This process has the potential not only to overcome several of the limitations associated with the machining processes traditionally applied to PM parts, but also to create a paradigm shift in the industry’s approach to forming certain complex, multi-level parts.
Stock removal processes, in near net shape manufacture in general and PM in particular, have traditionally been focused on “eliminating grinding and milling at almost any cost”. This view has forced the adoption of single point turning, which suffers from a number of drawbacks:
- It cannot cope well with the interrupted cuts associated with the machining of formed parts containing complex features, such as gear or sprocket teeth.
- It cannot be readily applied as a hard machining operation to heat treated materials. This often forces the introduction of an additional process step, with single point machining being applied before heat treatment and finish grinding after heat treatment.
- For similar reasons to the above, these operations cannot be readily applied to sinter hardened products and, of course, in this case machining prior to heat treatment is not an option.
- The forces involved in these operations create a problem in machining thin wall sections or axial features subject to dimensional distortions.
- The machining of high strength, ductile materials can create large burrs – often, deburring can cost as much as the machining operation itself.
- The machinability additives, often added to PM materials to aid these machining processes, increase costs and can also be detrimental to mechanical properties.
Super abrasive machining (SAM) has the capacity to attack all of these limitations.
The process is often referred to as “grinding at machining rates” as it employs an abrasive wheel but is performed on a platform that is more akin to a horizontal milling machine than a surface grinder. Its closest rival is probably creep fatigue grinding, but SAM achieves the high levels of stock removal to close finish tolerances at higher speeds, faster material removal rates and lower work piece loads.
Unlike single point turning operations, interrupted cuts actually benefit SAM, because the rotating action of an interrupted cut creates a “paddle” action that drives additional coolant to the machining interface. Fig. 1 quantifies this advantage by comparing the wheel cost/life of a solid cut to that of an interrupted cut for SAM at equivalent depths of cut. Fig. 2 shows the cutting tool cost advantages of SAM over single point turning with interrupted cuts for as-sintered and, most significantly, heat treated work piece materials.
The key to successful implementation of SAM relies on the complex integration and optimisation of a number of factors:
- Wheel design, geometry, material and speed.
- Wheel feed rate
- Coolant make up, flow and application
- Machine action, platform and capability
In view of the higher platform capital, wheel and fixturing costs associated with SAM, it is important to choose the correct applications for the technology, in order to achieve a rapid return on investment.
A range of such applications was discussed in the presentation and selected examples were as follows:
- Replacement of multiple process steps by a single operation (Fig. 3).
- Machining of PM gears and sprockets with interrupted cuts (Fig. 4).
- Combining geometries and converting multiple pieces into one-piece constructions (Fig. 5).
- Producing features typically produced as multi-level compacts on more expensive forming presses (Fig. 6).
- Creation of machined geometries that extend PM’s shape capability and open up new conversion opportunities (Fig. 7).
The costs justifications for adopting SAM in the applications cited above extend well beyond the direct comparisons of machining operation costs associated with SAM and alternative technologies. Table 1 identifies a range of upstream and downstream cost reduction benefits arising from a properly applied SAM operation.
The higher up-front investment associated with SAM has already been referred to and, in many of the application examples cited, there is an increase in material weight and cost in the initial PM compact. However, these costs can be more than recovered from the upstream and downstream costs referred to in Table 1.
By way of example, a specific sprocket application (Fig. 8) was considered and process benefits were quantified in relation to:
- An increase in compaction rate of 25%, arising from a simpler part configuration (Fig. 9).
- A decrease in mean press set-up time of 30%, arising from the simpler tooling arrangement (Fig. 10).
- A decrease in toolmaking costs of 20%, again associated with the simpler tooling concept (Fig. 11).
- A reduction in tool maintenance costs of 35%, because of the avoidance of thin-section punches (Fig. 12).
PM has traditionally captured new market opportunities by continuing advances in shape capability. This paper emphasised that achieving this by the use of ever more complex tooling arrangements and of more sophisticated, multi-platen presses may not necessarily always be the right way to go – judicious application of SAM to a simpler-shaped compact might be a superior solution.