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Rough grinding began on Saturday the 19/4/97 using the ceramic tiled tool.
We began by covering the cell with plastic sheet to add additional protection
against water and then placed a disc of small cell bubble pack between the
cell and the blank. This would take out small variations and provide damping
of the mirror and help combat vibration. 60-grade carberundum was used to
start. The mirror was wetted down and 60 grade sprinkled on the surface.
Normal stokes with a 2 inch overhand where used to start moving the tool
back and forth for a few strokes, then turning the tool and taking a step
to the right. Geoff and I took turns to have a go and hopefully give some
randomness to the stoke. Reading information of the Internet and other books
we did some full 'W' stokes to also give some variation and try and smooth
the whole surface at once rather than deepen the centre even more.
The glaze on the tiles wore off after a few 'wets' and the tool bed into
the curve. But it was not to long before we realised that the tool did not
seem to be doing its job. The carberundum was collecting near the middle
and the tool was breaking down making a light coloured sludge on the surface.
The tool seemed to be riding over this and not doing much work. Adding more
carberundum did not work and we seemed to be throwing it down the grid when
we cleaned up between wets. Work was stopped and I decided to redesign the
tool. This was done by covering the surface with metal washers, (see, Making
the tools for grinding by hand) an idea off the internet which said they
worked quicker than ceramic tiles but must only be used on pre-generated
curves. This was done and the mirror and cell cleaned down It was while doing
this that I noticed that the cell had swelled at many separate points on
its surface. Water had got in under the plastic sheet and penetrated the
varnish on the surface causing the surface to swell. It looked at first that
the cell was ruined and another would have to be made. I left the cell to
dry out for a few days and the finely planed out the damaged areas. The cell
was then painted with 1 undercoat and 3 gloss coats then left to dry. All
nooks and crannies where treated to ensure water could not get in. The cell
was then covered with two sheets of plastic to make sure water would not
get through again.
Grinding was resumed on Saturday the 19/4/97 with the new tool and 60 grade
carberundum. A number of grooves from hogging out remained on the mirror
and we watched these disappear over a few hours, gradually moving out towards
the edge of the mirror. The 'W' stroke was used mostly with a few normal
strokes to ensure we had even coverage. Direction and length of stroke where
also changed. The 60 grade ran out and we changed to 80 grade. This seemed
to work faster than the 60 and after 3 hours work there were only a few very
faint irregularities on the very outer edge. Curiosity got the better of
us and we decided to test the mirror. The mirror was removed from the cell
and carefully propped up against a tree in my garden. We wet the surface
with clean water and stood near the focus point. As we varied position relative
to the focus we could see the roof of the house, chimney pots and TV aerials.
Over the next week I completed the 80 grade and moved onto the 120 grade.
This was well on the way by Saturday the 3/5/97. We completed the 120 grade
and moved onto 240 grade after cleaning up and replacing the bubble pack,
plastic sheets and cleaning tools (sponge etc) to ensure no contamination.
Geoff changed his shirt from a 120 to a 240. Care had to be taken when touching
surfaces to ensure no cross contamination. The supprising thing that was
apparent about grinding by hand was how quickly it seemed to be going. This
could be partially attributed to the fact that the mirror was plate glass
and probably softer and easier to work than harder Pyrex or other low expansion
materials. The table below shows the times of total work done for each grade.
At the end of work on the 3/5/97 it was near 9:30pm and was going dark. We
removed the mirror from its cell and propped it up against the tree at the
end of the garden and wet the surface. A very bright image of the inside
of my garage could be seen with details of light fittings and the reflected
light off the roof. The imaged was diffused a bit because of the water not
being a perfect reflecting and smooth surface but these images where very
encouraging and showed that the mirror was well on the way and would give
a reasonable image if maybe not a good one. Only time would tell and we planned
on starting polishing within a few weeks.
Grade used Total time of work done
Roughing out
60
80
120
240
320
400
20t (500)
12t (600)
9.0 Fine grinding
I went to Orion optics on the 9/5/97 to pick up some fine grades of 20t and
12t , two pounds of each. These would be used in the last stages of fine
grinding before polishing. Orion optics stated that there was no point in
going further than 600 grade as scrtching of the surface would be inevitable,
and in any case it is not necessary. Polishing may be started from this point.
This was confirmed by Andrew bates of our society (he makes mirrors also).
The 20t was mixed 6 heaped teaspoons to 3/4 full sauce bottle and shaken
well. A spiral was added to the mirror and both tool and mirror misted with
water from a hand spray bottle. This was all used up and the mirror took
on a very smooth surface with the pits so small they could hardly be seen
in the microscope. Great care had to be taken in moving the tool on and off
the mirror and cleanliness during grinding. This grade was completed in one
session.
We then moved onto 12t on the 31/5/97. This proved to be a problem. It appeared
that the tool was beginning to scratch the surface. The grinding powder was
too fine for the tool and scritching sounds could be heard. We stopped work,
cleaned down and inspected the surface. Some fine buff scratches could be
seen. We decided that no more fine grinding be attempted and that we go onto
polishing.
Picture above is a close-up of Dave Owen (His hands anyway!) using a sub-diameter tool during fine grinding. You can see the 'sweep' marks of the washers from the tool on the surface of the mirror.
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Picture above shows Steve Southern 'having a go'. Many Liverpool Astronomical members helped out. See the credits on the main Home page.