The mirror may be easily drilled
with any commercial power-driven drill available. Included are:
Portable drills, drill presses, lathes, or automatic
multiple-spindle drilling units.
Before drilling a hole in a
mirror, it is recommended to use a bit offered especially for
plastics. If a drill bit for plastics is not available, a
metal-working drill bit with a high -speed twist may be used
with some modification.
Because metal-working drill bits
are designed to push through metal the following modifications
must be made to ensure
no chipping or other damage to the mirror:
1. The tip angle is usually about 120 degrees, this is too flat
to cut through the mirror
products without damage and must be ground to a sharp angle of
60-90 degrees to allow the bit to
enter and exit easily without chipping.
2. The cutting edge must be ground to a rake angle of 0-4
degrees. This "flat" cutting edge will scrape
the mirror without gouging it.
3. The surface behind the cutting edge must be ground away to
clearance angles of 12-15 degrees. This will allow back relief
for reduced metal to plastic contact and heat build up.
Drill bits with tips larger than
5/8" should be ground to a point to reduce the amount of force
required to start a hole.
Drill bits must be true, or melting, burning and chipping may
occur. Correctly modified drill bits will create two continuous
spiral strips as the bit passes evenly through the mirror, when
operated at the proper speed.
When drilling the actual mirror
it would be wise to back up the surface with a durable surface,
such as
plywood, so the drill bit will continue into a solid material,
this will prevent chipping on the opposite side of the
mirror. A slow feed rate should be used when the bit enters or
exits the mirror.
Holes of 1" or more may be cut
with a circle cutter. To accommodate the material properties of
mirror, the
cutter bit must be modified so the tip scrapes the material
without gouging it. Use a cool air mist system to avoid heat
build up, leaving the walls of the hole with a smoother cutting
edge. Use a drill press for uniform pressure and
constant vertical positioning.
Routing
Many routers are available for
use in the fabrication process. The router should have a minimum
of one horse-power
and a no load speed of about 20,000 RPM. Routers are normally
used with a single or double fluted bit, but may
consist of one to four flutes. Router bits can be carbide
tipped, high-speed steel, solide carbide, or diamond tipped.
They may be one piece piloted, non-piloted, straight cutting,
multiple part, forming or specialty bits."
Hand Routing: A hand router is
generally used when making a prototype or a replacement part, by
using a
precut template pattern clamped to the mirror, the hand held
router may be
smoothly guided around the pattern. Move clamps whenever
necessary."
Circle Routing: A circle router
would be used when a 360 degree piece of the mirror is needed."
Pin Routing: Pin routers are very
flexible. A double-backed tape or vacuum holds the mirror in
place. Using
the mounted overarm router to hold the cutter over a guide pin
in the table, feed the mirror and pattern into the cutter and
rotate 360 degrees to form finished product."
Contour Routing: By using a
contour jig on a pin routing machine, multiple parts can be
manufactured. Cut the
desired pattern on the base of the jig to follow the base guide
pin. To secure several mirrors at one time, clamps should be
mounted on the top of the work. Be sure to raise and
lower clamp holders as necessary when the jig is rotated."
Computerized CNC routers are used
in the manufacture of high volume production. This type of
router is
Numerical Control designed for maximum use of the mirror.
Mirrors may be designed for stacking
(CNC) Routing: which eliminates much of the waste normally
produced."
Direction of Travel: This router
is designed to rotate counterclockwise for external cuts, and
clockwise for routing
the inside edges of the mirror. When properly fed in the
direction necessary, a smooth cut will result.
When operating a router, several
precautions are necessary to avoid mistakes to the mirror or the
tool in use. First routers are designed with a small diameter
and must be operated at high speeds. Avoid vibrations, even the
slightest vibration can cause crazing and fractures in the
mirror during routing. Second, watch RPM speeds, higher RPM
rates allow for faster feeding of the mirror, resulting in a
smoother finish. Recommended RPM speeds are 18,000 to 28,000
RPM. Third, for maximum production, operate the feed rate just
below chipping speed. Do not overload the motor. Fourth,
maintaining a sharp cutter is very important to avoid chipping
and decreased production. Finally, use a 1/2" or larger diameter
cutter whenever possible, this larger diameter provides a better
surface with less tendency to chip."
Edge and Surface Finishing
The extent of finishing needed to produce a smooth,
transparent edge is based upon the quality of the cutting
tool used to machine the edge. A properly designed cutting tool
with a sharp cutter will reduce the amount of finishing needed.
Finishing is also reduced when a spray coolant is used along
with the cutting tool to reduce excess heat build-up.
Polishing: A polished edge is the best possible finished
edge, but requires the most preparation. Prior sanding is
necessary if the edge is shaped from a saw-cut, sanding is not
necessary when there is a well milled edge. A jointer, shaper,
or hand-scraped edge can be used in place of sanding. A
stationary polishing head produces the best polished surface.
Bleached muslin wheels with a diameter of 8" to 14" with bias
strips is recommended. This gives the buffing wheel a pleated
appearance, and runs cooler than a stitched buffing wheel design
and will also do a fast job.
Polishing Compounds: The finished quality of the polished
edge is determined by the polishing compound used. To produce a
high luster finish, the use of a fast cutting compound first
will remove all sanding marks, followed by a high luster
compound for the final buffing. To achieve a fairly good finish
in one operation, a medium cutting compound would be best.
Polishing Surface: Prior sanding is not necessary when the
scratches or machining marks are not too deep. A
surface polishing wheel should be from 6" to 12" in diameter,
built up to a width for 1 1/2" to 2". For the initial polish,
use a soft, bleached muslin wheel, followed by a soft flannel
wheel
for the finishing.
Depending on the depth of the scratches, use a medium-course
polishing compound or a fine compound. Be sure to keep the
mirror in motion at all times during the polishing procedure.
