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Tip: Drilling plexiglass take 2

koda2

Well Known Member
Fellow Vanziens,
Okay I've read everything I could find and experimented a bit and I am ready to drill plexiglass. Well not quite. As usual, instructions and tips are all over the spectrum.

Perhaps the most daunting words were Tony Bingelis' warning, "In my estimation countersunk screws should not be used...." (page 214, SCT). Well that's great. Only 100 or so on the tipup.

I also looked into putting a washer between the screw head and the plexiglass in the thinking that it might distribute any stresses, but it requires more countersinking than for just the screw and maybe weakens the plexiglass too much.

No consensus on how much relief the hole has to have on the body of the screw for expansion concerns; the new plans say 5/32 but older posts say 3/16 and the aluminum to #23.

I wouldn't obsess about it but the post that stated that 35%, IIRC, of respondents had cracks develop in their canopy, either during fabrication or thereafter, is worth one's attention.

For those interested, here is a close up of the hardware I rounded up.
Top row is 6's, bottom row 8s. From left, AN507 screw, respective tinneman washer, NAS finishing washer, mockup of screw seated, hi-dollar zero flute countersink and 110 degree 3-flute countersink 3/8"dia. (edit: the NAS finishing washer is second, tinneman third. DA).

fasteners.jpg


As with every other non-piloted c-sink I've ever used, the zero flute consistently wandered off center when used freehand (by me), especially when trying to countersink before "drilling up".

The 110 degree c-sink leaves a much better fit for aluminum side skirts, but will not be wide enough for washers. It takes a .5 inch dia. size. Also I am assuming what you want is a "sloppy" fit, so the plexiglass can expand and contract a little everywhere, not just in the area of the screw body.

Finally, I got out a scrap of canopy, beat it on the workbench until my hand hurt and then bent it 200 degrees without it breaking. Took another piece and carefully drilled a 1/8" plexiglass hole. Halfway that is. Snapped like cheap window glass.

plexiglassbit.jpg


So much for the reassurance of plexiglass bits. The kicker here is that if you look closely, one side of the scrap of plexiglass was not polished. Also, the temp in the hangar was 68F. The crack appeared to originate at the bottom of the drill hole but could have come from the stress being transferred to the sharp edge.

So far all I am sure of is that plexiglass must be heated till it softens enough to work on it and everything gets polished. I have kept heaters on the canopy at all times during the fitting to keep its temp 80F or above before moving, cutting or polishing it.


Dave A.
RV-6A
 
Last edited:
Dave, since you're experimenting with technique (bravo for you!), grab an ordinary drill bit and a fine whetstone. Hone a small flat to replace the sharp cutting edge on each flute, said flat in plane with the long axis of the bit. Drill some sample plastic using high speed and low pressure, examine the hole with a 10X magnifier.
 
Dave,
I am curious on the finishing washers. Are you planning on using them on the "open" countersunk screws? That being those that are not covered by a skirt or other alum piece? Where do you aquire such? I do like the look, more finished. Are they "proud" of the surface by much? I used an 1/8 plexi drill initially and then used the permagrit w/pilot until nearly finished. Then used a wilton non-flute countersink to finalize the hole. I too could not get my non-piloted c/s to stay on center. I then drilled w/plexi bit to 3/16 behind skirts and 5/32 on the open screws. I was very pleased with the result and canopy was drilled on 80 degrees or higher temps.
 
Washers

Terry,
Look in the Aircraft spruce catalog on the washer page or try the web. I have not made a decision on using them for acrylic. They may work better in fiberglass situations. They require a bigger countersink and may be a problem but the "flush" style of the NAS washers almost get to the point of being flush. I read somewhere that someone made some kind of stainless washers work on the open plexiglas but I never found any details. Things are on hold here waiting for warmer weather.

Good site I found with lots of acrylic info.

http://www.rplastics.com/plexdesign.html

Dave A.
 
plexiglass bits

I think I have discovered part of the difficulty in drilling with the "plexiglas" bits.

browntool.jpg


The photo shows the two bits I got from Brown Tool. The right one is 3/16" and exhibits all the characteristics described in articles I found on the net: 60 degree included angle, approximately 0-4degree rake, flattened cutting surface, and 12-15 degrees of spiral relief. The bit on the left is 1/8" and has no flattened cutting surface. This was the one I that was cracking the plexiglas.

The 3/16 bit also shows the desired two spiral chips being made while cutting:

twospiral.jpg


The finished hole is pretty good, a little rough maybe. The sharp front and back edges should probably be chamfered.

316.jpg


The small holes in the second picture are made using a discarded #40 bit which I put on the lathe and used a homemade tool grinder to get the 60 degree angles. I filed the flat as Dan suggested using a Norton stone. The result was still too rough to use on the canopy but if I can improve my bit making, thats what I will use for the initial holes.

Dave A.
 
I've had good luck drilling initial holes in plexi with a #40 bit, sharp or dull (dull is probably better); I then enlarge the hole to the required size with a step drill (Unibit). Trying to enlarge an already drilled hole in plexi with a standard drill bit has a high probability of starting a crack when it emerges on the other side.


Drilling the #40 hole slowly without a lot of force is probably a good idea.

I've built two canopies using this method with no problems - the #40 can drill through the plexi and then into the steel tubing underneath; plexi drills have a hard time drilling into the steel once they get through the plastic.

