Widebelt Sanding for Woodworking: Frequently Asked Questions

Posted by Karen Koenig | Posted: 05/01/2012 12:28PM

 

Here are just a few of the frequently asked questions woodworkers have with regards to widebelt sander usage. 

Chatter & Other Marks

Q: What causes chatter?
A: Chatter marks on the product are most often caused by the sanding belt splice. A good way to check is to use a black crayon to mark the splice. Then, run a new, clean workpiece through the machine. If the belt splice is the problem, the black crayon will mark the workpiece with chatter marks. Most of the time, using more pressure or making a heavier cut into the product will cure chatter. An exception to this guideline is a product application on a machine with a hard contact drum, that is not designed for finishing. Causes . Belt splice · Worn contact drum bearing . Vibration in the machine . Worn drive motor bearings · Worn idler roll bearings . Loose or worn drive belts · Flat spots on the drum · Out of balance drum or idler · Conveyor bed not feeding at a constant rate submitted by Timesavers Inc.

Q: How do you solve chatter?
A: Actions to cure chatter marks: Use an abrasive belt with a butt splice · Replace the contact drum bearings · Identify and stop the machine vibration · Replace the drive motor bearings· Replace the idler roll bearings· Tighten the drive belts · Replace or dress the contact drum · Balance the contact drum · Check the conveyor bed drive coupler/drive belt · Relieve tension from the abrasive belt when it is not in use to avoid flat spots on drums. submitted by Timesavers Inc.

Q: How do I identify chatter marks and eliminate them from the operation?
A: Chatter marks are the most common sanding defect and problem. If the marks are uniformly spread across the board, the rollers being out of balance can cause the problem, or the bearings may be worn. The rollers may have become “oval” or the pad, where fitted, may have become stuck. Another cause can be vibration caused by poor machine mounting or loose foundation plates. The spacing between the chatter marks will indicate whether it is the contact roller or a defective abrasive belt. To determine whether the belt is causing the marks, one of two things can be done.

Measure the distance between the chatter marks, mark the belt with a wax or graphite stick and pass a trial piece though the machine. The wax or graphite mark will show up on the trial piece, to indicate if the distance between marks is the same. If the belt is suspected as the cause, try fitting another belt, preferably from a different supplier in case one batch was faulty. If this fails to cure the problem, then look for another cause i.e. the machine bearings. Listen with a stethoscope, or a screwdriver, placed on the bearing race and place your ear to the screwdriver handle to listen for bearing rattle, or place a coin on the bearing head to see the vibration. Also feel the bearing race with the hand for heat build-up during running.

Splice marks are similar to chatter marks but are caused by a poor belt splice. There is a difference between the frequency of marks caused by a splice and chatter marks from a roller. To see if the marks are from the drum or the splice, sand two work-pieces, 1 at a fast conveyor speed, and the other at a slow conveyor speed. Compare the marks; if the marks are similar it is chatter from the drum. If the spacing of the marks is different, the abrasive belt splice causes them.

One important note: Steel contact rollers will leave chatter marks – 99.9% of the time. The belt splice is usually not the same thickness as the rest of the belt and it will telegraph to the board until the contact roller is soft enough to absorb the difference. — submitted by Stiles Machinery Inc.

Q: What are snake lines or tramlines and I do eliminate them?
A: Nearly all tramlines will be caused by damage to the abrasive belt from the workpieces being sanded. Minerals or grit within the material are responsible and once the belt has been damaged it is then of little use. Tramlines appear as elevated lines on the surface of the sanded workpiece. Some of the cause is removed by having sufficient extraction, which will keep any loose particles of grit away from the surface of the workpiece. Sanding the area of the fault with a used piece of abrasive of the same grit can sometimes save the belts. The abrasive belt is the only component moving back and forth on the machine; therefore the belt, not the machine, causes snakes. These can sometime be reduced in magnitude by decreasing the down pressure on a segmented pad or air bladder and/or decreasing both conveyor speed and sanding belt head speed. Further causes can be irregular or jerky oscillation movements, which can cause the belt to crease. Checks should be made on the belt tension and that the oscillation movement is as even as possible. A pressure segment belt, commonly known as a chevron belt, inside the widebelt will totally eradicate these marks. submitted by Stiles Machinery Inc.

