October 2010 Feature Article
Critical Aerospace ID Spec Met with New Honing System
Fuse Pins Must Be Designed to Hold a Jet Engine on a
Wing, But to Break Away in Emergency Situations, Allowing
the Engine to Separate from the Wing to Prevent Catastrophic
Structural Failure and Fires
Knowing when to "hold 'em" and when to "fold 'em" takes
on new meaning when referring to the fuse pins designed to
hold a jet engine on a wing, but to break away in emergency
situations That is the performance dilemma faced by the
highly-engineered, precisely-manufactured aerospace
components produced by Sonic Industries. To achieve this
delicate "hold-or-fold" balance, Sonic relies on a new
Sunnen SV-1000 honing system to produce the 5-to-7-micron ID
tolerance and proprietary finish critical to the part's
performance. The CNC-controlled SV-1000 also allowed Sonic
to meet increased customer demand when it replaced a manual
honing system, reducing cycle times from 40 minutes to 10
and increasing productivity from nine to 40 parts per day.
Sonic Industries, based in Torrance, California, is part
of the Sargent Aerospace & Defense group. The ubiquity of
Sonic's fuse pins in today's commercial and military
aircraft, and the importance of their proper performance,
make the design and manufacture of these small parts as
important to air travel safety as the integrity of a wing or
soundness of an engine.
Sonic
used to hone fuse pins manually; the CNC-controlled
SV-1000 helped reduce cycle times by 75 percent, and
increased productivity from nine to 40 parts per
day.
The fuse pin, also known as a shear pin, affixes the
engine onto the wing via the pylon – the structural
component connecting the jet engine to the wing spar. When
necessary, it allows the engine to break away under an
impact load in the event of a crash or other hard landing,
protecting the fuselage from engine fire caused by a dragged
engine. Fuse pins serve a similar function for landing gear
assemblies. Located in a structural assembly nicknamed the
"doghouse fitting," fuse pins attach the landing gear to the
wing and are designed to "fail" in the event of an extreme
hard landing, allowing the main landing gear to safely break
away from the airplane and prevent rupture of the fuel tanks
inside the wing box.
Sonic
Industries must meet ID tolerances of 5 to 7
microns, with a surface finish in the range of 8-16
RMS, for its fuse pins. The pins attach jet engines
onto the wing and break away in emergency situations
to prevent catastrophic structural damage and/or
fires.
Previous versions of fuse pins were designed with a notch
that would act as a "weak spot" and facilitate them breaking
on impact. However, a cylindrical pin with a notch is more
vulnerable to excessive corrosion and fatigue damage.
Therefore, fuse pins were re-engineered without the notch,
making ID tolerance and finish the critical factors in their
performance.
Fuse pins are made of steel and stainless steel alloys
including 318 and 15-5; they have various diameters and
lengths up to 23 inches. The pins start as a bar forged to
specific geometry and are gun-drilled, then bored to a rough
preliminary hole size. The parts are then heat-treated and
tested to establish the shear value, and the entire lot is
processed to final machining and finish grind on the OD. The
heads are finish machined with slots or hexes. The pins are
then bored to a specified size and honed to establish the
critical ID size, geometry and surface finish required for
proper performance.
Prior to acquiring the SV-1000, Sonic honed fuse pins
manually with Sunnen MBB 1805 and CV-616 machines. "We
needed to increase productivity and decided automating the
honing process was the best way to accomplish it," said Roy
Franks, Facility Manager of Sonic Industries. "Before
purchasing the SV-1000 we conducted time studies with Sunnen,
and indications were we could achieve the production levels
we were looking for. The machine has since exceeded the time
study estimates and the finish is superior to the previous
manual-honing method."
Sonic's
fuse pins have various diameters and lengths up to
23 inches. The Sunnen SV-1000 handles diameters up
to 3 inches and stroke lengths to 31 inches.
While the OD is machined to standard dimensional
tolerances and is repeatable, the ID must meet tolerances of
5-to-7 microns (0.0002 in. to 0.0003 in.). The ID surface
finish of the fuse pins is also critical, and while the
precise surface finish specs are proprietary information,
Franks says they fall in the range of 8 to 16 RMS.
Consistent size and finish of the ID are very important, as
size variations or surface irregularities could affect the
performance of the pin. The ID geometry of the fuse pins can
vary from a thru hole to a blind hole with an angle and a
radius, or just a bottom radius.
By upgrading to the SV-1000 series machine, Sonic is able
to use the Sunnen MMT superabrasive tools to achieve the
required micron-level accuracy. MMT tools are specifically
designed to work with the SV-1000 series machines, and each
tool is custom-engineered to the application based on width,
length, expansion angle, and number and placement of stones.
This customization produces accuracies of 0.0006 mm
(0.000027 in.) for diameter, roundness, straightness and
taper. MMT tools are precision-machined with a body and feed
wedge made from hardened tool steel, and typically last five
times longer than conventional designs, reducing per-part
cost by 30 percent. More importantly, the custom design of
the tools allows placement of abrasives to cope with
challenges like blind holes.
Another advantage for Sonic going to the SV-1000 is the
machine's longer stroke length. "Our old machines were
labor-intensive and had a maximum stroke length of 9
inches," said Franks. "It limited the parts we could
produce, but now we're able to make more sizes, and we're
doing it faster." The SV-1000 handles diameters up to 3 in.
and has a 31-in. stroke length.
Sonic
uses Sunnen MMT superabrasive tools, which last five
times longer than conventional designs and can
reduce per-part cost by 30 percent.
Automating the honing process also freed up Sonic machine
operators to attend to more than one machine. "With the
manual hones our process was hone a little, then check the
part, hone a little more, then check the part again," said
Franks. "With the CNC hone we 'dial in' the settings on the
machine and 99 percent of the time the part comes out to
spec. That means the operator can have one eye on the honing
machine and one on another piece of equipment." After
honing, parts are measured using a scanning air gage.
Sonic was able to achieve a more than 300 percent
increase in productivity, but Franks thinks that number can
climb even higher. "We didn't get the rotary table with this
machine, and we could bump up productivity even more by
loading three parts in the rotary table and continuing to
hone parts while others are being checked. We're looking at
possibly purchasing another SV-1000 and we'll be considering
the rotary table with that one."
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