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.
