Wide belt sanding is not rocket science. I don’t want to go into why you have the machine you do. You just do and that is that! There are so many factors and configurations that I could write an encyclopedia on it and still not cover it all. What is important is that your machine will have a lot to tell you about what it can do well. Listening to it will give you better results and huge savings in abrasives and prep sanding. This article applies mostly to cabinetry and furniture on solid wood.
First I would like to talk about what the different types of heads do well and what can go wrong.
Hard drums are great for cutting things down flat and tearing the surface open for stain penetration and coating adhesion. They have a high shear angle and very little give, which means the deepest scratch pattern of any contact type. These heads are usually steel, or 75 to 95 shore rubber. The hardness of these heads means that any drum run out or belt splice issues are very apparent. This is why they are usually only used in combination with other heads or types of contact surfaces.
Over pressure on a hard drum will manifest in several different ways.
Heat build-up will cause wood dust and resin to melt and stick to the belt. This will result in brown loading of the belts and burn lines. Little brown flakes left inside the machine are evidence of extreme heat and burning.
Another way it shows is in something that was often called “static lines”, which are very tiny raised line tracts following the abrasive belt tracking on the product after sanding. They will often come in groups of two or three. These are very rarely actually related to static. They usually stem from having belts that are too fine to handle the pressures required for the stock removal. Grains will rip out of the belt and become deposited in the wood, where they will cut lines into the next belt. This is part of the reason why a hard drum (75 to 90 shore) in the second position is a very poor decision. The finer the belt the easier it is to damage the belts this way.
The last way over pressure manifests itself is though compression of the grain structure of the wood. The belt may not be able to keep up with the stock removal required. This will put more and more pressure against the resin on the belt and it will create heat and a compressed surface. The material that is supposed to be removed by the coarse belt will instead be crushed and smashed into the surface creating a dense layer of crushed wood that will not take stain well and will result in diminished life for the rest of the sanding sequence.
Hard drums work best at 100 grit and coarser.
Medium density drums are for removing the scratch from earlier, harder drums. They have a wider contact patch so the scratch left over is not as deep. These drums are usually in the 45 to 65 shore range.
Overpressure on medium density can still rip grains out of the belts, but usually the belts will dull quickly and start to burn and compress the surface. Once the abrasive belt can no longer cut properly it will start to create hard and soft grain structuring, or high and low spots in the surface corresponding to the hard and soft grain. Damage from over pressure can also cause rounding of the surface, which I call pillowing. These drums are easier to damage than the hard rubber drums, but the medium density drums are extremely important to removing the hard drum scratch without excess pressure.
Soft drums are for removing previous drum scratches, but they are also excellent for finishing work in applications where we are sanding “with the grain”. The scratch is longer and very shallow and not quite as noticeable as a platen scratch in some applications. These heads are in the 24 to 40 shore range. Flatness is easy to maintain with a soft drum if used correctly, but they do not create flatness.
Overpressure on soft drums causes closing of the surface and soft and hard grain structuring and also pillowing. These drums are easier to damage but are critical to a longer scratch pattern for a quality finish. Burning is also an issue with soft drums as it is with any type of drum.
Soft drums cannot grind away burn lines and bumpy spots from earlier drums. Often issues attributed to the soft drum is actually damage on earlier drums.
It is best to leave soft drums out of agressive sanding situations like running coarse belts.
Platens are the largest surface area so they leave the shallowest and easiest to remove scratch. They do best when they remove .002” or less. Because they are constructed with anti-friction graphite material, they wear constantly so they must be adjusted periodically as the finish requires.
Overpressure is manifested by polishing more than anything. The graphite cloth is easy to damage with high pressure in small areas. The felt behind the graphite can be crushed by pressure as well.
Platens do not like skipping grit. They are very limited in stock removal capacity.
Combi-heads are drum/platen combination heads. They have all the advantages of a drum and platen both. They are extremely versatile heads. I often run both together, the drum taking the bulk of the material, and the platen just cleaning up the scratch pattern without removing much material at all. The platen will often run much longer before the graphite needs changed in this type of head. The platen is a lot harder to damage in a combi-head. Skipping a grit going into this head is no problem.
So many companies have major issues because their machine is configured with the wrong combination of drum hardness and grit sequence. I hope this article will help prospective machine buyers to understand what the different heads do and what issues can arise with misuse.
