INTERFACE CONTROL DOCUMENT
NFM-AD-01-0103    Filter Wheel Assembly

1.       Introduction

This document specifies the interface between the Filter Wheel Assembly and other assemblies that compose the Optical Support Structure. It is comprised of two drawings and a written description.

As usual for a mechanical assembly ICD, we specify where certain things must go for integration with other assemblies (locating surfaces,  matching bolt patterns); and an envelope that the physical assembly must remain inside of, and that other assemblies must remain outside of. We also specify some interface requirements arising from the installation procedure for this assembly.

A novel feature of this interface specification is the demarcation of an additional, substantial envelope of open space around the Filter Wheel Assembly, into which neither it nor other assemblies shall intrude. This is to permit possible future installation of motor extensions, and insertion of individual filters by hand subsequent to instrument integration. This volume is more complex than the usual access space requirements so is treated in some detail.

2.  Interface specifications

Drawings are described briefly, and appended as separate items. Written description follows below. The design engineer contact for this interface is Ruben Dominguez, 520-318-8275, rdominguez@noao.edu.

2.1  Interface drawings

Drawing NFM-MD-06-0001 shows the bolt pattern for the interface, and the location of bosses and matching V-notch that locate the assembly in the X-Y plane for system integration. The fundamental reference point in the X-Y plane is the centerline of the filter wheel axis. For the bolt-up, the Filter Wheel Assembly has Keenserts for M8 screws, and the Upper Housing Assembly has clearance holes for M8 screws.

Drawing NFM-MD-06-0002 shows the Filter Wheel Assembly with reference to surrounding assemblies. Critical planes are located. Z axis location of the assembly is defined by the plane of contact with the Upper Housing Assembly. The access envelope is indicated in three views. This envelope is for access to the motors on top, and to individual filters from below.

The access envelope has also been added as a ghost structure to the SolidWorks 3D model of NEWFIRM. This is intended to aid in spotting any minor intrusions by physical parts, and assessment of whether this is permissible if it occurs.

2.2  Description

The Filter Wheel Assembly (FWA), NFM-MD-01-0012, fits into a large cutout in the Upper Housing Cover Assembly (UHCA),  NFM-MD-01-0035. It is located in the X-Y plane by three bosses on the FWA that contact a V-notch on the UHCA. It is located along the Z direction by planar surface-to-surface contact between these assemblies. It is held in place by seven M8-1.25 x 16 mm LG screws installed through the UHCA. After installation there is a small (~1/8 inch) Z axis clearance between the FWA and the Optical Bench Middle Housing (OBMH), NFM-MD-01-1081, below it. There is clearance all around the moving filter wheels within the UHC.

The fundamental reference for locating the bosses, V-notch, and bolt locations is the centerline of the filter wheel axis. This is chosen since it exists as a physical location on the principle fixed and rotating parts of the FWA, and will be provided as a physical location on the UHCA.

The FWA is estimated to weigh 64 pounds and is required to slide into a tight space with blind registration.  Therefore integration into the Optical Support Structure (OSS) must be considered in the interface definition. The fixed frame that supports the rotating filter wheels, and protrudes from the OSS after FWA installation, is quite robust. The FWA center of gravity (CG) is slightly outboard of the OSS in the installed position. The installation procedure is

1.     The OSS, previously assembled, is positioned with its Z axis vertical.

2.     The FWA is supported by two people hand-holding it near the motor locations on a line through the CG.

3.     The FWA is moved into its aperture in the UHCA until the bosses on the FWA contact the V-notch on the UHCA.

4.     Care will be taken to minimize incidental contact between the FWA and the assemblies above and below it. In particular, this motion shall not use a sliding contact between the FWA and the UHCA or OBMH.

5.     Once the bosses contact the V-notch, the FWA shall be lowered slightly to rest on a support bracket fastened to the OBMH. This takes the FWA weight while the bolts are installed from above.

6.     The bolts shall be tightened to pull the FWA upwards into registration against the UHCA.

7.     Once the FWA is lifted off the support bracket, slight sideways force shall be used to keep the bosses and V-notch in registration while tightening the bolts.

This procedure imposes these additional interface requirements:

1.     The contacting surfaces of the FWA and UHCA shall be plane and smooth. For example, any bolt heads in these surfaces shall be slightly recessed.

2.     An installation support bracket shall be provided on the OBMH. This may be removable.

3.     The contacting surfaces of the FWA and the support bracket beneath it shall be plane and smooth.

4.     Otherwise, assembly envelopes shall be maintained but there is no requirement on smoothness or regularity of shape within an envelope. For example, bolt heads may protrude as long as they do not extend outside of the relevant assembly’s envelope.

5.     Other assemblies shall not interfere with the ability to maneuver the FWA into place by hand. In particular, the tangent bar shape and placement shall be examined for compliance with this requirement.

A substantial clearance envelope is shown around the installed position of the FWA, on the drawings accompanying this document and in the SolidWorks 3D model. This envelope serves two purposes. A central portion above and below the FWA permits insertion and removal of individual filters by hand with the FWA installed in the OSS. Substantial clearance must be maintained so there is no possibility of contacting or scraping the filter on anything as it is moved smoothly in and out. The upper portion of the envelope also has a “wing” around each motor. These allow for possible future installation of cold motor encoders, including associated connectors and wiring, and access to do the installation in place. These parts are more robust against being held in arbitrary ways, incidental contact with other parts, and complex motions to position them.

The envelope has been geometrically described as a collection of boxes. Minor intrusions by other parts or assemblies may be permissible without violating their purpose. If it appears that this would be helpful for the design or integration of other assemblies, contact Ruben Dominguez to discuss the situation before taking extraordinary measures to avoid intruding into this clearance envelope.