Q: What is the best method of preventing chatter marks?
A:
Typically, chatter marks are caused by the abrasive belt splice. Older machines and entry-level machines with small diameters drums and light frames are more likely produce chatter marks. Larger and heavier machines, machines with quality-balanced drums (hard to soft) can operate perfectly without producing chatters. To eliminate the possibility of chatter marks from being created (by the sanding units), it is important to start with a sander with a good drum technology. In many occasions, pad units installed in the last position help in eliminating all chatters. submitted by Costa Sanders/Costa & Grissom

Q: What problems are attributable to belt damage or wear?
A: Three of the worst enemies of a belt are glue, wood resins and metal pins not recessed deep enough. A number of problems also can result from wear:

• Chatter (marks across the width of the part). The belt joint is thicker than the abrasive. This is especially common with hard contact rolls where the roll pounds a thick joint into the part. Damaged contact rolls can cause this, but the joint is the first place to look.

• Longitudinal lines in the finished surface: The belt has loaded or grit has been damaged. This is common in conventional sanding units where the Feed Direction and the Attack Angle from the abrasive are parallel. The belt picks up debris and the debris grows into a thick ball on the belt. Even with a belt cleaner the conventional unit can’t eliminate the debris. The debris overpowers abrasive and a defect line or burn is transferred to the surface.

• Belt creases or damage during belt changes will telegraph to parts and leave damage marks. Handle belts carefully during changes. Hang belts to let them relax rather than storing them.

Lines can also occur when the sanding unit is damaged, i.e., grooved contact rolls will leave lines, as will damaged graphite or foam/felt in a pad. — submitted by Holz-Her U.S., a division of Michael Weinig Inc.

Q: What are the sources of chatter marks?
A: Chatter marks can come from different sources. Most often they come from the splice on the abrasive belt. Belts are usually glued to hold them together which makes them a little thicker at this point. As the splice comes around, the splice is pushed into the wood by the contact roller sanding the wood just slightly deeper than the rest of the belt causing a deeper cut at that particular point. A few solutions to this common problem are:

• Make sure your contact roller has the proper rubber hardness (durometer) for your application. We recommend a durometer of 60 for most sanding applications. Harder contact rollers will take off more material but they are also less forgiving increasing the likelihood of unwanted marks.

• Use a platen for finish sanding. The platen will sand less per pass and also spreads the sanding over a larger surface area because it is actually wider than the contact roller. Running your material through a couple of times during the finishing process with the platen activated should give you a nice smooth finish.

• Slow it down – Running your conveyor belt slower will allow more time for your abrasive sanding to remove the chatter marks. submitted by Safety Speed Mfg.

Stock Removal

Q: How much stock can I remove in one pass?
A: Stock removal on a widebelt sander is determined more by the abrasive belt than by the machine. Each abrasive belt is designed to remove a certain amount of stock, and if that amount is surpassed, the life of the belt is affected. As a rule, you will need to use the lower grit belts for heavy stock removal (36-80 grit belts can remove approximately 1/8 inch to 1/32 inch respectively) and medium grit belts for lighter stock removal (100- 120 grit belts can remove approximately 1/32 inch to 1/64 inch respectively). Belts in grits from 150 on up should only be used for finishing and are not considered cutting belts. Other factors affecting stock removal are: abrasive belt speed, type of sanding head, feed speed and available horsepower. submitted by Timesavers Inc.

