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
