Emerald Group Publishing Limited
Copyright © 2008, Emerald Group Publishing Limited
Producing micron tolerance mirrors to study history of our universe
Article Type: Aerospace technology From: Aircraft Engineering and Aerospace Technology: An International Journal, Volume 80, Issue 2.
The largest lightweight beryllium optic that has ever been made that will form a central element in the James Webb infrared optimised Space Telescope (JWST) due for launch in June 2013 to study phases in the history of our universe, has been machined in the USA using a custom built Mitsui Seiki horizontal machining centre (HMC).
The Mitsui Seiki HS6A, represented in the UK by 600 Centre of Shepshed near Loughborough, is one of eight installed in a climate controlled factory at Axsys Technologies Inc; Cullman, Alabama and has the ability to position to microns within the 1,300 x 1,000 x 1,000mm machining envelope. Each of the 18 beryllium segments, some 1.5m across, take around a year each to process with each sequence of production involving rough and finish machining, chemical milling, heat treatment and inspection (Figure 6).
The components form one of the most critical areas of the segmented 6.5m diameter folding primary mirror that will be adjusted along with a sunshield, the size of a tennis court, and be positioned once the JWST is in orbit about 1 million miles from earth.
Figure 6 Beryllium segments for the 6.5m diameter mirror on the James Webb space telescope are produced using Mitsui Seiki HS6A HMCs at Axsys Technology
Said Scott Walker, President of Mitsui Seiki, USA: “Axsys Technologies chose the HS6A machines following an in-depth selection process that ensured all machines met the requirement of providing a consistent cutting operation over extremely long periods of time. The process can easily generate stress, so care is important and a considerable amount of expensive material is being removed. It is the stable conditions provided for cutting by the machine tool, the performance of the servo motor drives and feed back systems to create the correct toolpath according to the program that are key in what would be high risk cutting cycles to produce the mirror.”
Beryllium is very light but brittle and was chosen for its properties of strength and hardness with the ability to absorb lots of heat. It is often used in computer X-ray windows and cathode-ray tubes for converting short-wave rays to visible light.
Each of the 18 segments are produced from a 250Kg billet of beryllium set on an angle plate on the table of the horizontal spindle machine. The billet is 100mm thick, 1.4m across and 1.6m from point-to-point. The reverse side of each mirror segment has 600 pockets each 62mm2 and there are 22 mounting pads and 249 holes to reduce weight (Figure 7).
During pocket milling alone, some 120kg of material is removed and the segment is then heat treated to remove any stress. The mirror side is then rough machined and a further stress relieving operation performed. During the rough machining, the circular cutting path originates from the centre and progressively feeds outwards removing a further 27kg of material. The pocket side of the segment is then finish milled which takes some ten weeks to complete.
Figure 7 The Beryllium segments showing 600 pockets on the reverse side are all machined on Mitsui Seiki HS6A HMCs at Axsys Technology
After finish machining and a further chemical milling process, the thickness of the pocket wall varies in size between 0.5 and 7.6mm. The finish milling cycles for the mirror, pocket sides and location hubs then follow with tolerances having to be maintained within five microns and a true position held within 2.5mm between the inside and outside of hubs on each segment and 0.12mm around a 1.2m pitch circle for bolt holes. The mirror surface has a dimensional thickness of just 2.5mm that has to be maintained within a profile tolerance of +0.05mm.
During the final machining cycle 30, 6.35mm holes are produced, each with slots on the edge that are process toleranced to 5mm. This is to ensure location datums for tooling balls in order that the exact profile of the mirror can be maintained during final polishing.
The Mitsui Seiki HS6A is a travelling column machine developed for high precision, heavy duty applications. The bed is an unusual T-shaped single monocoque structure that weighs over 20ton in order to minimise deflection, bending and torsional stresses. It has hardened and ground rectangular steel slideways and any overhang in X- and Z-axes is totally eliminated giving a dynamically superior structure. The spindle head is mounted within a double walled column that weighs some 11ton and is able to uniformly distribute cutting forces into the sides of the structure.
The table is carried on a cast iron saddle with direct drive ballscrews and the rotary table uses a large diameter Hirth coupling with rotary Inductosyn scales and hydraulic clamping for maximum rigidity. The spindle temperature is maintained using chilled oil around the cartridge and throughout the gearbox and is powered by an 18.5kW motor delivering between 15 and 4,600revs/ min. The spindle taper is a very rigid ISO 50.
The building to house the mirror machining facility was custom designed by Axsys and included a concrete pad foundation 1m deep. Each pad is surrounded by a bed of sand that isolates each machine from any vibration generated by neighbouring equipment or processes. Each machine was then secured to its pad via 27 anchor plates requiring 108 fixing bolts.
The JWST is to be premier observatory of the next decade serving astronomers world-wide to study every phase in the history of our Universe ranging from the first luminous glows after the “Big Bang” to the formation of solar systems capable of supporting life on planets such as Earth, to the evolution of our own solar system.
The telescope is an international collaboration between NASA, the European Space Agency and the Canadian Space Agency with Northrop Grumman Space Technologies appointed as the prime contractor. JWST will be operated by the Space Telescope Science Institute following its launch.