I wrote this a few years ago for the sanding technicians. I, of course, had help from many different sources. Love it or Hate it, let me know what you think. I can expand on any of them if desired!
Thanks and have a good night.
Wide Belt Sanders:
Operation, Troubleshooting and Maintenance
Daily Care
Good House keeping should be a priority, as should ensuring that the correct tension is used. When the belt is running it is important to use the right tension. Equally at night, don’t forget to release the tension on the abrasive belt. Correct storage of abrasive belts can save a lot of problems, as belts are vulnerable to variations in relative humidity. Do not allow new belts to become cold and damp, always have 2 days supply of belts near to your machines. This allows the belts to normalize to the atmospheric conditions before being put onto the machine. Everyone should build a good widebelt storage rack for hanging you sanding belts. You can build it with simple pipes not less than 50 mm in diameter attached to the wall. You should space your pipes at least 6 to 8 inches apart. Hanging your belts relieves stress in the seams, makes them more supple, and easier to install on the machine. The belt rack can even be hung high on the wall to save floor space. Don’t allow the belts to hang in damp conditions. This will cause damp patches to form at the bottom of the belt, then when the belt is put on the machine it will be out of valance and cause vibration and will not track, or take a long time to settle creating other problems.
Pneumatics
A sander’s air system is designed to run dry, meaning without the presence of oil and/or water. Never mount a pneumatic oiler in line with the sander. Always keep the water trap drained. If you have any problems with water in your air lines, it needs to be addressed with the highest priority. If water gets into your air system the repair could easily cost you thousands of dollars.
Cleaning the Machine
Blow out the inside of the machine every day. Close the belt access doors and open the cover on the opposite side of the machine and clean around the infrared eyes and tracking mechanism. Some owners make a long (3 ft) blow gun nozzle by threading a piece of 1/8” pipe onto a regular blowgun. This allows access to dust and residue deeper inside the machine. It is also important to clean the very bottom of the machine at least weekly focusing on the height adjustment and spindles to keep them free of debris.
Conveyor cover
Cover the conveyor belt when you are not using the machine. Ultra violet rays from the fluorescent light deteriorate the natural rubber making it harder and very slippery. Most wood workers make covers out of wood but the nicest I have seen is canvas with a Velcro strip on the machine for easy removal.
Operations
Sanding Applications
Solid Wood
- The softer the wood, the harder the platen that needs to be used. (i.e. pine use 0.68 density or harder) Otherwise differences between early and late wood can be felt as waves in the wood. The harder felt evens out these differences.
Veneer and Veneer Sanding
Webster’s Definition : A thin sheet of a material. A layer of valuable or beautiful material for overlaying an inferior one. A thin layer of wood to be glued to a cheaper wood.
Veneer, in the woodworking industry, is applied to the surface of the following most common core type material. These cores are:
- MDF (Medium Density Fiberboard)
- Particleboard
- Veneer Plywood
The core material is generally calibrated prior to veneer being applied to first create a rough surface for glue penetration and second to remove the thick and thin nature of the core.
During the “lay-up” process, a thin layer of glue is applied to the surface of the core. The veneer is then placed on that surface in preparation for pressing. Pressing is generally done via a cold or hot press operation.
Veneer tape is applied to the surface of veneer to hold the veneer faces together during the press cycle. This is done to prevent any negative separation at the joints of the veneer.
Veneer Core Variation
The most stable core material is MDF, whereas, plywood core is the most unstable. The numerous layers of a plywood core contribute to the highest amount of thick and thin variations after the lay-up process.
Press Variation
Hot pressing is also a large factor in creating variation. When the veneer and its core are heated during the hot press cycle, the wood core will “swell” and change thickness. When allowed to cool to room temperature, thin and thick variations occur. Plywood core is the most unstable due to the multi-number of layers within the core. MDF is again the most stable. The advantage of cold press operations is to minimize these core variations.
In Summary
To list in the order of degrees of variation (thick and thin), from the least amount of variation to the highest amount, the following would be good to use as a rule of thumb:
- MDF cold pressed
- Particleboard cold pressed
- MDF hot pressed
- Particleboard hot pressed
- Plywood cold pressed
- Plywood hot pressed
Veneer Tape
The number of layers of veneer tape applied at one specific location and the type of veneer tape used, add to the degree of difficulty presented to the next sanding operation. Veneer tape has generally been seen in the industry of three different types, which are:
- Perforated / white in color
- Perforated / brown in color
- Solid / brown in color
The thinnest and easiest to sand would be that of the white perforated type. The most difficult to sand would be the solid brown type.
