures. All cutting operations generate heat, which will affect the tool¡¦s hardness and eventually its ability to cut.

Strength and Resistance to Shock

At the start of a cut the first bite of the tool into the work results in considerable shock loading on the tool. It must obviously be strong enough to withstand it.

Low Coefficient of Friction

The tool rubbing against the workpiece and the chip rubbing on the top face of the tool produce heat which must be kept to a minimum

There are three important angles in the construction of a cutting tool rake angle, clearance angle and plan approach angle.

Figure 5. Main Features of a Single Point Cutting Tool

Rake Angle

Rake angle is the angle between the top face of the tool and the normal to the work surface at the cutting edge. In general, the larger the rake angle, the smaller the cutting force on the tool, since for a given depth of cut the shear plane AB, shown in Figure 4 decreases as rake angle increases. A large rake angle will improve cutting action, but would lead to early tool failure, since the tool wedge angle is relatively weak. A compromise must therefore be made between adequate strength and good cutting action.

Metal Being Cut	Cast Iron	Hard Steel / Brass	Medium Carbon Steel	Mild Steel	Aluminium
Top Rake Angle	0°	8°	14°	20°	40°

Clearance Angle

Clearance angle is the angle between the flank or front face of the tool and a tangent to the work surface originating at the cutting edge. All cutting tools must have clearance to allow cutting to take place. Clearance should be kept to a minimum, as excessive clearance angle will not improve cutting efficiency and will merely weaken the tool. Typical value for front clearance angle is 6° in external turning.

Plan Profile of Tool

The plan shape of the tool is often dictated by the shape of the work, but it also has an effect on the tool life and the cutting process. Figure 6 shows two tools, one where a square edge is desired and the other where the steps in the work end with a chamfer or angle. The diagram shows that, for the same depth of cut, the angled tool has a much greater length of cutting edge in contact with the work and thus the load per unit length of the edge is reduced. The angle at which the edge approaches the work should in theory be as large as possible, but if too large, chatter may occur. This angle, known as the Plan Approach Angle, should therefore be as large as possible without causing chatter.

Figure 6. Plan Approach Angle

The trailing edge of the tool is ground backwards to give clearance and prevent rubbing and a good general guide is to grind the trailing edge at 90° to the cutting edge. Thus the Trail Angle or Relief Angle will depend upon the approach angle.

A small nose radius on the tool improves the cutting and reduces tool wear. If a sharp point is used it gives poor finish and wears rapidly.

The usual conception of cutting suggests clearing the substance apart with a thin knife or wedge. When metal is cut the action is rather different and although the tool will always be wedge shaped in the cutting area and the cutting edge should always be sharp the wedge angle will be far too great for it to be considered knife shaped. Consequently a shearing action takes place when the work moves against the tool.

Figure 4. Basic Metal Cutting Theory

Figure 4 shows a tool being moved against a fixed work piece. When the cut is in progress the chip presses heavily on the top face of the tool and continuous shearing takes place across the shear plane AB. Although the Figure shows a tool working in the horizontal plane with the workpiece stationary, the same action takes place with the work piece revolving and the tool stationary.

The tool used in a lathe is known as a single point cutting tool. It has one cutting edge or point whereas a drill has two cutting edges and a file has numerous points or teeth.

The lathe tool shears the metal rather than cuts as will be seen later and it can only do so if there is relative motion between the tool and the workpiece. For example, the work is rotating and the tool is moved into its path such that it forms an obstruction and shearing takes place. Of course the amount of movement is of paramount importance - too much at once could for instance result in breakage of the tool.

Figure 3. Types of Cutting Tool

The type and design of the tools selected will depend on the job in hand, the machining operation selected and the material to be cut. The correct tool especially the various face angles are essential if the operation is to be done in a cost-effective (i.e. productive) way. The tools used in a lathe are various, some of which are shown in figure 3.

