Exploring Advanced Manufacturing Technologies. Steve Krar
into the applicable sections of the machining cost model. This model reflects the machining costs of a gray cast iron cylinder boring application comparing silicon nitride (SiN) inserts with polycrystalline cubic boron nitride (PCBN) inserts. The use of PCBN tools in gray cast iron machining is limited to certain grades, depending upon the microstructure of the cast iron.
Total Machining Cost Evaluation
Application: Gray Cast Iron Cylinder Boring
An engine cylinder block is being semi-finished and finish bored dry using a single-point tool boring head. After the semi-finishing pass is completed, a single tool is extended from the boring head by an actuator; the finishing pass is completed as the head is extracted from the cylinder bore. A total of twelve inserts are required to complete this operation on the gray cast iron V-6 engine.
▪Insert - SNG-432 (15° X .004 in. chamfer)
▪Speed - 2600 SFM
▪Feed - .014 in./rev.
Table 1-2-1 Machining cost model. (Courtesy GE Superabrasives)
▪DOC - .015 in. semifinish
▪DOC - .005 in. finish
The average bore cylindricity (roundness) obtained with the SiN tooling was .0006 in. When the change was made to PCBN inserts, average bore cylindricity was reduced to .0004 in. Since PCBN inserts conduct heat away from the workpiece, less heat shrinkage occurred in the bores, resulting in an improvement in cylinder honing.
Tool Cost (Cost of tooling only), Fig. 1-2-8. This is the cost often used as the major criterion for determining the economic justification for tool selection. Regrinding is also important, because it can bring the tool cost/part down significantly in some applications. The nature of this cylinder boring application did not allow the regrinding of inserts. As seen from the model, the price per part is essentially the same despite the significantly higher initial price of the PCBN tool.
Fig. 1-2-8 Tool cost factors - Machining. (Courtesy GE Superabrasives)
On-Line Labor Cost (Cost of operator to run machine), Fig. 1-2-9. This cost in some cases will also include setup because it is done by the same person. On a per part basis, the cost model shows a reduction in cost when PCBN is used due to the increase in productivity on this cylinder boring application.
Fig. 1-2-9 On-Line labor cost factors - Machining. (Courtesy GE Superabrasives)
Tool Change Cost (Labor cost required to change tools), Fig. 1-2-10. This may be the same as on-line labor cost depending on who is authorized to change tools. In the cylinder boring application, PCBN requires a reduced number of tool changes, one every 12.5 shifts, compared to two per shift with SiN. Thus the tool change cost is significantly reduced.
Fig. 1-2-10 Tool change cost factors – Machining. (Courtesy GE Superabrasives)
Scrap Cost (Cost of scrapped parts), Fig. 1-2-11. PCBN produces a tighter part tolerance, resulting in a reduced scrap rate that is portrayed as a 61% scrap cost reduction shown in the model.
Fig. 1-2-11 Scrap cost factors – Machining. (Courtesy GE Superabrasives)
Setup Cost, Fig. 1-2-12, – This is the cost for labor to index tooling or prepare cutter for use, before it is actually delivered to the line. Since PCBN requires fewer tool changes, setup cost can be reduced with respect to conventional tooling. This model involves evaluating a cylinder boring application where no setup was required, however in some applications this cost can be significant.
Fig. 1-2-12 Setup cost factors – Machining. (Courtesy GE Superabrasives)
Rework Cost, Fig. 1-2-13, – The cost of reworking parts that do not meet specifications the first time they are machined. This cost will also be reduced due to the higher quality of parts produced by the PCBN machining process. The cylinder boring application however did not have statistics for this cost, but a reduction in scrap parts indicates a probable reduction in rework parts.
Fig. 1-2-13 Rework cost factors – Machining. (Courtesy GE Superabrasives)
Inspection Cost, Fig. 1-2-14, - The cost of labor for the inspection of parts to meet specifications. Once again with the tighter part tolerance that a PCBN tool produces, a higher confidence in product quality can be achieved, thus reducing inspection time. The inspection procedure for the cylinder boring application did not change despite the significant improvement in process capability. Consequently, no inspection cost savings have been realized to date.
Fig. 1-2-14 Inspection cost factors – Machining. (Courtesy GE Superabrasives)
Inventory Cost, Fig. 1-2-15, – The cost of carrying raw material and in-process parts before and/or after machining. This number is based on the scrap rate and predicted production rate. Since the scrap rate will be reduced using PCBN tools, the number of parts kept in inventory should be reduced accordingly. Increased productivity, however, may cause this cost to increase. No information on this cost was available for the cylinder boring application.
Fig. 1-2-15 Inventory cost factors – Machining. (Courtesy GE Superabrasives)
Total Machining Cost, Fig. 1-2-16, - This yields total cost of machining and is the sum of the above costs on a per part basis for the cylinder boring application. As can be seen from the results above, the PCBN tool reduces cost $.44 per block, or 38%. Using a traditional machining cost analysis that looks only at tooling cost per part, the silicon nitride and PCBN inserts appear equal, leading the engineer to uninformed go/no go decisions. In reality, they are very different. Certain costs were not attainable for this cost model, and this may be true for many applications. The purpose of the model is to include all relevant costs for any machining process. It is the responsibility of the engineer to determine which costs are pertinent to the particular application.
Fig. 1-2-16 Total machining cost/part. (Courtesy GE Superabrasives)
The full effect which superabrasives have on this operation can be appreciated more fully by annualizing expendable tool costs and comparing this to annualized savings.