Q: Are there advantages to a planer/sander over abrasive planing?
A:
Yes. They are mostly related to power consumption and media costs, which are lower with a planer/sander. An abrasive planer will use high horsepower motors with very coarse sanding belts. The latter are expensive, and usually need to be replaced two or three times a day (depending on the type of product being sanded). A planer/sander uses about half the horsepower of a conventional abrasive planer, on average, and the cutter inserts will last several months. These replaceable inserts have four sides, and one edge will produce around 150,000 board feet of product. Another advantage of the planer/sander is in media costs. An abrasive planer uses 24- or 36-grit belts, and leaves very deep scratches. To remove these, an additional sequence of 60-, 100- and 120-grit sanding belts must be run to achieve the same finish as that produced by a planer/sander running one knife head and one sanding belt. — submitted by Timesavers Inc.

Feeders & Hold-Downs

Q: What are hold-down shoes, and when do I need them?
A:
Hold-down shoes in a widebelt sander are similar to chip-breaker shoes in a planer. They are used to control the part as it passes through the machine, prevent dubbed or sniped lead and trailing edges, and to allow for shorter parts to be run. Specific uses are: short parts, narrow parts, parts under 1/4 inch thick, veneered panels, or any time you need to hold close tolerances. submitted by Timesavers Inc.

Q: When should a vacuum belt be used in feeding a sander?
A:
There are two circumstances for which the vacuum belt is appropriate. The first one occurs when you need to run parts that are shorter than the distance from the infeed pressure roll to the outfeed pressure roll. Without it, the parts to be sanded may slip or stall in the machine. A second situation is when thin products must be sanded. Parts less than one-half inch in thickness will have a tendency to bow and curl. The vacuum belt assists the pressure rolls to flatten and hold the parts during the sanding operation. If they are not held flat, too much sanding may take place on the corners that curl up. submitted by Timesavers Inc.

Q: What is the shortest part I can run through a sander?
A:
The shortest part length is determined by the distance between the infeed hold-down rolls and the outfeed hold-down rolls. These distances change from model to model, however, there are some alternatives like with vacuum beds can hold 6 inch x 6 inch on most materials. submitted by Timesavers Inc.

Platens

Q: What is a segmented platen?
A:
When sanding veneered panels or sealer/lacquer, utmost control is required. To accomplish this, the platen (or shoe) is made up of individual segments which receive sanding pressure individually (pneumatically or electronically). These segments are controlled by a CNC controller that, along with a sensing unit, can be programmed to activate only when needed. By doing this, you have the ability to conform to the irregularities of the panel and sand without the fear of sanding through the sealer or veneer. submitted by Timesavers Inc.

Q: Should we sand with a drum or a platen?
A:
Generally speaking, a drum is used for stock removal and a platen is used for finish sanding; however, drums are also used for finish sanding in some applications. A rule of thumb would be, if you need to remove more than 0.003 to 0.004 inch, you should use a drum, otherwise a platen may be used. The difference between the two is also seen in the finish. A drum will produce a short scratch pattern, but it is deeper on a given grit. A platen will produce a longer scratch that is not as deep. You really need to determine stock removal requirements and desired finish to decide which will fit your individual needs. submitted by Timesavers Inc.

Finishing/Color Matching

Q: What causes irregular color matching on the sanded piece and how do I correct this?
A:
Much of the market today has moved from clear or natural finish tones on their products, to darker stains and coatings. This move has heightened the need for even color matching and consistent finish quality. Poor sanding quality could be hidden when applying natural or light stains, but darker stains and finishes will appear blotchy or uneven when the sanding operations are not performed properly.

