Cleaning rooms gain efficiency.
"There aren't many MBAs working on the economics of foundry cleaning and finishing operations, but there sure should be. Cleaning offers more opportunities to save money than any place in a foundry, but too often it gets the short end."
"Cleaning castings is difficult to automate, but we've come a long way. Maintenance plays a big role for us and so does employee education. It's made us profitable."
"We engineer castings to use our cleaning room capabilities better. This up-front work makes the cleaning job easier and a lot cheaper."
These comments at a recent AFS foundry management seminar are indicative of the growing interest in knowing and controlling casting cleaning costs. It has been estimated that of the total labor cost for an average foundry, up to 40% can go to cleaning, finishing and related activities.
Considering rising operating costs and price-sensitive casting markets, cleaning operations are an area ripe for the introduction of new methods and materials that reduce internal costs, improve customer satisfaction and, ultimately, lift profit margins.
Most cleaning room veterans, though, will be quick to point out that their work is highly dependent upon what happens before a casting gets to them.
"It all starts in the pattern shop, where the gates and risers are set, and it carries over to molding, coremaking and casting," said one foundry expert. "When one of these operations experiences difficulties, cleaning work and costs increase."
Cleaning & Finishing
It is common in the foundry industry to use the terms cleaning and finishing interchangeably, when in practice, there is a difference. Cleaning begins immediately after shakeout. In this stage, castings are separated from the mold and cores as quickly as possible to reduce wear on shot wheel vanes and cups, shotblast liners and dust collection systems.
Finishing, in contrast, encompasses welding and other types of metal replacement or surface enhancements like buffing, plating, painting, finish grinding, hole filling or surface treatments.
The main purposes of cleaning room activities are to:
* remove excess metal from castings (sprues, gates, risers, fins and flashing);
* open holes or passageways, shotblast, chip and grind smaller unwanted metal protrusions;
* remove penetration defects, fused or penetration sand cracks and subsurface defects;
* prepare castings for welding repair and surface finishing.
These operations are performed by various abrasive and nonabrasive methods, using vibration, grinding wheels, abrasive belts and wheels, hammers, chipping tools, chemical leaching and metal or grit shotblasting.
Cleaning operations were long thought of as the point in a foundry where all of the mistakes of other departments got fixed. The cleaning room, however, is increasingly being considered a foundry profit center--an active player contributing to better product design, quality and casting competence. Cleaning room difficulties are now recognized in many instances as indicators that upstream operations need attention.
The object of the cleaning room is not to salvage what is unsalvageable or even just to make a cleaner casting. Rather, it is one of economics. How good should a casting be to meet customer specifications? When should a casting be rejected for further rework? These are decisions grounded in profitability, reliability and casting acceptance.
Until a method is found to make castings ready for use as they come from the mold, the cleaning room will remain an essential foundry component worthy of labor and capital investments.
Removing the Unnecessary
Ideally, mold and core sand, risers, gates, runners and other excess molding by-products are removed during shakeout or on vibrating cooling conveyors. This is often not the case, however.
Gray iron risers, sprues and fins often don't snap off in shakeout. Tough ductile iron castings require more severe break-off procedures and nonferrous castings usually need more careful shearing or cutoff operations. Sand can cling to cooling castings and cores can get stuck and become immovable within a cast part. Cleaning requires intense and skillful use of both labor and equipment.
Cleaning room tasks are complicated by the broad mix of product sizes and shapes that make up a foundry's production. For many years, machines were fairly rudimentary, limited to chain falls, hoists, band saws, punch presses, rollover devices, vibrating and oscillating conveyors and assorted power trucks, conveyors and trolleys--and lots of hand labor.
Changes began to appear in the form of simple fixtures that could orient a part for easier access by various pneumatic and power saws, grinding wheels, chippers and hammers. One breaking system variation located a casting in a fixture mounted on a rotating table. As the table turned, the riser was forced against a pressure roller, breaking the riser off the casting.
Another machine still in use is a hydraulically powered wedge that is forced between a riser and the casting to break the riser free. The wedge is supported from overhead by a counterweight or is spring balanced for quick positioning and less operator fatigue.
