![]() |
Drilling plexi: enlarging an existing hole
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 |
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.
|
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. |
Quote:
|
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 |
Quote:
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.... |
Here is a link to numbered plexi bit which produces excellent result.
https://www.abbeon.com/Item--i-2118 |
Quote:
|
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 |
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. |
| All times are GMT -6. The time now is 02:49 AM. |