All,
Here is my take on the
TKR-Grid interfaces related to the size of the cut-out in the top flange,
profile tolerancing of the Grid, and flex'ing of the flexures due to
installation tolerances.
Datum holes
Datums A and B are holes with collinear centerlines on opposing
diagonal corner chamfers
Datum A is at the -X/-Y corner, Datum B is at the +X/+Y corner
Datum C is at the +X/-Y corner and establishes the 3rd point of
the plane and locks the TKR on the A-B line
Grid dimensions from
DS-01579 Grid drawing
Bay opening size: 346.600
mm (13.6457")
Diagonal flat-to-flat distance:
469.570 mm (18.487")
Assumed tolerance of Grid
features (with respect to Bay datum pin features in bottom of Grid)
Profile tolerance on cut-out:
0.006", one-sided (allowing for growth away from MMC--this is on
Grid dwg))
TKR datum hole position tolerance: 0.005" wrt Bay datum pin (rational but tight position tol)
TKR datum hole pattern parallelism to Grid bottom: 0.002" (3 holes form a plane that is
parallel to 0.002")
Analysis of worst-case
flexure flex'ing at hole B:
Tolerance on hole A and B in the Grid causes rotation of normal
to A-B line wrt theoretical: +/-
0.0025"
Tolerance on hole C location affects position of TKR on A-B
line: +/- 0.0025"
Profile tolerance of corner chamfer in Grid: +0.006" / -0.000"
>>Max flexure flex'ing due to Grid tolerances: +0.011" / -0.005"
The current design of the
flexures is that they nominally line up with the Grid top flange, so the
-0.005" tolerance says that there would be an interference between the
two. This would require that we flex
the flexure by 0.005" while installing the TKR module. Also, since the +0.011" gap is larger
than the allowed 0.005" flex'ing of the flexure, we would need to shim
behind some of the flexures to prevent damage to them.
Given that we will need to
shim <some> of the flexures, I propose that we plan to shim <all>
of them. There are two reasons for
this. First, given the very limited
range of these flexures, it seems imprudent to plan on using up much of the
flex to solve tolerance stack-up problems.
Using shims to zero-out all tolerance effects will maximize the
available travel for the flexures.
Second, allowing for an interference fit will mean that we will have to
physically flex the flexures during installation, and fight the flexure force
when shimming the opposing side (which presumably will have a gap). While this could be done, shimming all
joints would allow us to position and install the TKR with no external loading
required. This seems to be a much more
benign approach.
To implement all-around
shimming, the flexure mating face will need to move away from the Grid flange
sidewall. The minimum offset is
0.005". This keeps the shim
thickness and the post cantilever length to a minimum, while ensuring that the
TKR can be installed with no applied force.
I propose we offset the TKR interface by 0.15 mm (0.00591"), since
all TKR dimensions are in even mm and we avoid introducing round-off error in
their drawings. New dimensions for the
separation of opposing TKR flexure interfaces would be:
Mid-sides: 346.3 mm
(compared to 346.6 Grid bay opening minimum dimension)
Corner: 469.27 mm
(compared to 469.57 Grid bay opening mimimum dimension)
Note that these are
dimensions to the extremities of the TKR stay-clear and not to TKR
hardware. How far back the flexure face
is depends on internal tolerances of the TKR flexure positioning and hole
drilling tolerance. Judging from the
assembly plans, you probably only need to recess the flexure face by a few
mils.
Please consider this
plan. The shimming technique I
described in my earlier e-mail would be used for establishing shim thicknesses,
and you would likely need to do something similar to mount modules to the vibe
test fixture. As far as I know, this is
the last remaining issue that affects this interface. Hopefully we can close on this along with updating the flexure
design for next Tuesday.
Martin