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Custom-built horizontal machining centers are machining the James Webb Space Telescope mirror segments positioning them within a few microns anywhere in a machining envelope

The James Webb Space Telescope (JWST) will study the earliest galaxies and some of the first stars formed after the 'Big Bang'. The JWST has a 6.5m mirror, which is comprised of 18 beryllium segments recently completed by Axsys Technologies in its climate-controlled factory in Cullman, Alabama, USA.

Each segment is about 1.5m across.

It's believed to be the largest lightweight beryllium optic that has ever been made.

After a thorough selection process, Axsys chose Mitsui Seiki USA (Franklin Lakes, New Jersey) to supply the technology to machine the segments.

Northrop Grumman Corporation is the prime contractor for the JWST.

Mitsui Seiki custom-built the HMC

The HMCs have a massive structure: for example, the column weighs 11 tons and the bed is 20 tons, yet are able to position a workpiece to within a few microns anywhere in the machining envelope.

President, Mitsui Seiki USA, Scott Walker, said: 'These are very large, heavy machines; the components are substantial.

However, they are also elegant in that they have to position and travel in a very specific, ultra precise manner'.

* Billet machining - a 540 lb beryllium billet is fixtured vertically onto an angle plate on one of the eight Mitsui machines.

The six-sided billet is 4in thick, 54in across, and 62in from point to point.

The reverse side of each mirror segment has 600 pockets - about 2.5in square each.

There are also 22 mounting pads and 249 light-weighting holes.

Producing the pockets, pads and holes takes about two months to complete.

After pocket milling on the reverse side, the billet weighs about 275 lb at this stage.

The segment is then heat treated to relieve machining stresses, and then the mirror side is roughed before another stress-relief treatment.

Roughing is a circular cut that starts in the center and works its way out, and removes another 50 to 60 lb of material.

The next operation is finish milling of the pocket side, which is another 10 weeks of machining time.

'Consistent cutting is paramount in this application,' said Walker.

'Machining generates stress and a lot of material is being removed on these mirror segments.

The cutting conditions, the toolpath, and how the servo motors control the toolpath to provide consistency of machine load are the keys to successful cutting of the mirror'.

* Finish machining - after finish machining and chemical milling, the final dimensions on the pocket wall thicknesses range from 0.020in to 0.299in.

Finish milling of the mirror side, pocket side, and locator hubs is next.

Tolerances for many of the finished elements of the mirror structure are +0.0002/-0.00in, and true position is 0.001in from the inside to the outside of the hubs and 0.005in all the way around a 48in bolt circle.

The mirror surface itself has a specified thickness of 0.098in with a profile tolerance of +/-0.002in.

The final machining operation is for 30 x 0.250in diameter holes and slots on the edge and tolerances for those are +0.0002/-0.00in.

These holes are for tooling balls used to maintain a profile of the mirror surface during polishing.

* One year of processing - at Axsys, each segment requires almost one year of processing, which includes machine time, heat treatment, and inspection.

Parts are inspected at each step of the process for dimensional accuracy and for residual stresses.

'Obviously the goal here was not speed, but accuracy,' said Walker.

'Contributing to machine precision is the foundation we recommended built under each machine.

When Axsys designed the new building to house mirror production, they included a concrete pad under every machine at 39in thick.

Each pad is surrounded by a bed of sand to isolate the machine from any vibration generated by neighboring equipment.

Each machine is anchored to its pad using 27 anchor plates with four anchor bolts per plate for a total of 108 bolts'.

* Project control - John Mather, winner of the 2006 Nobel Prize in Physics and project scientist for the James Webb Space Telescope, is one of many overseeing the project at NASA's Goddard Space Flight Center.

The JWST is scheduled for launch in June 2013.

JWST is designed to study the earliest galaxies and some of the first stars formed after the 'Big Bang'.