The range of cutting tool types is extensive and a few examples only are shown in this handout. Nonetheless you should take every opportunity to look deeper into the types of tools available.

The term Centre Lathe is derived from the fact that in its operation the lathe holds a piece of material between two rigid supports called centres, or by some other device such as a chuck or faceplate which revolves about the centre line of the lathe.

The lathe shown above is a typical example. This machine is usually used in a jobbing (one off) situation or for small batch work where it would be too expensive to specially 'tool up' for just a few items.

The lathe on which you will work is a machine used to cut metal. The spindle carrying the work is rotated whilst a cutting tool, which is supported in a tool post, is made to travel in a certain direction depending on the form of surface required. If the tool moves parallel to the axis of the rotation of the work a cylindrical surface is produced as in Fig 2 (a) , whilst if it moves at right angles to this axis it produces a flat surface as in Fig 2 (b).

Figure 2a. Producing a Cylindrical Surface	Figure 2b. Producing a Flat Surface

The lathe can also be used for the purposes shown in Fig 2c, 2d, 2e and 2f.

Figure 2c. Taper Turning	Figure 2d. Parting Off / Under Cutting
Figure 2e. Radius Turning Attachment	Figure 2f. Drilling on a Lathe

This training module is designed to give you 'hands-on' experience through which you can gain a good appreciation of this well-known type of machine tool. In particular your attention will be directed towards its operational uses and parameters, the general layout of controls, accessories, associated tooling, and the maintenance factors related to lathes.

In order that you can make the most use of the limited time available on lathes it is essential that you use every chance to consolidate what you observe. This type of work is largely self-motivated and the drive and desire to find out must come from you.

It takes a considerable time to become a skilled lathe operator and to possess all the skill of hand that goes with it. Therefore it is not expected that you will be manually skilled on completion of the module but you will have gained intellectually and without doubt, by practical involvement, some skill of hand will be achieved.

Figure 1. Example of a Typical Centre Lathe

Idealized Chip Formation

The figure below depicts an idealized, two dimensional view of the metal cutting process. The assumptions in this model are that the tool is perfectly sharp, that the cut depth t and the cutting speed V are constant, and that the cut depth is small compared to the cut width. In this idealized model, the material layer at the top is formed into a chip by a shearing process in the primary shear zone at AB. The chip slides up the rake face undergoing some secondary plastic flow due to the forces of friction. This idealized model correctly predicts that cutting force increases with cut depth, material hardness, and friction coefficient. Cutting forces are inversely propoortional to rake angle. Power required increases with the feed rate.

Idealized Chip Formation

Built Up Edge

One important effect that is not considered in the model above is built up edge. Under most cutting conditions, some of the cut material will attach to the cutting point. This tends to cause the cut to be deeper than the tip of the cutting tool and degrades surface finish. Also, periodically the built up edge will break off and remove some of the cutting tool. Thus, tool life is reduced. In general, built up edge can be reduced by:

• Increasing cutting speed
• Decreasing feed rate
• Increasing ambient workpiece temperature
• Increasing rake angle
• Reducing friction (by applying cutting fluid)

Built Up Edge

Calculating Speeds and Feeds

Cutting speed refers to the speed at which the tool point of the cutter moves with respect to the work measured in feet per minute. Feed is the rate at which the work moves into the cutter measured in feed per tooth revolution. Feeds and speeds affect the time to finish a cut, tool life, finish of the machined surface, and power required of the machine.

The cutting speed is mostly determined by the material to be cut and the material of the tool. To find the right speed for any task, refer to the Machinery's Handbook or other reference. To calculate the proper spindle speed, divide the desired cutting speed by the circumference of the tool (or of the part if it is rotating) expressed in feet. The feed rate depends on the width and depth of cut, finish desired and many other variables. To calculate the desired feed setting from the feed rate, multiply feed per tooth per revolution by number of teeth and rpm of the spindle.

