A Systems Approach to Dual-Action Sanding.

For quality random-orbital finishing results aim for harmony among discs, backing pads, air flow systems and sanding tools.

DUAL-ACTION (DA) SANDING has become almost universal in woodworking plants for touch-up and repair work. The technique, also known as random-orbital sanding, is used to create a uniform surface finish and remove cross-grain scratches prior to staining.

The terms "random-orbital" and "dual action" refer to the sanding motion, which is created by combining the free-spinning rotation of a disc with an eccentric motion rotation of the motor shaft. The resulting sanding motion removes stock quickly, and leaves very short, random scratches.

Air-powered and electric DA hand tools are used in these applications. A circular, coated abrasive disc is mounted on a backup pad, which is then fixed to the face of the tool. Some DA sanders are designed with a built-in dust evacuation system, which operates through the face of the disc. The standard industrial vacuum-hole design is a 5-inch diameter with five vacuum holes.

Random-orbital sanding is one of the final quality stages for wood "in the white" prior to staining. Therefore, the quality of the surface finish has a great effect on the final appearance of the piece. For best results, users should take a "systems approach" to this application by carefully considering the choice of discs, backing pads, air flow systems and sanding tools, and the interaction between these components.

Sanding Discs and Abrasives

As mentioned, a 5-inch-diameter disc is the most popular size for furniture applications. Two main types of discs are used: pressure sensitive adhesive (PSA) discs and hook-and-loop discs, which use a hoop-and-loop design to adhere to the back-up pad.

Each type has its benefits depending on the application. PSA discs are less expensive than the hook-and-loop type, and have a slight advantage when sanding sharp corners, which can damage the "hooks" of a hook-and-loop backing pad face. However, hook-and-loop discs generally last longer than PSA discs in many applications, due to greater tear resistance and less heat being generated during use.

The type of abrasive grain specified should also be considered carefully. In general, aluminum oxide is a good choice for whitewood (bare wood) sanding. For sealer sanding, silicon carbide abrasive is an excellent alternative.

One of the best performing abrasive products on the market is premium aluminum oxide. Recommended as a first choice for DA sanding in furniture plants, this product uses an extremely durable grain that cuts fast and stays sharp longer than conventional products. A special treatment, called "stearate," is coated onto some abrasive products to reduce clogging and loading of the grit surface. Stearate is a soap-like release agent that prevents any soft or gummy sanding residue from sticking to the sanding disc.

Grit sizes for DA disc sanding generally range from P80 to Pl200B. The grit size chosen depends on the criteria for the particular job. Coarse grits remove more stock, have a faster cut rate, and last longer than finer grits. However, a grit that is too coarse will leave scratches that may remain in the work surface without extensive sanding with finer grits. Use the coarsest grit allowable for the job.

The most popular grits are: 80 or 100 for stock removal, 120 or 150 for intermediate upgrade of coarser scratches and softer woods, and 180 or 220 for finish sanding on bare wood, especially harder woods. Do not skip more than one grit size when moving from coarser to finer grits.

Remember, also, that DA disc sanders create a finish that is one to two grit sizes finer than a coated abrasive belt finish. For example, a grit 180 DA sander will generate a grit 220-240 finish.

One of the reasons DA sanders are so popular is because they cut fast, yet leave a good finish because the scratch lines are short and random. Most DA sanders have a "free running" speed, with the tool not in contact with workpiece, of 10,000 rpm, with some newer models operating at 12,000 rpm.

In addition to the rotation of the sanding pad, there is also an "eccentric" motion to the pad. The size of the eccentric "orbit" is a standard 3/16 inch, but some manufacturers offer an optional, tighter orbit pattern of 3/32 inch, which reduces the likelihood of swirl marks, also known as "fish tails."

To reap the full benefits of DA sanding, proper setup of the DA tool and the air system is essential to have the "total package" of performance.

