There seems to be a lot of guess work on how best to utilize 2 head
wide belt machines. I hope to clear some of that up for the reader, as
well as establish some basic processes that will help them to
achieve
better results. All belts discussed are paper belts unless noted. Stock
removal should be the same on either paper or cloth.
There are three basic types of two head machines; Drum/Drum,
Drum/Platen, and Drum/Combi-head. For the purposes of this article we
are going to focus on the two most common for smaller cabinet and
furniture shops; Drum/Platen and Drum/Combi-head.
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
doesn’t 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.
Grit max
80 .024”
100 .017”
120 .010”
150 .006”
180 .004”
220 .002”
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.
Grit minimum to remove
80 .008”
100 .006”
120 .004”
150 .002”
180 .001”
220 .0005”
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 it’s 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.
Drum/Platen
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 don’t 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.
Drum/ Combi-head
There are two types of Drum/ Combi-head machines.
Some machines have dead shafts and air exclusion that allows the drums
or platen to be moved up out of the cut at will. The heads automatically
drop back into the working position upon activation. This type of
machine is the most versatile. It easily allows only one head to be used
at a time. This is a huge advantage that will be discussed further.
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.
Written by Adam West, Abrasive Specialist/ Wide Belt Technician
