Turning

Turning

زهرة شوبينج اونلاين    April 16, 2009   

Turning: Engine Lathe

Turning is another of the basic machining processes. Information in this section is organized according to the subcategory links in the menu bar to the left.

Turning produces solids of revolution which can be tightly toleranced because of the specialized nature of the operation. Turning is performed on a machine called a lathe in which the tool is stationary and the part is rotated. The figure below illustrates an engine lathe. Lathes are designed solely for turning operations, so that precise control of the cutting results in tight tolerances. The work piece is mounted on the chuck, which rotates relative to the stationary tool.

Turning

Turning refers to cutting as shown below.

The term "facing" is used to describe removal of material from the flat end of a cylindrical part, as shown below. Facing is often used to improve the finish of surfaces that have been parted.

Turning: Engine Lathe Detail

Engine Lathe Carriage

The figure below illustrates the carriage of an engine lathe. The carriage allows cross-feed and diagonal movements in addition to axial movement.

Turning: Chucks

The chuck is integral to a lathe's functioning because it fixtures the part to the spindle axis of the machine. Below is shown a three-jaw chuck with jaws that are all driven by the same chuck key. This arrangement provides convenience in that parts can be mounted and dismounted quickly.

Turning: Engine Lathe Tool Post

Since the tool is stationary on a lathe, there is great flexibility for mounting the tool to best advantage. The tool post and carriage of a lathe provide several ways of positioning and feeding the tool. Below is shown the work area of an engine lathe.

The cutting tool is fixtured on the tool post, which sits atop the carriage assembly. The carriage can move the tool post along the axis of part rotation, perpendicular to the axial direction, and on a diagonal.

The tool post is shown below. The tool post can pivot the tool about a vertical axis and the cutting tool can be moved in and out along its long axis. The cutting tool is held in by the vertical screws, the heads of which can be seen above the cutting tool groove.

Engine Lathe Tail Stock

The tail stock of an engine lathe is used to provide a fixture at the end of the part opposite from the chuck. The tail stock can be used to support a long, thin part so that more radial cutting force can be applied and higher rotational speeds can be attained without a "whipping" instability effect. Below is illustrated another use for the tail stock. Drill bits can be fixtured in the tail stock to cut axial holes in a turned part. These central holes are more accurate than a drill press or mill could provide since the lathe is dedicated to operations in which an axially-symmetric part is rotated about its central axis. The fixturing is more accurate since all fixturing is based upon surfaces of revolution about the central axis, and the machining is more rigidly supported for the same reason.

Boring

Boring can be accomplished on a mill or even a drill press, but is most accurate on a lathe. The boring tool is fixtured in the tail stock. Again, since all fixturing is relative to the central spindle axis, boring on a lathe is more accurate than most other boring methods, an exception being jig boring on a dedicated jig boring machine. The length of the boring bar is of critical importance because of its tendency to bend. The figure below illustrates a boring tool which is double-ended so that it is less prone to the cantilever "diving board" effect.

For design guidelines for bored holes in parts, please check the design for boring section.

Below are illustrated some of the many types of machining that can be accomplished on a lathe.

Turning: Standard Tool Post Tool
The tool inserted in the tool holder is shown below:

CUTTING TOOLS FOR LATHES

Tool Geometry. For cutting tools, geometry depends mainly on the properties of the tool material and the work material. The standard terminology is shown in the following figure. For single point tools, the most important angles are the rake angles and the end and side relief angles.

The back rake angle affects the ability of the tool to shear the work material and form the chip. It can be positive or negative. Positive rake angles reduce the cutting forces resulting in smaller deflections of the workpiece, tool holder, and machine. If the back rake angle is too large, the strength of the tool is reduced as well as its capacity to conduct heat. In machining hard work materials, the back rake angle must be small, even negative for carbide and diamond tools. The higher the hardness, the smaller the back rake angle. For high-speed steels, back rake angle is normally chosen in the positive range. There are two basic requirements for thread cutting. An accurately shaped and properly mounted tool is needed because thread cutting is a form-cutting operation. The resulting thread profile is determined by the shape of the tool and its position relative to the workpiece. The second by requirement is that the tool must move longitudinally in a specific relationship to the rotation of the workpiece, because this determines the lead of the thread. This requirement is met through the use of the lead screw and the split unit, which provide positive motion of the carriage relative to the rotation of the spindle.

