Читать книгу Exploring Advanced Manufacturing Technologies - Steve Krar - Страница 18
Оглавление(Dirk Smits, President – Bethel Technologies, Inc.)
Grinding is a metal-removal process that uses an abrasive cutting tool to finish a part to an accurate size and produce a high surface finish. The most common abrasive tool used is a grinding wheel that consists of many thousands of abrasive grain bonded together. In a grinding process, a revolving grinding wheel is brought into contact with the surface of the part to be ground. As each abrasive grain on the periphery of the wheel contacts the part surface, it acts as a cutting tool and removes a minute (very small) chip of metal, Fig. 2-3-1.
Cylindrical grinding may be defined as grinding the periphery of a rigidly supported, revolving part. Cylindrical grinders fall into three general classes: plain cylindrical, universal cylindrical, and special cylindrical grinders. The centerless grinder, one of the special grinders, makes it possible to grind cylindrical parts without supporting the part between centers or holding it in some from of fixture, Fig. 2-3-2. Centerless grinders are precision machine tools capable of mass-producing countless numbers of parts held to close tolerances of size, shape, and surface finish. The modern grinding machine is capable of finishing soft or hardened parts to tolerances of .0002 in. (0.005 mm) or less on high-production machines, while producing very fine surface finishes.
The goal of every manufacturing operation is to produce quality products as quickly and accurately as possible. To accomplish this goal, it is important that every component in the manufacturing process be in top condition so that inaccurate parts are not produced. Inaccuracies in manufacturing result in parts that may have to be repaired, replaced, or scrapped, which affects the productivity and profitability of any operation.
Virtual Reality and certain software programs can be used to simulate a manufacturing operation on a computer before starting to actually manufacture a product. This allows any potential manufacturing errors or operational sequences to be corrected before spending time, material, and labor on a process that may not produce satisfactory results.
Fig. 2-3-1 The cutting action of abrasive grains in a grinding wheel. (Carborundum Abrasives, Div. Saint-Gobain Abrasives.)
CYLINDRICAL GRINDING SIMULATOR
The Grinding Simulator is a software package that can predict production rates for the cylindrical part to be ground in a mass production grinding operation. The calculations are based on macroscopic grinding principles and not based on microscopic principles. A macroscopic method is an averaging effect of combining data from many grinding operations. The advantage of this method is that it can be used to predict productivity. A microscopic approach is to calculate certain grinding parameters from an abrasive cutting into the metal. The disadvantage of a microscopic approach is that it is unable to predict productivity since each abrasive grain in the wheel is different and requires a different calculation.
Before testing is done on any machine tool, the machine spindle and slides should be checked to see that they are in good condition, otherwise the test results would be not be accurate.
Fig. 2-3-2 Parts pass between the grinding and regulating wheels during Thrufeed grinding. (Cincinnati Machine, A UNOVA Co.)
GRINDING PARAMETERS DEFINITIONS
Before writing any code, the grinding parameters must be defined. The difficulty with grinding is that every abrasive grain has its own geometry.
Specific Metal-Removal Rate
The parameter that combines all grinding operations, no matter what size and length, is the Specific Metal-Removal Rate. The Specific Metal-Removal Rate, defined as Q prime, is
| (1) |
The unit for this parameter is in.3/min/in. This equation can be simplified to
| (2) |
In center-type infeed grinding, the effective wheel width is equal to the part length, and the stock divided by time equals the infeed rate. This results in
| (3) |
The importance of Q′ is that one has a parameter that can compare with different operations, of which the part geometry and wheel width is different. As can be seen from Equation 3, if the part diameter is changed by a factor of 2 and the infeed rate is the same, It is possible to grind twice as aggressively.
Surface Finish Calculations
Knowing the definition of Q′ and having a method of measuring its value, it is necessary to obtain a relationship between the specific metal-removal rate and the surface finish (Ra). Whenever data is taken, be sure that equilibrium has been reached before making any analytical conclusions. The relationship between Q’ and surface finish (f) is logarithmic.
| fQ-n3 = C3 | (4) |
where C3 and n3 are depending on the conductivity coefficient of the material, material hardness, and fluid type. For the influence of metal-working fluids on the grinding process, see Reference 1. If requested surface finish is known, it is possible to calculate the Q′ from which the infeed rate or thrufeed rate in the finishing operation are calculated. When the type of material or its hardness is changed, and the infeed rate is left the same, the surface finish will change according to Equation 4.
Specific Energy Relationship
Another relationship that needs to be found is that of power. Whether grinding with a two-inch wide wheel or a one-inch wide wheel makes a big difference in thrufeed grinding. To have a parameter that is related to Q′ and to power, the Specific Energy is defined as:
| (5) |
There is a logarithmic relationship that is obtained when the Specific Energy versus Q’ is plotted:
| Qn2U = C2 | (6) |
where n2 and C2 are depending on material hardness, material type and on metal-working fluid. For the influence of metalworking fluids on the grinding process see Reference 1.
