Abrasives Lesson 12: Basic Types and Applications for Woodworking

Lesson 12

Basic Machine Types and Applications for Woodworking

The furniture industry ranks in the top five (5) positions of major coated abrasive users. A modern furniture plant uses a greater variety of products, grits, and shapes than any other industry, and an average plant will use approximately $60,000 worth of coated products annually. Success in the furniture industry depends largely upon style, quality, and finish. They depend heavily on coated abrasives in many forms to achieve the desired finished result. Selling in this market requires a thorough understanding of the many types of applications and products used to arrive at this all-important finish.

Coated abrasives are used in almost every segment of the woodworking industry and each segment has its own unique application. Furniture making has always been dependent upon coated abrasives for generating smooth, flat surfaces, and contours. Panel products such as particleboard and fibreboard also depend upon coated abrasives for the finish as well as dimensions.

In recent years, dimension stock mills, furniture plants, and dimension lumber mills have been producing planed boards with the use of coated abrasives - an operation that was dominated by the use of knife planers. The woodworking industry uses the widest variety of coated abrasive product shapes and types ranging from heavy-duty wide belts for abrasive planing down to 2" slotted discs for intricate shape sanding.

Principally, there are three types of sanding done to prepare a wood surface for the finishing stage. The first is primary dimensioning to create a flat, uniform surface. The resulting finish is not important other than the ability of subsequent sanding operations to remove the primary scratches. Coarse grit (40 and coarser) abrasives are usually used for primary dimensioning. Secondly, intermediate sanding is done to remove the coarse grit scratches from preceding operations and bring the board or panel to a uniform flatness that would require only a few thousandths of an inch to complete the sanding. Finally, finish sanding removes the secondary scratches, and brings the board or panel to the condition required before the application of finishing materials.

To become acquainted with sanding operations in woodworking, the discussion will start with abrasive planing operations, both in furniture and panel manufacturing, and progress through the various sanding operations to finish.

Abrasive Planing Operations

Rough Lumber Planing:

Rough lumber planing is the initial surfacing operation for lumber. Furniture plants, hardwood lumber mills, and softwood dimension mills utilize rough lumber planers to remove some, if not all, of the saw, marks as well as bring the lumber to a predetermin­ed thickness.

Rough lumber planers are large, high horsepower, heavy-duty wide belt sanders. They are equipped with 75-250 HP motors with variable feed speeds to 200'/ min. Abrasives used are either 20, 24, or 36 grit, although 40 grit abrasives are used by some. The contact rolls are normally smooth steel, and in some cases, serrated steel. Abrasive belt speeds range from 6.000 - 9,000 SFPM.

Rough lumber planers exhibit several distinct advantages versus knife planing:

a. Elimination of grain and knot tear out and resulting downgrade;
b. The optimum gluing surface where required;
c. Minimum downtime – change of abrasive belt requires but a few minutes;
d. Snipping from overcutting on the ends is eliminated because of equal cut on both sides of the lumber with a thickness tolerance of .005"
e. Roller splits are eliminated with centerline feed works.

Abrasive planers are capable of removing equal amounts of stock from both surfaces of the lumber, main­taining uniform thickness. This allows clean up of both surfaces where thinner boards are encountered.

There are basically two types of rough lumber planing. They are referred to as batch surfacing and in-line surfacing.
§ Batch surfacing deals with the batch method of surfacing any number of pieces up to 48" wide.
§ In-line surfacing deals with the in-line or single-board method with a maximum width of 28". Common machine widths are 18", 24" and 30"

Resin bond open coat zirconia alumina is abrasive on a heavy-duty Y wt. synthetic backing is best suited for this operation.

Finish Abrasive Planers - Finish abrasive planers are large, heavy-duty, high HP wide belt sanders used in furniture plants to perform what is called finish planing.

