I decided to try my hand at making a Maksutov corrector lens. I had
another reason also. When making the secondary mirror for the Cassegrain
Optics I was making 1 found that small diameter convex surfaces was
a different game to long radius concave surfaces. How was I to know
if the mirror was zone free. What 1 needed was a test plate to check
them by interference. As I intended making a corrector lens later I
wanted to use the same tooling. I therefore made the radius of the secondary
the same as the radius of the convex surface on the corrector. I would
make matching convex and concave test plates, the latter to be used
to test the Cassegrain secondaries. I was not interested in making a
Maksutov telescope, only in making the corrector lens.
Getting blanks for the corrector lens was going to be a problem. As
it didn't have to be optical glass I decided to use plate. First I had
to cut out four 150mm. diametes discs from a piece of 15mm. plate. My
practice is to always make more than one. This allows for accidents
and second tries. I thought of cutting the discs out with a biscuit
cutter and carborundurn powder. But I found a place Jet-Cut that had
a machine using high pressure water and garnet abrasive who could cut
them out. This was quite fast as the finish on the outside was not fussy.
I intended grinding this later. Most of the time went in programming
the computor that controls the machine.
Next I cast and machined two sets of cast iron tools, concave and convex.
one set for the concave survace and the other for the convex. The machining
and channelling of one of these sets is described in "Cast Iron Tools".
The other was machined to use as formers for slumping the plate glass
to form a meniscus lens. Before risking one of the glass discs I obtained
from The local branch of Pilkington a piece of square piece of glass
6" across corners and 1/2" thick. His is the thickest carried in Calms.
I was given pamphlets on the float process of producing plate glass
and was told that when the glass came off the tin bath it was at 600°
C. and was in a plastic state. This I reckoned would be the temperature
that the glass needed to be heated to to slump.
What follows could be consinered a case of "fools rushing in where angels
fear to tread" or not enough though was put into it. Years ago I passed
over my small foundry set-up to Peter Tolputt who has made my aluminium
and bronze castings since. I took the glass and the formers round to
Peter to cant' out the operation at his convenience. Fig.1. illustrates
how we set it up in the furnace. The formers were raised up from the
furnace bottom so that the flame would be directed under the formers.
A steel band protected the glass from direct contact with the flame.
A weight on top of the glass was to press the glass down as it softened.
Peter waited until he finished pouring a pot of aluminium before placing
the set-up in the furnace. This would give a gradual warm-up before
turning on the burner. He did not have a pyrometer. We have been melting
aluminium and bronze for thirty years and judge when the metal is hot
enough to pour by looking at it. When he considred that heat had soaked
through everything, he lit the bumer and ran it at its lowest. The lot
came up to a red heat before the weight sunk down. He shut the burner
off and put a cover over the too of the furnace to stop air passing
through and causing too rapid cooling. He left it overnight. Next morning
he lifted it out.....DISASTER!
We had got it too hot and had too much weight. The glass had melted
and the glass fatened out to a 6" circle. Also it had fused to the iron
former in places and when the formers were pulled apart the glass was
Now Debie Evans who manages my son's office makes dolls as a sideline
and has a electric kiln. I asked her if she would have a go at it. She
was reluctant at first in case she ruined my glass but as I had four
pieces of glass and only needed two I assured her I could afford to
loose a couple. Her furnace is computor controlled and the heating and
cooling cycle is programmed in. The glass was placed in the furnace
resting on the concave former, with no weigth on it. The temperature
was set at 600° C switched on. It takes hours to come up to temperature
and when it did. the glass hadn't moved and looked as if never would.
Now they say "When everything else fails, read the Instruction Book".
Referring to her instruction book she found a little about glass slumping
and the program. A temperature of 1350° F. was required. It also said
that when the glass slumped the lid of the furnace should be lifted
to drop the temperature rapidly to 1000° F She entered this program
and switched on. I went home. On the way I called on Peter to tell him
of progress. Since I had last seen him he had been talking to Judy,
a woman he knows and who does glass slumping. She said we had got it
too hot in the gas furnace and should not have put a weight on it. Also
we were going about it the wrong way. We should be slumping the glass
over the convex tool, not into the concave former. She said that she
had only slumper glass up to %." in thickness and had no experience
with anything thicker. She said though that the lid of the furnace had
to be lifted when the glass slumped to cool it down before the edges
began to flow. Her furnace has the heating elements in the lid and the
heat is directed down on the glass. Why didn't I asked her to do it
for me? Because doing it oneself is the "Name of the Game". Judy makes
her own glass and could if required mould a piece between my formers.
I On leaving Peter I phoned Debbie and told her to switch off the furnace.
She picked up the convex former and took it home for another go. Before
she went to bed that night she had a look at the kiln. The temperature
was up to 1350°F. She lid for a look. The glass had slumped. I don't
know if lifting the lid to have a look was the same thing as doing so
to drop the temperature. but she closed the lid and left it in the hands
of the computor control. On removing it from the kiln the next morning
it was beautiful to see: no cracks. SUCCESS !.
I asked her to do another one and to bring them to work the next morning.
I then phoned Peter to give him the good news. He said to wait `till
we had the second one before celebrating. The first may have been a
fluke. Next morning we had the second one. Debbie said she put it in
the kiln, switched it on and left it to the computor. The cycle took
seven hours. Should the credit go to the program. No! says Debbie: to
her for being able to key it in. She could be right. Below is the program.
It makes no sence to me. She was as excited as anyone at the success
of the operation and when she said she would slump the other two pieces
of glass I accepted. Fig.2. illustrates the glass slumped over
the former. By the time the outside diameter is squared it will be down
to 5-3/4". This will not matter.
The former had tool marks on it and these came out on the glass. Also
it was black from being in the gas furnace. This was picked up on the
glass also. So I polished the surface up to remove both. But the glass
slumped over it on the third time had blemishes. It appeared as if on
cooling the top surface hardened before the bottom surface and contraction
pulled the glass up in places. See photo. As the depressed areas were
only 1/16" deep they would come out with diamond milling.
We decided we would slump the forth piece of glass into the concave
former to find out how it would go. The former was glass blasted to
give a smooth surface. It slumped OK, but it had a deeper depression
and in a neat circle about 2" in diameter. See photo. We could not explain
this and destroys the above theory.
Fig. 1. shows the set up we tried for slumping in the crucible
Fig.2. & 3. shows how the glass disc was placed on the formers
in the electric kiln.
Fig.4. shows the dimensions of the blank after slumping.
All the above has taken place over a period of time. It must be two
years since I had the formers cast. Now there will be another pause.
I have to build a machine to diamond mill the outside diameters.