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Overview of abrasive-belt grinding.

Throughout the world, coated-abrasive grinding machines have been developed for a variety of metalworking applications. In the past 50 years, coated-abrasive-belt machines have served for snag grinding, deburring, heavy stock removal, tolerance grinding, improved microfinishing, cosmetic finishing, centerless grinding, and contour grinding.

Coated-abrasive belts have several advantages over other types of perishable tools used for single-point turning, milling, bonded-wheel grinding, etc. An abrasive belt can be manufactured to many specifications for operations ranging from rough grinding to fine finishing. The same belt machine can handle several different grinding requirements by simple belt changing. A single machine can be changed from one abrasive grit size to another in less than a minute.

Coarse-grit belts with abrasive-grain sizes ranging from No 24 to No 100 can provide fast, heavy stock removal on either manual or automatic grinding machines, processing stainless steel, titanium, carbon steel, cast iron, brass, aluminum, and other materials.

Grain and belt selection

Belts can be made with several different basic types of abrasive grain, including aluminum oxide, silicon carbide, and zirconia alumina. All abrasive media have certain qualities and properties to meet specific grinding requirements.

Aluminum oxide is probably the most commonly used grain for general belt grinding. It provides greater belt life on ferrous materials than silicon carbide, which has harder and sharper grain characteristics and is more commonly used on aluminum, brass, and copper alloys. Also, for certain cosmetic finishes on stainless steel, silicon-carbide belts help produce an established deep-blue color tone for final decorative finishes.

Zirconia-alumina grain was developed over the past 10 to 15 years for heavy stock removal and longer belt life. Normally, zirconia-alumina belts are used on automatic machines with higher horsepower than manual machines. Here, it is necessary to apply heavy grinding foreces to break down the abrasive grain with new sharp cutting edges to prolong belt life and maintain heavy stock removal. High stock removal also demands belts with heavier backing, providing tensile strengths up to 100 lb per inch of belt width. The backing material nromally used is made of polyester, rather than cotton or paper, to withstand the heavier grinding loads.

Abrasive belts can serve for wet or dry grinding. In wet grinding using straight oil, water-soluble oil, or synthetic oil, the belt construction, backing, and overall specifications must be well considered to gain best overall performance and economics.

There are several types of splices available for joining belt ends. Each splicing method has an effect on grinding quality and belt efficiency. The most common method is the lap splice, where belt ends are cut on a bias and overlapped for joining. A straight butt splice commonly is used on polyester belting material and today offers the extra strength needed for high-pressure belt grinding. Belt-splice thickness also must be controlled to suit a given grinding operation.

Machine designs

There are many machine concepts for using coated abrasives, starting with a simple hand grinding and polishing lathe for snagging, deburring, chamfering, and microfinishing relatively low volumes of parts. As an example, Figure 1 shows a manual grinding and polishign lathe with the following basic components:

* Main spindle drive. The drive can be single speed, or variable speed for greater flexibility. Belt speeds vary from 2000 fpm to 10,000 fpm for typical grinding work.

* Abrasive-belt-takeup device. This item keeps proper tension on the belt for grinding and tracking. Mechanisms can vary from simple mechanical or spring-tension devices to pneumatic and hydraulic tensioning systems for wider and heavier machines. For maximum productivity, it is necessary to hold belt-change time to a minimum. Normally, an operator can change belts in less than 60 sec.

* Abrasive-belt-tracking device. This component can be manual or automatic, depending on belt width. The idler or tensioning pulley can be adjusted to assist with proper belt positioning and and centering while the belt is running. Many types of tracking mechanisms have been developed to control belt position for a given belt size. When belt width is greater than 6" or 8", automatic tracking devices often are used to control and center the belt between the contact wheel and the idler pulley or pulleys.

* Contact wheel. The contact wheel supports the belt at the point of grinding and can be rubber covered, cloth, or metal. Rubber wheels come in various densities, depending on requirements. Wheels can be serrated in various patterns, Figure 2, or made with solid faces. Various rubber-covering compounds are used for contact wheels and rolls to meet given grinding conditions. The compounds include Neoprene, Buna N, butyl, vinyl, silicone, polyurethane, Hypalon, and Viton.

Certain variations of manual and automatic belt grinders use a platen behind the belt instead of a contact wheel. A platen grinder performs edging, squaring, facing, flattening, and sizing. Figure 3 shows a 10"-wide platen grinder set up for dry grinding. It's commonly used in toolrooms and polishing departments for general-purpose metal grinding. A smaller 6" model is available, and both can be set up for wet grinding as well as dry.

A platen machine can be designed with preset automatic cycles for tolerance grinding to [plus or minus]0.001". The machine employs a hydraulic work table that automatically feeds the fixtured workpiece into the vertical belt and oscillates the table from side to side to use the full belt width. This action also improves workpiece flatness, tolerance, and surface finish. When the machine is set on an automatic cycle, the operator can run more than one machine or perform other jobs during the grinding cycle.

