By Adam West
Reprinted with permission from SurfPrep website.
Does your Sanding Make Sense?
My job as a technician for Dixon Abrasives is to make sanding make sense. This is not as simple a task as most people would think. Every part of the sanding process must work together to accomplish a few simple tasks. The sanded wood must have the right surface texture, it must be able to accept the desired finish, and it must look pleasing to the eye of the customer. Each step of the process must complement and improve the process before it, but each step of the process has impact on the later processes. The wide belt machine is the first step in the process for most companies so it is always one of the first things I want to see as a tech. This article covers most of what I look at in an initial visit to a shop.
Lets start with what we are trying to accomplish with a wide belt machine.
The coarser belts should tear open and flatten the surface. The goal is to create tiny mountain ranges of scratches that cover the entire surface evenly.
The depth of these scratches should be very consistent, making them easier to remove by subsequent operations.
The belts creating this scratch should be sharp and cut cool because compression of the grain structure is very bad for color consistency, and that starts at this step.
Over taxing the first belt will start to smash the wood closed at the very first contact surface. Loss of color and consistency is the result.
A nice hard drum with a coarse belt is just right for creating a flat, open surface ready to take stain properly. The scratches might be too rough at this point, but the surface is right. The first belt in contact with the product sanded has as much to do with the final color and openness of the surface as the last, and all the subsequent sanding as well.
The sanding heads after the first head will break down the little mountain ranges of scratch pattern, without ever reaching below the bottom of the original valleys. The subsequent contact surfaces are often softer drums to give a slightly bigger foot print with a more shallow scratch, or platens to give huge surface area with much less grain penetration into the wood. The removal of previous scratch pattern is exceptionally easy for a sanding belt. Nearly 50% of the thickness removed, to get rid of a scratch pattern by a sanding belt, is empty space between the little mountains of wood. The belts are not removing solid wood. When this is realized it is easier to understand why skipping a belt grit between heads is often okay, and even preferred in some situations.
Trouble starts when taking off more than a belt can handle, or taking off more than the scratch pattern of the previous head. These are the most common mistakes in sanding.
Overtaxing a sanding belt will allow the material being sanded to push against the grains and resin coat, heating up and degrading the belt. This not only destroys the belt, but it also starts the heat and compression that destroys the surface of the wood.
When enough material is removed by subsequent heads to get past the existing scratch pattern, the head is driving the grains through solid wood below the original peaks and valleys. Tremendous heat and pressure is the result. This is very important to understanding proper use of abrasives in a wide belt machine.
If a wood surface has been sanded with 120, 150, 180 grit, and the sander was properly set up, the scratch pattern afterward will be very shallow. This scratch pattern only requires about .001 to be removed to get to the bottom of the valleys. If the part is then run back through that same grit sequence and another .010 is removed, .009 of that material is solid wood. This represents a huge amount of mass being removed.
If the surface was sanded to 80 grit, .008 stock removal is required to touch the bottom of the valleys to get rid of the scratch. If the 120, 150, 180 grit sequence is then used to remove an additional .014 (the real depth of the 80 grit scratch) (120/.008, 150/.004, 180/.002) most of the stock removed is just scratch pattern and air...
Below is a chart that gives the maximum amount of stock a particular grit belt can remove and how much to remove to get to the bottom of the valleys. This should not be misconstrued as the scratch depth. Every time I tell someone the real scratch depth of a sanding belt they go about removing that much material with the next belt, never thinking about the fact they are removing way too much material and smashing the wood. Everyone measures off the top of the mountain peaks to the bottom of the part, not to the bottom of the valleys. So I never give scratch depth, only give the amount to remove from the mountain peak to touch the bottom of the valley.
Grit Max Minimum to remove
A quick example grit sequence would be 120, 150, 180. 120 grit can remove up to .010. The scratch left behind requires the next belt to remove .004 to get rid of it. The 150 maxes out at .006, but it only needs to remove .004. The 150 leaves a scratch that requires .002 to remove. The 180 can remove up to .004, but it should only remove the .002 required. This belt sequence can remove up to .016 maximum with good results. The final scratch requires .001 stock removal.
Another example: 100, 150, 180. 100 max is .017. The 150 must remove .006 and the 180 must remove .002, so the grit sequence can remove up to .025. Final scratch requires .001 stock removal.
One more example: 80, 120, 180. 80 grits max is .024. The 120 must remove .008, and the 180 must remove .004. The grit sequence can remove up to .036 with good results. The final scratch only requires .001 stock removal.
I am not going into how the design of the machine affects the ability of the machine to use these sequences. That is a different article.
If the product coming through the wide belt machine is polished or smashed, it will feel smooth and shiny. It will not accept stain properly. In wipe stains certain pigments will wipe off. In spray stains the pigments will pool up on the surface and a darker color will result. You will notice that parts that have been hand sanded will often be patchy because the sanders will miss parts or not cover the whole surface evenly.
Swirls are also a result of smooth and shiny wide belt sanding. It is impossible to hand orbital sand a smooth and shiny surface without leaving scratch marks that show. This is the rotary swirl mark everyone tries to avoid. When wood is properly wide belt sanded, the scratch pattern sands away easily. I always say it melts away like butter. The hand sander should stop sanding once the little mountains of scratch are removed to avoid driving grains into the flat plane that has just been created. The more the flat plane is sanded the more swirls will result. If the hand sander is polishing the surface they will cause their own smooth and shiny surface to create swirls in. This concept is the real way to eliminate swirls.
If a sanding process breaks any of these principles it will cause issues. Many companies base their entire staining and coating process on very poor sanding. They fight forever to maintain color with an ever changing result. It can be incredibly frustrating to see a company trapped in a prison of their own design because they really never understood how to sand.
The concepts in this article are the bases of consistency in raw wood sanding.
Reprinted with permission from SurfPrep website.