(2) Reaming. A reaming operation is used to
laminates. Dense fabrics tend to be easier to drill
enlarge an undersized hole to very accurate dimen-
or machine than loose fabrics. The unique physical
sions in laminates with metallic material in the stack
properties of aramid ibers call for different cutting
up. Fluted reamers cut primarily on the periphery of
tools than those used to drill or machine carbon or
the hole removing 0.004 to 0.008 inch on the bore.
glass composites. Sawing, drilling or machining of
The removal of a small amount of material reduces
any kind generally cannot be accomplished using dia-
the amount of metal chips pulled through laminate
mond or carbide abrasive or resin coated abrasive
holes as well as allowing high quality close toler-
cutting tools. The relatively low compressive strength
ance hole patterns to be produced. During reaming
of aramid allows deformation to occur at relatively
operations, holes are irst drilled in the patch mate-
low machining loads. Consequently, the ibers have
rial 1/64 inch undersized using a twist drill. These
a tendency to move away from the cutter within the
holes are then transferred into the laminate using the
patch as a guide. The inner and outer patches are
installed and the patches and laminate holes reamed
a. Machining. Laminate structure, iber orienta-
simultaneously to inal size. This allows both patch
tion, iber content, and material process can signii-
and laminate holes to be line reamed ensuring hole
cantly affect machining quality and tool life. To attain
high machining quality and avoid fuzzing, the aramid
material must be pulled taut and the ibers cut in a
(a) Ensure pilot on reamer engages inner
shearing motion. For a rotating tool, this requires that
patch hole before beginning reaming.
particular attention be given to the outermost layers
Use straight lute, piloted carbide ream-
and that the outermost layers are restrained and the
shearing action is directed toward the center of the
laminate. Tools that have a radical C-shaped (positive)
(b) Perform reaming operation by hand
cutting edge and a centering point to maintain con-
using a drill motor operating at a maxi-
centricity perform best on most laminate thicknesses.
mum of 250 RPM and a feed rate of 4-6
Use of water coolant when cutting aramids helps to
inches per minute. Limit holes reamed
reduce cutter friction and adds resilience to the iber,
to no more than 10 per reamer. Ream-
permitting cleaner cuts while reducing resin buildup on
ers which exceed this limit must be
the cutter surfaces. In general, tool selection, feeds,
sharpened or discarded.
and speeds are more critical in cutting aramids than
in cutting carbon or iberglass composites.
(3) Countersinking. Following drilling, lami-
nate holes must be countersunk if lush head fasteners
b. Penetration Damage Removal. Use the tem-
are to be installed. To ensure countersinks are cut
to the proper depth and are correctly oriented with
split helix router. Routing with conventional aircraft
the part surface, use a countersink micro stop cage.
tool steel routers is very dificult and special routing
bits have been developed which use the principle of
(a) Select the carbide countersink cutter
shearing the outermost ibers towards the interior of
(or HSS cutter body with carbide chips)
the laminate. The split helix cutter is especially useful
with the pilot diameter that corresponds
for severing laminates into sections, cutting slots and
to the drilled hole being countersunk.
notches, and trimming honeycomb panels. Inspect
Insert the cutter in a micro stop cage
the router bit after every few inches of cut for clog-
and adjust depth.
ging of the bit with KevlarŪ ibrils. Keep bits clean
to improve quality of the cut. Some back sanding of
(b) Install the micro stop cage in a 2,000
RPM drill motor. Center cage over hole
may be required to remove fuzzing. Do not exceed
to be countersunk and ensure piloted
a feed rate of 15 inches per minute.
cutter is aligned with hole.
c. Partial Thickness Damage Removal. Remove
(c) Cut the countersink to the depth set in
damage by sanding with an 80 grit silicon carbide
the micro stop cage. Use a feed rate of
abrasive sanding disk and a 90 degree router motor.
4-6 inches per minute. Ensure counter-
Change the sanding disk frequently as it tends to load
sink cutter is rotating before contacting
up with KevlarŪ sanding residue. Some fuzzing may
laminate surface to prevent splintering.
occur. Carefully remove damage to keep fuzzing to a
minimum. Fuzzing can be removed by back sanding
6-10. KEVLARŪ-EPOXY REPAIRS PROCESS. W-
oven aramid laminates are easier to drill or machine
than unidirectional laminates, and plain-weave lami-
nates are easier to drill or machine than satin weave