[LargeFormat] Some Experiences with Re-cementing Lenses - part 1
Richard Knoppow
dickburk at ix.netcom.com
Thu Jul 21 16:44:24 EDT 2005
Some Experiences with Re-cementing Lenses
I recently re-cemented two lenses. There was a learning
curve and I want to share what I found out.
The two lenses were a Kodak Ektar f/4.5, 152mm and a
Wollensak Raptar Series 1a, 13", f/6.8
The Ektar is a Tessar type lens, the Raptar is a convertible
Protar type lens.
There were several false starts on both lenses. Initially, I
tried a two part heat curing cement. At the suggestion of a
correspondent, John Hendry (pict at pict.co.uk)
I tried using an Ultra-Violet curing cement, I wound up
using the latter. Both cements were obtained from Summers
Optical.
John also suggested using Methylene Chloride to separate the
lenses. I was a little sceptacle of this but tried it and
found it superior to the high temperature solvent also sold
by Summers.
Here are some of the things I found:
1, Clamping of the lens edges must be done using a machinist's
V-block. The makeshift methods suggested on various web
sites, such as using prisms, simply does not work reliably.
Proper edge clamping is imperative for proper centering.
2, UV curing cement is much easier to use than the binary
heat curing type, at least for small numbers of lenses.
The cement I used is Summers UV-74. This cement is one of
the few that can be shipped as a non-hazardous material. The
hazardous materials shipping on the other cements just about
doubles their prices. UV-74 costs about $15 US for an ounce,
enough to cement many lenses. Its shelf life is very long if
stored in the refrigerator.
3, The official method for separating lenses cemented with
synthetic cements is to boil them apart in a special solvent
at 340 degrees F. It takes about five minutes for the lenses
to come apart. However, its necessary to have a
thermostatically controlled hot plate for this. I have a
small laboratory hotplate but an electric frying pan can
also be used.
There is a definite hazard of damaging the elements when
using this method. At this high temperature the glass is
very sensitive to shock. I got edge damage on nearly all the
elements I cooked apart and will never use this method
again. In some correspondence with John Hendry he suggested
the use of Methylene chloride for de-cementing. I was at
first a little spectacle but I obtained a good supply from
Tri-S Sciences before they closed forever and tried it. The
lens is simply immersed in the solvent for several days. I
found that the UV cement would separate in about three days.
This is all done at room temperature and there is no damage
to the elements. I re-cemented both lenses several times
while learning how to do it. The edge damage from the
initial boiling apart does not seem to interfere with the
lens performance.
I obtained Methylene Chloride from Tri-S Sciences before
they closed but the same compound is available as paint
stripper from the hardware store.
For older lenses cemented with Canada Balsam decementing is
easily done with gentle heating. The traditional method is
to heat the cemented elements on a hot plate but I've found
that hot water works fine. Start with warm water and gently
heat it until the elements slide apart. I don't know the
temperature, and it probably varies, but its well below
boiling.
Canada Balsam is cleaned with Acetone and alcohol as is
described for synthetic cement below.
In the past I have tried recementing with Balsam but its
actually more difficult than with a synthetic and is not a
satisfactory cement.
Machinist's V-blocks are rather expensive new. I was lucky
to find a used one, they seem to be very difficult to find
used, but there is no substitute for one for reliably
clamping the lenses.
Method of applying cement.
I followed pretty much the instructions on the Summers
Optical web site. I also came on two books at the public
library on lens manufacture published by Adam Hilger Co., an
English maker of precision optical instruments. The methods
described were about the same. Actually, the method does not
differ substantially from that used for Canada Balsam.
Cleaning the elements.
I soaked the lenses in Acetone first. Then carefully wiped
them and rinsed them in more Acetone. Then I cleaned them
with Anhydrous Isopropyl Alcohol. This is obtainable at many
drugstores and is not expensive. I then washed the elements
in water with dishwashing detergent and rinsed them well. I
used a final rinse of alcohol just before cementing. The
room I used is reasonable dust free. Keeping the surfaces
free of dust is very important. Probably the best solution
if one has a dust problem is to build some sort of closed
chamber with access holes for your arms and a window. I did
not need anything this elaborate.
Small particles can be removed from the lens surfaces by
using sticky tape, like Scotch tape, to pick them up. This
tape does not seem to leave any residue.
I applied the cement with a wire made from a paper clip. Do
not be stingy with the cement. Make a small pool in the cup
of the concave surface. Then put the two elements together
and press down gently to spread the cement out until it
begins to leak out the edges. Air bubbles can be worked out
by gently sliding the upper lens over the lower one. Slide
them back and forth and vary the direction but do not use a
rotary motion.
After as much excess cement comes out as can be expelled
with gentle pressure, scrape off most of the excess with a
razor blade or some other convenient tool Some excess should
be left in place.
Now, place the lens, concave side down, in the V block and
clamp it. I do not have a machinist's flat so I use a small
sheet of plate glass. I clamp the V-block to the glass using
a C-clamp. The clamp for the lens is part of the V-block.
Inspect the lens to make sure its flat on the block and that
the edges are aligned. Then apply the UV light for about a
minute.
Summers UV-71 cures with an ordinary "black light" bulb. I
have a type BLB but a BL should work as well. This is a
tubular type lamp but there are now "compact" screw base UV
lamps which will fit into a desk lamp or other convenient
fixture.
After about one minute the cement will have
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