Metal Cutting

Metal cutting applications span the entire range from mass production to mass customization to high-precision, fully customized designs. The careful balance between precision and efficiency is maintained only through intimate knowledge of the physical processes, material characteristics, and technological capabilities of the equipment and workpieces involved. The  Metal Cutting Theory and Practice provides knowledge, integrating timely research with current industry practice. This brilliant reference enters its later concepts with fully updated coverage, new sections, and the inclusion of examples and problems.

Supplying complete, up-to-date information on machine tools, tooling, and work holding technologies, it stresses a physical understanding of machining processes including forces, temperatures, and surface finish. This provides a practical basis for troubleshooting and evaluating vendor claims. In addition to updates three new areas on cutting fluids, agile and high-throughput machining, and design for machining. Rounding out the treatment, an entire thing is devoted to machining economics and optimization.

Endowing you with practical knowledge and a fundamental understanding of underlying physical concepts, Metal Cutting Theory and Practice is a necessity for designing, evaluating, purchasing, and using machine tools.

Techniques To Combat Chatter

Following are some of the techniques commonly used to combat chatter. Use these guidelines to establish a good foundation for optimizing your moldmaking processes. 

1.The Right Tool holder: 

 Common tool holders (side-lock, double-angle collets and standard ER collets) do not provide the accuracy or stiffness needed for high-performance machining. Better options are tool holder shanks that incorporate face and taper contact to deliver high accuracy and rigidity. This type of holder engages the precision ground face of the spindle with simultaneous contact with the taper, which provides the additional rigidity required, and also aids in damping. All tooling should be evaluated for balancing, which provides improvements in surface finish even at lower RPMs. 

2.Cutter Tooling Selection: 

Cutter tooling can greatly influence chatter. Considerations include correct substrate, geometry, coating and length-diameter ratio. Programmers often gravitate to using the largest tool that can fit, but that may not be the ideal tool size. Incorporating multiple tools with variable flute geometries is an effective way to reduce vibrations.

3.Proper Work holding:

 If the part is not properly secured, the part itself can vibrate and induce chatter. There are many excellent systems available to clamp your workpieces. Criteria to look for include high precision, high clamping force, ease of use and flexibility (allowing use across multiple CNC machine tool platforms).

4.Machine Maintenance:

If you’re trying to hold fine finishes and tight tolerances on a poorly maintained machine, you’ll need to overcome mechanical challenges well beyond the issues listed here. Keep your equipment on a regular maintenance schedule to ensure the best performance.

5.Control Solution: 

The above can correct some causes of chatter, but there are limitations to these methods. The use of new control technology—smart control systems—that navigates processes and eliminates potentials for costly surface finish problems is another method. One such technology1 is designed to eliminate chatter and take the guesswork out of the trial-and-error process typically used to find the correct spindle speed, allowing the cutting tool and machine tool to continuously operate at the highest performance. It uses a single processor intelligent numerical control2 and vibration sensors to monitor chatter noise and automatically adjust spindle speed. No longer does an operator need to babysit a cut. With this technology in place, your mold shop can be more profitable and gain a competitive advantage.

Optimization Of Machining Techniques

First of all, machining models are required to determine the optimum machining parameters including cutting speed, feed rate and depth of cut, in order to minimize unit production cost. Unit production cost can be divided into four basic cost elements:

• Cutting cost by actual cut in time
• Machine idle cost due to loading and unloading operation and idling tool motion cost
• Tool replacement cost
• Tool cost

For the optimization of unit production cost, practical constraints which present the state of machining processes need to be considered. The constraints imposed during machining operations are:

 
• Parameter constraint – Ranges of cutting speed, feed rate and depth of cut.
• Tool life constraint – Allowable values of flank wear width and crater wear depth.
• Operating constraint – Maximum allowable cutting force, power available on machine tool and surface finish requirement.

An optimization model for multi-pass turning operation can be formulated. The multipass turning model is a constrained nonlinear programming problem with multiple variables(machining variables). The initial solution for SS is picked in a random way. The user-specified parameters have to be given. The experimentation can be run on a PC with Pentium800Mhz processor. The computational results validate the advantage of SS in terms of solution quality and computational requirement.

Future as a machinist looks bright

Everything you see that is not natural is manufactured and we need machinists to manufacture,Machinists use machine tools that are either conventionally controlled or by computer numerical controls (CNC), such as lathes, milling machines and grinders, to produce precision metal parts.The parts range from simple bolts of steel or brass and titanium bone screws for orthopedic implants to hydraulic parts, anti-lock brakes and automobile pistons.

Opportunities are vast for graduate. It's a high demand area and companies want people who are trained in the technology but still have the basics of the manual work and a foundation of safety.Machinists typically work from blueprints, sketches or computer aided design (CAD) or computer aided manufacturing (CAM) files; set up, operate and tear down CNC machine tools; install, align, secure, and adjust cutting tools and work pieces; monitor the feed and speed of machines; turn, mill, drill, shape and grind machine parts to specifications; and examine and test completed products for defects and ensure all products conform to specifications.Because the technology of machining is changing rapidly, machinists must learn to operate a wide range of machines, and as engineers create new types of machine tools, machinists must learn new machining properties and techniques.As jobs come back to the U.S.,we need to train a well-prepared workforce for the industry and that's where junior college comes into play.

