Small chips when drilling canopy

[iamtheari]

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.

[sf3543]

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

[F1Boss]

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.

[iamtheari]

Originally Posted by F1Boss

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.)

[DHeal]

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.

[iamtheari]

Originally Posted by DHeal

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.

[scrollF4]

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.

[iamtheari]

Originally Posted by sf3543

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!

[Tom Martin]

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!!

[RV Jerry]

I?m with Tom and Mark #40 intial hole then step drill, have many canopy?s without cracks.