Festool RO 125 FEQ Plus Supplemental User's Manual - page 7
Supplemental Owner’s Manual
7
Sanding Action/Mode
Sanding is defined as scratching or abrading a surface. So
when we discuss scratches and scratch patterns, the goal is
to minimize them or make them less visible to the eye, but
they will always be present.
The Festool Rotex sander is not only a dual-action sander,
but is also a dual-mode sander. Even though those two
terms sound similar, they do not mean the same thing.
Because both of the dual modes are variations of dual-
action, it is best to describe dual-action first.
Dual-Action
Dual-action means that the sanding pad moves in two
distinct motions simultaneously. The primary motion is an
eccentric orbit—similar to the motion of a bicycle pedal. The
center of the sanding disk moves in a small eccentric circle
about the center of the
sander. The diameter of this
orbital path ranges from
3mm to 5mm, depending
on the model sander (Refer
The second motion is the
rotation of the disk about
the disk’s center. Because
the sanding disk is eccentric
to the machine center, so is
this path of rotation.
The effect of this dual
motion is that each point
on the sanding pad traces out a geometric shape called
a roulette. You may be familiar with this shape from the
children’s drawing toy, “Spirograph”. The exact shape of this
roulette depends on the ratio between the orbital radius and
the radius of the pad rotation. This means that each part of
the sanding pad traces out a slightly different shape. This
minimizes the visibility of the scratch patterns on the wood
because each part of the sanding disk leaves slightly differ-
ent scratch shapes. Dual action sanders don’t leave fewer
scratches, but the scratches are less visible because each
one is different.
Gear Driven (Rotex) Mode
In the gear driven mode, the dual-action rotation of the
sanding pad is directly coupled to the orbital motion. For
each of the Rotex sander models, the pad rotates once for
approximately 10 orbits of the eccentric center. (In the
image below-left, this ratio is 9.5 to 1.)
This gear driven mode
provides some of the
aggressiveness of a
rotary grinder, but the
high ratio (10:1) of
the rotation-to-orbit
minimizes the vis-
ible scratches that a
grinder would impart
on the wood.
Random Orbit
This name random orbit is a slight misnomer because it
isn’t the eccentric orbit that is random, but the disk rota-
tion that is random. The motion of the sanding pad is still
considered dual-action, but the rotational component of the
motion is not constrained to follow a strict ratio of the orbits
like a gear-driven sander. The disk rotation is permitted to
freewheel about the orbit, and this includes even rotating
backward at times.
It is this freewheel motion that permits the sander to be
both fairly aggressive but to also leave very few visible
scratches. Again, it isn’t because there are fewer scratches,
but that the scratches are even less visible.
What makes the rotation of the disk somewhat random is
friction with the sanding surface. Inertia causes the disk to
want to rotate in unison with the orbital motion, but friction
with the workpiece tempers this motion, and causes the
disk to speed up, slow down, or even turn backward.
You may have noticed that when hand-sanding wood, there
is more resistance to sanding across the grain than there is
with the grain. The same is true for a random orbit sander.
As a result, when the orbital motion is moving across-the-
grain, the freewheel rotational motion will resist moving
across the grain. The net result will be more scratches
being parallel to the wood grain than across the grain.
Unlike a singular orbital motion (orbital sander) or a singu-
lar rotational motion (grinder), whose scratch patterns will
be circular, a random orbital scratch pattern will be more
“V” shaped and in the direction of the wood grain. Because
these are less geometric in shape, they are less visible to
the eye. The tighter this “V” shape, the less noticeable the
scratches will be.
Orbital Scratch
Patterns
Grinder Scratch
Patterns
Random Orbit
Scratch Patterns
Optimizing Random Orbit
Optimizing the sanding doesn’t remove the number of
scratches, it just makes them less visible to the eye. The
least visible scratches are those that are not geometric and
are in-line with the wood grain. Both of these conditions are
optimized by letting the friction between the sanding pad
and sanded surface control the rotation of the sanding disk.
Many operators will apply excessive downward pressure to
the sander to slow down the freewheel motion of the pad,
but this will increase the amount of friction and make the
scratches more geometric and more visible. Instead, using
a light pressure allows the cross-grain friction to be larger
than the in-line friction, and the ideal scratch pattern shown
above will be the result.
The best random orbit motion is when the sanding pad has
very little pressure, and is permitted to freewheel on its own.
Eccentric
Orbit
Rotation