Welcome to another Lghtblade learning lab,
today we're going to be talking about
lenses again but lenses are very
important because they focus the light
energy or the beam energy that we've got
coming out of this machine down into a
very small point which is where the work
is done the beam itself is as we found
out maybe eight nine ten millimeters
diameter, it's quite large
most of the energy is concentrated in
the central section probably within the
central fifty percent of the beam, all the
energy that we can collect is
concentrated down through the lens into
a very very small area here I've got some of my
hairs which you can hardly see now if I
measure one of those hairs we can see
that it is point oh four point oh five
millimeters diameter double that up to 0.1 and
that's the size of the laser beam
that we are probably going to get on the
best lens that we can probably find for
this machine and that's a one and a half inch
Lens for this machine, now when you start getting up to a
two inch. two-and-a-half inch and a four inch lens
those numbers start growing considerably
maybe up to 0.2 or 0.3 even now 0.3
is still not very thick but it makes a
huge difference to the way in which the
power is delivered onto the surface now
look here I've got
a tool that I've just picked up out of my workshop
it's got a very sharp point on one end
and a blunt section on the other end. Here
I'm going to press as hard as I possibly
can on that card and wobble it around
with that end
how much of an impression have I made? Not
very much
now i'm going to press the same amount
of energy because I haven't changed my
strength and I was doing exactly the
same thing with the other end
now there's a pretty significant hole in
there
that's nearly through to the other side
this one
well it's hardly marked the surface there's a
mark on there but I can feel that
it's just the smallest dent and this is
a very soft card so I would've expected
to have caused more damage than that but
the reason why I haven't caused more
damage is because the energy that I'm
able to put into that is spread over a
much much larger area where as when I do
it this way around the energy is
concentrated into that point
so the point is an energy concentrator
basically what we've got we've got a
much higher and you'll hear me using
this term a lot the energy density the
energy per square millimeter is a lot
greater when i do that then it is when i
do that
and that's the principle of why we
use different lenses and you say well ok
if we use a lens that produces a shape
like that a very thin hair like beam
we've got huge energy density we can do
a lot of damage to the product
why don't we just use one of those? The
answer is in this very crude diagram that I've put
here we may be able to concentrate the
energy in a very small area as opposed
to the larger area that we saw
demonstrated in my mechanical
demonstration but the 1.5 inch lens
focuses the light like this it's
a very sharp shaped lens which goes down
to a sharp point but it goes down to
sharp point quite quickly and expands
again very quickly so it's useful
working length is actually very short
now this one which is the 2-inch lens
okay it's got a bigger footprint
but it's working length where the energy
density remains reasonably constant is
larger and this one which is the
two-and-a-half inch has got an even
shallower beam shape and it has a
working length which is much longer
so you've got these strange properties
associated with the focused light you
can either have lots of energy over a very small
length or you can have less energy over
much longer lingth and this is the
trade-off you've got between lenses. Now
what we're going to do today is i'm
going to try and turn these drawings into
a real picture in other words I've got
my focus gauge here which runs from the
focal point plus 4 millimeters to the
focal point minus 4 millimeters
ok now my intention is to draw a line
along here and hopefully if I get it
right
I will manage to get the focal point in
the middle and so we can see that the
change of line thickness and power or
density, energy density changes as we
move along the line because at this
point here we've got a big footprint and
at this and we've got a big footprint
we've got a very small one in the middle
so that means their energy density at the
top here is very small, in the middle
it's very high and we get less of a
change with these because although we
might be starting off at the same length
the same diameter we don't get we come
down in a much shallower curve. Right now
I'm just going to do a quick pulse test
to make sure that my beam is
approximately in the right place to give
me good consistent power
that looks pretty good now I'm not going
to bore you to death with hundreds and
hundreds of results what I'm going to do
is to show you my method and then we'll
work on from there, i will carry on
working and show you the end results now
what I've got here is my focus ramp
which takes me from zero in one
millimeter steps out to 4 millimeters plus and
4 millimeters minus. Now I'm gonna pop
that in there and actually what I've got
this time I've got some card and it's
one millimeter thick card so it's nice
and stiff so that when I pop it in here
there is no chance that it's going to
flex in any way at all
in other words this surface here is
going to be as flat as the reference that
i can produce on here and i've also used
my metal surface here my flat metal
surface because i know that that is true
well I can see that I've got a 2-inch
lens here and a two and a half inch
lens so by default that one must be a
one-and-a-half inch lens and what we're
going to try and do to put to start with
that I'm going to turn this over the
other way like this and I'm going to try and
set this so that the power just burns
through somewhere around about the focal
point
ok so we're starting off these tests with
a speed of a hundred millimeters per
second and a power of 25% now I've got no
idea what that is in terms of watts but
