Ultimate Primer on 2 Head Wide Belt Sanders
Reprinted with permission from SurfPrep website.
Ultimate Primer on 2 Head Wide Belt Sanders
Let us start by talking about what the different types of heads are good for.
The drum is great for leveling out a part and inducing a short, deep scratch that also opens up the surface of the wood. Softer drums have a larger contact patch so they leave longer, shallower scratch pattern.
The platen is good for leaving a longer, shallow scratch that is helpful because it is easier to remove. These heads can only remove a few thousandths of an inch of material so they are only for finishing applications. These heads do not do well when you skip a grit. Platen heads also wear, making adjustments necessary to maintain stock removal. Platens like to run paper belts because it limits the amount of wear from the rough backing material on the graphite cloth that covers the felt.
A combination drum/platen head is the best of both worlds. It can remove some material and also leave a very shallow scratch. This type of head has a huge advantage over the normal platen because a vast majority of the work can be done by the drum part of the head. This type of head doesnt mind skipping a grit at all. Because the platen is doing much less of the actual stock removal, the head needs a lot less adjustment and there is much less wear on the graphite over time.
What is your goal with the wide belt sander?
You want to tear open the surface of the wood and level it with the first contact drum. Then you want to remove that scratch pattern with the subsequent belts and hand sanding without closing that surface back up. This is best accomplished by not removing more than the little mountains of scratch pattern from the first head, without going further down into the valleys at the base of that scratch pattern. Removing more than just the scratches will result in lots of heat and pressure, which are the enemies of color consistency and surface quality.
There are maximum amounts of material a belt can remove.
If you desire to just remove the scratch pattern then there are minimums you must remove to get rid of a particular scratch. It is best not to go over these numbers.
If you sand with an 80 grit, you want the next belt to remove about .008 to get rid of the scratch. A 100 grit belt can remove up to .017 max, but if it follows an 80 grit belt it only needs to remove .008.
The following suggested sequences are all two passes per side. I am doing it this way because I rarely find situations where the machine has the stock removal capability to remove all the necessary material and not over tax the belts. Occasionally very light amounts of material are removed and then its fine to do one pass.
This is where it is very important not to lie to yourself. If you are taking .015 off the average thickness of the stiles and rails, but your mismatch is .010, then you are removing .025 at some points on the surface. The overall stock removal is one of the most important factors in protecting your belts and machine. Heat and pressure is the result of sanding off too much material.
There are thousands of older drum/platen two head machines floating around in the industry. These machines are the most limited of the two designs discussed in this article. The platen cannot remove a lot of material. Skipping a grit between the drum and platen is not sustainable because of the pressure on the platen and normal wear that is accelerated by excessive pressure. If you want to end at 180, then the drum should run a 150 grit. This combination can only remove about .008 maximum, so a pass with coarser belts is almost always a must.
It is possible to skip a grit going into a platen. The accelerated wear from extra pressure means more adjustments to the platen height. The extra pressure means more polishing and heat.
This is where things get tricky. There are several factors at play.
If a drum/platen machine is set up right, it is set for the finishing grit sequence. This means the platen will be removing only a couple thousandths of an inch. If the finishing grit sequence is 150/180, the platen is set to take off exactly .002.
If 100 and 120 grit belts were installed on the machine set up this way, the 120 will only remove around .002. This is not enough to remove the 100 grit scratch, but as it only removes part of the scratch, it actually polishes the surface a bit and makes more heat and pressure on the next pass.
It is in the best interest of the operator to use only the first head with a 100 grit to knock down the surface, leaving the easily damaged platen out of the cut, or removed from the machine entirely on the roughing pass. The result of the 100 grit alone is definitely a rougher, more broken up scratch pattern that represents sharper peaks and valleys. The scratch is actually easier for the 150 grit, running on the drum during the second pass, to remove. The surface stays more open for consistency.
If we stick with the 150/180 grit sequence on the final pass, and use the 100 grit to knock down both sides first, taking no more than .017 per side. On the finishing passes you want to take .006 with the drum and .002 with the platen for a total of .008 per side finishing passes. This is a total of .025 per side. I would try to work it out to stay around .020 per side to keep from maxing out the 100 grit all the time.
People ask me all the time about just running multiple passes with the 150/180 combination, but they honestly dont realize this is the worst sanding practice EVER. The scratch pattern left by a 150/180 combination only requires about .001 to remove it in hand sanding. Those are very tiny peaks and valleys. If you them come back with the same sequence and remove another .008, you are removing .001 of scratch and another .007 of solid wood. Compared to removing a coarse scratch where the actual mass removed from the surface is dramatically less, this creates a ton of heat and pressure.
If you follow a 100 grit scratch you are removing .006 with the 150 and .002 with the 180, but 50% or more of the material you removed was air. The 100 grit drum scratch pattern is a broken surface. There is nothing solid about it. You just knocked off the tops of the peaks of the mountains without ever touching the solid wood in the valleys below.
There are two types of Drum/ Combi-head machines.
Other machines have manual adjustments on each head but no way to easily move the head out of the cut. Moving the heads once a proper set up is achieved is very undesirable. These machines necessitate running more than one belt at a time.
Air exclusion machines
After our earlier discussion on scratch pattern, we know we want a very open and soft scratch coming into a second pass. For these machines I like to run 80, 120, and 150 or 180 for a very easy to hand sand result. Because I can retract the softer drum and platen of the second head out of the way, I run an 80 grit belt by itself to cut down, flatten, and open the surface, while protecting the second head. The first drum is always harder so it gives the maximum scratch depth and openness. The maximum for the 80 grit is .024 per side. On the finishing pass, the 120 needs to remove .008 and the 150 or 180 must remove .004. This means .012 per side for the two finishing passes. This process can remove .036 per side max. I try to limit it to .030 per side to avoid maxing out the grit sequence.
Non air exclusion machines
If we are going for the same grit sequence in a machine that does not have air exclusion we have two options.
One option is to run 80/100, and pull the platen out of the machine entirely for the first pass. The 100 grit is really only there to protect the drum as it only removes a few thousandths of an inch off the 80 grit scratch. The next sequence would be 120 and 150 or 180 used as above with the platen, but maybe around .010 per side for the final pass.
The other option is to run a cloth 80 grit belt and leave the final belt (150 or 180) on the drum, but leave that head motor off. The reason for this is the greater thickness of the cloth belt will automatically take the second head out of the cut. The platen would be pulled out to avoid damage or incidental contact. This is often a pretty good option. The final sequence of 120, 150 or 180 would be used as normal with the platen when coming after an 80 grit belt.
Each of these configurations will create a scratch that will melt off like butter during hand sanding. How much you remove with the second pass and the structure of the wood is what really sets the final processes up for success or failure. Avoiding smashing and polishing is the key to success.
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