Tool Geometry. For cutting tools, geometry depends mainly on the properties of the tool material and the work material. The standard terminology is shown in the following figure. For single point tools, the most important angles are the rake angles and the end and side relief angles.

The back rake angle affects the ability of the tool to shear the work material and form the chip. It can be positive or negative. Positive rake angles reduce the cutting forces resulting in smaller deflections of the workpiece, tool holder, and machine. If the back rake angle is too large, the strength of the tool is reduced as well as its capacity to conduct heat. In machining hard work materials, the back rake angle must be small, even negative for carbide and diamond tools. The higher the hardness, the smaller the back rake angle. For high-speed steels, back rake angle is normally chosen in the positive range.

Most lathe operations are done with relatively simple, single-point cutting tools. On right-hand and left-hand turning and facing tools, the cutting takes place on the side of the tool; therefore the side rake angle is of primary importance and deep cuts can be made. On the round-nose turning tools, cutoff tools, finishing tools, and some threading tools, cutting takes place on or near the end of the tool, and the back rake is therefore of importance. Such tools are used with relatively light depths of cut. Because tool materials are expensive, it is desirable to use as little as possible. It is essential, at the same, that the cutting tool be supported in a strong, rigid manner to minimize deflection and possible vibration. Consequently, lathe tools are supported in various types of heavy, forged steel tool holders, as shown in the figure.

The tool bit should be clamped in the tool holder with minimum overhang. Otherwise, tool chatter and a poor surface finish may result. In the use of carbide, ceramic, or coated carbides for mass production work, throwaway inserts are used; these can be purchased in great variety of shapes, geometrics (nose radius, tool angle, and groove geometry), and sizes.

Single-Point Cutting Tool Variety

There are many types of cutting tools for different operations. Below is shown a few of the variety, here shown with a tool holder adapter that fits into a larger tool post fixture.

Below is shown how single-point lathe tools can be used.

Parting Tool

The illustration below shows how a parting tool is fixtured and used. Parting is important at the end of a turning process in order to separate the part from the raw material. Parting must be carried out slowly and carefully since the tool is quite long and is prone to chattering. Parting is not very accurate, and a finishing cut must often be undertaken after parting if the parted surface is to be accurate.

Knurling
Knurling is an operation used to produce a texture on a turned machine part. Handles are often knurled in order to provide a gripping surface. The two wheel inserts shown on the tool below contact the work piece, and with pressure, cold-form a pattern into the surface of the part.

Screw Machines

Screw machines are automated lathes which can quickly mass-produce turned parts. A screw machine uses cutting methods similar to that of a lathe but is highly automated. Screw machines are typically used for high-volume, low-cost turned parts. Feed stock for a screw machine is a long cylindrical rod of material. The screw machine automatically turns/faces the part, parts it off, and advances the rod for the next part. A screw machine is illustrated below.

Cross Slide Simultaneous Operation

Below is illustrated an on-axis view of how cross slides in a screw machine sequentially cut the work piece. Simultaneous action increases throughput. The view is down the axis of the spindle and shows how tools on cross slides can cut the work piece, some simultaneously with others.

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E-BUSINESS and E-commerce

E-BUSINESS and E-commerce

زهرة شوبينج اونلاين    April 15, 2009   

DEFINITION

E-commerce (electronic commerce or EC) is the buying and selling of goods and services on the Internet, especially the World Wide Web. In practice, this term and a newer term, e-business, are often used interchangably. For online retail selling, the term e-tailing is sometimes used.

