Optical finish for acrylic -- vapor polishing and other techniques----make money online

hey everyone I wanted to talk about

polishing acrylic today so I've got

these two lenses here and these were cut

on a CNC lathe and didn't have an

optical finish when they came off the

lathe so let me tell you what I found

today before starting on the lenses

themselves I wanted to have a few test

coupons to see what these techniques

would actually do no use wasting the

lenses on things that aren't going to

work so I chose some cast acrylic that's

most almost certainly the same material

that the lenses are made out of

I bought the lens material from McMaster

in its cast acrylic and this is also

cast acrylic from McMaster but it's

possible it's slightly different because

the lenses were cut from a rod and this

is obviously a sheet so there's a chance

that it was a different blend of acrylic

a note about different kinds of acrylic

the stuff that you get with brown paper

on it like this is almost certainly sell

cast meaning that the acrylic is formed

in basically a big tub I think the tub

is probably made of glass so it's nice

and flat and the polymer polymerizes

inside the tubs that it takes the form

of the cell this is sort of the best

stuff out there there's also a

continuous cast and extruded acrylic

which often has like a blue plastic

cover on it one of my friends said that

those that that's actually better for

laser cutting but they're for pretty

much every other operation like you know

milling drilling tapping machining

sanding it's not as good because the

melting temperature is lower and some of

its other properties aren't as desirable

so anyway so we got our sell cast

acrylic here and to start with a

baseline I use the fly cutter in my mill

just to flatten all the coupons down and

then for some of them I sanded to 600

grit using water so my favorite

technique is to put a piece of glass

down on the bench and then put the

sandpaper down on that and then move the

piece over the sandpaper while keeping

plenty of water going so this technique

is good for doing flat things because

the glass will make sure the sandpaper

is evenly supported on a truly flat

surface one of the coupons I sanded only

up to 600 grit and one of the others I

sanded to 600 and then content

you'd on to 2,000 grit I also cut one on

the table saw just as a reference point

I put a microscope objective in front of

my camera in place of the lens and took

some close-up shots so here's a close-up

of the fly cut surface here's a close-up

of the 600 grit this is the 2000 grit

and this is the saw cut I wanted to try

three different polishing methods the

first method is probably the most

standard I used just a Novus plastic

polish and this is just a very fine grit

suspended in a water-soluble binder

basically and each lens or each coupon

only took maybe a few minutes maybe four

minutes of polishing and normally I

would start with Novus number three

which is the coarsest and then move to

Novus 2 which is the finer one and Novus

1 is actually a cleaner that's not

actually at a polish but I didn't have

any Novus 3 on hand so I just went right

to to another method of polishing is to

use the flame of propane or map gas

torch and the trick is that you have to

move the acrylic at just the right speed

so that the surface will melt and form a

a smooth surface via surface tension but

not hang on it so long it starts to burn

and bubble so I'm a little rusty I

actually haven't done this in a little

while but I managed to get a few pieces

done fairly well this technique works a

whole lot better on the edge of a piece

of acrylic that you've finished with a

router or with a table saw but but

generally it's not known for having

great optical finish a new technique

that I haven't tried until today was the

use of methylene chloride vapor to

smooth the surface down so the idea here

is that when the methylene chloride

vapor touches the surface it actually

melts the acrylic or dissolves it a

little bit and then again surface

tension will pull the surface flat and

that the vapor leaves and so you're left

with just a flat surface of acrylic

there's a pretty interesting video on

YouTube that I'll link to in the

description that shows this being done

to a custom acrylic windshield for a

race car I ran into a few difficulties

one the vapor you know

you have to start with a fairly large

amount of liquid so I had to refill my

flask here a few times because it ran

out it only bubbles it only boils for

maybe five or ten minutes with that

amount in there second it seems like the

optical quality increases rapidly and

then actually declines a little bit and

holding the piece of acrylic in the

vapor stream for a longer amount of time

doesn't yield any additional improvement

so sort of like the flame polishing it

doesn't really get all the way to

optical quality it seems to sort of go

past the point very easily maybe I'm

definitely going to experiment with this

a bit more but at least at first blush

it seems like this technique is not

quite as good as the polish which we'll

see later but you know it's definitely

worth experimenting more even though it

seems like initially it's not quite as

interesting okay so here are the results

this is the fly cut surface with the

Novas polish on the left the flame

polish in the middle and the vapor

polish on the right here's the 600 grit

sanded surface with the novus on left

flame in the middle and vapor on the

right and here's the 2,000 grit surface

novus on the Left flame in the middle

and vapor on the right so as you can see

it's really no contest the 2,000 grit

sandpaper is really the magic ingredient

in this whole mess here so when I sat

down to do the lenses that's basically

exactly what I did I I did 600 grit

first just to get rid of the tooling

marks and one of the CNC pieces had

deeper tooling marks than the other so I

sat down with a light focused very

closely on the surface that I was

working on I could actually see the tool

marks disappearing as I wore that wore

it down with a 600 grit then I moved up

to 2,000 grit and the Novus number two

and ended up with these lenses here it's

sort of difficult to use the camera to

give an accurate representation of how

good the optical quality of these lenses

are I would say that they're not quite

commercial quality because you can still

perceive some scratches in the surface

and the contrast is just not quite as

good but this is a picture of a

150 line per inch code strip just to

give you an idea of what sort of

resolution is possible of course

chromatic aberration is one of the

biggest problems with these things since

they're there uncorrect it's just one

index of refraction here the spherical

aberration shouldn't be quite so bad

because these are actually aspheric

lenses and they've been designed to

reduce that aberration also I should add

I I would have used 1200 grit between

the 600 and 2000 if I had it I just I

checked my shelf and realized that was

out so I just went from 600 right to

2,000 if you're planning to glue two

pieces of acrylic together and want an

optical junction between the two it's

actually not necessary in fact

undesirable to polish the surface to a

an optical finish before applying the

solvent weld I've found that it's really

much better just to go with a sanded

edge or even just a really fine to

machined surface you'll get a better if

there's more tooth on the surface the

solvent will have more surface area to

grab on to and you'll still get an

optical joint because the solvent will

form a bridge between the two pieces and

will still be a continuous piece of

acrylic through there sometimes a

polishing it will leave a small amount

of polish in the joint and that is not

good for optical clarity I'll probably

do a video on solvent welding and

acrylic at some point I actually posted

some stuff to my website long ago before

I was posting to youtube okay see you

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