Improved grinding and cutoff technology for today's foundry.

Cleaning demands a significant portion of production costs. A better understanding of available technology can result in a more productive and cost-effective cleaning room.

Perhaps the most significant change in tomorrow's foundry will be the transition of the cleaning and finishing operations. What once was an operation in which castings were repaired through labor-intensive manual cutting and grinding will be one in which only minimum grinding and finishing are needed for cosmetic and quality purposes.

In analyzing casting cleaning process operations, it is imperative that the total cost of necessary cutoff and grinding functions be weighed against all other foundry processes. In the final analysis, the additional labor required to correct a condition caused by out-of-control precleaning operations must be separated from the cost of simple cutoff and grinding operations.

Why Castings Need Grinding

One of the final steps in finishing a casting before shipment is grinding to remove excess metal. Grinding has been estimated to account for 15-30% of the total cost of casting production in many metalcasting operations, particularly in sand foundries. The actual costs of grinding operations vary with the following factors:

* type of metal;

* production volume;

* finish quality requirements;

* casting size and weight;

* casting complexity.

Not only do grinding costs vary with the metal type, but costs may also increase due to the extra grinding required in the following areas:

* removal of gate, feeder and riser stubs;

* removal of mold parting lines;

* removal of core fins;

* removal of sand burn-in;

* blend of mismatch at parting lines;

* removal of extra metal caused by broken cores;

* removal of extra metal caused by pieces of molding sand dropping away during pouring;

* preparation of casting surface for machine locators;

* improving surface finish.

Because grinding wheels have existed in foundry cleaning rooms for more than 100 years, foundrymen tend to rely on grinding to correct many other precleaning faults.

Advanced grinding and cutoff wheel technology available to foundries today offer special abrasive grains, new bonding materials and improvements in the grinding wheel manufacturing process. Manufacturers now produce wheels that are better balanced, more consistent, stronger and safer.

The development of zirconia-alumina grains for foundry grinding wheels has provided the capability to improve productivity over the more traditional aluminum oxide and silicon carbide abrasive grains. While aluminum oxides and silicon carbide wheels are friable (break away with use), use of the more durable zirconia-alumina abrasive has led to a reduction in abrasive use by a factor of one-half to one-third.

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One type of zirconia abrasive used in floor stand snagging wheels, swing frame grinding wheels and larger wheels is widely available and very efficient. Tests conducted on ductile iron castings indicate zirconia-alumina wheels to be 8 times as efficient as aluminium oxide, 2.5 times as efficient on gray iron and 1.25 times as efficient on stainless steel.

While the zirconia-alumina grinding wheel may be higher in cost, the return by faster metal removal and longer wheel life will, in many cases, offset the higher cost of the grinding wheel.

A newer development in abrasive grains is that of cubic boron nitride. These wheels prove to be very durable and particularly applicable for hard-to-grind steel alloys and precision grinding applications.

Coated Abrasive Belts

The most rapidly advancing area in cleaning room grinding technology has been the development and use of abrasive belts and belt systems. The developments in both belts and machines make this method an attractive alternative to grinding wheels, particularly for automatic and semiautomatic grinding operations.

High metal removal rates are the primary advantage of abrasive belts, with reported removal rates of 0.5 cu in. per second per in. of belt width. These high metal removal rates are competitive with other heavy grinding operations and are a direct result of improvements in belt materials.

Similar to that of grinding wheels, the cutting rate of an abrasive belt is dependent on machine horsepower, contact pressure between casting and belt, and the design of the belt work wheel. Since work wheel diameter for the belt doesn't wear, correction isn't normally required for work wheel wear - unlike the grinding wheel.

Cooler cutting is also obtained, despite the increased metal removal rates. This is particularly useful in finishing heat-sensitive materials such as alloyed ferrous metals that might otherwise crack or distort when heat is generated. Generally, abrasive belts are also considered to be safer than abrasive grinding wheels.

Some applications of abrasive belts using controlled feed systems that can exceed metal removal rates of most grinding wheel operations also provide excellent cycle times. To illustrate this point, a system for finishing cast iron pump impellers is described below.

The system uses a microprocessor-control rotary feed system. About 3/4 in. of metal was removed from the diameter of the part in the finishing operation. The coated abrasive belt accomplished this operation in two minutes and can process 30 parts per belt. A rotary cutter required about eight minutes and one set of inserts every two or three parts.

Due to the fast cutting of the abrasive belt, it is often necessary to provide holding fixtures for the casting. If more than one surface of the casting requires grinding, a multi-position fixture may be most efficient for this application.

Abrasive Cutoff Wheels

Abrasive cutoff wheels are the fastest and most economical method of cutting materials in the foundry. Unlike most grinding wheels that abrade away the metal surface of the casting, cutoff wheels actually cut metallic materials.

