Grinding Glass: Opps. A change in the curve.

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A new curve.

I have run into a slight new curve in the road, or in the lens lap to be more accurate. I finished grinding a lens and moved on to polishing the curved surface. When I checked the polish I noticed that the edge of the lens was taking a polish, but the center was not. This is a challenge. Because to get that center to take a polish I will have to polish down the edges so much that the center of the lens starts to touch the lap and take a polish. This may take a long time with the polishing powder, so it isn't a good situation. The Amateur Telescope Maker volume 3 says, start over.

In the photos above the first few are the rough shaped lens, then the second half shows the lens with the edge polished, notice the darker red, that is the sealing wax seen through the polished glass.

How does this happen?

Well, as the grit is grinding away at the glass to shape the lens, it is also grinding away at the metal of the lens lap. This grinding at the surface of the spherical dimple enlarges the radius of the spherical shape. This widens the curve of the lap and results in a wider curve to the lens surface. Then when I do the polishing I use a second lens lap because I don't want to mix my 220 grit and my polish. This polishing lap has not been used as much so its curve is smaller. When the wider curved lens is placed in the smaller curved lap, it does not touch at the center of the lens. Thus no polishing in the center of the lens. Here are some illustrations that I put together to show this lens lap gap.

I think that I am going to work at it a little more and see what happens. I have not been able to work on the lens too much in the last week. But maybe this week!

In the future I will go with a harder metal than the aluminum that we went with this time. I went with aluminum because it was mentioned in one of the articles that I had read. Softness was an advantage in that situation, but with a lathe it should be possible to work with something else.

September 23, 2012

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