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Welcome to another Lightblade learning lab,
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today we’re going to be talking about
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lenses again but lenses are very
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important because they focus the light
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energy or the beam energy that we’ve got
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coming out of this machine down into a
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very small point which is where the work
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is done the beam itself is as we found
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out maybe eight nine ten millimeters
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diameter, it’s quite large
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most of the energy is concentrated in
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the central section probably within the
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central fifty percent of the beam, all the
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energy that we can collect is
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concentrated down through the lens into
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a very very small area here I’ve got some of my
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hairs which you can hardly see now if I
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measure one of those hairs we can see
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that it is point oh four point oh five
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millimeters diameter double that up to 0.1 and
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that’s the size of the laser beam
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that we are probably going to get on the
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best lens that we can probably find for
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this machine and that’s a one and a half inch
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Lens for this machine, now when you start getting up to a
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two inch. two-and-a-half inch and a four inch lens
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those numbers start growing considerably
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maybe up to 0.2 or 0.3 even now 0.3
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is still not very thick but it makes a
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huge difference to the way in which the
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power is delivered onto the surface now
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look here I’ve got
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a tool that I’ve just picked up out of my workshop
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it’s got a very sharp point on one end
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and a blunt section on the other end. Here
Transcript For Laser Focusing Lenses (Cont…)
02:02
I’m going to press as hard as I possibly
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can on that card and wobble it around
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with that end
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how much of an impression have I made? Not
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very much
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now i’m going to press the same amount
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of energy because I haven’t changed my
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strength and I was doing exactly the
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same thing with the other end
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now there’s a pretty significant hole in
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there
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that’s nearly through to the other side
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this one
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well it’s hardly marked the surface there’s a
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mark on there but I can feel that
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it’s just the smallest dent and this is
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a very soft card so I would’ve expected
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to have caused more damage than that but
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the reason why I haven’t caused more
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damage is because the energy that I’m
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able to put into that is spread over a
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much much larger area where as when I do
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it this way around the energy is
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concentrated into that point
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so the point is an energy concentrator
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basically what we’ve got we’ve got a
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much higher and you’ll hear me using
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this term a lot the energy density the
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energy per square millimeter is a lot
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greater when i do that then it is when i
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do that
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and that’s the principle of why we
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use different lenses and you say well ok
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if we use a lens that produces a shape
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like that a very thin hair like beam
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we’ve got huge energy density we can do
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a lot of damage to the product
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why don’t we just use one of those? The
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answer is in this very crude diagram that I’ve put
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here we may be able to concentrate the
Transcript For Laser Focusing Lenses (Cont…)
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energy in a very small area as opposed
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to the larger area that we saw
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demonstrated in my mechanical
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demonstration but the 1.5 inch lens
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focuses the light like this it’s
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a very sharp shaped lens which goes down
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to a sharp point but it goes down to
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sharp point quite quickly and expands
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again very quickly so it’s useful
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working length is actually very short
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now this one which is the 2-inch lens
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okay it’s got a bigger footprint
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but it’s working length where the energy
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density remains reasonably constant is
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larger and this one which is the
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two-and-a-half inch has got an even
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shallower beam shape and it has a
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working length which is much longer
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so you’ve got these strange properties
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associated with the focused light you
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can either have lots of energy over a very small
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length or you can have less energy over
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much longer lingth and this is the
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trade-off you’ve got between lenses. Now
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what we’re going to do today is i’m
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going to try and turn these drawings into
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a real picture in other words I’ve got
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my focus gauge here which runs from the
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focal point plus 4 millimeters to the
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focal point minus 4 millimeters
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ok now my intention is to draw a line
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along here and hopefully if I get it
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right
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I will manage to get the focal point in
Transcript For Laser Focusing Lenses (Cont…)
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the middle and so we can see that the
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change of line thickness and power or
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density, energy density changes as we
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move along the line because at this
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point here we’ve got a big footprint and
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at this and we’ve got a big footprint
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we’ve got a very small one in the middle
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so that means their energy density at the
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top here is very small, in the middle
