Sawing Plastics (was Re: Primary support -update)

Hi Terry, Brian, all,

> Original Poster: Terry Fritz <twftesla-at-uswest-dot-net>
> I have one of those small Makita table saws.  I have always been
>fairly afraid to cut anything in it but wood.  It would really be nice
>to use it for cutting plastics and such for my Tesla coil projects.
> However, with the big direct drive brush type motor, the thing is
>too dangerous if anything hangs up.  The high-speed direct-drive
>high torque motor does not stall gracefully at all!!  It is great for
>wood but odd materials may go nuclear...  I was wondering if
>simply putting the variac in line with it (universal motor) would
>reduce the speed and torque to sane levels and if that would be
>of use for cutting plastics and such?  I was going to give
> this a try but thought I would check around first for tips...

This CAN be done, but......

1.) Youīll need a good sized variac. I donīt know what small
in VA or HP means to you. My 9" Craftsman has a (rewound
for 220V/50Hz) 1HP motor and my 12" Elektra-Beckum has
a 9HP 3-phase motor. Of course these are induction motors.
When you start stalling or loading a universal motor, it will
start pulling hefty currents. This will also reduce brush life. It
might be better/wiser to build a small PWM for this to control
the speed and torque. As it is a brush motor, you can run it
on DC. Getting a rectifier and power MOSFet bridge shouldnīt
present a real problem either. As a matter of fact YOU should
be able to "fish them" right out of a scrap bin at work ;ö).

2.) The cut will become rougher as you go down in cutting
speed for a certain tooth-numbered blade. This is because
the teeth per time is lower (velocity goes down).

3.) When using the width stop (rip fence, instead of the
miter gauge) and cutting small strips of plastic ALWAYS
(!!!) use a chicken stick and stand to the side of the saw
(i.e.: 90° to what you are cutting). IF something goes
wrong, you can just "let it fly" and you wonīt get hurt. My
9HP tablesaw is more or less impossible to stop and I
ALWAYS use this method. Things like this will happen to
anyone, even the most skilled machinist. Some machines
have a set of "anti-kickback" pawls. NEVER depend on
these to stop the workpiece from being flung back in case
the saw jams. As a matter of fact, I always remove these
pawls, so I am never tempted to trust them.  Respect power
tools! They can and WILL injure you, if you are not careful!
Once you arenīt "afraid" of your tools anymore, it can
become dangerous (to oneīs health) ;o((.

4.) Use a thin rim blade with "X" shaped teeth. This
double-tooth arrangement will help preventing the saw
blade from sticking to the plastic. I think they are called
veener blades (Gee, my machining english is getting
a little rough. I think I need to come back to the States,
preferably California... lots of sunshine, nice Coil-A-Thons,
pretty girls, etc). Brian: thanks for letting me get an "inside"
view of the So-Cal Teslathon!! The pictures and all really
reminded me of my days in CA and it made me a little
homesick, too.

5.) Cutting acrylic (Plexiglass) this way is NOT a good
idea. Usually, it will shatter. For Plexi you need a fine
toothed blade running at relatively high speed and a
slow, but smooth forward cutting movement. Drilling is
very similar (high speed, low pressure).

6.) You donīt need a carbide tipped blade. These blades
are usually used when you encounter hard materials (nails,
particle board [the glue is a saw killer], G-10 and so on).
For equal tooth number, the carbide blade will make a
rougher cut (esp. if you plan on reducing the cutting

7.) DONīT go too low on rpms (esp) for a rough blade.
Doing so will lead to chipped edges, which esp. holds
true for brittle material, like Plexiglass.

Depending on your machine shop, there are other ways to cut
plastic. Depending on the exact piece to be cut, I use (aside
from the table saw) either a band saw or a jigsaw with a speed
control and an (adjustable) back and fro movement (instead of
just a simple up and down movement). This is a real help in
cutting things that tend to "smear".  I use a pretty rough blade
with "X" teeth and set the cutting speed to medium (3-4 on a
dial from 0-10). I move along slowly and donīt apply any real
pressure to the saw. Let the SAW do the work and not the
pressure. I usually have very little problems with jammed saws.
As a matter of fact, there is hardly any (melted) residue on the
saw blade after cutting. With a little practice, you can cut very
straight. Avoid the jig saws with a turnable blade (to aid cutting
around corners w/o having to turn the saw body). While these
will work great for a while, this mechanism starts to get wobbly
and it is next to impossible to get a straight cut thereafter.

While straight, the above cuts are usually too rough for my taste,
so I put them on my lathe/mill and take a cleaning cut afterwards.
Of course, if you are making precision fit parts, you will need to
account for this during construction ;o)).

While talking about cutting and machining, I would like to add
an interesting idea a Germany coiling friend of mine had. Using
a CAD program, make a 1:1 layout of all your designs. Plot
them on a suitable plotter and fasten them to the part you will
be machining. This will insure a good fit w/o having to think while
machining. I do like the idea, but since I donīt have a plotter, Iīll
go the old-fashioned way of thinking, measuring, testing and then
machining. After all, what do I have calipers, measuring "T", etc
for? ;o))

Coiler greets from Germany,

P.S.: To all: I would like to add to Bill Wysockīs plea. If you
change the body of the mail, please also change the header.
While I read ALL the posts, that are sent, (which is one
reason, why I am not the fastest answerer) it is somewhat
helpful if the subject matches the contents of the mail.