From: Tim Shoppa <shoppa@trailing-edge.com>
Newsgroups: rec.crafts.metalworking
Subject: Re: Plastics Fabrication Web Site
Date: Thu, 18 Feb 1999 13:20:56 -0400

Rick Legge wrote:
>
> I'd like to make bent acrylic guards for my mill and lathe but need
> information about bending acrylic.
>
> Can anyone recomend informative web sites?

I've been unable to find any plastic manufacturers who put up
any useful information on their web sites.  I've even looked
for simple information, like which color codes of plexiglass
correspond to what color, and I've been very unsuccesful.

On the other hand, all the plastic distributors I've been to have
flyers and booklets from the acrylic manufacturers with very good
recommendations on tools and techniques.  They also sell saw
blades specifically intended for cutting acrylic and drill bits
intended for acrylic, should you not want to regrind metal tools
yourself.

As for bending, it's pretty easy.  If it's a very simple bend,
heat the area to be bent with a propane torch (not in the direct
flame!  Burning plexiglass doesn't smell too nice!)  and wrap around
a dowel of the radius you want.  For more complicated
bends, heating the entire piece in an oven is necessary.  If
you're going to be doing this repeatedly, the plastics suppliers
sell temperature-regulated nichrome strip heaters that are really
handy.

One warning for those who aren't already aware: 1/8th inch plexiglass
is *not* 1/8 of an inch thick.  It's not even 3mm thick.  I've
seen actual dimensions as skinny as 0.105 inches for stuff sold
as "1/8th inch thickness".  Same
goes for the other sizes.  It hasn't gotten as bad as lumber sizes,
but it's getting there.  If you need precisely dimensioned acrylic,
it's generally special-order stuff.

Other plastics (i.e. Delrin rods) generally come oversized by
5 or 10 thousandths.

Tim.

From: Tim Shoppa <shoppa@trailing-edge.com>
Newsgroups: rec.crafts.metalworking
Subject: Re: Plastics Fabrication Web Site
Date: Thu, 18 Feb 1999 23:06:49 -0400

Spehro Pefhany wrote:
>
> the renowned Tim Shoppa <shoppa@trailing-edge.com> wrote:
>
> > One warning for those who aren't already aware: 1/8th inch plexiglass
> > is *not* 1/8 of an inch thick.  It's not even 3mm thick.  I've
> > seen actual dimensions as skinny as 0.105 inches for stuff sold
> > as "1/8th inch thickness".
>
> That's amusing.

It wasn't awfully amusing to me when I found it out.  I'd already
done all the dimensioning for the other members, under the
assumption that 1/8 = 0.125.  I've heard that the under-dimensioning
of thickness is a tradition by the glazers, but I don't believe
it... :-)

I either had to mill the 3/16" stuff down to 0.125 or start all over.
I chose to start over.

> 1/8" polycarbonate (GE lexan) is actually 3mm thick

Yep.  1/16" = .060, 1/8" = .118, 3/16" = .177, and 1/4" = .236.
(Or 1.5mm, 3.0mm, 4.5mm, and 6.0mm for those Celsius-heads.)
Strangely enough, though, 3/8" = .375 and 1/2" = .500 again.

I've also discovered that when a supplier claims that PVC
structural members (tubes, channels, angles) are 10 feet long,
they're really 118" to 119".  The school of hard knocks!

Tim.