Many of the methods and equipment used in cleaning departments are improvements on in-place processes. Cleaning processes have been upgraded and made more flexible with new technologies, fixtures and machines; their output has been boosted through automation and computerization;they have been implemented by new ideas for training and are manned by new generations of supervisors and workers for whom mechanization is a way of life.
The shotblast machine, or blast cleaner, uses high-velocity steel shot or grit directed at a casting to provide the final surface finish and clean internal casting passages. It is one of the most important steps in the cleaning process. A properly operated and maintained shotblast machine minimizes the cost of successive cleaning and finishing operations.
The objective of blast equipment design is to enable the blast stream to strike all surfaces of a casting with sufficient force to thoroughly remove contaminants without etching the casting surface. Newer blastcleaning systems offer greater efficiency by reducing parts handling, increasing production, lowering maintenance and requiring fewer replacement parts.
Small castings are cleaned by being rotated or tumbled on an endless belt under blasts of grit or shot. Larger castings are placed on revolving or stationary tables and are sometimes suspended from conveyor hooks where shot or grit is thrown at them as they slowly turn to expose all the casting surfaces to the blast.
Grit dulls the casting surface and, if not controlled, can remove metal from the surface. Steel shot flattens a casting surface with a peening action that causes the metal to flow, leaving a smoother and sometimes shiny surface. Nonferrous metals are too soft to withstand the materials used to blast ferrous metals, and instead use soft steel, copper or bronze shot or glass beads.
All blastcleaning machines consist of a material handling system for positioning the work in the blast stream(s); a cabinet to house the operation and safely contain the blasting medium; an abrasive throwing mechanism (usually a centrifugal throwing wheel); and an abrasive recycling system for cleaning the abrasive and returning it to the wheel for reuse.
A most efficient, effective method for cleaning castings when properly used, shotblasting is often misused as a shakeout substitute. As such, it creates high maintenance costs, production bottlenecks and can adversely affect finished casting quality.
The blast machine directs abrasive projectiles at the casting in a concentrated area at high velocity. Figures 3, 4 and 5 illustrate typical placements of shot wheels to assure complete coverage of a casting; the use of a robot to manipulate a multi-surfaced casting in blast cabinet; and a conveyorized line for continuous blastcleaning operation.
The abrasive material is flung at the casting through the centrifugal action of wheels fitted with vanes spinning at speeds up to 3400 rpm. Compressed air also is used as an abrasive propellant.
The shotblast system is composed of up to a dozen or more abrasive throwing wheel assemblies, depending on casting configurations or cleaning rate required. Parts move through the abrasive stream on tables, conveyors, rotating fixtures or are simply tumbled within the cabinet. Figure 6 illustrates a new direct-drive blast wheel that offers economical and flexible operation.
Tonnage cleaned per hour or day can vary because of the size and shape of the castings. Surface condition of the part to be cleaned also varies, affecting the blast cycle and lengthening or shortening the time required to remove contaminants.
According to one manufacturer, a new generation of blastcleaning equipment promises to be more automatic and offers more versatile, programmable manipulating devices to direct abrasive patterns at specific target areas of individual castings.
These units are multistation, multiwheel machines and are computer-controlled for part axis orientation, time per part exposure to blast stream and more accurate shot throw weight. They may even offer variable shot size selection. They also will have higher safety factors, require less maintenance and provide better sound control.
Chipping & Grinding
Chipping and grinding in the cleaning room are processes that remove and smooth fins, gating/risering stubs, parting line flashing, scabs, welded areas and burned-in sand. Chipping is also used to remove impacted core sand remaining after shakeout and shotblasting.
Grinding and chipping can be done before or after shotblasting but are best done as soon as possible to make castings easier to handle. Removing fins and contact stubs can improve shotblast efficiency and, in some cases, eliminate grinding marks.
Manual grinding is done on larger castings as they move along a conveyor line in a holding fixture or as they are held stationary in a multiaxis positioner. Stand grinders are used to remove metal from smaller castings that can be hand-held.