Timeline

* 1990: Tim Berners-Lee writes the first web browser, WorldWideWeb, using a NeXT computer.
* 1992: J.H. Snider and Terra Ziporyn publish Future Shop: How New Technologies Will Change the Way We Shop and What We Buy. St. Martin's Press. ISBN 0312063598.
* 1994: Netscape releases the Navigator browser in October under the code name Mozilla. Pizza Hut offers pizza ordering on its Web page. The first online bank opens. Attempts to offer flower delivery and magazine subscriptions online. Adult materials also becomes commercially available, as do cars and bikes. Netscape 1.0 is introduced in late 1994 SSL encryption that made transactions secure.
* 1995: Jeff Bezos launches Amazon.com and the first commercial-free 24 hour, internet-only radio stations, Radio HK and NetRadio start broadcasting. Dell and Cisco begin to aggressively use Internet for commercial transactions. eBay is founded by computer programmer Pierre Omidyar as AuctionWeb.
* 1998: Electronic postal stamps can be purchased and downloaded for printing from the Web.
* 1999: Business.com sold for US $7.5 million to eCompanies, which was purchased in 1997 for US $149,000. The peer-to-peer filesharing software Napster launches.
* 2000: The dot-com bust.
* 2002: eBay acquires PayPal for $1.5 billion [2]. Niche retail companies CSN Stores and NetShops are founded with the concept of selling products through several targeted domains, rather than a central portal.
* 2003: Amazon.com posts first yearly profit.
* 2007: Business.com acquired by R.H. Donnelley for $345 million[3].
* 2008: US eCommerce and Online Retail sales projected to reach $204 billion, an increase of 17 percent over 2007

Business applications

Some common applications related to electronic commerce are the following:

* Email
* Enterprise content management
* Instant messaging
* Newsgroups
* Online shopping and order tracking
* Online banking
* Online office suites
* Domestic and international payment systems
* Shopping cart software
* Teleconferencing
* Electronic tickets

Market Research

In early 1999, it was widely recognized that because of the interactive nature of the Internet, companies could gather data about prospects and customers in unprecedented amounts -through site registration, questionnaires, and as part of taking orders. The issue of whether data was being collected with the knowledge and permission of market subjects had been raised. (Microsoft referred to its policy of data collection as "profiling" and a proposed standard has been developed that allows Internet users to decide who can have what personal information.)

Electronic Data Interchange (EDI)

EDI is the exchange of business data using an understood data format. It predates today's Internet. EDI involves data exchange among parties that know each other well and make arrangements for one-to-one (or point-to-point) connection, usually dial-up. EDI is expected to be replaced by one or more standard XML formats, such as ebXML.

E-Mail, Fax, and Internet Telephony

E-commerce is also conducted through the more limited electronic forms of communication called e-mail, facsimile or fax, and the emerging use of telephone calls over the Internet. Most of this is business-to-business, with some companies attempting to use e-mail and fax for unsolicited ads (usually viewed as online junk mail or spam) to consumers and other business prospects. An increasing number of business Web sites offer e-mail newsletters for subscribers. A new trend is opt-in e-mail in which Web users voluntarily sign up to receive e-mail, usually sponsored or containing ads, about product categories or other subjects they are interested in.

Business-to-Business Buying and Selling

Thousands of companies that sell products to other companies have discovered that the Web provides not only a 24-hour-a-day showcase for their products but a quick way to reach the right people in a company for more information.

The Security of Business Transactions

Security includes authenticating business transactors, controlling access to resources such as Web pages for registered or selected users, encrypting communications, and, in general, ensuring the privacy and effectiveness of transactions. Among the most widely-used security technologies is the Secure Sockets Layer (SSL), which is built into both of the leading Web browsers.















E-commerce MAN..











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Drilling

Drilling

زهرة شوبينج اونلاين    April 15, 2009   

Drilling is easily the most common machining process. One estimate is that 75% of all metal-cutting material removed comes from drilling operations.

Drilling involves the creation of holes that are right circular cylinders. This is accomplished most typically by using a twist drill, something most readers will have seen before. The figure below illustrates a cross section of a hole being cut by a common twist drill:

The chips must exit through the flutes to the outside of the tool. As can be seen in the figure, the cutting front is embedded within the workpiece, making cooling difficult. The cutting area can be flooded, coolant spray mist can be applied, or coolant can be delivered through the drill bit shaft.

