Re: primary coil stand off construction question.

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Original poster: Jim Lux <jimlux@xxxxxxxxxxxxx>

At 09:44 PM 2/16/2007, you wrote:
> Original poster: "Barton B. Anderson" <bartb@xxxxxxxxxxxxxxxx>
>
> Hi Vinnie,
>
> Lexan, delrin, or even hard readily available acrylic can be used 
> for the supports (standoffs not required). I also use a hand drill 
> (a nice Dewalt. Maybe next Christmas, a drill press?). Some of the 
> harder materials such as acrylic are more difficult to machine.

White plastic cutting boards are your friend.  They're high density 
polyethylene and are about as far from brittle as you can imagine.

Polycarbonate (lexan) is less brittle than Plexiglas (acrylic)

There's a nifty thing called a "doweling jig" that you can use with a 
hand drill to make sure the holes go in at 90 degrees. It's basically 
a metal guide with a hole the diameter of the drill bit that clamps 
onto the work piece.

>  For primary's, something between 1/4" to 3/8" thick makes drilling 
> less fragile. The drill bit itself is the key. You want a bit with 
> a shallow angle (between 60 and 90 degrees). The standard 120 
> degree angle for metal work is designed to bite into the metal. 
> This 120 degree angle will often chip or crack acrylic. With a 
> shallow bit angle, the work done "scrapes" a nice hole without 
> chipping and cracking.

I wonder how a "spade bit" for wood would work?  I haven't ever tried 
it, but your description of scraping the hole triggered the idea.

Some coolant when drilling helps (water)


> Cutting the acrylic is more of a challenge than drilling. The first 
> basic cut is easy. But after the holes are drilled, you'll need to 
> mark a line to cut through the drilled holes slightly above center 
> so that there is enough material to keep the primary snapped in 
> place. When you cut that particular line, do it as precise as 
> possible. I've done it with a hacksaw, but it's not easy to keep 
> precision with a hand tool like that. Best to use a bandsaw with a 
> guide edge preset into position. Then try snapping a length of 
> tubing into it. Do you need more or less cut towards the center? 
> Experiment is the key to finding what "you" feel is the right feel 
> for the snap action.

If you have a belt sander, or a nice coarse rasp (like a surform 
plane) it works real well to take the material off evenly.  Not like 
you need to save the scrap you're cutting off, so planes, grinders, 
etc, are not a bad way to go.  You can finish with finer grit to get 
a polished edge.   (I scrape with a single edge razor blade to get a 
smooth edge, then firepolish with a torch)