Belts are usually 50" wide, running on steel contact rolls, driven by 150 HP motors (3 HP/inch of belt width). The majority of furniture plants use abrasive planers to dimension plain or edge banded particleboard and glued up panels, removing approximately 1/16" per side. Thru-feed rates tend to average 45'/min. and, overall tolerance on the finished panels is as close as +/-.0025. In both planing operations, the finish abrasive planer has the advantage over knife planers in the reduction of rejects and/or losses. Resin-bonded open coat aluminum oxide or zirconia alumina abrasives are used for finish abrasive planing.

Wide Belt Sanders - Wide belt sanders are among the more common wood sanding machines and represent the largest segment of the wood­working industry abrasives business. Although machines using 50" wide belts are most common, other machines use belts from 18" to as wide as 106". They come in both top sanding and bottom sanding designs or a combination of both. Multiple head top sanding machines are used most prevalently.

Wide belt machines consist of a horizontal conveyor belt to carry the workpieces through with an abrasive belt running over a contact roll or pressure shoe (platen) and one or more idler rolls. Multiple head machines with two or three sanding heads almost always have the abrasive belt running over a graphite canvas-covered shoe rather than a contact roll on the final head.

Wide belt machines may be used for primary sanding operations using coarse grit abrasives (usually 40-60 grit), or for finishing operations where 100 grit and finer abrasive belts are used. All of these may be used on one multi-head machine.

Because these machines operate in the speed range of 5600 SFPM, aluminum oxide paper, and cloth belts are used almost exclusively, and are almost always open coat. An exception is print lines where 320 and 400 grit SiC paper belts are used at about 1000 SFPM belt speed. Among the most commonly used machine manufacturers are Time-savers, A.E.M., Sanding Master and Ramco

Narrow Belt Sanders

Stroke Sander - Stroke sanders are among the oldest and most common machines that use coated abrasives. A 6" or 7" wide abrasive belt, usually 335" long and longer, runs horizontally above a table that supports the workpiece. The table on hand machines is manually reciprocated under the belt to sand the full width of the workpiece, while on automatic machines, a conveyor belt moves the workpiece through the machine under the belt.

Stroke sanders are used for final sanding on furniture parts in preparation for finishing. Usually, the parts are sanded with two grits in a "cut down" and "polish" san­ding sequence. Some stroke sanders have double belts so that both operations can be performed on the same machine. Aluminum oxide resin paper is the principle abrasive used on stroke sanders, but occasionally garnet is used. On rare occasions, glue bond paper belts are used. White aluminum oxide is becoming increasingly popular for use on stroke sanders because of its superior sharpness. The common grit sequence used for "cut down" and "polishing" is a 100 and 150 grit combination or 120 and 180 grit. The choice depends on the wood specie sanded, and/or the quality level of the furniture manufactured. Most parts that are stroke sanded have had prior sanding either on a drum sander or a wide belt sander. There is a trend toward using the finish obtained on a wide belt sander as the final sanding, particularly in low price furniture. However, it is impossible to obtain the quality of sanding on a wide belt machine that can be obtained on a stroke sander.

There are four (4) types of stroke sanders in common use:

1. Hand Block
2. Hand Lever
3. Automatic Stroke
4. Thru-feed Automatic Platen

Hand Block - On this barest and simplest type of stroke sander, the operator depresses the belt with graphite, canvas-covered, handheld padded block. The operator strokes the hand block longitudinally along the belt, and manually reciprocates the table under the belt to sand the width of the part. This is the crudest type of stroke sander, but, surprisingly, it is frequently used in the production of high-quality furniture.

Hand Lever - An improvement over the hand block stroke sander, in that the hand block is replaced by a shoe or pad that travels on a horizontal rail parallel to the belt. The operator applies sanding pressure by depressing the shoe with the hand lever and manually strokes the shoe along the belt to sand the length of the workpiece. As in the hand block machine, he manually reciprocates the table under the belt to sand the full width of the panel.

Automatic Hand Lever - This machine is similar to the hand lever stroke sander ex­cept that the lateral movement of the pressure shoe along the belt is hydraulically powered. As in the hand lever stroke sander, the sanding pressure is applied by the operator depressing the shoe with a hand lever. Table reciprocation is manual.

