Re: [CAD_CAM_EDM_DRO] drilling techniques for plastic to get specific sized hole?

Hi All:
I had a job once making some insect habitats for the space station.
Lots of precision bores in Delrin.
I tried every damned method under the sun to get the bores straight,
parallel, round and on size.
Best way I found, bar none, is to helical mill them.
Single point boring trashes the hole because the chips pile up and scuff the
finish.
Reamers cut undersize and create weird bell shaped holes.
Poking down a cutter; same problem.
Drilling; rough, non round holes of all kinds of sizes from the same drill.
Coolant helps a bit, but wasn't permitted for this application. (risk of
killing the critters).
Oh yeah; these were blind holes flat bottomed 3.25" deep and 0.700" dia.
They were drilled, flat bottom drilled, rough helicaled, semifinish
helicaled, and finally finished with a cut of 0.002" and double filtered air
blast.
I tore my hair out for weeks before I figured this technique out.
Good luck on your application.
Cheers

Marcus

----- Original Message -----
From: "Smoke" <smoke@...>
To: <CAD_CAM_EDM_DRO@yahoogroups.com>
Sent: Friday, February 01, 2002 8:24 AM
Subject: Re: [CAD_CAM_EDM_DRO] drilling techniques for plastic to get
specific sized hole?


> I have SEVERAL tools that will help you machine PRECISION holes in
plastic.
> The first is a wood boring auger bit. You'd have to alter the center
point
> so it wouldn't try to screw into the plastic. (This would be my LAST
> choice)
> The second would be a Forstner wood boring bit. This tool bores the OD of
> the hole and scrapes away the inner part.
> The third would be a "regular" drill bit with the point ground to an angle
> suitable for plastic.
> Fourth would be an end mill.
> Last would be a boring tool after predrilling a pilot hole to just under
> final diameter. With this tool you CAN bore accurate holes in soft
> materials. The secret is not to try machining the hole to rapidly.
>
> I'd feed all of these tools thru the work slowly and use water to cool the
> plastic so it won't melt around the edges while being machined.
>
> If you make ANY hole the same size as the part that supposed to fit into
the
> hole, you won't get the part in the hole. You will need to allow about
> ..0002 oversize for the parts to fit. This can be accomplished by either
> polishing the inside of the hole or the outside of the shaft/pin. The
tool
> I would use to polish the hole (made this in gunsmithing tool for
polishing
> chambers) is a 1/4" diameter steel rod with a 1" long slit sawn down the
> middle
> with a hack saw. Sandpaper is inserted in the slot and the tool is
rotated
> in a hand drill ot in the drill press. If you braze a cross piece on the
> end you can hold it in your hand for use while the work is turned.
>
> Smoke
>
> > > I want a 1/2" hole... however, when I slide it over my 1/2" precision
> > > ground rod, the hole is too small...
> >
>
>
>
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