Drilling Characteristics

The characteristics of drilling that set it apart from other powered metal cutting operations are:

• The chips must exit out of the hole created by the cutting.
• Chip exit can cause problems when chips are large and/or continuous.
• The drill can wander upon entrance and for deep holes.
• For deep holes in large workpieces, coolant may need to be delivered through the drill shaft to the cutting front.
• Of the powered metal cutting processes, drilling on a drill press is the most likely to be performed by someone who is not a machinist.

Drill Press Work Area

A view of the metal-cutting area of a drill press is shown below. The workpiece is held in place by a C-clamp since cutting forces can be quite large. It is dangerous to hold a workpiece by hand during drilling since cutting forces can unpredictably get quite large and wrench the part away. Wood is often used underneath the part so that the drill bit can overshoot without damaging the table. The table also has holes for drill overshoot as well as weight reduction. A three-jaw chuck is used since three points determine a circle in two dimensions. Four-jaw chucks are rarely seen since offset of the bit is not necessary.

























Twist Drill Bit

Definition:

Drill: Drill can be defined as a rotary end cutting tool having one or more cutting lips, and having one or more helical or straight flutes for the passage of chips and the admission of a cutting fluid.

The figure below labels the important angles for a typical twist drill bit.

Point Angle: THe angle included between the cutting lips projected upon a plane parallel to the drill axis and parallel to the two cutting lips
Relative Lip Height: The difference in indicator reading on the cutting lip of the drill; it is measured at a right angle to the cutting lip at a specific distance from the axis of the tool
Relief: The result of the removal of tool material behind or adjacent to the cutting lip and leading edge of the land to provide clearance and prevent rubbing (heel drag)
Shank: The part of the drill by which it is held and driven
Sleeve: A tapered shell designed to fit into a specified socket and to receive a taper shank smaller than the socket
Socket: The tapered hole in a spindle, adaptor, or sleeve, designed to receive, hold, and drive a tapered shank
Step Drill: A multiple diameter drill with one set of drill lands which are ground to different diameters
Straight Flutes: Flutes which form lands lying in an axial plane
Subland Drill: A type of multiple diameter drill which has independent sets of lands in the same body section for each diameter
Tang: The flattened end of a taper shank, intended to fit into a driving slot in a socket
Tang Drive: Two opposite parallel driving flats on the extreme end of a straight shank
Taper Drill: A drill with part or all of its cutting flute length ground with a specific taper to produce tapered holes; they are used for drilling the original hole or enlarging an existing hole
Taper Square Shank: A taper shank whose cross section is square
Web: The central portion of the body that joins the lands; the extreme end of the web forms the chisel edge on a two-flute drill
Web Thickness: The thickness of the web at the point, unless another specific locationis indicated
Web Thinning: The operation of reducing the web thickness at the point to reduce drilling thrust

Drill Bit Variety

The figures below illustrate various drill bits and their cut hole configurations.

Drill Chucks

Drill chucks can be of several types, but are typically three-jaw since three points on the circumference define a circle in two dimensions. A standard three-jaw and a multi-jaw chuck are shown in the figures below.

Turning: Chucks

The chuck is integral to a lathe's functioning because it fixtures the part to the spindle axis of the machine. Below is shown a three-jaw chuck with jaws that are all driven by the same chuck key. This arrangement provides convenience in that parts can be mounted and dismounted quickly.

Three-Jaw Chuck

The inner construction of the three-jaw chuck is shown below. A spiral gear meshes with cog teeth on the jaws to move all three jaws in or out simultaneously. Parts can be fixtured on outer or inner surfaces since there are gripping surfaces on the inner and outer surfaces of the chuck jaws.

Four-Jaw Chuck

If the part needs to be off center or is not a solid of revolution (axially symmetric), a four-jaw chuck with independently-actuated jaws needs to be used. Such a chuck is depicted below.

Drill Press Detail


A typical manual drill press is shown in the figure below. Compared to other powered metal cutting tools, a drill press is fairly simple, but it has evolved into a versatile necessity for every machine shop.

The most adjustable part of the drill press is the vertical movement of the drill bit, since this is the motion that is used in production.

• The capstan wheel (A) moves the drill head up and down.

