Exploring Advanced Manufacturing Technologies. Steve Krar
alt="image"/>
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