Some words of widebelt sanding wisdom can lead to better-quality workpieces.

 

  Sanding is a delicate operation, with very little standing between a perfectly finished piece and a scuffed piece of wood that needs rework. In this month’s debut of CWB’s new “Technology 2x4” column, we pose 2 questions to 4 representatives from leading machinery manufacturers regarding how custom woodworkers can keep their widebelt sanders running smoothly. Their responses are below:

Q. What is the best way to maintain the conveyor belt on a widebelt sander to obtain maximum accuracy and consistent performance?

A. Tim Mueller, marketing manager, Timesavers Inc.: In most instances, conveyor belts will not require a lot of maintenance; however, there are some things that you can do to keep them in good condition.

Most conveyors have a rubber cover to help them “grip” the part as it is passed through the machine. Because of the environment they are exposed to, and since rubber is a porous material, they are susceptible to air and fine dust particles. Air and fine dust particles by nature will dry out the rubber and make it hard and slippery. A conveyor that becomes slippery will not feed parts as well as is needed, and the end result will be marks across your parts as they hesitate under the sanding heads.

You cannot completely prevent this from occurring. But by maintaining a level moisture content in your plant air, making sure your dust collection system is adequate and by actually running your conveyor on a regular basis (to expose all parts of the belt evenly to moist air), you can reduce this problem considerably.

The good news is that even if the belt becomes a little hard and slippery, you can remedy this easily by “dressing” the top of the conveyor belt. This is done with an 80 abrasive belt on one of the sanding heads (only use the contact drum, not the platen, if equipped with one). Simply sand the surface of the belt a little to expose new fresh rubber and you will get your “grip” back.

However, not all machines are capable of doing this (at least, not without making some changes to your equipment settings). So check your owner’s manual or contact the manufacturer for instructions prior to trying this. Remember, you must first make sure the machine is in perfect alignment or you will grind a taper into your belt and that will cost you quite a bit of money to replace.

Another common problem comes from only running parts in the middle of the machine. Operators have to be trained to use the entire width of the belt area. This will aid in keeping an even wear across the entire width, which in turn will provide you will better tolerances over the life of the belt. Again, there will come a time that the middle three-quarters of the belt will be worn thinner than the rest and you will have to “dress” it to bring it back into tolerance.

A. Tim Middleton, sanding product manager, Stiles Machinery Inc.: When the machine is being used for sealer-sanding applications, conveyor belt blow-off is an excellent way to extend the life of the conveyor belt. Dust from the sealer leaves residue on the belt that compromises the belt life.

When the machine is at rest for an extended period of time, the belt should be covered with a protective layer, whether it be canvas or cardboard. This protects the conveyor belt from UV lighting. Ultraviolet lights glaze the surface of the belt faster than anything else. When the belt is glazed, it becomes slippery. If the belt does become glazed over or slippery, you can dress the conveyor belt with a relatively coarse abrasive sanding grit. This is done for two reasons: Number one is to level it, and Number two is to produce a surface with better friction for the workpieces.

A. Keith Paxton, product manager, industrial sanding machines, Holz-Her Inc.: With use, rubber covers on conveyor belts harden or glaze and lose the ability to grip parts as well as when new. This is especially noticeable on smaller parts. Different approaches have been used to overcome part slippage after the belt becomes glazed. Manufacturers have used shoes to create more hold-down pressure, softer conveyor belts to maintain the grip on parts, or vacuum conveyors.

A common solution is to break the glaze using an abrasive. When properly executed, using a hard contact roll in the sander works well. Caution must be exercised, because the abrasive belt is going to grind away material from the conveyor belt and make it parallel to the sanding head. If the drum is not perfectly parallel (within 0.002 inch) to the actual steel conveyor bed, an angled conveyor belt develops. If the drum is not concentric, the conveyor belt will not be flat. Using this method the conveyor belt will take the shape of the contact roll.

We have specific procedures used with our machines to create a perfect work surface.



a. A new 60- or 80-grit belt is used on a hard contact drum

b. Set parallel position from drum to steel conveyor bed within 0.002 inch.

c. Check contact roll for concentricity.

d. Use an accurately machined thin plate (0.250 inch to 0.375 inch) to elevate the conveyor belt above the conveyor bed. The plate must be locked directly under the contact drum.

e. When grinding the conveyor belt, use adequate dust collection and blow the sanded rubber into the dust hood. Cleanliness cannot be over emphasized.

f. Grind no more of the rubber cover than necessary.

g. Re-set the measuring system of the sander after completion.

h. Re–set all heads parallel to the flat conveyor belt.

i. Check hold-down rolls or shoe position after completion.

j. Don’t rush it. Don’t cut too heavily. Take enough time to complete the job with no interruptions. An error can be expensive. If you are uncomfortable with this procedure, hire an experienced technician.

