Drilling plexi: enlarging an existing hole

[AeroDog]

Van's advice for enlarging an existing hole in Plexiglas is to never use a drill bit but instead use either a reamer or Unibit. I need to drill #40 through the Plexiglas and roll bar, then enlarge to #36. I've done tests with scrap Plexiglas using a regular #40 (I've never seen a plexi-specific #40) followed by a bit made for plastic from McMaster-Carr, the really pointy kind, with great results. These bits make the cleanest holes, either as on original hole or enlarging a hole, I've seen. They're noticeably better than those made using a "modified" bit (drilled into concrete). Am I tempting fate using this method (#40 followed by #36 plexi) on my rear window?

Jerre

[DHeal]

I have always had good results using a Unibit to enlarge holes in plexiglass and Lexan. I usually have no concern if the Unibit hole is slightly larger than the plans-specified hole size -- the slightly larger hole provides a tad more room for material expansion/contraction/misalignment. The Unibit can also be used to very lightly chamfer/deburr the hole edges. Practice on some scrap material to build your confidence and hone your technique.

[JonJay]

I did exactly the same as you. Are you tempting fate? Can’t answer that one.
A “plastic bit”‘won’t go through the steel roll bar. If there is a better process, let’s hear it.

[JonJay]

Originally Posted by DHeal

I have always had good results using a Unibit to enlarge holes in plexiglass and Lexan. I usually have no concern if the Unibit hole is slightly larger than the plans-specified hole size -- the slightly larger hole provides a tad more room for material expansion/contraction/misalignment. The Unibit can also be used to very lightly chamfer/deburr the hole edges. Practice on some scrap material to build your confidence and hone your technique.

Won?t work for the OP?s question. The initial hole is too small.

[Carl Froehlich]

I don?t drill through the plexi and steel roll bar. This is what I do:
- Trim the glass to fit.
- Lightly run 3/4? masking tape over the steel roll bar and/canopy frame (RV-8).
- Place the glass back on the roll bar/frame and lightly clamp. You will end up with a perfect glass to roll bar/canopy frame line in the masking tape.
- Remove the glass, take a Sharpy and draw in this tangent line.
- Mark off the fastener holes on the tape and drill with standard #40 bit.
- Remove tape. Put the glass back on and lightly clamp.
- Drill #40 through the glass into the hole you drilled in the roll bar/canopy frame.
- Use a #30 plexi bit to enlarge the holes.
- Remove the glass. Use a unibit to enlarge the hole in the glass. For the RV-8 roll bar I use a large hole in the glass and slightly countersink it to accept a #6 tinnerman. The thought being that the tinnerman will spread out the load.

The reason for doing the tape is to create perfectly tangent holes so you don?t put a side load on the glass.

Carl

[JonJay]

Originally Posted by Carl Froehlich

I don?t drill through the plexi and steel roll bar. This is what I do:
- Trim the glass to fit.
- Lightly run 3/4? masking tape over the steel roll bar and/canopy frame (RV-8).
- Place the glass back on the roll bar/frame and lightly clamp. You will end up with a perfect glass to roll bar/canopy frame line in the masking tape.
- Remove the glass, take a Sharpy and draw in this tangent line.
- Mark off the fastener holes on the tape and drill with standard #40 bit.
- Remove tape. Put the glass back on and lightly clamp.
- Drill #40 through the glass into the hole you drilled in the roll bar/canopy frame.
- Use a #30 plexi bit to enlarge the holes.
- Remove the glass. Use a unibit to enlarge the hole in the glass. For the RV-8 roll bar I use a large hole in the glass and slightly countersink it to accept a #6 tinnerman. The thought being that the tinnerman will spread out the load.

The reason for doing the tape is to create perfectly tangent holes so you don?t put a side load on the glass.

Carl

I did the same with the tape to find where to drill, but drilled the plexi and roll bar in one step. With your method you are still drilling through the plexi with a #40. Not sure what the multistep process is gaining you here, but if it works, it works.
I think the OP's question is are we at risk using a #40 drill through the plastic. I do not know this, but you and I both did it and enlarged the hole later with an appropriate plastic drill or unibit.
My guess is most do the same....

[Bavafa]

Here is a link to numbered plexi bit which produces excellent result.

https://www.abbeon.com/Item--i-2118

[JonJay]

Originally Posted by Bavafa

Here is a link to numbered plexi bit which produces excellent result.

https://www.abbeon.com/Item--i-2118

There ya go. Now you can use Carl's method and not drill through the plexi with a standard #40 split point. Problem solved, if there ever really was one....

[Marc Bourget]

The steps provided by Carl repeats previous posts from other threads, but I am concerned about any regularly ground drill bit used to drill plexiglass - particularly the "far side" exit. Moreso when drilling to a larger size drill. The bite of a regularly ground drill bit is accentuated when there is no resistance to the point of the drill, (providing a "back force") to limit the aggressiveness of the advance through the material.

I'd recommend using a drill ground to a lesser rake any time drilling plexiglass.

I can't give attribution, but I learned most of my fabrication skills from guys who learned their trade in the WW2 aircraft factories. The "guru" of that group was the head of Lockheed-Vega division's prototype shop. But, I recall the advice of never drilling "through" pilot holes when you could drill "almost" and then back drill. The plexi fractures (micro fractures) and provides a stress concentration when you go to enlarge the hole.

Combine this with an "aggressive rake" of a standard ground drill bit and you're asking for complications.

These guys (especially the prototype shop - which was pro level homebuilding ) got more "practical experience" in a month or two than most of us will gain in years.

FWIW - YMMV

[wnplt]

From my A&P Mechanic Oral...

Plexiglass should be backed with wood and the feed slowed as the drill point breaks through the underside of the sheet. A drill bit should be modified to a 60 degree tip angle, the cutting edge to zero rake angle and the back lip clearance angle increased to 12-15 degrees.