I’m writing this article to start a revolution in the minds of the readers and the industry in general. I’ve been blessed in my career to have seen literally thousands of sanding processes and nearly the same number of variations on why things are done certain ways. This has taught me humility when I first approach a new customer because you never really know what inspired them to do things that, on the outset, seem rather silly and counterproductive. Often times you really have to ask the right questions so you can understand why they have come to such radical and strange ideas on how best to sand products for consistency. You have to listen before you can teach.
This article will not cover simply hand orbital sanding a raw piece off the planer. If you have not orbital sanded something that was properly abraded with a wide belt or drum sander you really might not understand what I am getting at with this article.
Where does your consistency start; at your wide belt or hand sanding table?
Answering this question accurately is the key to understanding why your sanding lacks consistency. Let me start by relaying an experience from a recent customer visit. This was an extreme situation but it is proof of concept and a vivid illustration of what I need to get across.
This customer has a two head DMC widebelt sander with a drum and combination head (Combination heads have a drum and a platen in one head). The parts coming to this machine were sanded to 80 grit in a planer sander and they ran 180-220 to finish sand out the scratch and make the parts ready for hand sanding.
Already an astute individual can spot a problem. Those two grits cannot remove the 80 grit scratch without abusing the belts dramatically. Very short belt life, heat, and polishing would all be huge issues. The maximum stock removal for a 180 grit belt is .004" and the 220 can remove .002". The machine needed to remove at least .012" to get to the bottom of the 80 grit scratch.
They fixed this by sanding everything with 120 grit on an orbital sander. This broke the surface back open and gave them most of what they wanted in the finishing area. Most of the surface took stain well after this onslaught....but at a price.
I set the machine up with 120-150. For most readers this would seem a good choice for getting rid of the 80 grit scratch, but seem a poor one in terms of removing the final scratch with the hand orbital machine. Most wood workers with wide belts think that the finer the last belt, the easier the hand sanding. This is where they lose consistency and perspective.
The scratch going to the hand sanding table or orbital machine must be long, shallow, and soft for the best results and easiest removal. Shallow does not just come from smaller grains, but also from the type and structure of the head producing it. Longer contact area means longer, more shallow scratches. Soft is by far the most important factor. Soft is achieved by very low pressure, low heat, and very little compression of the wood fibers. A long, shallow, and soft 150 grit beats a hard polished 220 grit any day of the week.
This part is not as simple as it may seem. Properly explaining it is the hardest part.
We put some cherry and soft maple panels through the machine and hand sanded them to remove the scratch pattern from the wide belt. The scratch took much less work to remove than the original sequence with very little swirls.
We sanded half of each part with 150 grit as I suggested and 120 grit because that is what they’ve used all along. We sanded just enough off to get rid of the scratch. I knew what was going to happen before we stained them, but it’s nice to show the customer instead of telling them.
Both sides of each panel matched color almost perfectly, within the normal variations for wood color and grain structure. The only difference was that the 120 grit side had way more swirls than the 150 grit. There would have been almost no difference with 180 grit either.
How is this possible?
The first head of a wide belt machine has as much to do with the final color as everything that follows. The wood was broken open by the first head in the wide belt sanding process. That openness survived the properly set subsequent heads and the hand orbital sanding process as well. This is part of the reason I only sand enough to remove scratch pattern. There are less swirls and less risk of color shift from extensive over sanding.
The wide belt set up becomes more critical with the addition of more sanding stations. The more heads the more chances to start heating up the wood and polishing it closed. Each station on the wide belt machine removes just enough of the earlier scratch to touch the valleys but not intrude into the solid wood below.
Often smaller machines fall prey to operators who want to sand much more than a belt can handle to avoid a belt change. Multiple steps are often extremely important for good results. A couple belt changes and correct set up can yield much less work at the sanding table and better consistency.
They had inconsistency because they hard polished the surface closed and tried to break it back open with their hand orbital sanding. The doors already in the stain room showed the classic signs of the results of too much polish and the attempt to open it back up by an orbital machine. Light and dark streaks are easy to recognize as inconsistency in the hand sanding process. The dark streaks really are just the mass of scratch pattern that opened the wood up, while the polish from the 180-220 remained on large parts of the product.
This is a very important concept to grasp because it makes all the difference in quality and consistency. Consistency is no mystery. The less it depends on the human element the better. There is nothing at all wrong with properly trained people running hand orbital sanders. To rely on them to be perfect every time will lead to disappointment.
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