Automatic grinding equipment is becoming more common as cleaning room operations search for ways to increase efficiency. A schematic of one such CNC-controlled snag grinding machine is illustrated in Fig. 8. The machine's six numerically controlled casting positioning axes can be programmed to automatically grind parting line or core print fins, gate and riser stubs.
For instance, the Z axis (grinding spindle) uses a casting sensory system that scans and adjusts for the position of the casting in the work fixture, accounts for any dimensional deviation and compensates for grinding wheel wear. The machine processes castings weighing up to 660 lb in five operating planes.
High-speed cutoff machines, like the one illustrated in Fig. 9 working with a 2-axis positioner, provide a fast and economical way to remove gates and risers. The swing frame allows 180-degree maneuverability through 90-degree swings to the right or left. A depressed center wheel with a recessed flange allows flush cuts up to a fraction of an inch from the casting face, reducing the time required for secondary grinding.
Using lasers to align the abrasive saw with the correct cut line on the casting eliminates the typical trial-and-error method of making a cut. The laser system uses a four-axis manipulator equipped with proportional valves that rotate or move the part laterally until the laser line is on the correct cut line. This degree of accuracy for laser-assisted positioning greatly reduces grinding time.
Positioning devices are relatively new to the cleaning equipment list. A floor-mounted example of one is shown in Fig. 10, but positioning devices can be mounted on pedestals and machines as well. They can rotate a casting mounted on their fixture plate through 360 degrees horizontally and tilt to a vertical position. This allows part orientation that uses the full range of movement of articulated cutoff saws and grinders as well as presenting the part for touch-up hand grinding and inspection.
Cutoff and Grinding Cells
Figure 11 is a schematic of an integrated cleaning operation, a self-contained, enclosed cutoff and grinding cell centered around a floor-mounted multiaxis casting positioner. Side by side in the cell are a swing frame cutoff machine and a swing frame grinder. Both are suspended from bridge cranes powered for up/down and right/left movements. Each crane and swing frame is controlled by the operator from switches mounted on the swing frames' handlebars.
A casting is secured to the positioner and the operator aligns a particular gate or riser for cutting. Using the crane to locate the cutoff machine over the cut line, the user makes the cut, automatically repositioning by rotating or tilting the part until all gates and risers are removed.
The user then "parks" the cutoff machine to one side, moves the swing frame grinder into position using the second powered crane and performs the necessary grinding operations. The casting is now set for internal grinding with small hand grinders and chippers, and prepared for final inspection while still clamped in the articulating positioner.
Relatively new to cleaning operations are robot-controlled manipulators that can be custom designed for special jobs such as grinding, hammering, shearing and handling heavy castings. Figure 12 illustrates a cleaning operation using a robot to snag-grind a large steel casting. Housed in an air-conditioned cab, the operator controls 4000 separate programmable movements of the robot.
A variety of robot sizes and tooling configurations is possible to pick and place small or large castings, perform hazardous, repetitive and precise tasks, and can be engineered to do successive tasks with custom-designed tooling. Robots can be mounted overhead in fixed locations or rail-mounted for rapid movement to perform successive cleaning operations.
School Provides Basics of Blastcleaning Machines
"By their very nature, blastcleaning machines are self-destructive, but skillful operation and planned maintenance programs will assure maximum productivity and utility for years," said Eugene Tarabek, national service manager for Wheelabrator Corp.
Long, useful shotblast chine production. blast machinery life is the mission of the company's successful maintenance and operations school. Under the direction of Tarabek, the school has trained more than 6000 shotblast supervisors in seminars. The aim of the training is to coach them in the fundamentals of blast equipment design, and key operating principles and procedures.
Instructed by specialists, attendees learn that even aged machines can be productive for many years if properly maintained. During the two-day training sessions, participants are given hands-on experience and classroom instruction involving specific blast machine problems, equipment modernization, and take part in troubleshooting production simulations.
"Foundries are asking their machines to exceed design specs and our job as manufacturers is to help them maximize their production and profitability within the limits available to us," Tarabek said.
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|Title Annotation:||cleaning metal foundries|
|Date:||Nov 1, 1992|
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