While there are many aspects of sanding that can influence the final quality, two of the most common problems we see are incorrect stock removal amounts per abrasive grit being used, and extreme pressure being applied to material causing the cell structure to crush. This crushing of the cell structure leads to uneven penetration of stain into the wood surface. All companies should strictly follow the recommended stock removal amounts given by their abrasive supplier. In addition, all sanding operations should be evaluated to ensure that proper feed speeds are being maintained in line with the amount of stock being removed. Current machine configurations also should be re-evaluated to ensure they are correct for the applications being performed today. Properly configured machines and working units are the most direct way of maintain the correct pressure of the abrasive on the work surface, ensuring a more consistent finish quality for the demands of today’s market.submitted by Biesse America

Q: Why does my stain look blotchy?
A:
This can be for several reasons, but normally because the final grit is too fine and will not let the stain penetrate evenly into the wood. Another reason is that the final sanding unit has abrasives that are clogged and need to be changed. Clogged belts will burnish the wood, close the cell structure, and not allow the stain to penetrate correctly. submitted by Costa Sanders/Costa & Grissom 

Q: How do I get a better finish?
A:
In order to maintain a good finish be sure to find a speed rate for your conveyor table that allows for the sanding belt to do its work and give you a quality finish. A way you can speed up the process is by sanding multiple parts at one time, spread them evenly across the conveyor belt. submitted by Safety Speed Mfg.

Q: How can I ensure a “true” end grit finish?
A:
“True” End Grit Finish - Today using the abrasive grits of 120 – 180 and 220, we must improve the sanded scratch pattern by properly removing the depth of the scratch created by the previous abrasive belt. Such as; a 120 grit abrasive belt produces a scratch pattern approximately .010 deep into the work piece. A 180 grit abrasive belt produces a scratch pattern approximately .005 deep into the work piece. A 220 grit abrasive belt produces a scratch pattern approximately .002 deep into the workpiece.
A:
Quality of End Finish – The end finish is produced by the polishing platen. The graphite polishing platen today consists of a metal bar with machined felt backing wrapped by graphite. The surface condition of the polishing platen will affect the end sanded finish. It is recommended that periodically the polishing platen be removed and inspected for “grooves” or worn areas. It is also recommended that if grooves or slight damage are seen that the graphite of the polishing platen be repaired or replaced. The pressure applied with the polishing platen is also important. The quickest way to gauge the proper pressure being applied would be to feel the surface of the work piece as it exits the sander. The work piece should feel “warm” to the touch. Hot would be excessive polishing platen pressure. This can also roll the outside edges and cause grain relief. Cold to the touch would be insufficient pressure being applied. submitted by Apex Machine Group, division of SlipCon

Q: Can I skip gits (say P60, P100, and P150) and achieve the same finish?
A:
Skipping grits is not advisable. Finer grits cannot clean well the scratch and imperfections of rougher abrasives. This often requires machines with multiple heads. When these are not available, the only option is to find a compromise that works best. submitted by Costa Sanders/Costa & Grissom

Q: What is sealer sanding?
A: Sealer (lacquer) is the first coating applied to your product after finish sanding or staining. The purpose of this is to fill in or "seal" the wood pores and protect the wood. A negative effect to applying sealer is, that being a liquid, it raises the grain of the wood, producing a rough surface. Sealer sanding is performed to create a flat, smooth surface with the proper texture, so that the next coat (also known as the top coat) will adhere properly. Methods used for sealer sanding are: hand sanding, widebelt sanding, brush sanders (both hand held and feedthrough), and with feed- through orbital sanders. submitted by Timesavers Inc.

Veneer Sanding

Q: When should a crossbelt sander be used?
A:
A cross belt sander is used primarily in veneer tape removal applications. The cross belt sander is designed to run across the grain of the wood, which is an aggressive sanding method. Because of this design, veneer tape is removed with one head, whereas two heads need to be used with other wide belt methods. Cross belt sanders also are used on long panels in which the grain runs in the narrow direction, such as desktops and front panels. In processing these, the cross belt is located on the out feed of the machine, and the scratch pattern produced by the belt goes with the grain. submitted by Timesavers Inc.

Proper Setup

Q: How do I eliminate improper setup?
A:
Test the machine’s set up regularly. Sand three parts of equal thickness, length and width located at both edges and the center of the conveyor. Label the three parts for easy reference. Hit the Emergency Stop when all parts are being sanded. Open the machine and remove the parts. Measure stock removal from Unit to Unit and Side to Center to Side.