Given the use of the solid brown veneer tape and multi-layers applied on top of one, and other pressed via a hot press, creates the hardest sanding scenario. The multi-layers of tape are embossed into the thin layer of the veneer face.
Veneer Sanding of Taped Veneer Product
- Cannot remove all tape and/or very rough surface (tear out) with one pass without a crossbelt?
- Higher end companies need a crossbelt with two wide belts (optimal).
- All high quality needs a crossbelt if in a finishing line or if stand-alone and doing solid wood or veneer, first pass should be against grain on wide belt.
A crossbelt sanding head is generally used when three or more layers of veneer tape are present. The crossbelt is very effective removing the veneer tape that runs left to right across a given panel. The crossbelt is in full contact with the left to right tape and has the longest “dwell” time. A widebelt has a very short “dwell” time on tape running left to right. On the other hand, a widebelt does have a long “dwell” time on tape running infeed to outfeed. For the best results on multilayers of tape, a crossbelt followed by a widebelt is recommended. Generally, abrasive grits of 120 on the crossbelt and 150 on the widebelt are used. This is then followed by abrasive grits of 180 and 220 for a final polish.
If one or two layers of veneer tape are used, a two-head or three-head sander is recommended. The first sanding head is used to remove the veneer tape and the following sanding heads are used to polish. Abrasive grits of 120 or 150 are used to remove the tape. Abrasive grits of 180 and/or 220 are used to polish the product.
Veneer Cut Down
Veneer cut down is terminology used in the industry referring to the sanding of a veneer surface. It has been stated that most veneer sanding requires the removal of one-half the thickness of the veneer to achieve proper cut down. This amount of stock removal should remove any and all defects in the veneer used regardless of the core used.
Sealer and Sealer Sanding
Webster definition of sealer is: “Any device to prevent the passage or return of gas, liquid or air into a product.”
Sealer used in the woodworking industry can come in various forms. They may be: a primer paint coat, lacquer, sealer, UV fill, or water-based sealer. All of these sealers are used to seal the wood and prevent outside elements from penetrating the surface. In most all cases, the seal coat is applied prior to a final topcoat. Sealer is also used to “freeze” the wood fibers to become a stronger structure. The key element prior to a seal coat being applied, is preparation of the wood surface. Sanding to a very smooth surface (180 grit or higher) and denibbing via brush machines or orbital sanding, are very important factors. Denibbing will remove loose and/or broken wood fibers prior to the seal coat being applied. In the modern world of water base sealers, it is even more important that the wood surface be prepared properly. By the nature of “water base” sealers, grain raise can become a problem. In most cases, customers converting to water base sealers are sanding to a finer grit.
Seal Sanding
Seal sanding or a form of seal sanding has been used since the early 1960’s. On most print and/or laminating lines, UV fill is applied to the surface (180 or 220 grit finish sanded) of particleboard or MDF. The product is then sent through an oven for drying and curing. After the oven, a finishing sander is used. This process creates a very smooth solid surface for printing and/or a thin laminate to be applied. The requirements for a seal sander (flat stock) are:
- A segmented pad with the smallest segments possible.
- Variable edge pressure.
- An electronically controlled “rise” and “fall” time.
- A very fine abrasive grit (280, 320, 400, 600, ….etc.)
- A very slow running speed of the abrasive belt (1.5 – 4.5 meters per second), controlled by a frequency inverter.
- Abrasive air-blast blow-off systems to clean the fine grit abrasive belts while in production.
- A fast running speed of the conveyor belt (10 to 15 meters per minute), controlled by a frequency inverter.
- A vacuum bed conveyor feed system is recommended when product to be processed is “slippery” on the bottom surface or the material is coated on both sides.
Generally, after a product has been seal sanded via a widebelt, the material is run through a grit impregnated bristle brush to sand very small areas that were not sanded by the widebelt. All areas must be sanded to prevent what is called “orange peel”. Orange peel would be a given area that did not receive ample amounts of seal sanding. This area will turn to a dark look or orange look as to the remaining properly sanded areas. Thus, the name “orange peel”.
On non-flat products such as cabinet doors, a brush machine equipped with grit impregnated bristle brushes are used.
Operations (cont.)
Sanding Belt Choices
- Stock Removal – 60, 80, and 100 grit
- Finishing – 120, 150, 180, 220…etc.