Knowing the required surface finish, it is possible to obtain the Q’ from which the infeed rate and the power required is calculated. If the required power is larger than the available power of the machine, then the Q’ must be lowered which can result in a lower surface finish.
Static Stiffness and Tolerance
One important relationship that has not yet been discussed is related to tolerances and static stiffness. It is generally known that the tighter the tolerances, the stiffer the grinding process needs to be. Requirement on tight tolerance can require a change in the grinding process such as higher wheel speed or machine rebuild, or it can require the purchase of a new machine. There are two static stiffness that are important: part stiffness and machine stiffness. If a part is weak, no matter how stiff the machine might be, the weakest link is the part and the part stiffness then determines the overall system stiffness. The relationship between static stiffness and tolerance is:
| (7) |
where µ is grinding force ratio that is dependent on material hardness and material type. The 33000 is a factor because of the British unit system. Knowing the power required for a requested surface finish, and knowing the tolerance and the wheel speed, it can be calculated from Equation 7 what static stiffness is required. If the system static stiffness is too low, then it can be calculated backwards from Equation 7 to determine the power. From the power, the Q′ is calculated, which gives the actual surface finish that will be obtained and the infeed rate needed to stay within tolerances.
Static Stiffness Test
To determine the system static stiffness, both the machine static stiffness and the part stiffness need to be known. To measure part stiffness, a load must be put on the part and then the part deflection must be measured. The static stiffness built into the machine at the time of manufacture deteriorates over time. The Grinding Software contains instructions on how to measure the static stiffness of the machine.
Machine Static Stiffness Test
Always take a part that is solid, rigid, and has a very high stiffness -3,000,000. lbs/in. to ensure that the actual stiffness of the machine is being measured and not the part stiffness.
Note: If a very stiff machine (i.e. 1,000,000.-lbs/in.) is being used and the part has a weak stiffness (10,000.-lbs/in.), the machine static stiffness result will be very close to that of the part. In measuring the machine static stiffness, it is very important to have a solid rigid part to ensure the correct machine stiffness. It is suggested that a machine static stiffness test be done every year to ensure the accuracy of the stiffness test. The machine stiffness will deteriorate over time.
The basic philosophy behind the machine static stiffness test is that the deflection of the machine affects the grinding process and therefore it should be measurable with how much stock is removed during a loaded machine and unloaded machine.
There are two types of machine static stiffness tests: thrufeed, and infeed.
1. Thrufeed Instruction
Grind enough components with total stack length larger than twice the wheel width. Operate the grinder at a power consumption that is larger than 60% of the available power. Take one component in the middle of the stack out of the flow. Measure its diameter accurately and record this number for the first measurement input of the program.
After taking the part out of the flow, the flow can be stopped, however leave the machine running but do not change any settings. Use the measured component and feed it through the grinder again. Measure its diameter and record it for the second measured input of this program.
2. Infeed and Internal Instruction
▪Grind one component at an infeed rate with high enough power consumption (60% or more of available power); keep spark out time to zero. Measure the diameter of the component and record the net actual power consumption.
▪Grind a second component in the same setup with spark out set to 10 seconds or more. Measure the diameter of component.
There is more to grinding than only the basic principles covered, but this will serve as an introduction to the capabilities of the software.
GRINDING SIMULATOR SOFTWARE
There are eight different types of grinding operations built into the Grinding Simulator, Fig. 2-3-3A to D:
1.Thrufeed grinding with one machine (A)
2.Thrufeed grinding with multiple machines in line
3.Centerless Infeed (B)
4.Centertype Infeed
5.Microcentric
6.Angular (C)
7.Multiple-Diameter Centerless Infeed (D)
8.Multiple-Diameter Centertype.
It would be difficult to discuss all these grinding operations in detail since the same basic principles apply to all. Therefore for this example, the focus will be on Thrufeed Grinding for one machine with multiple passes.
Part Information
Figure 2-3-4 shows the Part Information page, which lists various material groups, each containing several specific materials. There are a total of eighty different types of materials and other materials can be added, provided that its thermal conductivity at 300°C is known.
There are different types of tolerances that play a role in the grinding process and are related to each other. In the Part Information Page, the print final size tolerance is an input, and automatically the expected roundness tolerance and cylindricity tolerances are calculated. The software allows users to type in their own roundness or cylindricity tolerance. This would then initiate a reverse calculation for the final size tolerance. Another important input is the CPK (Capability Index). The print tolerance and the shop tolerance are not the same when the CPK is larger than one. The relationship between shop tolerance and print final size tolerance is
| (8) |
For each input where there are metric and inch units, each unit system can be used. The green label shows the converted value from one unit to the other.
The part’s information can be saved in a database and be reloaded or modified at any time.