To break a light glaze: If the conveyor belt is flat and has no low area in the center, the conveyor belt can be run at its lowest speed and the glaze can be broken with a hand-held sander using 60 or 80 grit. Use caution. Do not dwell in one location. Check conveyor belt flatness with a straight edge.

Use the entire conveyor belt — not just the center. Start at the belt loading side and work to motor side, then back. Do not feed parts only in the center of the conveyor. Do not feed narrow parts, such as 3/4-inch-wide edges of face frames, into the sander unless there is a group of them bunched together. Let your belt and conveyor speed work together. Give the belt enough time to actually remove material.

We’ve heard of chemicals that break the glaze, but have not found one that works well enough to recommend at this time.

A. Dan McClellan, sales manager/Metal Division, MidwestGroupone: Keep your conveyor belt clean, and make certain that the conveyor is level (parallel) to the abrasive belt head. Always use the correct grit for the work being performed, and use the complete conveyor belt when running parts. If narrow parts are being run, stagger parts across the conveyor belt.

Q. What’s your best tip for getting the maximum longevity from abrasive belts?

A. Mueller: Abrasive belt life is dependant on a myriad of items, too many to cover in just a few sentences. But if I had to choose one single item, I would have to pick “improper stock removal” to be the most common factor in short belt life.

The stock removal of an abrasive belt is one of the most misunderstood (and underestimated) problems in the sanding world. Just because a certain grit belt “can” remove a certain amount of stock doesn’t mean that it should. Every grit is designed to remove a specified amount of stock at a specific feed rate on a specific product (wood vs. metal vs. plastic and so on, or oak vs. maple vs. pine and so on). Changing any one of these will (not may, will) affect the life of the belt.

The main tip to help with this is “know exactly how much stock each of your sanding heads is removing.” Many plants have no idea how much stock is being removed from each head (obviously this applies to multiple head machines). They only know that the part went in at one thickness and comes out at another. Because of improper alignment, sanding belts can easily be forced to take more stock than they are designed to take, and when this happens, not only does belt life suffer, the finish is not what it is supposed to be.

Even on single-head machines this can be a problem, unless the operator knows how to adjust the machine opening to compensate for the varying belt thicknesses when moving from a rougher grit to a finer one. Contact your abrasive belt manufacturers for a stock removal chart and teach your employees how to use them, along with a caliper, and you will take a giant step towards maximizing your abrasive belt life.

A. Middleton: The best way to extend the longevity of any abrasive belt is to minimize heat, however possible. Heat is the number one enemy to an abrasive belt. Proper stock removal with the proper grit at the proper speed, and belt blow-off both help to reduce heat buildup.

A. Paxton: Use the correct grit for the desired stock removal, and match your conveyor speed to the desired stock removal. Slow conveyor speed extends the abrasives’ cutting time and is more aggressive, while fast conveyor speed shortens the abrasives’ cutting time and is less aggressive.

Don’t overwork your grit. Each sanding grain has a specific size. When it is overworked, it heats, breaks down prematurely and life is reduced. Watch heat when using the pad or platen. When sanding with a wide pad, the sanded part immediately exiting the sanding cabinet should be about the same temperature as your forehead. (This guide does not apply to contact rolls or narrow pads.)

Use the correct belt. A cloth-backed belt works fine for heavy calibration because it is stronger. A paper-backed belt generally works much better in intermediate to finish applications. If you have questions, contact an abrasive belt professional who really understands various applications. He can save you money and improve your finish.

Use the correct air pressure to your tension roll. Over-tensioning shortens belt life. Too little tension may not apply the proper power to the abrasive belt to cut properly. The sander manufacturer should have guidelines for this.

Check belt joints. If you see chatter across your part, measure the joint thickness versus the abrasive thickness. A thick joint will create chatter. As a belt works, the abrasive grains wear out and can become thinner than the joint.

A. McClellan: Always stagger feed parts across the conveyor path so you wear the abrasives as evenly as you can. Make sure that you use coarse enough grit to do the work that you want to accomplish, and make sure the abrasive belt is parallel to the conveyor.

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