Another canopy question-drilling

[trib]

Section 39 has you initially drill the canopy holes using a #30 plexiglass drill bit. It then states after canopy removal from the frame to enlarge the holes using a #27 plexiglass drill bit in step 4 of page 39-09. No issues here. Reading section 5-19 on the installation of the canopy it states that using a regular or plexi twist drill bit to enlarge holes in the canopy will practically guarantee a cracked canopy!!!

Any advice or experience here is appreciated. The general section seems to directly conflict with the specific instructions of section 39.

 

[Bob Y]

I questioned the same inconsistency during my build. Van’s support confirmed using reamers was the safest way to go. And I oversized them slightly to give more margin for expansion and contraction.

 

[DHeal]

I have always had very good success using a "step-drill" (aka: Unibit) to final-enlarge and very slightly chamfer the holes in plexiglas. Test drill in scrap pieces of plexiglass and as always proceed with caution. I have also found that having a slightly larger diameter hole in plexiglas is acceptable in that it provides additional room for plexiglas expansion/contraction/movement.

 

[trib]

I had thought about the unibit. A #27 drill is 0.144". Next size up is 5/32 which is 0.156" . Just seems like a really glaring conflict in instructions and with the number of RV-12's built it's hard to see that a bunch of people didn't have a problem with this that some clarification would result.

 

[CF86301]

Section 39 has you initially drill the canopy holes using a #30 plexiglass drill bit. It then states after canopy removal from the frame to enlarge the holes using a #27 plexiglass drill bit in step 4 of page 39-09. No issues here. Reading section 5-19 on the installation of the canopy it states that using a regular or plexi twist drill bit to enlarge holes in the canopy will practically guarantee a cracked canopy!!!

Any advice or experience here is appreciated. The general section seems to directly conflict with the specific instructions of section 39.

I had exactly the same question and observation. The rear window (RV-14A) is even more interesting because they initially have you drill the window and Roll Bar Assembly #40, then "final drill" #36 and tap them both for a 6-32 screw and insert screws. Stress risers, perhaps? After you get everything fit together, they have you final-final drill the window holes to #27 (clearance for #6 screw). See page 38-03 of the RV-14 KAI.

But if you look at the instructions on one of the aircraft canopy manufacturer's (LP Aero Plastics Inc) website, they say drill the acrylic canopy material about 1-1/3x the size of the fastener, to allow for the different thermal coefficient of expansion of the aluminum frame and the acrylic canopy. For a #6 screw, that would be about a #10 drill, or preferably, as Bob Y suggests, a #10 reamer. I would probably go up in steps with at least a couple of intermediate-size reamers. A step drill/Uni-Bit also seems to work well, but the hole size selection is limited. Great Lakes Aero Products sells drill bits for acrylic in numbered sizes. A 3/16" Uni-Bit with a 1/8" pilot might be pretty good. You could match drill #40 with a bit designed for acrylic, "final-drill" #36 (preferably with a straight-fluted reamer) and tap, ream again to #30, then use the Uni-Bit to get to 0.1875", or use straight-fluted reamers to go bigger.

My personal preference would be to drill larger holes and use Tinnerman washers to spread the load. The countersink has to be big enough to allow the washer to move a bit, according to LP Aero. Tighten screws until you start to see a little distortion in the window or canopy and then back off slightly.

Buy some 1/8" and 1/4" acrylic sheet from Jeff Bezos and experiment before you start making holes in something expensive.

Carl Froehlich should chime in here....

I'm sure several people will chime in and say just do it like the instructions say... but whose instructions?

And be very, very careful about what kinds of chemicals (including adhesives on tape) that you put on your expensive acrylic.

 

[mc607]

Section 39 has you initially drill the canopy holes using a #30 plexiglass drill bit. It then states after canopy removal from the frame to enlarge the holes using a #27 plexiglass drill bit in step 4 of page 39-09. No issues here. Reading section 5-19 on the installation of the canopy it states that using a regular or plexi twist drill bit to enlarge holes in the canopy will practically guarantee a cracked canopy!!!

Any advice or experience here is appreciated. The general section seems to directly conflict with the specific instructions of section 39.

I used these tapered reamers from Superior Tool Service for my RV14A canopy. Bit pricey but they worked flawlessly.

.1440" (#27) Dia x 3.0" OAL, 4 Flt, 1.0" LOF, Straight Flt, Solid Carbide Tapered Drill Reamer - Superior Tool Service

Ready to Ship

superiortoolservice.com

 

[lr172]

I started all holes with a standard #40 bit, though i used specific steps to avoid cracking. All holes were then enlarged with a unibit. Imho unibits are pretty safe on acrylic if used correctly. They cut VERY differently than a traditional drill bit. Specifically, they don’t have angled corners at the outer edges and that is what chips the plexi as they come out of the bottom of the hole. They cut more like a reamer than a drill bit.

 

[ravenstar]

I just finished this task yesterday. Before I started I made a number of test holes in a scrap of canopy material I had been given. I tried various combinations of jobber bits, reamers, and plexiglass bits with different initial hole sizes and enlarging by various steps, all driven by a hand held electric drill. I even placed the holes absurdly close together to see what kind of cracks I might see when I drilled my canopy. The result? Nothing caused cracks. I think it's more important to use a solid backing block to support the material, sharp drill bits, and light pressure when drilling.

I made sure to keep the drill perpendicular to the surface, and I started slowly so the bits didn't walk. It just didn't seem to matter if I started with a #40 jobber bit or #40 plexiglass bit, and whether I incrementally increased the size of the hole or jumped all the way to a #12 bit. I even drilled holes a couple of mm apart then cut the material between the holes with a Dremel.

Having done that, I then moved on to my canopy and had no issues drilling and enlarging the holes. I even chose to revisit a number of holes and make them slightly larger because I wasn't happy with the tight fit of the screws.

I don't mean to suggest getting careless, or that anyone else's suggestions aren't better than my technique. Maybe I was just lucky, but it sure seemed like the material was very tolerant of my amateur drilling job. I'm more worried about whether there'll be too much tension on the screws when the weather gets 100 degrees colder. All of my work was done around 75°F.

 

[Breezy]

Section 39 has you initially drill the canopy holes using a #30 plexiglass drill bit. It then states after canopy removal from the frame to enlarge the holes using a #27 plexiglass drill bit in step 4 of page 39-09. No issues here. Reading section 5-19 on the installation of the canopy it states that using a regular or plexi twist drill bit to enlarge holes in the canopy will practically guarantee a cracked canopy!!!

Any advice or experience here is appreciated. The general section seems to directly conflict with the specific instructions of section 39.

Trib,

The initial mandrel pull on pop rivets cause the shank to expand and will put unwanted side force on the hole in the plexiglass. There also needs to be room for movement as temperature induced expansion/contraction occurs. The #30 hole is the correct grip size for the rivet so that is used in the frame. This is why the multi-step drilling is used.

The drill bits for metal will grab plastic material and twist in like a screw garrenting a crack. Plexiglass drill bits are available from all aircraft tool sources.
They are pitched to prevent augering in. They are not very expensive, I highly recommend using them. I have also had success with taking a used metal bit and running it in concrete to intentionally dull the edge. Do some test holes in scrap before using this method. Also, use a backup block like a hardwood block to prevent punching through hole. I also lightly deburr the hole to help prevent stress risors as the edge.

The specified rivets have aluminum mandrels, be sure to used them. The lower breaking tension reduces the clamping force of the rivet to minimize pressure on the plexiglass.

Good Luck!