Air Flow Requirements

Random orbital sanders come in either electric or pneumatic models. Electric tools are generally less expensive and have the advantage of being used wherever an electrical outlet can be found. Pneumatic models are generally smaller, lighter and have longer service lives than electric tools. Pneumatic tools, however, require air compressors that can be expensive to operate and maintain for production use.

Air systems used for DA sanding require a compressor that generates a constant air volume of 14 to 20 cubic feet per minute (cfm) at a pressure of 85 to 90 pounds per square inch (psi). The optimal air pressure to maintain is generally 90 psi. It is important to note that a decrease in air pressure from 90 psi to 60 psi will decrease the free-running speed of the sander about 12% and reduce maximum power by more than 40%. The result of reduced speed or power is swirl marks and inconsistent finishes.

Air delivered to the tool must be free of dirt, moisture and excess oil. (No oil is preferred.) Fixed air line (metal pipe) layouts should be designed to maximize flow and eliminate condensation.

Flexible air lines should be kept as short as practical because rubber hose or coiled hose creates friction to the air flow, and thus creates a pressure drop. A maximum length of 20 feet is usually recommended. Keep in mind that 5/16 or 3/8 inch I.D. hose is better than 1/4 inch I.D. hose because the large diameter will allow a greater volume of air to pass through the hose in a given time period. A greater volume of air will aid in maintaining the air pressure during use.

For maximum air flow, it is best to use a direct connection from the main air line to the air hose, and from the air hose to the DA tool. Typically, however, a quick-disconnect coupling is used to connect the tool to the air hose, so that the tool can be moved to different areas of the workshop or plant, or to allow the same hose to be used for different tools.

There are three primary types of quick-disconnect coupling designs in use today: Aro, Industrial/Milton and Automotive/TruFlate. (Each type is made by a variety of coupling manufacturers.) All are known as 1/4-inch basic types or body style. (This refers to the thread size on the threaded ends.) Of the three, the Aro coupling design seems to be the most popular in furniture plants.

The standard I.D. on the male couplings of all types is 3/16 inch. However, there is now available a "maxi-flow" male coupling with 1/4-inch I.D. Note, also, that the female coupling is sometimes interchangeable with all types of male couplings, but most commonly is usable only with the matching male coupling design.

Each coupling can reduce air flow by the equivalent of approximately 15 feet of flexible air hose. For that reason, keep quick-change couplings to a minimum on your DA setup.

Backing Pads

The backing pad chosen for the job has a great impact on sanding quality and operator comfort. Among the best backing pads are those made of molded polyurethane foam with a fiberglass backing plate and a riveted plate holding the inch 24-threaded shank.

Backing pads should be correctly balanced to reduce vibration to the operator, excessive wear on the tool and poor sanding finish. An out-of-balance pad will "duck walk" on the workpiece.

To check a tool for improper pad balance, hold the air hose a few inches behind the tool inlet. Push the trigger of the tool with one finger of the other hand, and allow the tool to sand on a flat surface. A properly balanced pad will allow your finger to guide the tool with ease; an out-of-balance tool will "walk" uncontrollably.

Operating Techniques

The most important thing for operators to remember is "start on, stop off." In other words, the tool should be on the workpiece when it is started, and then removed from the surface before it is stopped. A tool running at 10,000 rpm will easily gouge the workpiece if allowed to come in contact with the surface at full speed.

Other tips for best results include:

* Use slow, even sanding strokes with an overlapping pattern.

* Avoid tipping the tool on edge. Try to keep the pad as flat on the work as possible. Let the weight of the tool and the abrasive do the work.

* Sand with the wood grain whenever possible.

* Keep tools clean and well-maintained. Clogged tool mufflers and exhaust channels will reduce sanding efficiency.

* Keep backing pads in good condition and clean on the surface.

* Remove PSA discs for overnight storage of tools. The vinyl face on most pads will permanently bond with the PSA if left attached for long periods.

* Oil the DA sander daily as recommended by the tool manufacturer. Three to four drops per shift is typical.

Doug Washer is Regional Product Engineer for the Norton Co., Troy, NY.