WoodDigest.com |

Hand, widebelt, orbital, edge, mould, abrasive planing or fine grit sanding all may produce defects of varying magnitudes. The inability to see defects until a stain or finish has been applied is one of the most troubling aspects of defects attributed to sanding. In fact, applying a stain is one method of locating a defect well before further work is done to the part.

Sanding defects may be reduced dramatically by calibrating the equipment, maintaining a sandpaper usage schedule, good preventive maintenance, use of inspection lights and good common sense.

Please understand that sandpaper is constantly changing while it is being used. As the grain breaks down and the space between the grain particles load with sawdust, the scratch pattern changes and the amount of material removal reduces. Burn marks may be seen as dark streaks on the wood surface as additional heat is generated at the point of contact of sandpaper and wood. The following defect list is attributed to sanding operations:

1. Chatter marks
2. Streaks
3. Grooves
4. Glaze marks
5. Edge roll-over
6. End roll-over (dubbing)
7. Excessive roll-over on profiles
8. Snipe
9. Burn
10. Thickness variability
11. Belt splice marks
12. Sand-through
13. Uneven sanding
14. Dwell (Hesitation) marks
15. Wavy scratch pattern
16. Fuzzy grain
17. Raised grain
18. Orbital sanding marks
19. Cross-grain sanding
20. Uneven stain penetration
21. Blotchy finish
22. Out of square edges

Although there may be more, identification and determination of the cause of the defect needs to be a top priority in a shop. Although sanding operations and equipment may cause the defects on this list, other issues may exacerbate the situation. Excessive humidity, moisture content, glue, sawdust or lack of training may compound the reason for the defect.

In a widebelt operation, the roll-over of the ends (dubbing) and edges of a panel or part is caused by several different reasons. Each reason illustrates lack of operator judgment, lack of training, an inability to set up the machine or improper sanding equipment. If it is determined that the hardness of the contact drum and the alignment of the contact drum(s) and holddown bars or rollers are correct, the next probable cause is the removal of too much material by the platen head. Roll-over or dubbing is caused from the removal of excessive material, usually combined with poorly adjusted hold-down bars, rolls or soft contact drums or platens. Dubbing is an arc-shaped roll-over of the edge or end of a panel. The roll-over can easily be seen by taking a straight edge and placing it directly over the edge or end of the part. Personally, another indication is the temperature of the panel. By touch, an experienced operator may be able to notice if the panel is warmer than usual. A panel that is warmer than normal indicates excessive material removal or the belts are starting to wear out.

Every head must be aligned to remove only the scratch pattern of the preceding operation. The platen head is no exception. If proper alignment is maintained, the platen felt and graphite will perform for a longer period. A good practice is to utilize the entire width of the sandpaper belt. This will produce machine parts to wear evenly, such as the sandpaper, the rubber feed belt, contact drum and any other wear surfaces with which parts have contact. The operator may notice thickness variation (wavy) across the width of wide panels. Attention to using the entire width of the sandpaper will reduce the thickness variation across the panel.

The most common problem in sanding is the removal of too much material too fast. Both situations are not good, and only poor quality will result.