Maintained by 2.670adm@mit.edu

Most CNC milling machines or machining centers are computer controlled vertical mills with the ability to move the spindle vertically along the Z-axis. This extra degree of freedom permits their use in diesinking, engraving applications, and 2.5D surfaces such as relief sculptures. When combined with the use of conical tools or a ball nose cutter, it also significantly improves milling precision without impacting speed, providing a cost-efficient alternative to most flat-surface hand-engraving work.

CNC machines can exist in virtually any of the forms of manual machinery, like horizontal mills. The most advanced CNC milling-machines, the 5-axis machines, add two more axes in addition to the three normal axes (XYZ). Horizontal milling machines also have a C or Q axis, allowing the horizontally mounted workpiece to be rotated, essentially allowing asymmetric and eccentric turning. The fifth axis (B axis) controls the tilt of the tool itself. When all of these axes are used in conjunction with each other, extremely complicated geometries, even organic geometries such as a human head can be made with relative ease with these machines. But the skill to program such geometries is beyond that of most humans. Therefore, 5-axis milling machines are practically always programmed with CAM.

With the declining price of computers, free operating systems such as Linux, and open source CNC software, the entry price of CNC machines has plummeted. For example, Sherline, Prazi, and others make desktop CNC milling machines that are affordable by hobbyists.

Milling machine tooling

There is some degree of standardization of the tooling used with CNC Milling Machines and to a much lesser degree with manual milling machines.

CNC Milling machines will nearly always use SK (or ISO), CAT, BT or HSK tooling. SK tooling is the most common in Europe, while CAT tooling, sometimes called V-Flange Tooling, is the oldest variation and is probably still the most common in the USA. CAT tooling was invented by Caterpillar Inc. of Peoria, Illinois in order to standardize the tooling used on their machinery. CAT tooling comes in a range of sizes designated as CAT-30, CAT-40, CAT-50, etc. The number refers to the Association for Manufacturing Technology (formerly the National Machine Tool Builders Association (NMTB)) Taper size of the tool.

An improvement on CAT Tooling is BT Tooling, which looks very similar and can easily be confused with CAT tooling. Like CAT Tooling, BT Tooling comes in a range of sizes and uses the same NMTB body taper. However, BT tooling is symmetrical about the spindle axis, which CAT tooling is not. This gives BT tooling greater stability and balance at high speeds. One other subtle difference between these two toolholders is the thread used to hold the pull stud. CAT Tooling is all Imperial thread and BT Tooling is all Metric thread. Note that this affects the pull stud only, it does not affect the tool that they can hold, both types of tooling are sold to accept both Imperial and metric sized tools.

SK and HSK tooling, sometimes called "Hollow Shank Tooling", is much more common in Europe where it was invented than it is in the United States. It is claimed that HSK tooling is even better than BT Tooling at high speeds. The holding mechanism for HSK tooling is placed within the (hollow) body of the tool and, as spindle speed increases, it expands, gripping the tool more tightly with increasing spindle speed. There is no pull stud with this type of tooling.

The situation is quite different for manual milling machines — there is little standardization. Newer and larger manual machines usually use NMTB tooling. This tooling is somewhat similar to CAT tooling but requires a drawbar within the milling machine. Furthermore, there are a number of variations with NMTB tooling that make interchangeability troublesome.

Two other tool holding systems for manual machines are worthy of note: They are the R8 collet and the Morse Taper #2 collet. Bridgeport Machines of Bridgeport, Connecticut so dominated the milling machine market for such a long time that their machine "The Bridgeport" is virtually synonymous with "Manual milling machine." The bulk of the machines that Bridgeport made from about 1965 onward used an R8 collet system. Prior to that, the bulk of the machines used a Morse Taper #2 collet system.

As an historical footnote: Bridgeport is now owned by Hardinge Brothers of Elmira, New York.

