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.