DeWalt DW441 Instruction Manual page 8

Emery, even though harder than garnet (Mohs' Scale 9), has blunt
edges making it a poor sanding abrasive. Its primary use is for polish-
ing metal.
* Mohs' Scale is the mineral hardness scale. It rates diamond at 15,
the hardest, and talc at 1.
Manufactured Abrasives
White Aluminum Oxide (AlO) is extremely hard (Mohs' Scale 12) and
durable which makes it a great abrasive for use with the Random Orbit
Sander. "White" describes the particular way the abrasive is manufac-
tured, not its color. This abrasive is widely available and comes in a
variety of colors due to the increasing use of dyes. Stearate lubricants
on the paper also enhance performance by reducing heat and the rate
at which the paper clogs.
Silicon Carbide (SiC) is the hardest abrasive (Mohs' Scale 13) com-
monly available next to industrial diamond but is not as tough as
Aluminum Oxide abrasives. This abrasive easily fractures and pro-
vides sharp faces to the work throughout the life of the sanding paper.
This "self-sharpening" feature makes SiC an outstanding abrasive for
material removal but limits its life.
OPEN COAT VERSUS CLOSED COAT
Abrasives are applied to the paper with glue. When the grit is large
(say 60 or 80 grit) the percent of coverage is usually reduced to 60%
to 70% of the backing paper surface.This increases the life of the
paper by reducing the rate at which the paper clogs. This is called an
open coat and almost the only way you can buy sandpaper in heavier
grits. With finer grits the grit is applied at rates of 90% or more and
are labeled closed coat. Since the dust particle is much smaller the
tendency of the paper to load up and clog is greatly reduced.
GRIT SIZES
There are several grading systems used. The system most popular
today is sieve size. The number on the back of your sandpaper disc
refers to the screen size the particle can pass through. 120 grit paper
for example will sift through a screen with 120 holes per linear inch.
Every square inch of screen has 14,400 holes (120 x 120). The high-
er the number the finer the particle size. Common sizes are from 36
to 600 with the recent introduction of grit sizes up to 1200.
PAPER AND FILM: THE BACKING
Paper: The material most commonly used to carry the abrasive is
paper. Paper is graded by weight. "A" weight paper which is the most
prevalent is the lightest paper used and gives good durability and flex-
ibility in most sanding applications. Other paper weights that are avail-
able are C, D, E, and F weight. The disadvantage of paper is its low
mechanical stiffness. Since the paper fiber gives during sanding, the
abrasive doesn't stand up and some of the cutting edges do not
engage the work. The advantage is cost.
Film: Film backed sanding discs address the disadvantage of paper.
The film is mechanically stiffer than paper. Film is also more expen-
sive but when all other things are held constant improves the cutting
performance of the abrasive.
Cloth and Vulcanized Fiber: These backings are typically hard to
find and are used in specialty applications. Most sandpaper you will
use will have the paper or film backing.
HOOK AND LOOP AND PSA: WHAT HOLDS THE SANDING
DISK TO THE PAD.
Hook & Loop : The sanding disk as backed with a fabric nap (loops)
that interlocks with a grid of posts (hooks) on the sander's pad. The
advantages are paper reusability and cooler interface between paper
and work.
PSA: Pressure Sensitive Adhesive is applied to the back of the sand-
ing disc and adheres to the sanding pad (Not the same pad that car-
ries Hook & Loop paper). The advantage is cost. The major disad-
vantage is the tendency of PSA backed papers to adhere permanent-
ly to the pad if left on after sanding. Why does this happen? The main
culprit is heat. During sanding the pad and paper heat up. This caus-
es the adhesive to flow into all the ridges on the vinyl pad and form a
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