• This movement can be locked (D)
• and there are point-to-point stops (B) for maintaining a specific length of travel.
• Gradations marked on the stationary part of the drill press (C) let the operator know where he is vertically.
• Both the drilling head and table can move vertically and rotate about the vertical guide post (b).
• The base of the drill press incorporates a work surface similar to the table's for oversize workpieces. The base can be bolted down, but often is not since forces on a drill press do not typically cause it to tip over.
  The drill is powered by an electric motor (I)

Drill Press Work Area Detail

A detail of the work area of the drill press is shown below.

Drilling can be accomplished more accurately on alath or mill , although drill presses are much cheaper and more accessible machines.

Jig Boring

Jig boring is used to accurately enlarge existing holes and make their diameters highly accurate. Jig boring is used for holes that need to have diameter and total runout controlled to a high degree. Typically, a part has holes machined on regular equipment and then the part is transferred to a dedicated jig boring machine for final operations on the especially accurate holes. Jig boring can also maintain high accuracy between multiple holes or holes and surfaces. Tolerances can be held readily within ±.005 mm (±0.0002 inches). Dedicated jig boring machines are designed to machine holes with the tightest tolerances possible with a machine tool.

When designing a part with holes, it is important to determine what holes must be jig bored. The reason for this is that jig boring requires extra time and attention, and the jig boring machine at the machine shop may have a back log of jobs. Jig boring can therefore have a big impact on the lead time of a part. A cross section of a hole being jig bored is shown below.

Standard boring can be carried out on a mill fitted with a boring head or on a lathe. Boring is most accurate on a lathe since a lathe is dedicated to solids of revolution (axially symmetric parts).

Gun Drilling


For long holes such as those found in gun bores, gun drills are used. The length of the hole requires that coolant be delivered through the shaft of the gun drill to the cutting front. The coolant also serves to eject chips from the cutting area and to move them back and out of the hole entrance. The figures below illustrate a gun drill and the cutting/cooling configuration.

Computer Numerical Control (CNC) Drilling

Computer Numerical Control (CNC) Drilling is commonly implemented for mass production. The drilling machine, however, is often a multi-function machining center that also mills and sometimes turns. The largest time sink for CNC drilling is with tool changes, so for speed, variation of hole diameters should be minimized. The fastest machines for drilling varying hole sizes have multiple spindles in turrets with drills of varying diameters already mounted for drilling. The appropriate drill is brought into position through movement of the turret, so that bits do not need to be removed and replaced. A turret-type CNC drilling machine is shown below.

A variety of semi-automated drilling machines are also used. An example is a simple drill press which, on command, drills a hole of a set depth into a part set up beneath it.

In order to be cost-effective, the appropriate type of CNC drilling machine needs to be applied to a particular part geometry. For low-volume jobs, manual or semi-automated drilling may suffice. For hole patterns with large differences in sizes and high volume, a geared head is most appropriate. If holes are close to each other and high throughput is desired, a gearless head can locate spindles close together so that the hole pattern can be completed in one pass. For further reference for CNC processes, please refer to the

The Computer Numerical Control (CNC) fabrication process offers flexible manufacturing runs without high capital expenditure dies and stamping presses. High volumes are not required to justify the use of this equipment.

Tooling is mounted on a turret which can be as little as 10 sets to as much as 100 sets. This turret is mounted on the upper part of the press, which can range in capacity from 10 tons to 100 tons in capacity.

The turret travels on lead screws, which travel in the X and Y direction and are computer controlled. Alternatively, the workpiece can travel on the lead screws, and move relative to the fixed turret. The tooling is located over the sheet metal, the punch is activated, and performs the operation, and the turret is indexed to the next location of the workpiece. After the first stage of tooling is deployed over the entire workpiece, the second stage is rotated into place and the whole process is repeated. This entire process is repeated until all the tooling positions of the turret are deployed.

Drilled Part Design

The following are guidelines for drilled part design.

1. Advantages of drilled holes include accuracy and sharpness of edges. Since machining is expensive compared to other manufacturing processes, drilling to create a hole should be justified by looking at alternatives. Before adding drilled holes to a design, ask yourself whether the hole is needed and/or whether it can be cast, molded, or pierced with sufficient accuracy instead of drilled.
   
