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Small chips when drilling canopy

iamtheari

Well Known Member
We managed to drill the entire canopy without making any cracks. However, there are three holes where a small chip came out of the interior surface of the acrylic at the edge of the hole. By "small chip" I mean that they are big enough that deburring the holes did not eliminate them but countersinking the wrong side of the holes would eliminate them.

All of them are along the aft edge, where a screw will sit directly in the countersunk hole in the acrylic. If a crack develops from any of them, it should go toward the aft edge of the canopy rather than to the fore.

The next step is countersinking the holes in the canopy. Before I do that, I want to check in here in case anyone has hints or tricks that I can apply to these small chips to reduce the chance of a crack developing later on.
 
I usually take a pencil soldering gun and touch the edges of the chip to knock down the edge
Has worked for me many times with no issues
 
Maybe use a different drill bit?

Standard bits pull when turning in plexi - causing this blow out on the back side. Why not use a step drill, which does not pull? I have tried plexi drill bits, and the step drill works better than those do also.

Are there any others using the step drill on plexi?

Be sure to chamfer both sides of the hole too..less on the backside, of course.
 
Standard bits pull when turning in plexi - causing this blow out on the back side. Why not use a step drill, which does not pull? I have tried plexi drill bits, and the step drill works better than those do also.

Are there any others using the step drill on plexi?

Be sure to chamfer both sides of the hole too..less on the backside, of course.

We used a pointy plexi bit. It's too late for me to change bits, but if I had it to do over again I would probably do the same thing because I think it would be harder to keep a step drill from walking when starting the hole.

That being said, I did use a step drill to enlarge a similar (but larger) chip that occurred on the aft edge of the aft window. That worked because a dimpled skin sits on top of the window there and I can just fill the larger hole with a rubber washer and between that and the proseal it should be fine. The canopy problems are all along its aft edge, where a screw sits directly in the countersunk plexiglass, and therefore the bigger-hole solution won't work there. (Unless I use much bigger screws, but that seems like it would be overkill and even more obvious to the naked eye than a crack would be.)
 
You can also use the step-drill as a chamfer tool to touch up the edge of the hole -- just use the next higher diameter on the step-drill to lightly chamfer the hole's edge. In my experience it is better to have a slightly oversize hole in plexiglass or Lexan than a too-tight hole.
 
You can also use the step-drill as a chamfer tool to touch up the edge of the hole -- just use the next higher diameter on the step-drill to lightly chamfer the hole's edge. In my experience it is better to have a slightly oversize hole in plexiglass or Lexan than a too-tight hole.

That's an idea. My concern would be over-chamfering the inner surface combined with countersinking the outer surface, which could result in a knife edge in the middle of the hole.
 
I offer two pieces of advice:

- Remember that when you drill through the canopy, your objective really is to create enough friction to heat and melt through the material, not to cut a hole. I kept my drill speed up, but didn't push as hard.

- For chamfering and countersinking, try googling "edgeless countersink" for bits that don't have individual blades. My favorite is the diamond-encrusted conical bit. A countersink bit with individual blades will leave an edge at the point that pressure and rotation ended: These can contribute to a possible crack point.
 
I usually take a pencil soldering gun and touch the edges of the chip to knock down the edge
Has worked for me many times with no issues

Thanks for this suggestion. The chips were small and a soldering iron was the perfect tool to remove any sharp edges from them. Onward and upward!
 
I am with F1 Boss on this one. I use a standard #40 dull bit for the initial hole then enlarge with the step drill. To finish the edges of the hole I use the standard deburring tool, Light Pressure! It makes a very secure hole.
The key is gentle. Gentle initial hole, gentle exit of the drill in the opposite side of the material. Gentle enlargement of hole with the step drill. The hole MUST be larger the the final fastener to allow for movement of the canopy. The canopy frame flexes in use and holes need to be oversized to allow for this. Finally gently chamfer the edge of the hole with your deburring tool

Practice on scraps!!
 
#40

Hi Folks,

My finish kit will be arriving within the next 2 weeks so I'm watching this thread with great interest.

Regarding drilling with a #40 bit, did you use a special #40 plexiglass bit or just a standard #40 bit that had been dulled?

Thanks
 
Hi Folks,

My finish kit will be arriving within the next 2 weeks so I'm watching this thread with great interest.

Regarding drilling with a #40 bit, did you use a special #40 plexiglass bit or just a standard #40 bit that had been dulled?

Thanks
I used a special (pointy) #40 plexiglass bit. When we did the window, my friend who was running the drill was just a hair off center on one of the holes into the roll bar, resulting in a sideways force that broke out a chunk of plexiglass when the bit centered in the pilot hole in the roll bar. We had 100% perfect results with the replacement window by drilling each hole with the plexiglass bit in a Dremel tool, which made it lighter and easier to drill straight with minimal downward force. As soon as the bit was up to the pilot hole, we drilled the metal with a regular #40 bit in an air drill. That worked out great on the window and canopy. We did not have any damage at all from this part of the process.

As far as upsizing holes goes, the front edge of the window calls for you to upside to #36, then tap through the plexiglass into the roll bar, and then upsize just the window to #27 (which obliterates the threads you tapped when it was #36). The aft edge of the window has you remove the window and then final drill #27 the skins and window separately. The canopy just gets a #27 final drill after the initial #40 match drilling. We used pointy plexiglass bits size #36 and #27 for all of this and had almost no problems.

Of the 66 holes in the canopy and 55 holes in the window, we had chips occur in 4 of them. One was the center aft hole in the window, which chipped when we drilled it up to #27. That damage was easy to remove with a step drill since the skin will cover the oversized hole. The other three were on the part of the canopy that attaches to the rear frame, and occurred when we were final drilling #27 through the canopy and rear frame. One of those was so small that I had trouble finding it again yesterday. The other two were small enough that a dulled soldering iron tip just barely fit into the chipped area to "cauterize" the damage.

Read the other posts here and absorb their advice. Everyone here has more experience and skill than I ever will. The process I wrote out above is just one way to do it and the results that I experienced as a first-timer with help from an experienced A&P.

My only solid advice is this: When you are drilling plexiglass, whether match-drilling or final-drilling, be 10 times more patient than you thought possible and then slow down another 50%. It is much faster to drill 121 holes at 2 minutes each (4 hours of drilling) than to wait for a new window to show up, even if you have a money tree to pay for the new window.
 
Dull bits

I just finished Sikaflex. I experimented with scrap quite a bit and tried several bits and cutting methods. Plexi bits worked best followed be dull. Reamers worked best for the bigger holes. Ultimately only one hole was drilled. I wrote up a big document on Sika and posted it to my bog if anyone is interested.
 
Excellent information....thanks

Ari,

Thank you for the detailed information. It will be very helpful. I'm a first time builder, and must say, I'm a bit intimidated by the canopy assembly process.

Thanks,
 
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