Friday, February 20, 2009

Generating Some Work!

Haven't posted in an age! But I'm glad that at least Melinda's colonoscopy post finally moved off the front page of the blog - I was getting grief from her for taking advantage of her fuzzy mental state when she approved posting those pictures!

I've also not posted about work in nearly a month - a lot has happened! My friend and workmate Pat retired 3 weeks ago now and as a result, I'm back up to full time helping direct the backplate diamond generating on LSST. I've posted previously about the project - gave some background back in early December , as well as posted updates on January 7th, the 15th,and the 22nd. Since the last post, we've been preparing for diamond machining and are now well along on that task.

Like metals, glass can also be machined, but instead of hardened tools, diamond coated tools are used to cut away the glass, a process called diamond generating. So on a large scale, with the huge mirror substrates we are producing, we use large machines and tools designed to shape them.

The photo at left shows our workhorse, the Large Optical Generator, with the LSST mirror mounted to work on the flat rear surface. It is a "busy" photograph, with lots of features to point out, but of course, the large circular white thing is the 8.4 meter diameter mirror. The bulk of the machine is mostly hidden, but the red object is the spindle and 15 horsepower motor that turns the diamond wheels. The black accordion covers protect the horizontal and vertical slides and drives - 5 meters of horizontal motion and 1.5 meters of vertical with resolution of nearly a thousandth of a mm. The yellow fence-looking object is actually a safety rail to keep us from falling the long distance to the floor while we are working on the potentially slippery mirror surface.

The last thing I want to point out is the ventilation system - the coolant used to keep the diamond tooling cool is particularly dangerous when in a mist form. Since it can contain minute glass particles that can be inhaled deep into your lungs, it is instead contained around the cutting tools and exhausted to the roof where it is drawn through filters, allowing us to work without respirators in a shirtsleeve environment. Because of the ventilation system, the cutting point of the the diamond wheels cannot be monitored very closely visually. The operators depend a lot on how it sounds, and watch the computer outputs of the machine, including the spindle motor current, to judge how well the process is going. Mostly all you can see is the back of the mirror flooded with coolant - mostly water with a few percent of a cutting fluid specially designed for diamond machining operations.

The first order of business is to center and level the substrate as accurately as possible from the cast surface, then generate the backplate round, so that if the mirror needs removal, we will now have a machined surface to center accurately. This picture shows a diamond tool with diamond pellets around the periphery, with lots of coolant making that initial first diamond contact. You can see how a dangerous mist can form with the rapidly rotating wheel. I pulled back part of the ventilation system to take the picture, normally it would be fully enclosed.

After machining a big honking bevel on the outside diameter, to prevent a fragile sharp edge that might fracture or chip off, it was time to start the back surface machining. For this we use a "cup" wheel with diamond pellets along the bottom edge. In a view down the ventilation system, this is the tool we use the most for removing bulk amounts of glass on the large mirror surfaces. Diamonds only work well when they stay cool (removing glass up to 10 cubic inches per minute will make your diamond red hot very quickly without cooling), so we use about a liter of coolant per second from a tank where it is recirculated. Coolant is fed directly to the pellets, through the wheel and also under the wheel in a fan pattern in order to flood the contact point. To the right of the wheel is a white safety roller - connected to an Linear Variable Differential Transformer (LVDT), the height of the roller, thus the glass surface can be monitored by the computer monitoring the machine.

This next view shows the backplate now edged and partially generated with the back surface cleaned off. In a couple weeks after the machining is complete, we will also grind and polish the back surface, making for a stronger surface, as well as allowing visual inspection of the mirror interior. Come visit again and check out the latest activities!

Today I brought my macro lens in to work - I've been in conversation with our tooling manufacturer and wanted to know how a product was working, so shown here are closeups of our diamond tools. Likely not like the diamonds you were expecting that you see in jewelry, these are industrial diamonds that come in different sizes and concentrations, depending on the application. Currently we are still in the bulk removal stage, so we want to maximize removal. For that we use large diamonds that can remove that 10 cubic inches per minute that I mentioned. Unfortunately, the price you pay for that is the subsurface damage that large diamonds leave in the surface that must be removed in subsequent smoothing operations. So you follow up the coarse diamonds with finer diamonds that leave correspondingly less damage. Shown here are the pellets from our coarse and fine diamond cup wheel. Especially in the one with large diamonds (40/60 mesh, or up to about a half millimeter in size), you can see voids where dull diamonds have fallen out, exposing fresh, sharper cutting edges. The metal bond material erodes away slowly - you can see what look like small specks of diamonds, but are actually the tips of larger ones still buried in the bond matrix. While the large diamonds are mostly colorless, in the fine diamond pellet shown (80/100 mesh) you can see the yellow-green tint of some of the diamonds - click to enlarge. The pellets are about 8mm wide from top to bottom, and on the fine pellet, the top portion has not fully worn-in to full contact.

While I've been working with this stuff for over 20 years, there is still more to learn and apply to modern materials and products that manufacturers are now supplying. I suspect that the diamond tooling business is just about the largest it has ever been - not only from glass work and jewelry applications, but you see it every day in concrete and asphalt sawing and coring too.


Cut off wheels said...

Nice blog keep it up


nicholas said...

hi dean, it was great seeing you at the mirror lab, after so many years. the lsst mirror is an amazing achievement. nick