Thru-feed Automatic Platen Sanders - Automatic sanders are efficient, high production machines that differ considerably from the manual stroke sanders. Workpieces are carried through the machine under the abrasive belts on an endless conveyor belt of variable speed. Sanding pressure is applied by a platen that lowers when the workpiece is in a standing position and applies pressure on the abrasive belt. The platen incorporates an airbag that can be inflated to varying degrees of air pressure to regulate sanding pressure. The length of the inflated area can also be adjusted to effect sanding pressure only on the length required.

Production rates on thru-feed automatic sanders are very high. A single machine can easily replace 6-8 hand lever stroke sanders. They are available in single and double belt models, the double belt being most common. Heeseman, Bottcher-Gessner, and Tannewitz are popular makes of thru-feed automatics.

Edge Sander - On this machine, a 6-8" wide abrasive belt travels laterally over two vertical pulleys. In some areas, it is called a side stroke sander. The sanding belt is backed up by a vertical platen, with a horizontal work rest table in front of the belt. These machines often have a platen and work resttable on both the front and back of the machine as well as a half round table at one end. An edge sander is primarily used for sanding square edges on panels or square table legs. Most have provision for the belt to oscillate while in use. The work rest table height is adjustable so that the full width of the abrasive belt can be utilized. Abrasive belt speeds range from 2500 to 4000 SFPM. For sanding inside radii, the half round table at the end is often used with the sanding being done by the belt running over the idler pulley. Often the idler pulley is replaced by a formed wooden block suitably padded and covered with graphite canvas. This is called a nose block and manufacturers refer to the sanding operation performed on it as "dead head" sanding.

Open coat resin bonded garnet or aluminum oxide X weight cloth belts are commonly used on edge sanders, usually in the 80-120 grit range. Garnet is preferred to sand end grain because it has less of a tendency to burn than aluminum oxide. Lubricating the back of the belts with stick graphite and using graphite coated canvas on the platens will reduce the heat buildup on edge sanders.

Mold Sanding - As the name implies, mold sanding involves the sanding of straight furniture parts that have a machined shape usually on the edge. This is accomplished through the use of a handheld formed block, a stationary formed block, or an automatic thru-feed mold sanding machine where the work is automatically conveyed past a stationary formed block. In all cases, the abrasive belt is made to bear on the workpiece by a formed block that has been machined to match the shape of the molding. On the hand block machine, the formed block is manually stroked along the length of the molded edge. On a stationary block mold sander, the form block is mounted in a fixed position and the workpiece is pushed past the form block.

Garnet is by far the most commonly used abrasive for mold sanding, but occasionally aluminum oxide is used. Almost invariably, open coat materials are used for mold sanding and, although J wt. resin over glue construction has been most popular, there is a big increase in the use of resin/resin X weight products. Many furniture companies make their own belts from roll goods, but many also buy 14" and 28" wide belts and tear them down to the various narrow widths required. Mold sanders usually operate at relatively low speeds, 2500-4000 SFPM, and 100 or 120 grit are the most commonly used grit sizes. Although maximum product flexibility is of key importance for mold sanding, customers often purchases materials with a 90 flex to ensure that a fold or "break line" in a belt will track through the form block.

Pneumatic Drum Sander - The pneumatic or "pump drum" sander consists of an inflated rubber drum, rotating on the horizontal shaft of a floor stand mounted electric motor. An abrasive sleeve, usually 9" wide, is slid over the drum which is then inflated to the hardness or firmness preferred. The pneumatic drum sander is used for sanding contoured parts and for breaking edges. Conversion of this machine to a belt machine by the addition of a back stand idler greatly improves abrasive efficiency. Open coat resin/resin garnet X or J weight abrasives are usually used, with the popularity of the flexible X weight increasing. 100 or 120 grit are the common grits used

Vonnegut Brush Back Sander - This sander is essentially a rotating wheel, 4" wide x 14-16" in diameter, with either 16 or 32 brushes (not unlike paint brushes) mounted around the periphery. Slashed loadings are coiled inside the hub, and the individual strips are fed out in front of each brush. The brush back sander is commonly used to sand beaded or fluted parts like chair arms and is a finish sander only. 1/8", 1/4", and 1/2" wide slashed loadings are commonly used. When ordering, specify whether parallel or staggered slashing is needed.

