What is a
does it work?
Plaster Disaster to
-- Aft Pylon Fairing --
I chose this job as
a case study because it has the full range of reasons, that are
common in the industry, for a part to need reverse engineering.
Some of these reasons are very universal to other industries as
well. Some of the reasons include:
- The part was created by hand, by master tool-builders, as no
CNC machines were used to cut the profiles.
- Engineering data for the profiles was hand drafted on Mylar
at 1:1 scale for visual inspection of the rib templates.
- Rib templates are placed on flat tooling plate and set to
specific distances apart as parallel as possible to each other
with a grid of thin supports between them
- Plaster poured into cavities between rib templates with a
thin sheet of material stretched tightly over the assembly to
help form and retain the plaster.
- After curing, the plaster is hand blended in between the rib
templates to form the final contours.
- There is an incredible amount of man hours and talent
invested in this procedure, and this plaster master
engineering tool has made successful parts for many years.
Although there is a fair amount of hand fitting required to
get the individual components to fit to each other because of
the tolerance build-up in each of the master tools and
additional error in the parts themselves.
(Very damaged tool)
here it is fresh out of the box, and oh isn't it lovely.;-)
Don't mind the missing pieces, we'll digitally fill those in
later. Notice all the different colors of plaster, indicating
repaired, replaced and or modified sections in the tool. This tool
was part of a joint effort program between the United States and
France, so it's probably seen more miles in transit back an forth
than most Sky miles platinum members, and I'm afraid it's
beginning to show.
individual aluminum "loft plates" that basically control
or create the shape, were first hand drafted onto Mylar sheets at
a 1 to 1 scale. The aluminum pieces are cut and trimmed to
visually match these Mylar "loft curves". The Mylar now
becomes the actual inspection tool for the plates, and thereby
transfers the engineering authority to the "loft plate".
This, of course, cannot be done completely accurately, so in this
case we also want to inspect not only the plaster surfaces, but
also the aluminum loft plates themselves, against the Mylar, this
will show the error in the "loft" plates, which are the
basis of the plaster model. To do this we must first digitize the
Mylar sheets. With the sheets being 4' x 16' in length, using a
traditional scanner would result in a great deal of error, so the
Laser Tracker is a clean and accurate method of capturing this
legacy data. Once the data is captured, we then model the data in
3-D CAD. This is now the most accurate representation of the
original design intent of the part.
We then digitize the aluminum
sections of the master tool and compare them with the nominal
(Close up of
(CAD model of digitized
Mylar loft data)
you can see in the picture below, the deviation error (shown in red
+, and blue -) between the
nominal Mylar data and the actual part is very inconsistent. This
is as expected when you consider the method of production, and
that the shaping and inspection of the aluminum "loft"
plates was done visually, by hand. These errors are now forever
part of the master inspection tool and subsequent parts. This is a
case where it is not desirable to simply just reproduce the actual
surfaces of the plaster model, this would also reproduce all the
errors and damage that the plaster mould contains, and make those
errors a permanent part of every actual finished part that is made
from here on out. Rather, we should attempt to capture the
original intent of the designers, which in this case is
represented by the hand drafted Mylar.
| If there are
large errors between the plaster and the Mylar, we must add
a third artifact to help determine which geometry is correct. This
would need to be one of the following:
- An actual part (pylon fairing) that has been proven to fit
well on the aircraft.
- A CAD model of the mating part(s) geometry, or again we
could digitize the mating part(s) if no CAD data were
- Data from other hard tooling or jigs that actually were used
to make the most recent successful part.
As no actual parts or
mating part data was available for this project, we turn to the
hard tooling that holds the actual part, and drills all the 5 axis
mounting holes that secure this fairing to the mating pieces. This
is called a drill or jig fixture. Once again, the Laser
Tracker shows excellent capability as a portable co-ordinate
measurement machine, or "portable CMM."
(Actual error between
plates and Mylar)
( Interface drilling
careful evaluation of all the digitized data, we compile an
accurate CAD model that represents the original engineering intent
and also incorporates all the updates and changes that have been
made to the hard tooling and plaster master since the original
Mylar were drawn. This is a far better solution than simply using
any one of the original tools, and now all of the storage
for all of the associated inspection tools can be reduced to a
simple CD, not to mention we can now modify, reproduce, and create
any of the required geometry at will.
Finished CAD model