Originally published November 1997
The following discussion comes from experience working with an RV-6A sliding canopy. However, the information should be relevant to anyone working with acrylic material.
Second, when you need to fit an overlying aluminum washer strip or skirt, you can enlarge every other hole to 1/8" thru the acrylic only (not thru the steel) using a drill bit ground for plastic drilling. Then tap the steel and countersink the acrylic for a #4-40 flat head screw. Although the #40 holes in the steel are a bit oversize for a 4-40 tap, you will get enough threads for the screws to hold temporarily. This way you can get a flush surface while the acrylic stays firmly in place. The alternate holes that do not contain screws can be used to cleco the overlying part.
A third advantage to using #40 pilot holes is that blind hole-finding (described below) is more accurate than it would be for larger holes.
CAUTION! If you anticipate significant temperature changes (more than ±10°F) in your workshop between work sessions, I suggest that you do not leave tight-fitting screws and clecoes in place. The expansion and contraction of the acrylic will generate stresses that could possibly cause cracks. You can simply remove the fasteners. Or you can drill the clecoed holes in the acrylic somewhat oversize. Oversize drilling will not help much for flat head screws, because their heads prevent the acrylic from moving freely. Either loosen or remove the screws, or replace them with clecoes.
Begin drilling the canopy by first making the 5/8" hole for the latch tube. I am very cautious about drilling acrylic, and so I did my drilling in this order:
1. Pilot drill 1/8" and enlarge to 1/4" using plastic-drilling drills.
2. Gradually enlarge the hole with a Unibit. When each step of the bit breaks out the opposite side, it is likely that the acrylic will chip at the exit. You can avoid this problem by alternately drilling from one side and then the other. To get a 5/8" hole thru 3/16" acrylic with a Unibit, you have to drill from both sides. This is because most Unibits can only go 1/8" deep at a given diameter. (The Unibit 2 can drill almost 3/8" deep at each diameter, but it is limited to 1/16" increments and 1/2" maximum diameter. It is very useful for enlarging the screw/rivet holes.)
If you place masking tape (lightly, do not press down) over the tubular steel parts and then press the acrylic against the tape, the tape will darken at the touch line. Aim at this dark line when drilling.
Continue drilling the canopy aft along the center spine. Because these holes are hard to reach, and because you probably want them to line up in a perfectly straight line, start by pilot drilling or punching the top center washer strip on the bench, where you can get good accuracy. Also drill the hole for the latch tube. Place the washer strip on top of the canopy, carefully center it, and tape it down. The latch tube will center the front end, but you must manually center the aft end. Then drill thru the pilot holes, the acrylic, and into the steel tube. If you hold the drill correctly, you should get a very accurate line of holes.
CAUTION! If you plan to insert anything between the acrylic and the underlying steel frame (such as spacers, rivet nut heads, large washers, washer strips, etc.), insert temporary shims of the same thickness as the final assembly as you drill and cleco. It is not necessary to insert these shims along the center spine as you first drill it. But after it has been drilled, insert the shims before you drill anything else. Continue to insert appropriate shims hole by hole as you go down the sides of the canopy and the windscreen. Because the canopy and windscreen may both need shimming where they butt together, drill one hole thru the canopy and one just opposite thru the windscreen at the same time. Shim both as needed, then continue to the next hole. If you do not install shims as you go, the holes thru the acrylic will misalign with the holes in the steel when the shims are installed at a later time.
Two suggestions: First, use vinyl shields over the ends of your long reach clecoes to prevent scratching the acrylic. I made my own shields from standard protective vinyl caps (3/8" diameter by 1/2" long) by punching a hole in the end with a leather punch.
Second, you will need to do a lot of blind hole-finding to locate holes in the overlying aluminum parts after the acrylic and underlying steel have been pilot drilled. I used many strips of drafting vellum about 1 1/2" wide by 4" or 5" long, taped to the acrylic and overlapping each pilot hole. By lightly rubbing a pencil over the hole, the hole outline shows up clearly on the vellum. Where #4-40 screws are holding the acrylic, the outline of the Phillips screw slot marks the hole. The aluminum is slipped under the vellum strips. When the aluminum is correctly positioned, a sharp prick punch can be used to make a small nick in the aluminum by pressing the point thru the vellum in the center of the hole. When the aluminum is removed, the nicks can be enlarged with a regular center punch and then pilot holes can be punched. I like to use a Roper Whitney hand held metal punch to make these holes, because the punch has a centering point that fits into the center punch mark and keeps the hole exactly in position. It does not walk like a drill bit. If the holes are punched 1/8", they can be easily reamed out for pop rivets or screws, depending on what you plan to use.
My experience has been that plastic-drilling drills up thru 1/4" seem to drill thru the acrylic fairly safely, creating very little or no chipping where the drill exits (as long as you don’t press too hard). However, the expansion and contraction characteristics of acrylic demand that most holes be larger than 1/4". Larger plastic-drilling drills can be used by drilling first from one side part way thru, and then back thru from the other side. A Unibit can also be used the same way. The Unibit 2 can reach all the way thru the acrylic from one side, but it scares me when it breaks out the other side. So I always drill from both sides when going over 1/4".
You may plan to control the position of the acrylic during expansion and contraction. For example, you may want the butt joint between the canopy and windscreen to maintain a fairly constant gap over temperature to get a good weatherseal. In this case you may wish to elongate certain small holes rather than make them large and round. If you are using #6 screws, start with 5/32" round holes in the acrylic. Countersink them slightly to mark the center. Elongate the holes both ways the required amount with a 5/32" chain saw file. These files really work well, and it does not take long to elongate a row of holes.
After all holes are either drilled to size or elongated, deburr the insides and countersink the outsides to provide space for the dimples in the overlying aluminum. Countersinking the acrylic was the most demanding part of the job for me. Oddly enough, countersinking the elongated holes was easier than countersinking the large round holes.
First, the elongated holes. I used a multi-flute carbide countersink, 100° by 3/8", in a die grinder. I found that it must never touch both sides of the hole at the same time. If it does, the countersink will start to jump around and chew up the acrylic. It can ruin your whole day. Just lightly move the countersink around the periphery of the hole, touching only one edge at at a time. You can determine how much cutting to do by using a scrap of aluminum with an appropriate dimple as a gauge. Precision is not required. You are simply trying to remove enough material so the dimple will slide up and down the full length of the hole while the flat part of the aluminum stays against the acrylic. Do not remove too much material. You really want the dimple, not the screw shank, to constrain the movement of the acrylic in the sideways direction.
Now, the round holes. I used a multi-flute carbide countersink, 100° by 5/8", in a slow-turning (400 rpm) drill. It is necessary to stop frequently and clear the chips out of the countersink. When you are about deep enough, clean the countersink thoroughly and take one final, very light cut. This step cleans up any gummy residue in the hole caused by chip overload. The process was painfully slow. (Well, it took over an hour to do the canopy.) However, before you jump ahead and try something faster, consider how much time (and money) you would lose if you ruin your canopy or windscreen. Here are some of the things I tried (mostly in scrap acrylic) and the results:
MSC, various locations in the East, (800) 645-7270
McMaster-Carr Supply Co. Various locations. Los Angeles phone (562) 692-5911.
Contents of The Leading Edge and these web pages are the viewpoints of the authors. No claim is made and no liability is assumed, expressed or implied as to the technical accuracy or safety of the material presented. The viewpoints expressed are not necessarily those of Chapter 1000 or the Experimental Aircraft Association.
Revised -- 25 April 1998