Side to center to side tolerance should be > .004 inch. Unit to Unit should conform to good stock removal capabilities. Recalibrate the machine as necessary. submitted by Holz-Her U.S., a division of Michael Weinig Inc.

A: Use the right belt. Don’t overpower the belt’s cutting capacity. In conventional sanders, no more than one grit should be skipped: i.e., 80/120/180. submitted by Holz-Her U.S., a division of Michael Weinig Inc.

A: Check your speed: A high feed rate does not translate to high throughput. Give belts time to work. Finish improves and belt life improves. If an inverter driven variable sanding unit is used, set the abrasive speed to accomplish its task. submitted by Holz-Her U.S., a division of Michael Weinig Inc.

A: Too much pressure with pads/platens. This is seen when parts are over heated by the wide contact point of the pad in materials like flat cut red oak. The wide pad will remove the soft sapwood, leaving heartwood as a ridge. This results in a surface with pronounced peaks and valleys. Parts sanded with a 3-inch wide pad should exit the sanding cabinet the same temperature as your forehead. This indicates enough pressure has been applied, but excessive pressure has been avoided. (This test changes depending on wood species and density.) submitted by Holz-Her U.S., a division of Michael Weinig Inc.

A: Too much pressure with soft contact rolls. A soft contact roll will conform to a non-flat surface. Like the pad, it will remove sapwood and conform to heartwood leaving a surface with peaks and valleys. — submitted by submitted by Holz-Her U.S., a division of Michael Weinig Inc.

Q: How do I avoid parts slipping on the feed mat?
A:
The feed mat will become glazed due to oxidation over time. The rubber cover can regain its grip by grinding the mat with an abrasive belt on a precisely set hard contact roll. Extreme care must be taken in this process. Hold-downs may also be improperly set. Too little or too much pressure is applied to the part. Slippage or snipe can occur or parts can be stopped. submitted by Holz-Her U.S., a division of Michael Weinig Inc.

Other Questions

Q: Can I run this machine 50% faster and achieve the same results.
A:
The answer is typically always no. Higher speeds generally require additional sanding units, more horsepower, higher cutting speeds, and with calibrating machines, heavier frames. In the case of calibration sanders increased speeds of a given configuration is almost always problematic and will generally result in; less stock removal, shorter belt life, and/or poorer finish quality. The key here is to always error on the side of “more heads are better”. In the long run it is less expensive if you only consider abrasive belt usage. submitted by Costa Sanders/Costa & Grissom

Q: How do I calibrate the machine and check for adjustment?
A:
Check with your manufacturer to ensure the correct alignment of the heads so the machine sands perfectly and one head does not do more work than required. This also helps with belt life. submitted by SCM Group USA.

Q: How do I get longer life from my abrasives?
A:
Use the correct mineral and backing for job. Don't use paper backing for dimensioning. Zirconium and ceramic will last longer for stock removal and generate less heat (thus less loading from resinous and oily woods and glue) along with less chance of burning woods such as maple and cherry.

• Use the entire width of the sanding belt, stagger stock across width of belt.

• Clean abrasives, either in-house or professionally. There are many cleaning fluids (including several green products) to clean in-house.

• Reverse belt before the abrasive is worn (if the belt and splice allow).

• Do not remove more material than grit is designed. submitted by SuperMax Tools


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About the Author

Karen M. Koenig

Karen M. Koenig has more than 25 years of experience in the woodworking industry, including visits to wood products manufacturing facilities throughout North America, Europe and Asia. As Editor-in-Chief of Wood Products (formerly Wood & Wood Products), Karen’s primary responsibilities include spearheading the writing, editing and coordinating of the editorial content of the publication, along with the Red Book resource guide and the Red Book online source and supply directory (RedBookOnline.com). She is also a frequent contributor to other Woodworking Network online and print media. She can be reached at kkoenig@woodworkingnetwork.com or Google+.

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