On Fri, 19 Jun 2020 12:53:18 +0100, 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.

Thanks for that info Dave, my plan is to clamp the mirror to the kitchen
worktop and clamp two thin 'lats', one either side of the glass sheet, so
the applied force is distributed evenly over the width of 14". The mirror
corner is missing to a depth of 2" with a large crack propagated down to
about 4" . My plan is to take off 6". I shall do this with the break line
over the sharp edge of the worktop. I shall use the clamps as handles to
apply the breaking force.