Spool Sanding - Spool sanders are used for sanding molded parts, usually an inside radius. A felt or rubber spool is dressed to match the molding, and covered with a cloth back abrasives disc having 16 wing slots (also called DeLappe discs). The spool may be mounted on either a vertical or horizontal spindle. A different type of spool sander incorporates an idler pulley arm, and narrow belts run over the formed spool. More efficient abrasive usage is obtainable in this mode.

Spindle Sander - On a spindle sander, an oscillating rubber spindle or drum from 1-6" in diameter projects vertically through a round work table. 100-150 grit sleeves are used to sand inside radii.

Scroll Sander - A small sander, sometimes called tongue sander, has a blade or tongue projecting vertically about 6" from an opening in the work table. A drive pulley below the table powers a 1/2 to 1-1/2" wide belt that runs over the vertical blade and flat against the blade. The scroll sander is used to sand cutouts such as chair backs, music stands, grills, etc

Flutter Sanding - A cloth disc with eight (8) radial slots is mounted on a horizontal spindle. A corner of each wing of the disc is folded over similar to a pinwheel and secured with a nut and washer. Sanding is done with the unsupported periphery of the pinwheel. This flexible disc is used for sanding intricate carvings often found on chair arms or the feet on some chair legs.

Drawer Sander - A drawer sander is basically a horizon­tal platen sander with a very sharply radiused lip at the operator's end. 10" 12" and 14" wide belts are used to sand flush the dovetailed joints on assembled drawers. The machine operates at a relatively low speed from 3000-3800 SFPM. Two problems are encountered with drawer sanding belts:

1. Because of the extremely sharp angle at which the belt breaks over the lip of the platen, an exceptionally strong, flexible splice is required. The KB splice used exclusively for drawer sanding belts meets these requirements.
2. Usually, the drawers are assembled a few minutes before they are sanded, causing heavy belt loading from the excess glue. Because of this, all resin open coat aluminum oxide belts are used

Orbital Sanders (Jitterbug) -These portable air or electric sanders are used extensively in the cabinet room for overall clean-up or touchup prior to the final finish. Various cut-to-size abrasive sheet goods are used, usually in 100-150 grit. More recently, resin bond J weight cloth-backed abrasives are being used in place of paper because it is less susceptible to puncturing or tearing, and overall longer life makes it more economical to use.

Hand Sanding - A variety of sizes of C weight or D weight paper sheets are commonly used, with or without a felt or wood hand block, for overall touchup or cleanup, removing glue spots, pencil marks, dirt, etc. before the final finish. Although stearated papers were not intended for sanding wood in the white, they are frequently used in cabinet rooms for final cleanup. Usually, 100-150 grit garnet papers are used.

Finishing Room - The most common operation found in finishing rooms is sealer sanding, usually done with jitterbugs or random orbital sanders using A weight garnet or silicon carbide stearated papers in the 240-280 grit range. Occasionally, sealer coats are hand-sanded in the finishing room using garnet or silicon carbide fine-grit stearated paper.

Rub Room - The final finishing operation is done in the rub room where the conspicuous areas - dresser tops, table tops, etc., are polished with silicon carbide waterproof paper and a special lubricant or "rubbing oil" A heavy-duty, dual air-operated rubbing machine is used for this operation. 360 -600 grit are the most frequently used grit sizes.

Panel Products Sanding

Particle Board, Flakeboard, Medium Density Fibre Board, Oriented Strand Board

The panel products industry has become one of the major markets for coated abrasives in wide belt form. The above products are all manufactured in basically the same manner. Wood chip flakes or fibres are dried, blended with resin adhesives, deposited on a caul in a loosely formed mat, then pressed to uniform thickness and density. The panels are then cooled, trimmed to length and width and sanded. They are then inspected and graded.

