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Sometimes
lens elements are installed into their metal mounts
in a way that defies disassembly. These were intended
to be permanent assembly into the metal lensmount.
Good practice is always followed in the regular
optical industry and elements assembled this way
never seal in any air to glass surfaces.
Nonetheless, cemented lens groups can be a problem
if the cement fails. Sometimes such a lens, with
failed cement and permanent assembly, can be restored.
Often, to complicate matters, the assembled elements
may be frozen in place in the barrel or shutter.
Disassembly can be difficult but possible.
In the case of groups stuck in barrel, like the
old Collinear in this example, first fabricate one
or two aluminum slotted rings like this one. These
should be a close fit onto the cell mount and/or
barrel and have a section of approximatly 1/2" The
slot can be sawed on the bandsaw or by hand.
Next, clamp the lens in a lathe, using two three
jaw chucks bearing on the specially made clamping
rings so as not to mark the finish. Lock the spindle
and with the second chuck aligned by the tailstock
center unscrew the assembly. (You may want to use
a little solvent or penetrating oil depending on
the individual case) The adjustable wrench shown
on the chuck jaw is no joke: This can require
some real muscle
Once the two element groups and the mounting barrel
have been taken apart there still remains removing
the glass from its "rolled in" or "burnished" mount.
Use a keenly sharpened cutoff or "parting" tool
in the ordinary way to remove the turned in part
of the brass mount. Turn at about 300 RPM and advance
the tool slowly until the brass to be removed just
parts. Any technique should have been practiced
so as to avoid a burr on the inside of the cut.
A clean cut without a burr will make removal of
the glass easier. A burr on the inside of the cut
will make the glass bind in the sleeve and be even
more difficult to remove.
At this point the glass sometimes falls right
out and you should be prepared for this. Usually,
the glass is still stuck in the barrel either from
a burr at the cut, the cut not having been made
far enough back, old balsam or paint adhering the
lens to the side of the barrel or any combination
of the three. 
In this case the glass remained stuck in the mount
mostly due to old balsam and somewhat due to burring
of the cut. Use an electric hot plate to
warm the assembly to about 175 to 200 degrees F.
or about as warm as you can handle with bare hands.
(Like a cook can handle hot plates) If you set up
the hot-plate so that gravity can allow the element
to find a soft place to fall sometimes the balsam
will soften at the edges and gravity will allow
it to slide out.
A fair amount of heat and thumb pressure were
necessary to slide the lensgroup out. Great
care must be taken to push the lens evenly
and not allow it to become cocked to one side and
jammed in the mount. This becomes most important
at the very end of the removal where it must be
nice and straight. Pushing the element out cocked
at the last 1/8" will cause a flake/chip and spoil
the lens. (Like the method used to flake flint
into arrowheads)
The desired result (removing the cemented
group from the cell, from the barrel) is shown here.
I did this job to illustrate the risks and complexities
of this type of work. The end result illustrated
(not including the photography for this page)
took about three hours of concentrated work.
And this is for one cell. Remains to the amateur
owner to separate, re-cement, and solve the problem
of re-assembly, either by threading the cell metal
and fabricating a retainer, cementing the group
back into the sleeve, or making the retainer a part
of a mount to shutter adapter.
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