Machine Information
Figure 2-3-5 shows the Machine Information page. The speed ratio is defined as
| (9) |
The user can enter the Speed Ratio, or it can be selected from the general ranges. A high-speed ratio means a low workpiece speed that makes the chip length larger. This would then make the grinding process more sensitive for surface damage due to heat. A speed ratio higher than the High Speed Ratio selection will result in burning of the workpiece. The selection of High Speed Ratio should be used in cases where workpiece out-of-balance takes place or where the machine dynamic stiffness does not allow higher work speed.
A selection of Typical means an average moderate work speed that is used on most grinding operations. The selection of Low speed ratio means a high workpiece speed and can be selected when the machine has a high static stiffness. With weaker machines, a low speed ratio may cause chatter on the workpiece. The practical speed ratio has a correlation between the wheel speed, workpiece speed, and with the part diameter.
When the user enters in a wheel speed, the wheel r/min is automatically calculated. It is possible to enter the wheel r/min that would result in a wheel speed calculation. The workpiece r/min is automatically calculated when the speed ratio is selected. The user can enter the workpiece r/min at which time the speed ratio selection will go to None. There is an important correlation between workpiece r/min and wheel r/min. The ratio between wheel r/min and work r/min is called the Beta ratio. It is defined as
Fig. 2-3-3 Types of centerless grinding: A. Throughfeed (Cincinnati Machine, A UNOVA Co.) B. Centerless infeed (Carborundum Abrasives, Div. Saint-Gobain Abrasives.) C. Angular (Cincinnati Machine, A UNOVA Co.) D. Multi-Diameter Infeed (Carborundum Abrasives, Div. Saint Gobain Abrasives)
This relationship is important when the grinding process results in triangle or other non-round parts. When the Beta Ratio is exactly 3 or close to 3, then it could indicate a triangle part problem. Another important calculation that depends on the workpiece r/min and the speed ratio is the regulating wheel r/min. Whenever the workpiece r/min is changed, the regulating wheel r/min is changed accordingly.
Quick Results
Figure 2-3-6 shows the most important calculated parameters. The first item shown in the calculations is the surface finish. If no static stiffness constraints or available machine power constraint occurs, then the surface finish in the finishing operation should be the same as the requested surface finish. The next item is the Specific Metal Removal Rate Q’. For certain fluids, the Q’ cannot exceed a certain value. In case this occurs, it will appear in the messaging box. The other calculated values are important but need no detailed discussion.
The messaging box shows the factors that can occur that could limit the production rate. Whenever there is enough system static stiffness available, the software calculates at what CPK value the job should be able to run. The message box suggests changes to increase productivity whenever possible.
Machine Setup
The Machine Setup page is shown in Fig. 2-3-7. There are four items that are shown in this page.
1.If a machine picture is available, it would be shown in the upper left corner.
2.The upper right corner lists the machine manufacturer, the machine model, the static stiffness of the machine when built, and the date of the last static stiffness test.
Fig. 2-3-4 The Part Information page of the Thrufeed One machine with multiple passes.
Fig. 2-3-5 The Machine Information page of the Throughfeed One machine with multiple passes. (Bethel Technologies, Inc.)
3.The lower left corner shows a graph of the static stiffness of the machine as a function of time. In this example, the static stiffness of the machine is slowly deteriorating.
4.The lower right hand corner shows the machine setup.
▪The center height is the height above the centerline of the grinding wheel spindle and regulating wheel. The calculated center height is close to the actual center height that should be used in the actual machine setup.
▪The throughfeed angle is the angle at which the parts are fed through the machine.
▪The diamond setover is calculated for proper contact line across the regulating wheel.
Work Blade Setup
Most work is centerless ground with its center above the centerline of the grinding wheel, Fig. 2-3-8. However, factors such as the part diameter, its physical characteristics, type, and diameter of the grinding wheel influence this setting. Generally for parts up to 1.00 in. diameter, adjust the blade until the part center is above the wheel center about one-half the part diameter. For larger diameter parts, the part center should rarely exceed .50 in.
Fig. 2-3-6 Quick Result page of the Throughfeed One machine with multiple passes showing the calculated parameters and production rate. (Bethel Technologies, Inc.)
Fig. 2-3-7 Machine Setup page of the Throughfeed One machine with multiple passes. (Bethel Technologies, Inc.)
Fig. 2-3-8 Machine Setup Contact Geometry. (Bethel Technologies, Inc.)
Fig. 2-3-9 Machine Power Display. (Bethel Technologies, Inc.)
If the part being ground is too high above the centerline of the wheels, chatter generally occurs. This is caused by the tendency of the wheels to raise the part out of contact with the workrest blade. If the height of the part above the center is reduced too much, the part is ground with three high spots.
Fig. 2-3-10 Static Stiffness Display. (Bethel Technologies, Inc.)
The graph in Fig. 2-3-9 shows the Machine Power while the graph in Fig. 2-3-10 shows the Static Stiffness of the sample part ground in this example.
For more information on GRINDING SIMULATOR e-mail: dsmits@one.net
