Tip: Countersinking Plexiglass

Rick6a · #1 09-02-2009, 07:09 AM

I have a confession to make. As I read through many of the comments submitted by builders frustrated by causing a crack when countersinking Plexiglas, I simply did not make a crucial connection. Every time I read one of these posts, I ask myself...."Why do folks continue to insist on using a regular countersinking bit?" "That's crazy." All I know is I have worked with a lot of Plexiglas over the years and as far as RV's are concerned, have drilled and installed two canopies with nary a crack. I never obsessed over causing a crack or otherwise gave it much thought. To me the canopy is just another sub-assembly, no better, no worse than any other in the assembly process. So I hope you understand that when I see photographs of builder induced cracks in the 'glass caused by an inadvertent momentary side load on the countersink cutter pilot, I tend to consider that as a wholly unnecessary consequence of using an "improper" tool. I further confess that when it came to drilling the many holes through the canopy, I felt no need to refer to the written instructions feeling quite familiar and comfortable with Plexiglas' characteristics. Heck, when I installed the (slider) windshield portion on my -6A in the unheated hangar, I felt comfortable enough to drill and countersink through the 'glass and into the rollbar amid an ambient temperature of 65? although I certainly do not recommend anyone try it.

But after reading so many comments, some sounding rather huffy that so-called "experts" thought they knew better, I felt compelled to dig out the good old RV-6A builder's manual for review. Sure enough, I found what I was looking for and here is a direct quote from Van's: "By countersinking before enlarging the holes, you can use a countersinking bit with a standard pilot."

I see now. Certainly, I am not going to challenge the wisdom of Van's advice so I thought I would submit this post merely in the spirit of a "builders tip."

May I humbly submit an alternative way to easily countersink Plexiglas with very little risk of cracking the 'glass REGARDLESS of how large or small the hole is? I use a 100? no flute countersink bit, a type of countersinking bit sometimes used in the production environment to countersink graphite composite skins. Because there are no flutes, the countersink it produces will not chatter and is high quality. In composite work, the use of a standard 3 flute sheet metal countersinking bit is verboten. I cannot recall where I specifically learned to use this type bit on Plexiglas, so I guess I found out on my own. A no flute countersink bit works just as well on Plexiglas. The only way you might crack the Plexiglas using this type of countersinking bit is if you haul off and throw it!

http://www.skygeek.com/ats-zfc01.html

This type bit does not cost very much and also serves double duty. Perfectly suited for deburring holes, I routinely chuck it in a cordless drill and have at it. Let me suggest you buy one one of these inexpensive bits to use as a deburring tool. Then take a scrap piece of Plexiglas and drill several different size holes through it. Finally, use this bit to countersink those various sized holes. I do not think you will be disappointed. Nevertheless, good, bad, or indifferent, be sure to share your own findings with us.

One thing is clear to me. In a VAF poll "Has Your Canopy Cracked?," over 35% of the 211 respondents reported that their canopy had at some point, cracked. Over ONE THIRD of RV builders who participated in the poll have already experienced a significant problem with the canopy 'glass cracking. Those stark numbers speak for themselves. For sure, the poll only accounts for a tiny number of all RV's out there. Even though two thirds of the respondents reported no problems, if those percentages are anywhere near representative, that alone speaks to underlying quality control issues that beg to be addressed. Countersinking Plexiglas need not be a contributing factor in the overall number of canopy cracks that do seem to occur.

http://www.vansairforce.com/communit...ht=poll+canopy

hydroguy2 · #2 09-02-2009, 07:55 AM

nice post.

might have to get me one of those next time.

Crossbow · #3 09-02-2009, 08:45 AM

Thanks much Rick!

While I am still a good way from working on the canopy, but now now I have two ideas (both obtained from VAF) which I will steal when that time comes.

The first, is to use a vibrating cutting tool to actually cut the canopy.

And the second, is to use that no-flute counter-sink bit that you mention.

Many thanks again.

Rupester · #4 09-02-2009, 09:40 AM

In light of above:
a) I used the "vibrating tool" ...a Fein Multimaster ...for canopy cuts. It worked easily and flawlessly. Can't recommend it highly enough. HF now has an El Cheapo copy of the Fein tool for $40 ...w-a-y more attractive than the $200+ for the Fein, tho' it is lesser quality.
b) I used a plain 'ol microstop countersink on all the canopy holes w/o a problem. All canopy holes are drilled and countersunk ... but not enlarged to 5/32" yet.

Maxrate · #5 09-02-2009, 08:49 PM

Rick,

Thank's for the post. I got mine in the mail from Skygeek yesterday.......BTW what do you use to cut your canopy?

Thx,

Rick6a · #6 09-03-2009, 01:42 AM

Quote:

Originally Posted by Maxrate

.....what do you use to cut your canopy?...

Mark,

I used a common kit supplied fiber disk chucked to a (high RPM) die grinder, not a (low RPM) drill motor. The fiber disks perform just fine. When making the big cut, an important thing is to make sure that the canopy is well supported, especially the side by side models. You don't want it flopping around because the whole canopy gets pretty flimsy. If unsupported, you will locally stress the Plexigas as you proceed with the cut. I am aware of one builder who did just that and about 3/4 of the way through the cut, the stress became overwhelming and canopy glass cracked big time! So, before I made that big cut, I drilled 4 #40 holes on each side of the canopy through the waste flange that is later removed. I then threaded kite string through the holes and pulled it just taunt. As the cut proceeded, the Plexiglas had no where to go and when the two pieces finally separated, the end result was a gap no bigger than the kerf. I suppose you could use duck tape to do the same thing but the important thing is to keep those two canopy halfs well supported.

DanH · #7 09-03-2009, 06:57 AM

Strongly endorse Rick's recommendation. Drill a piece of scrap and deburr with a standard fluted bit, then look at the hole with a 10X magnifier. You'll quickly realize why cracking, now or later, can be a problem.

In larger diameters (like 1/4") dress a fine grit mounted stone to around 100 degrees and deburr by lightly grinding the hole.

Paul Thomas · #8 09-03-2009, 09:13 AM

Do they make those with threads? I looked but couldn't find them.

Maxrate · #9 09-03-2009, 09:26 AM

Rick,

Thanks for the great advice on the big cut. What about the trim work on the flange around the edge of the canopy. Did you use the same fiber disk in a die grinder?

carguy614 · #10 09-03-2009, 09:31 AM

Seach for them under the trade name, Weldon bit. Prefer them to all fluted bits for all applications. 100 degree piloted bits are also available, and are very aggressive, but chatter free. Absolutely the best for deburring if you use a non piloted version. Thier weakness is that the pilot breaks off easily with this design, so side loads on the bit should be avoided.

Hope this helps,
Chris