Comparing vertical with horizontal

In the vertical mill the spindle axis is vertically oriented. Milling cutters are held in the spindle and rotate on its axis. The spindle can generally be extended (or the table can be raised/lowered, giving the same effect), allowing plunge cuts and drilling. There are two subcategories of vertical mills: the bedmill and the turret mill. Turret mills, like the ubiquitous Bridgeport, are generally smaller than bedmills, and are considered by some to be more versatile. In a turret mill the spindle remains stationary during cutting operations and the table is moved both perpendicular to and parallel to the spindle axis to accomplish cutting. In the bedmill, however, the table moves only perpendicular to the spindle's axis, while the spindle itself moves parallel to its own axis. Also of note is a lighter machine, called a mill-drill. It is quite popular with hobbyists, due to its small size and lower price. These are frequently of lower quality than other types of machines, however.

A horizontal mill has the same sort of x–y table, but the cutters are mounted on a horizontal arbor across the table. A majority of horizontal mills also feature a +15/-15 degree rotary table that allows milling at shallow angles. While endmills and the other types of tools available to a vertical mill may be used in a horizontal mill, their real advantage lies in arbor-mounted cutters, called side and face mills, which have a cross section rather like a circular saw, but are generally wider and smaller in diameter. Because the cutters have good support from the arbor, quite heavy cuts can be taken, enabling rapid material removal rates. These are used to mill grooves and slots. Plain mills are used to shape flat surfaces. Several cutters may be ganged together on the arbor to mill a complex shape of slots and planes. Special cutters can also cut grooves, bevels, radii, or indeed any section desired. These specialty cutters tend to be expensive. Simplex mills have one spindle, and duplex mills have two. It is also easier to cut gears on a horizontal mill.

Other milling machine variants and terminology

• Box or column mills are very basic hobbyist bench-mounted milling machines that feature a head riding up and down on a column or box way.
• Turret or vertical ram mills are more commonly referred to as Bridgeport-type milling machines. The spindle can be aligned in many different positions for a very versatile, if somewhat less rigid machine.
• Knee mill or knee-and-column mill refers to any milling machine whose x-y table rides up and down the column on a vertically adjustable knee. This includes Bridgeports.
• C-Frame mills are larger, industrial production mills. They feature a knee and fixed spindle head that is only mobile vertically. They are typically much more powerful than a turret mill, featuring a separate hydraulic motor for integral hydraulic power feeds in all directions, and a twenty to fifty horsepower motor. Backlash eliminators are almost always standard equipment. They use large NMTB 40 or 50 tooling. The tables on C-frame mills are usually 18" by 68" or larger, to allow multiple parts to be machined at the same time.
• Planer-style mills are large mills built in the same configuration as planers except with a milling spindle instead of a planing head. This term is growing dated as planers themselves are largely a thing of the past.
• Bed mill refers to any milling machine where the spindle is on a pendant that moves up and down to move the cutter into the work. These are generally more rigid than a knee mill.
• Ram type mill refers to a mill that has a swiveling cutting head mounted on a sliding ram. The spindle can be oriented either vertically or horizontally, or anywhere in between. Van Norman specialized in ram type mills through most of the 20th century, but since the advent of CNC machines ram type mills are no longer made.
• Jig borers are vertical mills that are built to bore holes, and very light slot or face milling. They are typically bed mills with a long spindle throw. The beds are more accurate, and the handwheels are graduated down to .0001" for precise hole placement.
• Horizontal boring mills are large, accurate bed horizontal mills that incorporate many features from various machine tools. They are predominantly used to create large manufacturing jigs, or to modify large, high precision parts. They have a spindle stroke of several (usually between four and six) feet, and many are equipped with a tailstock to perform very long boring operations without losing accuracy as the bore increases in depth. A typical bed would have X and Y travel, and be between three and four feet square with a rotary table or a larger rectangle without said table. The pendant usually has between four and eight feet in vertical movement. Some mills have a large (30" or more) integral facing head. Right angle rotary tables and vertical milling attachments are available to further increase productivity.
• Floor mills have a row of rotary tables, and a horizontal pendant spindle mounted on a set of tracks that runs parallel to the table row. These mills have predominantly been converted to CNC, but some can still be found (if one can even find a used machine available) under manual control. The spindle carriage moves to each individual table, performs the machining operations, and moves to the next table while the previous table is being set up for the next operation. Unlike any other kind of mill, floor mills have floor units that are entirely movable. A crane will drop massive rotary tables , X-Y tables , and the like into position for machining, allowing the largest and most complex custom milling operations to take place.
• Portical mills It has the spindle mounted in a T structure