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it’s very high and we get less of a
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change with these because although we
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might be starting off at the same length
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the same diameter we don’t get we come
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down in a much shallower curve. Right now
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I’m just going to do a quick pulse test
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to make sure that my beam is
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approximately in the right place to give
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me good consistent power
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that looks pretty good now I’m not going
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to bore you to death with hundreds and
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hundreds of results what I’m going to do
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is to show you my method and then we’ll
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work on from there, i will carry on
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working and show you the end results now
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what I’ve got here is my focus ramp
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which takes me from zero in one
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millimeter steps out to 4 millimeters plus and
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4 millimeters minus. Now I’m gonna pop
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that in there and actually what I’ve got
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this time I’ve got some card and it’s
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one millimeter thick card so it’s nice
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and stiff so that when I pop it in here
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there is no chance that it’s going to
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flex in any way at all
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in other words this surface here is
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going to be as flat as the reference that
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i can produce on here and i’ve also used
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my metal surface here my flat metal
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surface because i know that that is true
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well I can see that I’ve got a 2-inch
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lens here and a two and a half inch
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lens so by default that one must be a
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one-and-a-half inch lens and what we’re
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going to try and do to put to start with
Transcript For Laser Focusing Lenses (Cont…)
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that I’m going to turn this over the
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other way like this and I’m going to try and
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set this so that the power just burns
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through somewhere around about the focal
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point
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ok so we’re starting off these tests with
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a speed of a hundred millimeters per
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second and a power of 25% now I’ve got no
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idea what that is in terms of watts but
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we’ll sort that out later, I’ve got the focal
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point set to 7.5 millimeters
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ok let’s look at our result
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have we got what we’re looking for yes
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we have now what we’ve got here we’ve
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got something that runs from two and a
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half plus to about 1 minus, it’s about a
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millimeter too high so what we’ve got to
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do to put that back in there again
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instead of seven half millimeters we
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really ought to drop that down to six
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and a half millimeters same speed same
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power
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and there we go that we’ve moved it down now
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we’ve gone from we’re roughly one and a
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one and three quarters to two and a
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quarter so we may well have gone too
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much but we’re now going to do is we’re going
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to change the power slightly because
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this has cut through as you can see this
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is cut through and what I want to do is
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try and decrease that length now ah there we go
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right up the middle
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fifteen percent we’re just about making
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it through just about making it through
Transcript For Laser Focusing Lenses (Cont…)
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and would you believe it looks as though it’s
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about minus one and just about +1 maybe
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one and a half, so we still might be a
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little bit out on centering but that’s not
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bad
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so we do a result on the front so we can
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see what the dimensions are now this is a
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two-and-a-half-inch lens test which is
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typical for the other tests that just
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been doing
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so the first thing I’m going to do is to
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set the power and the power has got to be
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set up to 17%
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now we do it this way just so that we’ve got
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something to measure on the front
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so that’s 10mm/s
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focus is 5.5
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here we are looking at the scorched line for the
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2 inch focal length lens now this is the
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line as it entered the bottom of the
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ramp this is four millimeters below the
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focal point now you’ll see that they
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look like a couple of tram lines along
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there but when we look a bit closer and
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tip the card up you can clearly see
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that it’s a V-groove cut by the laser
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itself into the card but i’d like you to
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look at the top of the V and I think
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you can see there’s a very small almost
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a filleted radius on the top of the V where
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the power has dropped off quite
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dramatically now I’m going to basically
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for these measurement purposes I’m
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going to ignore the little fillet radius
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on the top of the V because that
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basically has got no significant power
Transcript For Laser Focusing Lenses (Cont…)
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it’s enough to scorch the edge but that’s all
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it is doing so what I’m really
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interested in is the powerful cutting
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piece towards the center where the V is
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going all the way down from the vertical
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now what i’m going to do is measure
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all positions between minus 4 and plus 4 to
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get nine results and then we’ll take a
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look at the shape of the line now I have a
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glass graticule which is marked off in
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point 1 millimeter divisions it’s
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possible with this microscope to
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reasonably accurately measure the