Centerless grinders

Centerless-grinding machines use grinding ODs of cylinders, spools, cylindrical parts, tubing, and round bar stock. Machines of this type have served in many metalworking plants, factories, and steel mills for more than 100 years. The same basic machine concept has been applied using abrasive belts for the last 40 years. During this period, many significant machine developments have been made.

Today, abrasive-belt centerless-grinding equipment is available in many sizes and degrees of automation. Machines can be provided for small parts, finishing in the range of [plus or minus]0.0005" and microfinishing down to 1 to 10 microinches rms. A small model grinds parts from 1/8" to 2" dia. A single-head machine equipped for wet grinding-head machine equipped for wet grinding typically uses an abrasive belt 4" wide driven by a 5-hp motor. Manual part feeding and manual machine adjustments help hold down initial equipment costs.

In comparison, more sophisticated centerless belt grinders are available with automatic feeding, part gaging, automatic machine diagnostics, and 40-hp to 100-hp motors with electronic controls. These machines can take advantage of cutting belts that allow faster stock removal. For example, a large four-head tandem system with automatic load and feed tables has been built for grinding bar stock up to 6" OD and tubing up to 13" OD in mill lengths to 30 ft. The belt grinding heads selected employ electronic controls and a patented workrest that maintains a constant centerline to provide rapid machine changeover from one diameter to another. Figure 4 shows details of the centerless configuration.

More automation

Other designs include automated strip- and coil-grinding equipment for the steel industry. In one case, a six-head, 66"-wide coil-grinding machine was designed with 150-hp motors to provide stainless-steel condition grinding and final No 4 finish grinding. These machines use automatic belt tracking with hydraulic belt tensioning, and hydraulic billy rolls or support rolls opposite each contact roll to apply the grinding pressure, Figure 5.

Abrasive-belt grits are used progressively in sequence, and hydraulic breaker rolls are positioned between each grinding head for more efficient performance. The stainless-steel coil material is under tension of nearly 5000 psi. For good grinding performance, the coil should maintain a coil-break angle of 8 degrees to 20 degrees, depending on coil thickness and grinding parameters. Figure 6 illustrates a three-head line with typical coil break.

Abrasive-belt flat grinding and polishing equipment can provide cosmetic finishing using tandem grinding heads that work with conveyor-belt transfer systems. A six-head line for grinding butt hinges appears in Figure 7. The hinges are fixtured on the conveyor belt and ground as shown in Figure 8. The machine can belt grind the two flat areas adjacent to the hinge barrel at a rate of 1000 to 1500 pcs/hr.

In another application, a 10-head abrasive-belt conveyorized system grinds saw blades up to 28" long, Figure 9. The hard heat-treated blades automatically feed into the machine from a part-stacking double-rack system. The blades feed through five abrasive-belt heads before they are turned over, Figure 10, and passed through a second five-head system that grinds the bottom side. The blades exit through an automatic washer and oiler to preserve the final cosmetic finish. This system can produce 400 to 800 blades per hour.

Automatic rotary indexing machines are widely used for grinding and polishing stainless-steel cooking utensils, and hand tools. On the finishing machine in Figure 11, coated-abrasive belts in the first two work stations grind the OD of the stainless-steel body. A polishing operation wth abrasive flap wheels blends the outer surface after belt grinding. This equipment can produce 200 to 400 cookware bodies per hour, depending on required finish. Part condition before finishing also affects productivity.

For rough-forged hand tools such as wrenches, hammers, screwdrivers, and punches, belt grinding can remove a lot of material efficiently. It's common to use several setups to grind different surfaces of a given part with one machine. For example, hammers are ground on each side, face, and chamfer of one six-station indexing rotary automatic machine with five abrasive-belt heads at 300 to 400 pcs/hr.

Another four-station indexing rotary automatic with 16"-wide grinding heads rough grinds top and bottom wrench faces at about 1000 pcs/hr. The belt heads are equipped with pneumatic belt-tensioning and automatic belt-tracking devices.

Today, with CNC and servomotors, contouring can be done electronically without the need for mechanical cams. For example, a CNC belt grinding machine now rough grinds the profile of eight claw hammers, each of a different size. The basic CNC machine grinds five hammers simultaneously on each of two semiautomatic units. The two-axis positioners are programmed through a special CNC system with automatic teach-mode capability, Figure 12. Each machines uses a 10"-wide X 164"-long abrasive-belt grinder with 25-hp motors, steel contact wheels, pneumatic belt tensioning, and automatic abrasive-belt tracking.

This, then, is the latest abrasive-belt grinding technology. It should be investigated as another modern universal tool for manufacturing plants. The basic machines, in conjunction with new coated-abrasive products, cna provide efficient production concepts for future growth and prosperity--in China as well as the US. For more information, Circle E3.
COPYRIGHT 1985 Nelson Publishing
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Copyright 1985 Gale, Cengage Learning. All rights reserved.

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Author:Carlson, Glen A., Jr.
Publication:Tooling & Production
Date:May 1, 1985
Words:1772
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