There are many ways to classify milling machines, depending on which criteria are the focus:

Criterion	Example classification scheme	Comments
Control	Manual; Mechanically automated via cams; Digitally automated via NC/CNC	In the CNC era, a very basic distinction is manual versus CNC. Among manual machines, a worthwhile distinction is non-DRO-equipped versus DRO-equipped
Control (specifically among CNC machines)	Number of axes (e.g., 3-axis, 4-axis, or more); Within this scheme, also: Pallet-changing versus non-pallet-changing Full-auto tool-changing versus semi-auto or manual tool-changing
Spindle axis orientation	Vertical versus horizontal; Turret versus non-turret	Among vertical mills, "Bridgeport-style" is a whole class of mills inspired by the Bridgeport original
Purpose	General-purpose versus special-purpose or single-purpose
Purpose	Toolroom machine versus production machine	Overlaps with above
Purpose	"Plain" versus "universal"	A distinction whose meaning evolved over decades as technology progressed, and overlaps with other purpose classifications above; more historical interest than current
Size	Micro, mini, benchtop, standing on floor, large, very large, gigantic
Power source	Line-shaft-drive versus individual electric motor drive	Most line-shaft-drive machines, ubiquitous circa 1880-1930, have been scrapped by now
	Hand-crank-power versus electric	Hand-cranked not used in industry but suitable for hobbyist micromills

A milling machine is a machine tool used for the shaping of metal and other solid materials. Its basic form is that of a rotating cutter which rotates about the spindle axis (similar to a drill), and a table to which the workpiece is affixed. In contrast to drilling, where the drill is moved exclusively along its axis, the milling operation involves movement of the rotating cutter sideways as well as 'in and out'. The cutter and workpiece move relative to each other, generating a toolpath along which material is removed. The movement is precisely controlled, usually with slides and leadscrews or analogous technology. Often the movement is achieved by moving the table while the cutter rotates in one place, but regardless of how the parts of the machine slide, the result that matters is the relative motion between cutter and workpiece. Milling machines may be manually operated, mechanically automated, or digitally automated via CNC (computer numerical control).

Milling machines can perform a vast number of operations, some of them with quite complex toolpaths, such as slot cutting, planing, drilling, diesinking, rebating, routing, etc.

Cutting fluid is often pumped to the cutting site to cool and lubricate the cut, and to sluice away the resulting swarf.

Temperature Considerations

At high temperature zones crater wear occurs. The highest temperature of the tool can exceed 700 °C and occurs at the rake face whereas the lowest temperature can be 500 °C or lower depending on the tool.ram

Energy Considerations

Energy comes in the form of heat from tool friction. It is a reasonable assumption that 80% of energy from cutting is carried away in the chip. If not for this the workpiece and the tool would be much hotter than what is experienced. The tool and the workpiece each carry approximately 10% of the energy. The percent of energy carried away in the chip increases as the speed of the cutting operation increases. This somewhat offsets the tool wear from increased cutting speeds. In fact, if not for the energy taken away in the chip increasing as cutting speed is increased; the tool would wear more quickly than is found.

Effects of Tool Wear

Some General effects of tool wear include:

• increased cutting forces
• increased cutting temperatures
• poor surface finish
• decreased accuracy of finished part

Reduction in tool wear can be accomplished by using lubricants and coolants while machining. These reduce friction and temperature, thus reducing the tool wear.

