Does Your Sanding Make Sense?

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.

Let’s 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