In peripheral (or slab) milling, the milled surface is generated by teeth located on the periphery of the cutter body. The axis of cutter rotation is generally in a plane parallel to the workpiece surface to be machined.
In face milling, the cutter is mounted on a spindle having an axis of rotation perpendicular to the workpiece surface. The milled surface results from the action of cutting edges located on the periphery and face of the cutter.
The cutter in end milling generally rotates on an axis vertical to the workpiece. It can be tilted to machine tapered surfaces. Cutting teeth are located on both the end face of the cutter and the periphery of the cutter body.
METHODS OF MILLING
Down milling is also referred to as climb milling. The direction of cutter rotation is same as the feed motion. For example, if the cutter rotates counterclockwise , the workpiece is fed to the right in down milling.
Other milling operations are shown in the figure.
A shaping machine is used to machine surfaces. It can cut curves, angles and many other shapes. It is a popular machine in a workshop because its movement is very simple although it can produce a variety of work. Shaping machines come in a range of sizes but the most common size is seen opposite. | |||||||||||||||||||||||||||||||||
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| The main parts are indicated below: The tool feed handle can be turned to slowly feed the cutting tool into the material as the 'ram' moves forwards and backwards. The strong machine vice holds the material securely. A small vice would not be suitable as the work could quite easily be pulled out of position and be damaged. The vice rests on a steel table which can be adjusted so that it ca be moved up and down and then locked in position. Pulling back on the clutch handle starts the 'ram' moving forwards and backwards. | |||||||||||||||||||||||||||||||||
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EXAMPLE - QUICK RETURN CRANK MECHANISM The shaping machine is used to machine flat metal surfaces especially where a large amount of metal has to be removed. Other machines such as milling machines are much more expensive and are more suited to removing smaller amounts of metal, very accurately. The reciprocating motion of the mechanism inside the shaping machine can be seen in the diagram. As the disc rotates the top of the machine moves forwards and backwards, pushing a cutting tool. The cutting tool removes the metal from work which is carefully bolted down. prob: Draw a diagram of the shaping machine and explain how the quick return mechanism works?
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The pure analytical approches , in general, came out the average temperature on the shear plane and at the tool/chip interface. The temperature distridution along the shear plane and the tool/chip interface was also obtained some of the following approaches:
Here is the idealized diagram of shear plane moving heat source:
The temperature distribution along the shear plane was assumed as the same as that along a uniform band source of heat moving obliquely through an infinite solid. Similarly, the temperature distribution or average temperature at the chip/tool interface also can be approximatedly obtained.
To avoid giving rise to two different temperature on either side of a same plane due to partition principle, Chao and Trigger approximated the uniform plane heat source at the interface by a grid of point sources. Assuming that the angle between the rake and flank faces was right and that the tool surfaces were insulated, the grid of real and fictious image point sources is shown as follows:
Then they used an iterative procedure to obtain the final interfacial temperature distribution and later extended their work to include the frictional heat source at the tool/work interface.
Kronenberg postulated that the temperature at the tool/chip interface was dependent on the following important variables: chip area, cutting speed, a specific cutting force, a thermal conductivity, and the product of density and specific heat. Applying the principle of dimensional analysis, two dimensionless groups of variables were derived. Then the experimental results were fitted in the following:
Generally, only the average temperature at the tool/chip interface can be obtained. Sometimes, a tool-work thermocouple was also used.
The calibration of the tool-chip thermocouple is shown as:
Some limitations of this methods are mainly:
The following is a typical embedded thermocouple setup:
The first use of this technique was reported by Schwerd who developed a total Radiation Pyrometer for determining the temperature distribution at the surfaces of tool and workpiece.
Several researchers attempted to access to chip/tool interface by scanning through holes drilled either in the work or tool. Prins' pyrometer arrangement is shown as follows:
Limitation of the above measurements is that there is considerable interference of the contact zone which must influence the heat flow and the resulting temperature of either the chip or tool surface.
Infrared Photography technique proposed by Boothroyd can obtain a full temperature field in the chip and workpiece. A calibration strip, which was electrocally heated from one end,was simply used to calculate the real temperature. The following setup was used at that time:
Today, many disavantages of this technique are overcomed.
Under suitable cutting conditions, the metallographic method can be used to determining the temperature gradients in high speed steel cutting tools. The following picture is the etched rake face of tool used to cut nickel in normal dry atmosphere at 46 m/min, 0.25 mm/rev feed for 30 seconds(After E. F. Smart and E. M. Trent):
Using the thermosensitive paints for estimating cutting temperature is because that these paints change colors at different temperature.
Limitations are that there is a relatively long time lag for the colors to change and that small changes in temperature are not easy to be detected.
This technique is based on the fact that the temper colors of some metals are different at different temperatures. And it wasn't widely used.