   
2. Specify standard drill bit sizes. Unusual hole sizes bring up the cost of manufacturing through purchasing and inventory costs.
   
   
3. Through holes are preferred over blind holes. This has to do with the fact that a blind hole does not provide as much leeway for chip exit and cooling. Operations such as reaming and threading after drilling are more easily conducted on a through hole.
   
   
4. Do not specify flat-bottomed holes. Twist drills create cone-bottomed holes and flat-bottom holes cause problems with reaming, etc.
   
   
5. If possible, do not specify holes that are smaller than one-eighth inches in diameter. Drills for smaller holes tend to break and for convenient mass production, are not recommended.
   
   
6. For large holes, try to cast in a preliminary hole that must only be bored out to specification. This saves material, transportation cost, and drilling cost.
   
7. When dimensioning holes, it is better to use rectangular rather than angular (or polar) coordinates. Angular coordinates will require the machinist to set up a dividing head or to re-dimension the part, both of which take time.
   
   
8. Minimize the number of drilled hole sizes so that tool changes are minimized.
   
   
9. Minimize the number of directions on the part that holes must be drilled from.
   
   
10. The entrance and exit surfaces of a drilled hole should be perpendicular to the hole axis. The reasons for this are as follows:
    
    
    1. Upon entrance of the drill, the drill tip will wander if the surface that the tip contacts is not perpendicar to the drill axis.
    2. Exit burrs will be uneven around the circumference of the exit hole. This can make burr removal difficult.
    
    
    Bad and good examples of entrance and exit lands are shown in the figure below.
    
    
    
    
11. Intersections of drilled holes with other cavities should be avoided if at all possible. If interesection with a cavity is unavoidable, the drill axis should at least be outside of the cavity, as shown below.
    
    
    
12. On drawings, multiple holes in a flat surface should be located from the same horizontal and vertical datums.
    
    
13. If there are protrusions surrounding a drilled hole, it may be difficult to bring the drill press head close to the entrance surface, resulting in a drill bit that is prone to wandering, chatter, and other instabilities. This problem can be solved by providing a fixture with a drill bushing close to the drill bit. However, part design must allow for this fixture, as shown below.
    
    
    
14. Drilled Hole Depth:
    
    Deep, narrow holes with length to diameter ratios of larger than three should be avoided. Deeper holes are possible but the drill will tend to wander and possibly break. One way to avoid a deep, narrow hole is to use a stepped entrance. Blind holes should be drilled to a depth 25% deeper than the actual hole in order to provide space for chips.
    
    

Reaming: Summary

Reaming is a process which slightly enlarges a pre-existing hole to a tightly toleranced diameter. A reamer is similar to a mill bit in that it has several cutting edges arranged around a central shaft, as shown below. Because of the delicate nature of the operation and since little material is removed, reaming can be done by hand. Reaming is most accurate for axially symmetric parts produced and reamed on a lathe.

Detailed Nomenclature for a Reamer

A more complete listing of reamer nomenclature is provided below.

Reamed Part Design

Reamed holes should not intersect with drilled holes, so the configuration below should NOT be implemented:

As with a drilled hole, clearance for chips is needed at the bottom of a reamed hole. This is illustrated below:

Reaming should not be relied upon to correct the location or alignment of a hole. Its primary purpose is to fine-tune the diameter of the hole.

Honing

Honing is a final finishing operation conducted on a surface, typically of an inside cylinder, such as of an automotive engine block. Abrasive stones are used to remove minute amounts of material in order to tighten the tolerance on cylindricity. Honing is a surface finish operation, not a gross geometry-modifying operation. Hones can be of the multiple pedal type (pictured below) or the brush type. Either type applies a slight, uniform pressure to a light abrasive that wipes over the entire surface.

The figure below illustrates the configuration of the abrasive stones of an external hone.

Honed Part Design

Below are illustrated bad and good honed part geometries. Both pairs of figures show that a through hole design is always best for coolant flow, etc. The upper pair shows that a certain portion at the end of a blind hole is not completely honed. The lower pair shows that a right circular cylinder is the easiest to hone.

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