- Wide Belt multiple head machines can skip 1 grit size between belts or heads. (i.e. 80 to 120 to 180)
- Cross Belt multiple head machine sequence is as such: 80 grit crossbelt to 80 grit wide belt to 120 grit wide belt, or 120 grit cross belt to 120 grit wide belt to 180 grit wide belt.
From Wide belt to cross belt, 3 grit sizes can be skipped (i.e. 80 grit wide belt to 150 grit crossbelt).
Things that can and do affect quality.
1. Feed speed.
2. Abrasive belt speed.
3. Grit size.
4. Pressure – Surface and Edge.
5. Wood species.
6. Number of Heads.
It is always better to send a work piece through 1 or 2 degrees off from parallel to rollers.
- Control has a better or easier time seeing the work piece
- It also ensures segments are hitting the entire work piece due to falling and rising time cover all four edges.
Ensure Belt tensioning is set to the proper level.
- If you are having a problem with belt flare out on sanding surface but increasing rise time rounds the corners, try raising belt tension pressure.
4 to 5 bars is the nominal operating range.
Sanding Defects
1. Chatter Marks
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.
1) 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 chow 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 head 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 head with the hand for heat build-up during running.
2) 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 workpieces, 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.
2. Tramlines (Snaking longitudinal strip marks)
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 air 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.
3. Coarse Markings
This is often caused by the incorrect selection of grit size, i.e. too coarse. Where only one roller/pad is available the choices of grit size is critical. The answer is either change to a smaller grit size or use a machine with two heads, one with a pad, allowing for one coarse grit and one finer grit to be used in a single pass. It is good policy never to jump more than one grade between heads. With any bigger jumps than this, the follow-up belt will not remove marks from the first belt.
4. Straight longitudinal strip marks
These can be caused either by the machine or the belt. They can be positive or negative – heavily sanded or unsanded. Regarding the machine this fault is usually caused by neglect of the sanding pad or a build up of deposits on the platens or contact roller, and can be rectified with a new covering to the sanding pad and cleaning the platen and contact roller. If the problem still persists, the abrasive belt will have to be checked and signs of deposit build up on the back of the belt should be checked. If present this should be lightly sanded to remove and further checks made to the graphite pad.
5. Round Edges
Sanding with a very soft contact roller or pad usually causes this problem. On an air pad machine you can increase the pressure in the pad to make it stiffer/less flexible and carefully set the depth of the pad. On segmented pad machines you can select –1 to –4 pad segments on the edges. For delicate work, variable edge pressure is a must for precise control of the pressure on workpiece edge segments.
6. Shadows
These are unsanded areas on the workpiece. They are caused by badly cupped or warped workpiece, too little hold down pressure, too light sanding pressure, too much pressure in an air pad. Be sure to check calibration and pressure ranges for the machine model.
7. Dubbing
This is the rounding over or heavy sanding the front and/or rear edges of a workpiece.
1) Contact Roller – Typically caused by having the contact roller down too far below the pressure rollers, or not having the pressure rollers calibrated properly. Some dubbing can occur when the machine is set too narrow for the piece to be sanded. The piece is pressed down into the conveyor and springs up into the contact roller at the ends.
2) Pad – Older machines with a fixed pad will dub the front and back and there is little you can do. On Butfering machines with the air pad or segmented pad there is a fall and rise timer so the pad sands perfectly on the front and back edges.
8. Shiny Heat Marks
Shiny head marks, which have been reported on veneered panel work, again result from the same problems as the chatter marks and usually can be rectified by following the same procedures for elimination. In severe cases, burn marks are left. Using too fine a grit for too heavy a cut causes burning. Heat builds up as the belt gets clogged with dust. Try a coarser belt or a lighter cut or a combination of the two.
9. Bumpy finished board
This can be caused by the incorrect selection of roller hardness or wrong feed table setting (floating or fixed). Often machines are specified to perform one function when new, and then when demand changes, i.e. from machining solid pine to machining veneers and /or sealer in line, the same machine may still be used. However the requirements can vary tremendously, the chore hardness for one operation is totally different from the other. Also the type of grit and bond strength of the abrasive will change considerably.
10. Grooves
Another common problem is caused by the material being fed into the machine with dirt or foreign bodies on it, this being picked up either from the operators careless handling or whilst in storage. Grooves are like dotted or dashed lines running the length of the workpiece. Grooves can be positive or negative, by a defect or damage to the abrasive belt. Sanding a staple could cause a groove; the missing abrasive will leave raised dashes in the workpiece. Excess glue on the boards is another common cause. A negative groove is caused by a defect in the manufacture of the belt where there are some heavy spots of grit on the belt.
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