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dimensions
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I mean I’m once i get below point one of
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a millimeter which is the division that
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you can see on here you can easily
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estimate to half a tenth which is
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.05 point .025 well that’s a little bit
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flaky but I’ve attempted it in a few places
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well here we are measuring the dimension
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right at the end which is the minus 4 position
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then we’re moving along to the minus
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three position quickly taking it all the
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way through to zero over the course of four
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millimeters I think you can see how
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dramatically the the line has changed so
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just in case it’s confusing with the
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graticule in the way i’ll show you a
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series of pictures we start from the
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lens here at four millimeters and then
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three millimeters
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and then two millimeters and then one
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millimeter and then 0 that’s how the beam width
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is changing as it goes from four down to
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zero the focus point, well after a lot of
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tedious repetitive work we finish up
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with this rather daunting array of
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figures here now don’t get too upset
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because it’s actually very very simple
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let me just explain let’s start off here
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at this two-and-a-half inch focal length
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lens and what we find is we’ve got the
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focus point here at zero and then we’ve
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got plus or minus 4 millimeters above
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and below the focus point now the beam
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diameter translates to a beam area and
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this is the area in square millimeters
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in this next column and then what we’ve
Transcript For Laser Focusing Lenses (Cont…)
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got across here are 10 20 30 all the way
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through a 100 watts of power that could be
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going through the lens so if we had as
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in my case a 60 watt tube, that doesn’t
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mean to say there was 60 watts going
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through the lens but let’s just stay
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with 60 watts now right at the focal
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point we have the smallest possible beam
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that we can get and in this instance it
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was 0.18 of a millimeter now we
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translated that diameter into an area
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square millimeter area so if we squeezed
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10 watts into 0.25 square millimeters we
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will finish up with 393 Watts per square
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millimeter it’s just a standard
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definition and that is called energy
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density now we’ve calculated the energy
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density for different wattages for that
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same spot size and you can see how the
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energy density creeps up why do we and
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why are we interested in energy density
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every particular material will have a
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threshold of damage, burning damage
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because basically that’s what our beam
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is doing its burning the material now
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that threshold maybe a thousand watts
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per square millimeter it maybe 10,000
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watts per
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square millimeter it’s difficult to say
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because there is no information out
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there on the internet which tells me
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what this burning threshold is now for
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this exercise I used card which had
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some sort of substance or body to it, it
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was one millimeter thick and it required
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a noticeable amount of power to cut
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through it the amount of energy required
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to cut through that card will ultimately
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be defined by the energy density
Transcript For Laser Focusing Lenses (Cont…)
16:01
itself that it can resist but i’m
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suspecting that card the card that I
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used is probably sitting here with an
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energy density of probably somewhere in
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the region only maybe four or five
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hundred now if we were to draw a line a
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vertical line through those three graphs
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at roughly where my arrow is which is
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about 500 watts per square millimeter i
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suspect that at 500 watts per square
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millimeter i would probably be somewhere
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near the damage threshold for that card
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but the reason why I want to bring that
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to your attention is because each one of
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these lenses and we’re looking at the 60
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watt energy density for the lenses is
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capable of exceeding the energy density
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required to damage that card so that
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means that any of those lenses i could
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use to cut the card
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ok something else rather interesting
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about these pictures is the way that
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they actually describe the type of
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lens they are imagine these to be
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three different types of knife one of
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them very sharp like a scalpel this one
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being a bit like a pain knife and this
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one being more like a butter knife now i
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think if i give you that analogy you
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will clearly see that it doesn’t require
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much effort to cut something with a
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scalpel
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it will require more effort to cut the
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same thing with a penknife and a lot
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more effort to try and cut it with a
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blunt butter knife, that will help you to
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visualize how and why you would want to
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use a certain type of lens we’ve already
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seen this sharp lens can cut with as
Transcript For Laser Focusing Lenses (Cont…)
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little as 17 watts whereas in fact cutting
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the same material with a blunt lens takes
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54 watts and I hope that that clearly
