There is a lot to learn to really make wide belt machines really shine. I am not going to go into troubleshooting the entire machine, just into solid best wide belt sanding practices and processes for the best quality result possible.
For the purposes of this article I am going to assume your single head sanding machine has a combi-head, which is a drum/platen combination in the same head. This is the most common machine in this category. If you just have a single drum the rules all still apply.
Rules for single head machine sanding
1. Know what you want to achieve.
For most cabinet and furniture shops, you need to sand parts flat, but you also need them to accept stain. This means you need to fracture and tear open the wood with your rougher belts and sand to a very fine scratch pattern with your finer belts, leaving the parts open to accept stain. If you are doing cross grain you want the sharpest, softest scratch possible, so it will melt away in seconds with your hand sander, instead of taking minutes per door to grind the surface scratches away. You want the least amount of heat and compression of the wood fibers possible.
2. Know your sander is working right.
I won’t go into setting up hold down rollers and such, but there is a very easy way to check the level of your machine and get it right. I use two boards a few feet long. I prefer maple around 1.5” to 3” wide and ¾” thick. I will look closely for warp and set them on a table with the warp in the middle of the part up. I use a coarser belt and just my drum to take a light sanding pass on the boards run through the machine together in the center of the machine somewhere, just enough to hit both parts and true up the surface a bit. I then mark one part for left and the other right. Flip them over and take another .010” off , but run them on their perspective sides about 2” off the edge of the conveyor.
Measure only in the center of the boards as this will be the only truly accurate spot due to the warpage. If it matches, your machine is accurate. If not, almost every machine has a way to level the head to get the machine absolutely dead on side to side. Most have that adjustment on the block you remove to change belts.
3. Multiple passes
It will most likely take more than one pass to level out a part and give you the scratch you desire. I could write a book about this part of the process. Running multiple passes on the same grit is a very bad idea. If you use a 150 grit belt to remove a 100 grit scratch, you have to remove .005” to get to the bottom of the valleys in the original scratch. 50% of what you just sanded away was air. The rest was the little mountains of the scratch pattern. This is no problem.
If you sand with a 150 grit, you leave a scratch that only requires about .002” of stock removal to touch the bottom of those valleys. If you then take another .005” pass, you remove that .002” scratch pattern and .003” of solid wood under that pattern. This means you removed 100% to 150% more actual wood from the surface, than if you followed a 100 grit belt, and only removed the scratch pattern alone.
The issue is heat and pressure. The more of both the less consistency in your stain and the greater amount of hand sanding will be required to grind off that closed surface. You are better off taking 2 or 3 passes with the right removal rates than forcing one belt to do it alone. You will see huge advantages to doing it right at the sanding table and in finishing.
How do we do this right? There is a maximum for each grit when it comes to stock removal. This is well known. What is not well known is that there is a minimum to remove a scratch and that is the really important number. This is not the actual depth of scratch, but the amount that must be removed from the top of the mountain to touch the bottom of the valleys without going further into solid material.
Grit Maximum Minimum to remove
60 .035” .015”
80 .024” .008”
100 .017” .005”
120 .010” .004”
150 .006” .002”
180 .004” .001”
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