Exploring Advanced Manufacturing Technologies. Steve Krar
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The final bottom line on this application is that the actual total cost of expendable tools decreased very slightly and created a total production system cost reduction of 312,000 x $.44 X $137,000/year.
Total Grinding Cost Evaluation
As in the machining cost analysis, present grinding evaluations do not properly portray the potential savings associated with superabrasive grinding programs. Let’s take a simple example such as regrinding high-speed steel end mills. One-half inch end mills made of M-4 may take up to 15 minutes each to regrind with a conventional wheel costing about $10. These same tools may be reconditioned with a $200 CBN wheel in 5 to 8 minutes. The CBN wheel typically reconditions 20 to 50 times as many tools as the conventional wheel it replaces.
Table 1-2-2 Grinding cost model. (Courtesy GE Superabrasives)
In a traditional cost analysis, the CBN wheel is anywhere from break even to an actual cost advantage. The real advantage of the CBN wheel is in productivity and quality. The tools reground with CBN are more accurate and have retained surface integrity due to the cool grinding characteristics of CBN wheels. Thus, the tools ground with CBN may stay on the milling machine and produce twice as many operations or parts before it needs reconditioning.
To develop a more consistent and complete cost evaluation, a grinding model shown in Table 1-2-2 has been established. Manufacturing economic equations are used that were specially adapted to the grinding process. Process parameters used in the equations include tool life, machine and overhead costs, dressing roll costs, scrap, coolant and many other factors peculiar to grinding.
Application: Superalloy High Pressure Turbine Nozzle Assembly Internal Diameter (ID) Grinding
An application from the aerospace industry has been selected to demonstrate the effectiveness of the grinding cost model. The ID creep feed grinding of a gas turbine aircraft engine component is used to compare a conventional aluminum oxide (Al2O3) wheel and a vitrified bond CBN wheel. This qualification was performed on a 34 in. diameter high-pressure turbine nozzle shroud assembly made of a superalloy material.
Machine tool: Vertical Spindle CNC Grinder
Wheel speed: 7000 SFM
Table speed: 7 in./min.
Stock removal: .065 in./side
Operation Manpower Cost (Cost required for labor to operate the grinding machines), Fig. 1-2-17. This cost is dependent on labor rate and cycle time per part. In the model, CBN increases productivity from .67 to .76 parts per shift, thus decreasing labor cost by $123.85 per part.
Fig. 1-2-17 Operation manpower cost factors – Grinding. (Courtesy GE Superabrasives)
Wheel Cost (Cost of the grinding wheels), Fig. 1-2-18. The initial investment in a vitrified bond CBN wheel is 140% of that of the Al203 wheel. In many industries, this is the only cost used to calculate abrasive cost. The model shows nearly three times the wheel cost/part for CBN, however, productivity increased 13%. The rest of the model shows that there are other important factors that must be considered.
Fig. 1-2-18 Wheel cost factors – Grinding. (Courtesy GE Superabrasives)
Wheel Change Cost (Cost of labor to change grinding wheels), Fig. 1-2-19. Wheels containing CBN abrasive last much longer than aluminum oxide and require fewer wheel changes. CBN achieved a 98% wheel change cost reduction over Al203 in this application.
Fig. 1-2-19 Wheel change cost factors – Grinding. (Courtesy GE Superabrasives)
Dressing Roll Cost (Cost of the dressing roll used), Fig. 1-2-20. Vitrified bond CBN wheels require less frequent dressing than conventional grinding wheels; consequently, dressing rolls used in the CBN process outlast those used for Al203 grinding. This results in a lower cost per part, as shown by the 58% decrease in roller cost/part in the model.
Fig. 1-2-20 Dressing roll cost factors – Grinding. (Courtesy GE Superabrasives)
Dressing Roll Change Cost (Cost of labor to change dressing wheels), Fig. 1-2-21. Since fewer rolls are required with CBN, this cost will also decrease. Although in this application the cost savings are insignificant, in some applications greater dresser roll change cost savings can be achieved.
Fig. 1-2-21 Dressing roll cost factors – Grinding. (Courtesy GE Superabrasives)
Maintenance Labor Cost (Cost of maintenance performed on the machine during downtime), Fig. 1-2-22. Grinding machines equipped with CBN wheels require less frequent coolant changes and therefore downtime to change coolant is decreased. This cost also includes downtime for filter changes, machine maintenance and time required topping off the coolant tanks. The model reflects a 41% reduction in this cost.
Fig. 1-2-22 Maintenance labor cost factors – Grinding. (Courtesy GE Superabrasives)
Scrap Cost (Cost of scrapped parts), Fig. 1-2-23. A CBN grinding application produces a higher part-to-part consistency, resulting in a reduction in scrap cost. This is illustrated by the $391.80 per part savings or an 89% scrap reduction offered by the CBN wheel.
Fig. 1-2-23 Scrap cost factors – Grinding. (Courtesy GE Superabrasives)
Coolant Cost (Cost of keeping grinder supplied with coolant, free from excess swarf contaminant), Fig. 1-2-24. Wheels containing CBN abrasive require less dressing than conventional wheels resulting in less swarf contamination from the grinding wheel. This results in longer coolant life due to reduced amounts of swarf entering the system.
Fig. 1-2-24 Coolant cost factors – Grinding. (Courtesy GE Superabrasives)
Filter Cost (Cost of filter paper used in the coolant system), Fig. 1-2-25. This cost is also reduced due to less dressing of the CBN wheel, and a consequent reduction in swarf contamination due to the wheel. The model shows 56% savings per part as a result of using CBN abrasive.