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describe the difference between these
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sets of lenses now I, now I need you to
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do a little bit of mental gymnastics
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because i want to bring you back to the
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fact that i used a hundred and ten
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millimeters per second speed to do all
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of these tests i could have used this
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and lets call it a blunt lens which
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takes 54 wats to cut through the
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material at hundred and ten millimeters
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a second i could have used 15 watts to
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cut through the same material but i
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would have had to do it at very very
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much slower speed it certainly has more
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power than the damage threshold of the
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material so it will definitely cut the
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material but it won’t cut it as
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efficiently if we were to use the red
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lens with 15 watts of power we would
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probably have to run it maybe ten maybe
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even less than that millimeters per
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second to achieve a cut
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where as cutting that same cardboard
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with the sharp lens we can do it at a
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hundred and ten millimeters per second
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that again hopefully reinforces why you
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would want to use the right lens for the
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right job if you’re only ever cutting
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thin materials
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why would you ever go and use it two and
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a half inch lens or a two inch lens when you
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can slice through it at a much faster
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speed with a one-and-a-half inch lens so
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if you’re only using one of the half and
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two millimeter thick material it’s a
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no-brainer if you’ve only got a 60 watt
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machine
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so I hope these illustrations have
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demystified why you were supplied with
19:57
three lenses and why you might want to
Transcript For Laser Focusing Lenses (Cont…)
20:00
seek a fourth lens in your armory
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depending on the power of machine that
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you’ve decided to buy so longer length
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lens requires more powerful machines and
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more powerful machines means you can cut
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thicker materials so i hope this begins
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to remove some of the mystery about why
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you would want different types of lenses
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ok now there was another very good
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reason why I’ve approached the tests in
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the manner that I did, i used a thickish
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card i use a constant speed but what I
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did was to vary the power to just get a
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cut through the material for each one of
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these three lenses technically what that
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means is i was using the card as a
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measure of the energy density that was
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in the beam in other words i had to have
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the same amount of energy density
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available in each one of these lenses to
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just cut through the card and what we
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can do is we can look at the results the
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backward way round
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i was using seventy percent power which
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when I look it up on my calibration
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chart which I’ve got for the tube it was
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approximately 62 watts now we assume
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that we’re going to lose power through
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the mirrors there’s three mirrors each
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losing three percent let’s just assume
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and a lens which could be another three
21:29
percent so we could be losing as much as
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twelve percent through the transmission
21:34
system before we get down to the work so
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the available power at the work could be
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as little as 54 watts now I don’t
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know this for a fact because I didn’t
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measure it
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54 watts divided by 0.25 square
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millimeters gives us an energy density
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of 2160 watts per square millimeter
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when we do that same calculation for the
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2-inch lens where we were able to use
Transcript For Laser Focusing Lenses (Cont…)
22:02
twenty percent power
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which was 28 watts less the twelve
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percent brought it down to 25 watts so
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we divide 25 Watts by the area of the
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beam at the focal point we get 2080 watts
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and similarly when we do the same result
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here for the one-and-a-half inch lens
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we find ourselves using sixteen percent
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power which is equivalent to 17 watts
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less the twelve percent is 15 watts and
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when we do that calculation again with
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the area of the footprint we find that
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we get two thousand three hundred watts
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per square millimeter now I hope you can
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see that it’s not perfect but it’s in the
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right sort of region we’ve got a typical
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we’ve got a typical energy density here
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which we’re using to damage this material
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which is a round about 2,100 2,200
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watts per square millimeter now this
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was never meant to be a perfect
23:07
scientific experiment and I never
23:08
expected to get results may be quite as
23:11
good as this but it does demonstrate
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clearly that that material regardless of
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the lens that we fire at it has got the
23:20
same damage threshold it would be good
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to know what the damage threshold for
23:26
different materials is because then we
23:28
could exploit the lenses and can predict
23:30
what the lenses were going to be capable
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of now you may consider that’s going to
23:34
be a bit of a futile waste of effort
23:36
considering the next subject that we’re
23:38
going to be looking at is cutting
23:41
parameters cutting parameters are
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something that you do not predict
23:45
you laboriously sit down with your machine and
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work through the variables and once
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you’ve got an ideal set of data you log
23:52
that data and that then becomes your
23:55
cutting parameters until the next
23:57
session when we’ll be talking about
23:59
cutting parameters
Transcript For Laser Focusing Lenses (Cont…)
24:00
thank you very much for your attention
24:02
today and hope this has been of some
24:04
benefit to you
