Grinding operations for investment casting: keeping an eye on costs.

Grinding Operations for Investment Casting: Keeping an Eye on Costs

When reviewing the major expenses in an investment casting foundry, what immediately comes to mind? Metal? Power? Labor?

If you forgot finishing, don't feel badly. Finishing costs are often regarded as a necessary evil and given little consideration in cost analysis, even though they are usually among the five largest expense categories in an investment casting foundry. In fact, finishing costs can account for as much as 60-70% of nonmelting production costs.

Because finishing costs are considerable, the metalcaster interested in getting the most out of his foundry must take a hard look at finishing room efficiency in general and, more specifically, total grinding costs.

Reviewing grinding costs is not difficult and the results may be surprising. The analysis may suggest a need to upgrade the grinding process to make use of premium performance abrasive products, or, conversely, it may indicate that the current abrasives are perfectly adequate. Either way, foundry management gets a better understanding of its operations and expenses.

Abrasive Options

An examination of grinding costs requires a basic understanding of the abrasive product categories, the various abrasive materials, the metals used in investment casting and finishing room procedures. The first of these, product categories, includes:

* bonded abrasives, which consist of abrasive grains held by a vitrified, organic or metal bond in a particular structure (usually a wheel). Bonded abrasives used in investment casting include cut-off wheels, thin wheels and mounted points;

* coated abrasives, which have abrasive grains bonded by an adhesive to a flexible or semi-rigid cloth backing. Coated abrasives found in investment casting include narrow belts, discs, sheets and specialty items such as flap wheels, cartridge rolls, spirapoints, pencils and bands;

* loose abrasive grain used for blasting;

* nonwoven nylon abrasive-impregnated wheels used for finishing and polishing.

A variety of abrasive materials are available in each of these product categories (see Table 1). Among the more common abrasive materials are:

* aluminum oxide, used to grind ferrous materials. It is the softest of the conventional abrasives but is relatively resistant to impact;

* silicon carbide, the conventional abrasive commonly used to grind nonferrous materials. It is the hardest of the conventional abrasives, but is less resistant to impact than aluminum oxide;

* zirconia alumina, one of the premium abrasives for investment casting. Used for rough grinding metals (particularly ferrous metals), it has a high resistance to impact and works best under high pressure in high stock removal applications;

* SG[TM] abrasive, a ceramic aluminum oxide developed by Norton Co under a patented seeded gel process. SG abrasive is a durable product that is used in grinding wheels and belts. It contains micron-sized cutting edges that fracture, rather than becoming dull, thereby constantly exposing new cutting points.

The physical properties of the metal to be finished determine which abrasive materials are applicable. For grinding purposes, metals used in investment casting fall into two categories. The first consists of high-tensile, heat-resistant metals such as stainless steel superalloys and nimonics (nickel- or cobalt-based alloys). These materials are extremely difficult to grind.

The second group of metals are soft, gummy materials such as copper, aluminum and magnesium. These metals are easier to grind than the first group, but have a tendency to load an abrasive product by filling in the pores of the abrasive. This decreases the rate of stock removal.

Finishing Room Procedures

Compared to many other types of metalworking, investment casting requires relatively few finishing steps. This is in keeping with its objective of casting to shape in order to minimize the amount of traditional metal shaping. The finishing steps that are required are the same for both the hard and soft metal groups listed above, though in some cases the actual finishing methods are different.

After molds are poured and cooled, the individual castings must be removed from the cluster. A band saw or friction saw is used for softer metals and small cross-sectional gates. For hard metals, an abrasive cut-off wheel is required. The cut-off wheel functions as a saw, cutting through a workpiece instead of grinding its surface.

Following cut-off, the casting goes through a caustic wash or a shotblasting operation to remove any traces of the shell. It is then blasted with loose abrasive grain to remove discoloration. The blasting also may be used to give the casting its final, mottled appearance.

Abrasive coated narrow belts (under 14 in. wide) are used to remove the stub, or gate, left from the cut-off operation. After stub or gate removal, the casting may then go to a rework area where surface problems can be corrected, sometimes by repeating the aforementioned steps.

Additional grinding steps may be required, depending on the type of casting and the dimensional and finish requirements. These steps may call for other bonded abrasive products such as thin wheels and mounted points, and coated abrasive products such as finegrit belts, discs, sheets or specialty items.

The final blending or polishing of the casting surface is often done with nylon abrasive wheels or similar products.

Total Grinding Costs

Because approximately 70-80% of a foundry's finishing expenses are in cut-off and belt grinding, we will concentrate on these operations in discussing how to calculate total grinding costs and optimize finishing operations.

Total grinding cost is a combination of abrasive costs and labor and overhead costs. Abrasive cost per piece ground may be calculated using this formula: amount of abrasive product x cost of abrasive product/number of pieces ground. Labor and overhead costs per piece may be calculated as follows: labor and overhead rate x grinding time/number of pieces ground.

The combination of abrasive costs and labor and overhead costs per piece will equal total grinding cost per piece. After determining total grinding cost, compare it to production cost per work-piece. If grinding costs are high (60-70% of total production costs), finishing room processes and equipment should be reviewed.

One cost-conscious New England foundry used these methods to analyze the cut-off operations it had traditionally performed with conventional aluminum oxide wheels. By comparing aluminum oxide wheels with high-performance zirconia alumina wheels, this foundry discovered that the more expensive zirconia alumina product provided more cuts per wheel, resulting in fewer wheel changes (and less downtime), while leaving cleaner cuts that required less additional finishing.

The cut-off operation at this foundry was a push-through application that was a labor-intensive, physically exhausting job for operators. In addition to the previously mentioned benefits, management found that the freer-cutting zirconia alumina wheel caused less operator fatigue and less worry that the wheel would break.

These cost analyses showed that using the zirconia alumina product could save the foundry 25-30% in wheel and finishing costs.

Optimizing Grinding Systems

Like the foundry just described, the first step in cost analysis is reassessment of the abrasive products currently in use. If abrasives are purchased based on price alone, the productivity improvements possible with a premium abrasive, such as SG abrasive or zirconia alumina, may be overlooked.

If premium abrasives look promising, a foundry must make sure its equipment meets the horsepower(hp) and grinding force requirements needed to take advantage of these abrasives' performance benefits.

The abrasive product is a tool for removing unwanted metal whose productivity (i.e., the speed at which it removes metal) is directly related to the available horsepower. Here are two rules of thumb for determining the horsepower requirements of premium abrasives: cut-off wheels need one hp per inch of grinding wheel diameter; and, coated products (e.g., narrow belts) require three to five hp per inch of belt width in off-hand grinding applications and 10-15 hp per inch in pressure-assist grinding.

The second key to optimizing abrasives is grinding force. The two alternatives are manpower or mechanical devices, such as a lever bar or hydraulic or pneumatic infeed.

A human being applies less force than a mechanical device, and, what's more, human power varies over the course of the workday as the operator tires. Power assist devices can increase grinding productivity (in terms of stock removal) by a factor of ten.

Because finishing operations are expensive, investment casters who ignore them do so at a risk to their profitability. A simple analysis of grinding costs can help keep finishing costs under control while maximizing productivity.