The numerical methods were successfully applied in calculating the temperature distribution and thermal deformation in tool, chip and workpiece. Especially,the finite element and boundary element methods can deal with very complicated geometry in machining, they have great potential to slove the problems in practice. These methods are listed in the following:
Generally, the pure theoretical methods only gave us very approximate results, such as average temperature and temperature distribution along the shear plane and the tool/chip interface.
Finite difference method can be used to calculate the temperature distributions in the chip, tool and workpiece. And better results can be expected because the geometry and boundary conditions of chip, tool and workpiece, as well as the shape of distributed heat sources can be descripted well.
Finite element method(FEM) has great potential to calculate the temperature distributions in the chip, tool and workpiece if the geometry, boundary conditions and the shape of distributed heat sources become very complicate.The following diagram is the mesh used by O. A. Tay:
In addition, FEM can be used to calculate the temerature distribution in either toolholder or machined parts and then to obtain the thermal deformation.
Finally, because an accurate distributed heat source model is needed in order to obtain a better result in the temperature distribution. FEM can be naturally coupled with some mechanics model, therefore, predicts the intensity and distribution of the heat sources.
Boundary Element Method(BEM) was used in calculating the temperature distribution in the tool by O. A. Tay. BEM has great potential in reducing solid modeling to surface modeling. A wide application in this field is undoubtable.
In this class of methods, some information such as chip surface temperature or temperature distribution in workpiece is first obtained experimentally. Then the temperature distribution and/or thermal deformation in chip, and sometimes in the tool and workpiece as well are calculated analytically. The inverse heat transfer problem in machining is an example of these methods.
Almost all of the heat generation model were established under orthogonal cutting condition. But in practice, there are various machining operations which cannot satisfy this condition, such as oblique turnning, boring, drilling, milling, grinding, etc.
Generally, the intensity of heat sources in real machining operations can be determined approximatedly by the external work applied, however, the distribution of the heat sources are hard to obtained by either theoretical or experimental methods.
The following listed are the simplified heat source model in real operations:
There are several types of heat source in machining:
Heat generated in this zone is mainly due to plastic deformation and viscous dissipation. But in classical machining theory, the rate of heat generated is the product of the shear plane component, Fs, of the resultant force and the shear velocity, Vs, i.e., the shear energy is completedly converted into heat.
If heat source is uniformly distributed along the shear plane, the intesity of shear plane heat source, Ip, satisfies the following relation:
where b is the cutting width and t1 the uncut depth.
In this region, because of the complexity of plastic deformation, this part of heat was ignored in many prevoius theoretical research.
Boothroyd has shown that the secondary plastic zone is roughly triangular in shape and that strain rate, E., in this region varies linearly from an approximatedly constant value along the tool/chip interface given by
Where Vc is the chip velocity, dt the maximum thickness of the zone.
Hence the maximum intensity of heat source in this zone is proportional to the strain rate.
Heat is generated at the tool/chip interface by friction. The intensity,Ic, of the frictional heat source is approximatedly by
where F is the friction force, Vx the sliding velocity of the chip along the interface, and h is the plastic contact length.
Heat generation is not well investigated in the following areas:
The three types of heat transfer, conduction, convection and radiation, all exist in the machining operations.
Heat transfer inside the chip and workpiece, the tool and toolholder is by conduction.
Heat transfer between coolant/air and the chip/tool/workpiece is by convection.
Radiation is rarely investigated in traditonal machining operations. But radiation techniques are widely applied in measuring the temperature distribution in various machining operations.
For more plots of temperature distrbutions, please click here.
Cutting fluids' effects on heat transfer are, in gerneral, classified as:
In practice, there are other types of heat source involved in machining, such as ambient heat sources. They may cause some thermal deformation in the lathe and so on.
Heat influence on the cutting forces is mainly because that:
Variations of tool life with workpiece bulk temperature when milling Cr-Ni-Mo steel at speeds of (1) 150 fpm and (2) 200 fpm. (After krabacher and Merchant 1951)
Heat gives rise to thermal deformatiom in the workpiece, which finally takes on the form of surface toughness.
Thermal deformation in the lathe is the so-called thermal error in precision machining.
Interesting? please take a Health issue in Enviromentally Conscious Machining.
Predictive heat generation models in either orthogonal cutting or other various operations
A Heat Transfer Performance Module, which can predict the convective heat transfer coeffients of several kinds of coolants used in some typical machining operations, can be accessible.
A energy and mass flow model of cutting fluid circulation system is a very important issue in environmentally conscious machining. Sometimes, the disposal of chips and coolants needs much more energy than that in real cutting operations. Developing an effective way to utilize energy should be under consideration.
Other than research issues mentioned above, there are still some areas listed here:
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.
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.
| Cross Slide Simultaneous Operation |
Introduction: There are many different types of machining operations, each of which can produce a different part geometry and surface te…
