Spheres can be made with a variety of drill holes to accommodate connectors, feedthroughs, and a vacuum port for connection to electronics and batteries inside, or releases, sensors or other packages on the outside. With the ability to exchange a single hemisphere, one can easily be exchanged by another with a different arrangement of drill holes. Usually, each drill hole is accompanied by a circular polished spot face (also called ground flat) as a sealing surface.
The following types of drill holes can be applied:
Round drill hole
- Drill hole with accuracy of +/- 0.2mm
Round high accuracy drill hole
- Drill hole with accuracy of +/- 0.05mm
Round oversized drill holes
- Drill hole with diameter of more than 5/8" (16.2mm) and accuracy of +/- 0.2mm
Conical drill holes
- Conical (1: 10) drill holes with accuracy of +/- 0.05mm
Please see below for more information.
Please also take account of the following information when applying drill holes:
- A completed drill hole specification form (DHS) is required prior the production of instrumentation housings. At least the position of the vacuum port has to be specified there.
- Drill holes can be positioned anywhere across the hemisphere with minimum 30mm away from the edge of the equator.
- A certain distance between drill holes is recommended in order to not weaken the glass at a specific region and to compromise the integrity of the enclosure. As a rule of thumb, more but smaller holes (diameter < = 5/8”) shall be preferred over few and large ones.
- If the glass housing will be installed in protective shells and the penetrators protrude more than 5 mm from the spherical surface, we propose to position the drill holes under the top of the protective shell. Most space is available 60° referenced to the equator.
- The positioning accuracy (azimuth and elevation) of the drill holes is ± 1° and the ground flat’s outer diameter has an accuracy of ± 4 mm. Due to production process and characteristics of glass, the curved surface may not be entirely hemispherical and very small imperfections may appear. Hence, the drill holes may need to be re-positioned within the above given specification to ensure a decent round shape of the ground flat and to avoid spalling during the drilling process.
- Each drill hole is completed by a chamfer with a length of approx. 0.2mm on both ends at approx. 45°. The greatest caution must therefore be exercised in checking tolerances of drill holes and should only be done with measuring gauges of non-metallic material to avoid damages around the chamfers at their the very sensitive edges.
|Bulkhead specification||diameter [mm]|
|bulkhead||Drill hole||Ground flat|
|7/16" - 20UNF||11.1||11.5||25|
|1/2" - 20UNF||12.7||13||30|
|9/16" - 18UNF||14.3||14.6||30|
|5/8" - 18UNF||15.7||16.2||32|
|3/4" - 20UNF||19.1||19.5||40|
|7/8" - 20UNF||22.3||22.6||40|
|3/8" - 20UNF||9.6||10||22|