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SDN 0013.08 – Validation of GNIRS Heat Treatment Procedures

1. Introduction

The preceding SDN (0013.07) described the heat treatment procedures to be used on the GNIRS aluminum parts manufactured from the 12” thick plate (see 2.2.) This note discusses follow-up information on the performance of the process in terms of dimensional stability throughout the manufacturing sequences.

2. Stress Relief Performance

2.1 NOAO Standard Uphill Quench

This procedure is described in SDN 0013.07 and references therein, and has been used extensively in the past. In particular, it has been applied to several optical substrates, including the Phoenix grating blank and the GNIRS Offner primary mirror. The Phoenix grating blank was tested cold, and has maintained its performance with repeated cold cycling. The results of the Offner tests have been written up and published (Vukobratovich et al, Proc SPIE 3435, 9 [1998]).

2.2 Procedure for Parts from 12” Plate

Most of the critical structural parts for GNIRS are being made from a single large 12” plate. Because of its thickness, the material in the plate is not in a true T6 condition (and definitely not in a T651 condition); a somewhat different treatment is therefore required. This treatment is specified in SDN 0013.07, Appendix A.

In order to verify the performance of this procedure, we (“we” = Roger Repp) undertook a series of measurements during the processing of one of the bench components, specifically the OIWFS Bench (89-NOAO-4200-0030).

The part was rough-machined to +1/8”/surface, inspected on the surface plate prior to heat treatment, and then following heat treatment. It was then further machined to +0.050”/surface and inspected again. Following this, it was cooled to liquid nitrogen temperature using the NOAO quench tank. The bench was placed flat in the quench tank with Datum -A- down. Liquid nitrogen was then poured over the part until it was completely submerged. The bench was left in the liquid nitrogen overnight, and removed from the quench tank the following morning. At this point, all the liquid nitrogen had evaporated. This cooling process was by far more abrupt than anything this or any other component in the instrument will undergo in practice. The bench was allowed to warm to room temperature prior to re-inspection. 

2.2.1 Inspection Results

The measurements taken after the initial rough machining showed significant distortion and lack of flatness, which is not surprising given the volume of material removed (about 90% of the initial chunk of aluminum). Also, since the machining was an intermediate step only, no attempt was made to hold tight tolerances. Departures from flatness were as much as 0.015 inches.

After the heat treatment, the bench was re-inspected. The largest changes appear to be associated with “rock” – lack of flatness on the underside – resulting in a systematic shift of ~0.013 inches. In the upside-down orientation, where rock was smaller, the changes were much less, 0.0025” or smaller. This suggests that the larger changes measured right side up had more to do with overall tilt and less with local distortion. This kind of large scale warping would be significant for a part that was cantilevered or end-mounted.

In any case, the effects were much smaller than the margin left by the 1/8”/surface specification.

The bench was then inspected once more after machining down to 0.050”/surface; one would expect this to have much smaller errors given the relatively modest amount of material removed. Note that for all other parts one would continue removing material, and not stop for extensive measurements.

The measurements showed this was in fact the case; most departures from flatness were 0.002 inches or less (the small number of exceptions were problems with the machining rather than distortions of the part; these will disappear with the finish machining).

After the liquid nitrogen “quench” and warm-up the part was re-inspected. With one exception, the changes were 0.0003 inches (about 8 microns) or less; this is close to the errors in the measurements themselves. The larger change may reflect a location error in the measurement. (The point in question is a deep pocket with thin walls, and Roger thinks it may well be a real shift, and adds, "Such pockets are difficult to manufacture, and not conducive to a structurally sound finished product.")

2.2.2 Evaluation

The consequences can best be evaluated by examining the size of the changes across the hole patterns for the various mechanisms mounted in the bench. One would expect similar effects for other mechanisms mounted in other bench components. The typical effects are ~0.0002 inches over a 6-inch baseline, or 0.03 mrad (30 mrad). If this occurred for a sensitive optical element, such as the collimator or one of the turrets, the change of alignment at the detector would be about 90 microns, which would be undetectable. The combined effect of similar tilts for all mechanisms on post-slit alignment would be roughly double this value, which is still quite satisfactory.

The actual performance could in fact be somewhat better, since any errors (uncertainties) in Roger’s inspection measurements are included in the estimates above.

One should note that the changes introduced by the initial heat treatment were at least 10 times larger, which suggests that this process is in fact quite necessary. That is, if we simply machined the benches with no heat treatment at all, the distortions upon cooling could lead to motions at the detector in excess of 1 mm, which is the tolerance on our baffle locations. In addition, one would not expect things to stabilize after being cooled once, so that the zero-points would evolve over subsequent thermal cycles, quite possibly by amounts in excess of 10 pixels (270 microns). Note that some effects are seen, even with the heat treatment, which suggests that radically scaling it back could be unproductive.

In summary, the heat treat process is required in order to produce the structural stability needed for GNIRS (or similar instruments); the resulting structure is stable to comfortably within the optical alignment tolerances.

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