On 19/06/2020 12:53, Dave Liquorice wrote:
On Fri, 19 Jun 2020 09:00:14 -0000 (UTC), Smolley wrote:

I need to cut some glass 4.5mm (a mirror), do I need to score both
sides or just one side.

One and I'm pretty sure the non-silvered side for a mirror.

Can I use washing up liquid for lubricant instead of oil ?

Glass cutting uses crack propagation to perfom the "cut". The score
starts and then guides the crack as it grows when the glass is flexed
to open the score line. For something 4.5 mm think score the glass
well supported then slide a thin (5 mm ish) lath under the glass,
align one edge of the lath with the score then starting near one end
of the score press down on the unsupported glass 6" or more from the
scrore. You should see a crack start to grow, keep gently pressing
and moving along the line. Thinner glass can be scored and lifted and
gently tapped with the cutter underneath the score line, the weight
of the glass and the impact causing the crack to grow.

The score needs to be done in one single and very firm pass with the
"cutter". The skill is in the firmness of that pass. Too light and
the score doesn't reliably guide the propagation. Too heavy and the
score has many cracks to propagate. More than one pass also
introduces extra cracks.

The oil isn't really a lubricant but a "wedge" to help the crack
propagate from the bottom of the score. ie the molecules of oil flow
into the growing crack keeping it open and the maximum stress
(hopefully) in the right direction.

Trying to trim less than an inch or two off can be tricky as it's
harder to apply the flexing pressure over a long enough section of
the score. There is a tendancy for bits to break off forming a
"scalloped" edge.

The other useful technique is, after making the score, to gently tap
along the line of it with something like the ball of a ball pein hammer.
Done correctly, it will cause the surface defect to propagate through
the thickness of the glass. You can see when this has happened by
viewing from different angles. Do this all or almost all along the
crack, then the glass will crack very easily (but still using the lath
technique). Masters of the technique make this look really easy (they
typically use the back of the cutter).