There are many ways to become a skilled machinist, but as the trade evolves, so does the training required.Courses result in several levels of qualification, from a certificate to an associate of applied sciences degree and can take from two semesters to two years to complete, depending on full- or part-time commitment and level of education desired.There is tremendous opportunity and the greater the education, the more opportunities that will be available to graduates.And varied experience the will allow them to move up. Even after completing a formal training program, machinists still need years of experience to become highly skilled.But it is getting hard to get on-the-job-training because of production schedules - there is little or no down time in order to train.

Thinks to Remember while designing Jigs and Fixtures

Important considerations while designing Jigs and Fixtures are based on certain parameters and design factors. These factors are analysed to get design inputs for jigs and fixtures. The list of such factors is mentioned below :

1.Study of workpiece and finished component size and geometry.

2.Type and capacity of the machine, its extent of automation.

3.Provision of locating devices in the machine.

4.Available clamping arrangements in the machine.

5.Available indexing devices, their accuracy.

6.Evaluation of variability in the performance results of the machine.

7.Rigidity and of the machine tool under consideration.

8.Study of ejecting devices, safety devices, etc.

9.Required level of the accuracy in the work and quality to be produced.

Purpose of Jigs and Fixtures

Following are the purpose and advantages of jigs and fixtures,

• It reduces or sometimes eliminates the efforts of marking, measuring and setting of workpiece on a machine and maintains the accuracy of performance.
• The workpiece and tool are relatively located at their exact positions before the operation automatically within negligible time. So it reduces product cycle time.
• Variability of dimension in mass production is very low so manufacturing processes supported by use of jigs and fixtures maintain a consistent quality.
• Due to low variability in dimension assembly operation becomes easy, low rejection due to les defective production is observed.
• It reduces the production cycle time so increases production capacity. Simultaneously working by more than one tool on the same workpiece is possible.
• The operating conditions like speed, feed rate and depth of cut can be set to higher values due to rigidity of clamping of workpiece by jigs and fixtures.
• Operators working becomes comfortable as his efforts in setting the workpiece can be eliminated.
•  Semi-skilled operators can be assigned the work so it saves the cost of manpower also.
•  There is no need to examine the quality of produce provided that quality of employed jigs and fixtures is ensured.

Jigs and Fixtures Design

Although many people have their own definitions for a jig or fixture, there is one universal distinction between the two. Both jigs and fixtures hold, support, and locate the workpiece. A jig, however, guides the cutting tool. A fixture references the cutting tool. The differentiation between these types of work holders is in their relation to the cutting tool.

Jigs

The most-common jigs are drill and boring jigs. These tools are fundamentally the same. The difference lies in the size, type, and placement of the drill bushings. Boring jigs usually have larger bushings. These bushings may also have internal oil grooves to keep the boring bar lubricated. Often, boring jigs use more than one bushing to support the boring bar throughout the machining cycle.

In the shop, drill jigs are the most-widely used form of jig. Drill jigs are used for drilling, tapping, reaming, chamfering, counterboring, countersinking, and similar operations. Occasionally, drill jigs are used to perform assembly work also. In these situations, the bushings guide pins, dowels, or other assembly elements.

Jigs are further identified by their basic construction. The two common forms of jigs are open and closed. Open jigs carry out operations on only one, or sometimes two, sides of a workpiece. Closed jigs, on the other hand, operate on two or more sides. The most-common open jigs are template jigs, plate jigs, table jigs, sandwich jigs, and angle plate jigs. Typical examples of closed jigs include box jigs, channel jigs, and leaf jigs. Other forms of jigs rely more on the application of the tool than on their construction for their identity. These include indexing jigs, trunnion jigs, and multi-station jigs.

Specialized industry applications have led to the development of specialized drill jigs. For example, the need to drill precisely located rivet holes in aircraft fuselages and wings led to the design of large jigs, with bushings and liners installed, contoured to the surface of the aircraft. A portable air-feed drill with a bushing attached to its nose is inserted through the liner in the jig and drilling is accomplished in each location.

Fixtures

Fixtures have a much-wider scope of application than jigs. These work holders are designed for applications where the cutting tools cannot be guided as easily as a drill. With fixtures, an edge finder, center finder or blocks position the cutter. Examples of the more-common fixtures include milling fixtures, lathe fixtures, sawing fixtures, and grinding fixtures. Moreover, a fixture can be used in almost any operation that requires a precise relationship in the position of a tool to a workpiece.

Fixtures are most often identified by the machine tool where they are used. Examples include mill fixtures or lathe fixtures. But the function of the fixture can also identify a fixture type. So can the basic construction of the tool. Thus, although a tool can be called simply a mill fixture, it could also be further defined as a straddle-milling, plate-type mill fixture. Moreover, a lathe fixture could also be defined as a radius-turning, angle-plate lathe fixture. The tool designer usually decides the specific identification of these tools.