we'll sort that out later, I've got the focal
point set to 7.5 millimeters
ok let's look at our result
have we got what we're looking for yes
we have now what we've got here we've
got something that runs from two and a
half plus to about 1 minus, it's about a
millimeter too high so what we've got to
do to put that back in there again
instead of seven half millimeters we
really ought to drop that down to six
and a half millimeters same speed same
power
and there we go that we've moved it down now
we've gone from we're roughly one and a
one and three quarters to two and a
quarter so we may well have gone too
much but we're now going to do is we're going
to change the power slightly because
this has cut through as you can see this
is cut through and what I want to do is
try and decrease that length now ah there we go
right up the middle
fifteen percent we're just about making
it through just about making it through
and would you believe it looks as though it's
about minus one and just about +1 maybe
one and a half, so we still might be a
little bit out on centering but that's not
bad
so we do a result on the front so we can
see what the dimensions are now this is a
two-and-a-half-inch lens test which is
typical for the other tests that just
been doing
so the first thing I'm going to do is to
set the power and the power has got to be
set up to 17%
now we do it this way just so that we've got
something to measure on the front
so that's 10mm/s
focus is 5.5
here we are looking at the scorched line for the
2 inch focal length lens now this is the
line as it entered the bottom of the
ramp this is four millimeters below the
focal point now you'll see that they
look like a couple of tram lines along
there but when we look a bit closer and
tip the card up you can clearly see
that it's a V-groove cut by the laser
itself into the card but i'd like you to
look at the top of the V and I think
you can see there's a very small almost
a filleted radius on the top of the V where
the power has dropped off quite
dramatically now I'm going to basically
for these measurement purposes I'm
going to ignore the little fillet radius
on the top of the V because that
basically has got no significant power
it's enough to scorch the edge but that's all
it is doing so what I'm really
interested in is the powerful cutting
piece towards the center where the V is
going all the way down from the vertical
now what i'm going to do is measure
all positions between minus 4 and plus 4 to
get nine results and then we'll take a
look at the shape of the line now I have a
glass graticule which is marked off in
point 1 millimeter divisions it's
possible with this microscope to
reasonably accurately measure the
dimensions
I mean I'm once i get below point one of
a millimeter which is the division that
you can see on here you can easily
estimate to half a tenth which is
.05 point .025 well that's a little bit
flaky but I've attempted it in a few places
well here we are measuring the dimension
right at the end which is the minus 4 position
then we're moving along to the minus
three position quickly taking it all the
way through to zero over the course of four
millimeters I think you can see how
dramatically the the line has changed so
just in case it's confusing with the
graticule in the way i'll show you a
series of pictures we start from the
lens here at four millimeters and then
three millimeters
and then two millimeters and then one
millimeter and then 0 that's how the beam width
is changing as it goes from four down to
zero the focus point, well after a lot of
tedious repetitive work we finish up
with this rather daunting array of
figures here now don't get too upset
because it's actually very very simple
let me just explain let's start off here
at this two-and-a-half inch focal length
lens and what we find is we've got the
focus point here at zero and then we've
got plus or minus 4 millimeters above
and below the focus point now the beam
diameter translates to a beam area and
this is the area in square millimeters
in this next column and then what we've
got across here are 10 20 30 all the way
through a 100 watts of power that could be
going through the lens so if we had as
in my case a 60 watt tube, that doesn't
mean to say there was 60 watts going
through the lens but let's just stay
with 60 watts now right at the focal
point we have the smallest possible beam
that we can get and in this instance it
was 0.18 of a millimeter now we
translated that diameter into an area
square millimeter area so if we squeezed
10 watts into 0.25 square millimeters we
will finish up with 393 Watts per square
millimeter it's just a standard
definition and that is called energy
density now we've calculated the energy
density for different wattages for that
same spot size and you can see how the
energy density creeps up why do we and
why are we interested in energy density
every particular material will have a
threshold of damage, burning damage
because basically that's what our beam
is doing its burning the material now
that threshold maybe a thousand watts
per square millimeter it maybe 10,000
watts per
square millimeter it's difficult to say
because there is no information out
there on the internet which tells me
what this burning threshold is now for
this exercise I used card which had
some sort of substance or body to it, it
was one millimeter thick and it required
a noticeable amount of power to cut
through it the amount of energy required
to cut through that card will ultimately
be defined by the energy density
itself that it can resist but i'm
suspecting that card the card that I
used is probably sitting here with an
energy density of probably somewhere in
the region only maybe four or five
hundred now if we were to draw a line a
vertical line through those three graphs
at roughly where my arrow is which is
about 500 watts per square millimeter i
suspect that at 500 watts per square
millimeter i would probably be somewhere
near the damage threshold for that card