One other trick I learned is how to find the correct place to drill to hit the center of the steel tubing underneath (round tube, square isn't a problem). Assuming the plexi is pushed against the tubing, take a small flashlight and hold it flat (light shining into the plexi) on the plexi just above or below where you want to drill the hole. You may have to slide the light further along the tubing to get the proper angle. What you will see is an arc where the light is shining on the tubing just through the plexi; mark the spot on the plexi that corresponds to the apex of the curve. This will be very close to the ideal position to drill.

As always, practice drilling plexi on scrap pieces to build confidence that you aren't going to crack your expensive canopy.
 
I found that if you place masking tape(I used the green painting tape) along the tubing and then press the plexi onto the tube you will see a line where they touch. I used two drills. An electric one with the plexi bit and my pneumatic drill with the regular bit for going into the tube...most important of all is TAKE YOUR TIME !!!! :D
 
Tapered Drill Reamers work well

I have been testing a Tapered Drill Reamer, often used for drilling composite materials, with good success on plexiglass. I have been trying to abuse them by pushing hard and moderate speeds, and even drilling plexi I just pulled from the freezer. With limited testing, I have yet to crack a test piece. The picture below is the tapered drill reamer. I do not have a good source for these unfortunately. Perhaps one of the RV tool suppliers can start carrying them. I did find them advertised at http://www.avitecnet.com/cutting_tools/tapered.htm, but I have no idea if they sell small quantities.

p1060537t.jpg


The picture below is taken about a minute after I pulled a scrap of plexi from a 10 minute cold soak in a freezer and immediately drilled two 5/32 holes at moderate force and speeds. No backup material was used. The condensation is still visible on the plexi even after wiping it once. So the material was cold when drilled.
p1060543s.jpg


The last picture is of the is from the same scrap after reaching rool temperature. The middle hole was drilled at room temp, pushing hard, with no backup. The left two used standard drill bts pushing moderatly with moderate speed on the drill with no backup. Both show rougher hole sides and some chipping on the backside.
p1060545u.jpg


In general, the holes sides are very clean, with no chipping or burrs on the entrance or exit of the hole. The material cuts like a reamer in a controlled fashion, with no apparent tendency for a flute to grab the edge of the hole on exit. I am planning on using these to open up the holes in the canopy when I get to that point. Your results may vary.
 
This is from AC 41-13.1B

While not addressing the problem exactly, it gave me a chance to cut and paste from my PAMA copy of the CD.


(a) The drill used on acrylics must be
carefully ground and free from nicks and burrs
that would affect the surface finish. Grind the
drill with a greater included angle than would
be used for soft metal. The rake angle should
be zero in order to scrape, not cut. (See figure
3-18.)
FIGURE 3-18. Drill having an included angle of
approximately 150?, used to drill acrylic plastics.
(b) The patented Unibit (see figure
3-19) is good for drilling small holes in aircraft
windshields and windows. It can cut
holes from 1/8-to 1/2-inch in 1/32-inch increments
and produces good smooth holes with
no stress cracks around their edges.
FIGURE 3-19. Unibit drill for drilling acrylic plastic
 
I received the tapered drill reamer that I ordered from Starlite Industries. I used it to drill a bunch of holes in 1/8" acrylic with no backing and with and without pilot holes. It make beautiful holes clean holes with no chipping that look just like the ones in Brice's post above. I really tried to get it to crack the plexi by getting way too close to the edge and pushing way too hard and never got it to crack. Thanks Brice for an excellent find and post. I highly recommend this tool.

-Chris
 
Followup

Well,
The canopy is drilled and the front installed and so far nary a hitch.

I never used the plexiglass bits I got from Brown Tool.

My technique:

Made the frame. I put in all the pilot holes, spacing, etc. before assembly and up drilled to #40. (The parts were replacement parts for a 6 and had no prepunched holes.)

Installed, cut, fitted and clamped the canopy on gently with soft clamps.

Back drilled the plexiglass with a homemade #40 bit made using instructions by www.plasticmag.com and Dan H. Its easy to get "dead on" the frame hole if you do it carefully and check the hole location a couple of times during the slow drill. I clecoed each hole as I did it to keep alignment. Chamfer both sides of canopy with a zero flute c-sink.

Countersunk with 110 degree 3-flute in a microstop while the canopy is installed and clecoed and the frame holes help keep the sink from wandering around. The edge of the countersink was made just slightly wider than the #6 dimple or the #6 Tinneman washers.

By this time I could no longer make a decision between 5/32 or 3/16 up drilling so I took a new 11/64 bit and converted it to plexiglass configuration and enlarged the canopy holes.

Rechamfered the back side of the plexi and drilled up the frame to #25

All screws installed in the front and no cracks yet.

If I had known about it in time, I would have used the tapered drill reamer shown by the other members. Would have saved a lot of time.

I still believe 80 F+ temp is the key. I warmed the canopy carefully with a small electric heater under the canopy and a blanket over the top and monitored the temp closely

Dave A.
6A rebuild
 
Yep

Chris

Pretty amazing little drill. Takes all the worries out of drilling the plexi. I tried to abuse it too without any cracks, chips, and the inside walls of the hole are very clean with no scratching.

Funny thing is when I started drilling my canopy, I was still very carefull despite being reckless on scraps with the same drill.
 
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