Tool Life Expectancy

The Taylor Equation for Tool Life Expectancy provides a good approximation.

V_cTⁿ = C

A more general form of the equation is

where

• V_c=cutting speed
• T=tool life
• D=depth of cut
• F=feed rate
• x and y are determined experimentally
• n and C are constants found by experimentation or published data; they are properties of tool material, workpiece and feed rate.

Tool wear describes the gradual failure of cutting tools due to regular operation. It is a term often associated with tipped tools, tool bits, or drill bits that are used with machine tools.

Types of wear include:

• flank wear in which the portion of the tool in contact with the finished part erodes. Can be described using the Tool Life Expectancy equation.
• crater wear in which contact with chips erodes the rake face. This is somewhat normal for tool wear, and does not seriously degrade the use of a tool until it becomes serious enough to cause a cutting edge failure.

Can be caused by spindle speed that is too low or a feed rate that is too high. In orthogonal cutting this typically occurs where the tool temperature is highest. Crater wear occurs approximately at a height equaling the cutting depth of the material. Crater wear depth ~ t₀ t₀= cutting depth

• built-up edge in which material being machined builds up on the cutting edge. Some materials (notably aluminum and copper) have a tendency to anneal themselves to the cutting edge of a tool. It occurs most frequently on softer metals, with a lower melting point. It can be prevented by increasing cutting speeds and using lubricant. When drilling it can be noticed as alternating dark and shiny rings.
• glazing occurs on grinding wheels, and occurs when the exposed abrasive becomes dulled. It is noticeable as a sheen while the wheel is in motion.
• edge wear, in drills, refers to wear to the outer edge of a drill bit around the cutting face caused by excessive cutting speed. It extends down the drill flutes, and requires a large volume of material to be removed from the drill bit before it can be corrected.

As every one knows, chips are formed during the machining of workpieces. The side of the chip in contact with the cutting tool is normally shiny, flat and smooth while the other side, which is the free workpiece surface, is jagged due to shear.

It is important to study the formation of chips during the machining process as the former affects the surface finish, cutting forces, temperature, tool life and dimensional tolerance. Understanding the chip formation during the machining process for the specific materials will allow us to determine the machining speeds, feed rates and depth of cuts for efficient machining and increased tool life in the specific actual machining operation.

During the machining process, three basic types of chips are formed. They are discontinuous chips, continuous chips, and continuous chips with built-up edge.

Discontinuous chip formation normally occurs during machining of brittle work material. This type of chips also occus in machining operation with small rake angles on cutting tools, coarse machining feeds, low cutting speeds. Discontinuous chip formation results in poor workpiece surface finish.

During continuous chip formation, a continuous "ribbon" of metal flows up the chip-tool zone. This is considered to be the ideal condition for efficient cutting action.

Continuous chip with built-up edge formation is basically the same process as continuous chip formation, except that as the metal flows up the chip-tool zone, small particles of the metal begin to adhere or weld themselves to the edge of the cutting tool. As the particles continue to weld to the tool, it affects the cutting action of the tool.

This type of chip formation is common in machining of softer non-ferrous metals and low carbon steels. Common problems are the built-up edges breaking off and being embedded in the workpiece during machining, decrease in tool-life and final poor surface finish of the workpiece.

Studies on the built-up edges have shown that the chip material is welded, deformed and then deposited onto the rake face of the tool layer by layer. It is thus possible to observe the presence of built-up edges by studying the back face of the chip during the machining process. This is normally used in micro or ultra precision machining operation.

To reduce built-up edges, improve the lubrication conditions, use sharp tools and better surface finish tool and also apply ultrasonic vibration during the machining process.

Author Ken Yap is a director of Suwa Precision Engineering in Singapore and represents niche manufacturers of metal stamping, swiss turning, pcb fabrication and gear hobbing from Suwa, Japan.