There seems to be some kind of detent at the end of the screw thread that is
preventing the screw from coming out.
The documentation said "captured screws". Does the word "captured" mean that
it is a special kind of screw?
There are several ways of doing it, but "captured screws" normally
means that there's a washer somewhere on the screw's body that stops them
falling out and getting lost after their hold on the body of the device has
been released.
Not having the machine to play with, there is likely a series of
catches built into the plastic/ metal casing which provide most of the
mechanical strength of the join, and the screws hold the corners together.
So,
- locate a thin blunt knife ("butter knife" ; the bluntness stops you gouging
the plastic in the next step) ;
- observe the *side* of the casing near each screw as you tighten and release
it, somewhere you'll see the casing flex and a joint be exposed as the
tensions change ;
- slacken off the screws fully (they rattle in their holders), then push and
prod the casing until the loosest joint you identified above allows you to
insert the butter knife ;
- work the knife along the casing gap, popping open each catch as you get to
it (sometimes using one knife in each direction from the initial entry point
helps) ; observe catches three times before contemplating using more force and
you'll soon get a feeling for how well-made the case is. IBM machines are
generally pretty-well made.
- proceed as your experience dictates.
This is getting embarrassing.
You've discovered an area in which you've not got the necessary skills
and experience ; you've asked for advice ; you've received it ; you're
practising on a machine where using a large hammer and a blow torch is
permissible (you said it's scrap) ; you're expanding your skill-set.
I don't see any grounds for embarrassment.