Panel plants utilize giant 4, 6, and 8 head sanders that sand both sides. These are huge, rugged machines that use high HP (150+) motors to drive smooth steel contact rolls on the primary heads and remove 0.090"-125" of material at feed rates ranging from 100-200'/min. The final or finishing heads use a graphite canvas-covered platen or shoe. Common belt widths are 50", 51", 52", 63", 65" and 67".

A large portion of the panel products manufactured can be classified as either Industrial or underlayment, each with different requirements regarding the abrasive used.

Underlayment must be sanded to close uniformity of thickness but finish is not par­ticularly important. In sanding industrial board, finish is of greater importance. The industrial board is the material used by many furniture companies as core stock for veneered dresser tops, tabletops and other furniture parts. Some industrial board is vinyl or paper coated, and some are filled with a urethane filler material painted with a color ground coat then printed with wood grain for use as furniture components.

A six-headed machine is the most commonly used for panel sanding and usually consists of 3 top heads and 3 bottom heads. The first 4 heads are almost always contacted rolls, while the last two heads are smoothing bars or platens with 1/4" felt and a graphite canvas cover.

Run sequences vary with different consumers and products. Some will use 36, 60, 80; others will use 40, 60, 80, or 100. Where finish is critical to the end-use customer of the panel, the grit range may go as high as 120. A common problem encountered in panel sanding is streaking. Usually, these are raised ridges, the result of the belt being scored by a piece of metal or stone included in the panel. There is no abrasive remedy for this problem other than replacing the belt. Panel sanding is a severe application for coated abrasives because of high stock removal, high horsepower, and high feed rates. Consequently, belt breakage runs high with more belts broken than are removed when worn out.

Another factor that affects abrasive belt life in panel sanding is the amount of resin used as the binder in the panels. This varies con­siderably from product to product, running from 7% to 15%. Belt life will be longer when sanding low resin con­tent panels versus high resin content panels. High panel temperature can also affect belt life adversely. Some plants sand shortly after pressing with the panels still hot, while others stockpile between the press and the sanders to allow the panels to cool before sanding.

Provision should be made in a plant to hang the belts for several hours after they have been removed from the box and before installing on the sander.

Resin bond open coat zirconia alumina on heavy-duty Y wt. backings are generally recommended for primary heads using 24, 36 or 40 grit where heavy stock removals are encountered. This product is limited to 52" wide belts. For all other widths and for the finer grits (60 and finer), resin bond silicon carbide abrasive on a heavy-duty Y wt. the backing is used exclusively. It can be manufactured into belts up to 67" wide.

Plywood Sanding

Plywood mills use the same high horsepower, heavy-duty machines that the panel pro­ducts industry uses - 4, 6, or 8 head sanders. Abrasive products are also the same resin bond open coat zirconia alumina in the coarse grits with resin bond silicon carbide in 60 grit and finer. Plywood sanding lines operate in the 90-150'/min. feed rate range, slightly lower than the speed on particleboard sanding lines. Also, because the thickness of veneer plies can be closely controlled, the total stock removal required is not as high on plywood sanding lines as it is on panel product lines. These factors, plus the fact that the wood veneer surface sands more easily than the part resin surface of a fibre board, contribute to considerably longer belt life on plywood sanding lines. In the coarse grits, a belt life of 5 days is not uncommon, barring accidental belt damage which is not infrequent. Abrasive grits used are similar to those used in panel sanding 40-120 grit depending upon the number of sanding heads and the grade of the board product.

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Product Selection Guide - Woodworking

Narrow Belt - Flat Sanding Operations (Belt width 12" and under)

Narrow Belt - Shape Sanding Operations (Belt width 12" and under)

Other Wood Sanding Operations

Orbital Sanders (Jitterbugs)

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Courtesy Carborundum Abrasives