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Chatter may be the most common wide belt problem encountered in the field. There are several reasons for the chatter to occur besides the belt itself.
First, check if the source of the issue is a multiple-head machine. If it is, isolate whichever head the chatter is occurring on. This is achieved by running a test piece through the machine with only one head at a time. If the first head runs clear, raise it for the next pass, lower the second head, and so on until the head is found.
If all the contact rolls are run with no chatter, then the platen is the cause. Chatter is most commonly found on the platen due to improper materials, joints, and grits that are run there. Platen sanding is designed to do 10% of the work on a piece moving through the machine. It is strictly for finishing grits (normally 150 & finer, although some 120 grit is seen). The purpose of platen sanding is to clean up any stray scratches left behind from the intermediate sanding grits before putting down stains or sealing.
Platens should be raised on single-head machines until the finishing grits are reached in the process sequence and lowered only. Also, platens should be covered in felt (to cushion) and graphite (to reduce heat). Both of these materials should be on a maintenance schedule for replacement. Gouges, raised or flat spots on either of these will telegraph through the belts and onto the workpieces.
KLINGSPOR recommends that all platen sanding on wide belts be used with paper materials made with a #1 joint where possible. The finish will be finer, and the belts will be less expensive. The tolerance on a #1 joint in paper belts should fall between +/- 1000. Joints outside this range are out of spec and may indeed be causing chatter.
If, due to machine age, poor maintenance, dust collection, or operator experience, cloth needs to be run on the platen sanding step. Make sure to put a T-joint on the belts. This will help with joint thickness and remove any chatter problems due to the thicker cloth backing.
Chatter originating on a contact roll tends to be a machine issue. It could be a contact roll that is too hard (steel) or, too aggressive (serrated), contaminated, or damaged. Poor dust collection results in dust accumulating where the heat is being generated, which is between the contact roll/belt/workpiece. Some other causes of chatter can be oscillation mechanisms that are out of adjustment, static electricity buildup, excessive PSI, or excessive outfeed hold-down shoe pressure.
Causes and Recommendations for Chatter Marks
- There are worn bearings or journals on either contact roll or idler roll. Normally, this appears as chatter on one side of panel only.
- Recommendation: Check bearing temperature and listen to bearings with mechanic's stethoscope if possible.
- Sanding pressure is too low.
- Recommendation: Increase pressure. Marks often appear in spots.
- Contact roll or idler roll is out of balance.
- Recommendation: Marks caused by vibration are regularly spaced and clearly defined. Have wheels/rolls checked periodically.
- Contact roll is too hard.
- Recommendation: Steel and/or serrated contact rolls should be used for removal applications only.
- Belt splice is defective or incorrect for the application.
- Recommendation: Marks are usually narrow, regularly spaced, and clearly defined. Try a different belt on the machine to compare.
- There is machine vibration due to improper anchoring of machine to floor.
- Recommendation: Regular maintenance should include checking mounting of machinery.
- Belt tension is too low.
- Recommendation: Results in belts to contact roll slipping. Increase PSI.
- Hesitation in feed or drive mechanisms.
- Recommendation: Chatter marks are irregularly spaced. Have machine technician evaluate the machanism.
- Grit is too fine for application or contact roll.
- Recommendation: Do not try to dimension with finishing grits, and do not use finishing grits on serrated or steel contact rolls.
- Drive belts are too tight or too loose.
- Recommendation: Check V-belts and adjust as needed.
- Abrasive belt is too stiff for application.
- Recommendation: Check for other materials in this grit available in lighter weight backings suitable for belts.
- Glazed conveyor belts.
- Recommendation: Results in workpiece to belt slipping. Replace or re-surface.
- Hold-down rolls or pressure shoes are incorrectly set up.
- Recommendation: Can cause work piece to move through machine unevenly. Have machine technician evaluate and adjust as needed.
- Dust build-up on conveyor rollers or underneath conveyor belt.
- Make sure draw on dust collection is operating at optimum capcity and collection containers, filters etc. are clean and regularly emptied.
- Loose motor coupling, guards, or cover plates.
- Recommendation: During routine maintenance check for any loose nuts, bolts, guards, or fittings resulting from machine vibration.