but the reason why I want to bring that
to your attention is because each one of
these lenses and we're looking at the 60
watt energy density for the lenses is
capable of exceeding the energy density
required to damage that card so that
means that any of those lenses i could
use to cut the card
ok something else rather interesting
about these pictures is the way that
they actually describe the type of
lens they are imagine these to be
three different types of knife one of
them very sharp like a scalpel this one
being a bit like a pain knife and this
one being more like a butter knife now i
think if i give you that analogy you
will clearly see that it doesn't require
much effort to cut something with a
scalpel
it will require more effort to cut the
same thing with a penknife and a lot
more effort to try and cut it with a
blunt butter knife, that will help you to
visualize how and why you would want to
use a certain type of lens we've already
seen this sharp lens can cut with as
little as 17 watts whereas in fact cutting
the same material with a blunt lens takes
54 watts and I hope that that clearly
describe the difference between these
sets of lenses now I, now I need you to
do a little bit of mental gymnastics
because i want to bring you back to the
fact that i used a hundred and ten
millimeters per second speed to do all
of these tests i could have used this
and lets call it a blunt lens which
takes 54 wats to cut through the
material at hundred and ten millimeters
a second i could have used 15 watts to
cut through the same material but i
would have had to do it at very very
much slower speed it certainly has more
power than the damage threshold of the
material so it will definitely cut the
material but it won't cut it as
efficiently if we were to use the red
lens with 15 watts of power we would
probably have to run it maybe ten maybe
even less than that millimeters per
second to achieve a cut
where as cutting that same cardboard
with the sharp lens we can do it at a
hundred and ten millimeters per second
that again hopefully reinforces why you
would want to use the right lens for the
right job if you're only ever cutting
thin materials
why would you ever go and use it two and
a half inch lens or a two inch lens when you
can slice through it at a much faster
speed with a one-and-a-half inch lens so
if you're only using one of the half and
two millimeter thick material it's a
no-brainer if you've only got a 60 watt
machine
so I hope these illustrations have
demystified why you were supplied with
three lenses and why you might want to
seek a fourth lens in your armory
depending on the power of machine that
you've decided to buy so longer length
lens requires more powerful machines and
more powerful machines means you can cut
thicker materials so i hope this begins
to remove some of the mystery about why
you would want different types of lenses
ok now there was another very good
reason why I've approached the tests in
the manner that I did, i used a thickish
card i use a constant speed but what I
did was to vary the power to just get a
cut through the material for each one of
these three lenses technically what that
means is i was using the card as a
measure of the energy density that was
in the beam in other words i had to have
the same amount of energy density
available in each one of these lenses to
just cut through the card and what we
can do is we can look at the results the
backward way round
i was using seventy percent power which
when I look it up on my calibration
chart which I've got for the tube it was
approximately 62 watts now we assume
that we're going to lose power through
the mirrors there's three mirrors each
losing three percent let's just assume
and a lens which could be another three
percent so we could be losing as much as
twelve percent through the transmission
system before we get down to the work so
the available power at the work could be
as little as 54 watts now I don't
know this for a fact because I didn't
measure it
54 watts divided by 0.25 square
millimeters gives us an energy density
of 2160 watts per square millimeter
when we do that same calculation for the
2-inch lens where we were able to use
twenty percent power
which was 28 watts less the twelve
percent brought it down to 25 watts so
we divide 25 Watts by the area of the
beam at the focal point we get 2080 watts
and similarly when we do the same result
here for the one-and-a-half inch lens
we find ourselves using sixteen percent
power which is equivalent to 17 watts
less the twelve percent is 15 watts and
when we do that calculation again with
the area of the footprint we find that
we get two thousand three hundred watts
per square millimeter now I hope you can
see that it's not perfect but it's in the
right sort of region we've got a typical
we've got a typical energy density here
which we're using to damage this material
which is a round about 2,100 2,200
watts per square millimeter now this
was never meant to be a perfect
scientific experiment and I never
expected to get results may be quite as
good as this but it does demonstrate
clearly that that material regardless of
the lens that we fire at it has got the
same damage threshold it would be good
to know what the damage threshold for
different materials is because then we
could exploit the lenses and can predict
what the lenses were going to be capable
of now you may consider that's going to
be a bit of a futile waste of effort
considering the next subject that we're
going to be looking at is cutting
parameters cutting parameters are
something that you do not predict
you laboriously sit down with your machine and
work through the variables and once
you've got an ideal set of data you log
that data and that then becomes your
cutting parameters until the next
session when we'll be talking about
cutting parameters
thank you very much for your attention
today and hope this has been of some
benefit to you
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