A variety of cutoff wheels is available for foundry operations. Proper selection of the right cutoff wheel can increase productivity in terms of wheel life and metal removal rates, significantly reducing the total cost of cutoff operations.

The cutoff wheel consists of three main elements: the abrasive grain, the bond and the pores.

The most common abrasive grain found in the cutoff operation is ceramic-coated aluminum oxide. It is the most cost effective grain for cutting most steels and steel alloys.

The zirconia-alumina alloyed abrasive is usually considered the premium abrasive grain for cutoff operations. Tougher and more ductile than aluminum oxide, it is best for cutting stainless steel and superalloys, including nickel-base super alloy castings. A third grain, silicon carbide, is recommended for cutting titanium and nonmetallics such as ceramics and concrete.

Cutoff Operations

Some of the major masons for the application of cutoff wheels in the foundry include:

* removal of runners, risers and gates;

* some grinding of castings with special contours that are difficult to reach with a normal grinding wheel;

* cutting metal samples for laboratory testing and spectrographic analysis. Removing gates, runners and risers from all metals, however, doesn't always require a cutoff operation. Gray iron casting gates, runners and risers will normally break off when the casting travels over a shakeout. They can also be removed using manipulators or other automatic methods of applying pressure to break the gates and runners from the casting.

With most steel, ductile and malleable iron, and most nonferrous castings, it is more difficult to remove the gates, runners and risers.

Gates, runners and risers may be designed with notches to help streamline breakoff operations. It is also important that the area of the ingates to the casting be designed in a rectangular section to improve the breakoff potential through the thin section of the rectangle. Both techniques can reduce the necessity of a cutoff wheel application to reduce cleaning costs.

Selecting Coated Abrasives for Wheel and Belt Grinding

                                       Abrasive Type
Metal               Zirconia-Alumina   Aluminum Oxide   Silicon Carbide

Gray Iron                   1                 2                NR
Chilled Cast Iron           2                 3                1
Carbon/Mild Steel           1                 2                NR
Stainless Steel             1                 2                NR

Aluminum                    1                 2                3
Brass/Bronze                1                 2                3
Copper                      1                 2                3

1 = Most Effective    2 = Moderate    3 = Least Effective

Courtesy of Norton Abrasives

Cutoff Machine Design

The most common design of a cutoff machine uses a pivot arm that holds the rotating cut-off wheel that is automatically or manually fed downward into the workpiece. Cutoff machine designs are also available with the following type of cutting wheel motions:

* chop stroke;

* chop stroke with oscillating head;

* rotational;

* traverse:

* portable or swing frame;

* locked head (workpiece is guided into wheel).

The majority of these designs should use reinforced wheels only. A side-reinforced wheel gives added rigidity, better control and straightness of cut. Several of these applications will require the casting to be securely fixtured.

In cutting off runners, risers and ingates, removal should occur as close to the casting as possible to reduce or eliminate grinding in this area.

Better Tools

With the development of special abrasive grains, bonding materials and improved manufacturing techniques, the new technology in cutoff wheels, grinding wheels and abrasive belts provides the opportunity to upgrade machinery with faster and safer grinding practices. New machines with hydraulic clamping fixtures and flexibility of rotating the casting into different positions offer more accurate and faster grinding and cutoff operations.

The new abrasive grains and bonding materials also have led to the development of advanced grinding wheel designs for floor-mounted grinding machines and portable grinders, which are capable of a variety of casting configurations produced for today's market.

Grinding wheel and machine manufacturers have provided the foundry industry with the capability for achieving higher productivity through technological advances in abrasive grains materials and bonds. Each of these can increase metal removal capabilities and provide more accurate finishing while reducing heat generation during the grinding operation. This results in safer grinding operations at low overall costs.

RELATED ARTICLE: Seven Factors for Selection of Grinding Wheel Specifications

1. Material to be ground

Soft Metal (Ductile iron): Coarse abrasive/hard wheel Durable Metal (Hard steel): finer abrasive/soft wheel

2. Amount of Material to Be Removed

Coarser Metal: Rapid stock removal Finer Metal: More cutting points for hard-to-grind materials

3. Wet or Dry

precision grinding or applications where heat buildup is critical: wet grinding general grinding and most rapid stock removal applications: dry grinding

4. Wheel Speed

Performance: higher speed yields greater metal removal: weight of wheel ratio

5. Area of Contact

Large: soft grade Small: hard grade

6. Severity of Grinding Operation

Severe Pressure: zirconia-alumina Light Pressure: friable abrasives

7. Horsepower

Affects abrasive, bond and grade selection Low Power: must use soft grades, friable abrasives and forgiving bond system High Power: must use hard grades, durable abrasive and durable bond system

Courtesy of Norton Abrasives