Overall Mechanical Design

The final design is made of two smaller assemblies and fulfills all the requirements requested. The first part of the design is the stand, shown below.
This stand consists of three arched legs arranged in a tripod with a connecting piece at the top. The chamber inside this top Y-shaped piece is continued in an attached cylindrical piece below. The legs are made by connecting two cut pieces of 1/8” aluminum with 1” threaded spacers and machine screws. Finally, these legs are attached with blocks to the circular base.
Nearly all parts of the stand are made of Alloy 6061 aluminum, including the hardware used to put the jig together. Because of this, the coefficients of thermal expansion (CTEs) are all the same for all components and therefore do not cause problems as the cryogenic temperatures cause parts to shrink. The one exception is the bearing inside the cylinder at the top, which could only be found in stainless steel. This is press fit at room temperature, and, because the CTEs are so close, the fit should be tighter at cryogenic temperatures, but not enough to damage the bearing. The tripod design allows the stand to sit level, as well as allow for some adjustment, while the arched design of the legs and the circular base provide added stiffness. The result is a structure that can handle significant loads at its center with little deflection and should provide a stable base for the jig.
The second component of the design is the arm, which hangs from the center of the stand. A picture is shown below.

This assembly consists of a semicircular arch over a 30-cm diameter hemisphere with a one centimeter lip around the outside. At the top of the arch, there is a block, which connects the arm assembly to the stand. Like the legs on the stand, the arch in this part is made by connecting two cut pieces of aluminum sheet with ¾” spacers and machine screws. Specially designed blocks are at either end of the arch that house bearings with a press fit in the same way as in the stand. These blocks also have arms designed to physically stop the hemisphere from rotating along the horizontal axis by more than 45 degrees by stopping the counterweights. A switch could also be attached here to coordinate with electronic systems. One side of the arch has an additional block attached that extends the chamber that houses the flexible shaft coupling for the shaft between the motor and the bearing. Shafts are press fit into both bearings and also into the brackets seen attached to the hemisphere. These brackets have supports and counterweights extending below them to balance the load on the motor, and also attach to a yoke, to which the hemisphere attaches.
The only components not made of aluminum are the counterweights, shafts, bearings, and hemisphere. The counterweights had to be made of lead to meet space limitations, but the CTEs of aluminum and lead are so close that this will not cause problems in our design. The shafts are made of stainless steel, and since they are only attached to other steel components, except for press fits into some aluminum blocks, this should not be a problem, either. The only result will be a tighter press fit at cryogenic temperature. Issue that may arise with respect to the connection between the arm and stand is that the steel shaft may become less wide than the hole in the arm joint when brought back to room temperature. This potential issue was solved by threading the shaft and tapping the arm joint, giving some flexibility. The issue of the bearings has already been dealt with in the discussion of the stand. Finally, the hemisphere will be made of an absorbing material and will be constructed separately as it is far beyond our machining capabilities. Its CTE is close enough to that of aluminum that no design problems should arise. The arch provides a stiff structure, and the yoke design ensures that hemispheres can be switched in and out without a change in center. The press fitting of the bearings and overall tightness of the design should constrain the hemisphere enough to attain the desired motion around the horizontal axis.
The overall design combines both of these parts to create a mechanism that moves in both the theta and phi axes. The image below shows this, as well as indicates the location of the motors.

The arm assembly hangs from the stand by a shaft connected to a motor. This motor controls the rotation of the entire arm apparatus through 360 degrees in the phi direction. The downward orientation of the hemisphere results in a load on the motor only in the axial direction. Therefore, the motor need only overcome the rotational inertia of the arm assembly. The motor controlling the hemisphere’s motion in the theta direction is mounted to the arm assembly. The counterweights aid in balancing the load, lessening the torque that the motors need to output to turn the hemisphere. The stops included in the arm design limit the movement in the theta direction to forty-five degrees from vertical in either direction.
Overall, the design should produce a precise and repeatable movement with a constant center for the measurement of the infrared scattering.

Machining Results and Future Changes

The limiting factors in the construction of the team’s prototypes were largely our abilities with respect to machining and the availability of the equipment. Unfortunately, the machine shop with the precise machines was only open one day each week, and that time was largely used for water jetting parts from large pieces of aluminum. While the waterjet cutter was convenient and much faster than other methods of machining, it was very imprecise, and the tapering and/or shifted cuts of parts led to many of the problems with the existing jig. Because of the availability issues with the larger machine shop, many components had to be finished and drilled without the use of precise measurement tools, meaning holes were measures and drilled by hand, greatly increasing the error in construction. This accounts for most of the sway between the stand and the arm, as the vertical bearing did not get a successful press fit. In addition, we would have added a tight-fitting spacer between the stand and the arm, to decrease the chance that the shaft would bend. If the team had been given one to two weeks to use the better mills in the larger machine shop, we believe that we could deliver a final product.
In order to make a final version of the FIR Jig, we recommend that no interior holes that can be drilled be cut with the waterjet cutter. This is especially true in the case of the holes that attach to blocks. While this will be tedious to drill, we believe that the precision gained will provide a more level and stable structure. In addition, we also recommend the use of accurate digital readouts on all machinery used so that measurements are precise. This will ensure that all parts fit together much more precisely than they do in the prototype.
In order to use the jig at cryogenic temperatures, several components will have to be changed. The most significant change will be the replacement of the existing motors with dry motors of the same size and power. The use of a dry lube is essential, as a wet lube will become a solid and cause the motor to fail when the motor is taken down to the needed temperatures. Also, a flexible strap should be attached to the hemisphere and the arm to conduct heat from these bodies as the environment cools. All other factors should be covered by the design, including expansion and contraction during temperature change, bearing lubrication, and any electronic concerns.