David Paule
Well Known Member
I've had a bit of trouble on my RV-3B leading edges and tanks, and as a result now have a fair bit of experience drilling blind holes. These are holes in which the part underneath isn't visible, and in which neither part has been pre-punched or pre-drilled. Here's what I've learned.
1. Establish some reference positions that are solid and can't move. Ideally, these will be on the bottom part and visible when the top part is in place. That is, these positions aren't hidden by the skin.
For the leading edges, I used the main skins and the mating edge of the other part, either the tank skin or the outboard skin, whichever was more convenient. Since I'm building an RV-3B, the wing is in a stiff jig and the jig itself could be used.
For the horizontal stabilizer, I taped pieces of mylar with extended centerlines to the substructure, and was able to use these to line up the drilling line. I made a mark one inch outboard of where I thought the skin would be, and knowing the distance from that to the outboard-most hole, could locate that one.
2. Sometimes it's necessary to lay out the drilling pattern on the substructure and transfer the pattern to the skin. Using the references, I made a sketch or a table of dimensions, and then after the skin was in place I was able to mark the skin to these.
For the tanks, I had an aft reference (the forward edge of the main skin) and a lateral reference (two of them, the outboard skin's inboard edge and the jig). With the tank skin clamped down in exact position, I marked the rib centerlines that were extended to the main skin and the lateral position of the centerlines. That let me draw the centerlines with some confidence, and I simply measured forward from the aft edge for the rivet holes.
3. You may have heard that sometimes, if you're careful, you can stop drilling as soon as you've broken through the skin and see the substructure underneath. Here's an example:
You can only do this if there's a slight gap between the skin and the structure. You can feel that. If the skin is tight to the substructure you can't feel the difference. Still, it's worth stopping and checking if you can, because it gives you a chance to move the centerline just a bit. That won't help the hole you're drilling but it'll help the next ones, and might save some parts.
Here is my right tank substructure showing the holes. Every one of the rib holes was drilled blind, and you can see that I didn't hit every location - but I was close enough.
Now that the skin can be clecoed to the ribs, I don't need the wooden pieces. They were in place to prevent the front of the ribs getting crushed.
Dave
1. Establish some reference positions that are solid and can't move. Ideally, these will be on the bottom part and visible when the top part is in place. That is, these positions aren't hidden by the skin.
For the leading edges, I used the main skins and the mating edge of the other part, either the tank skin or the outboard skin, whichever was more convenient. Since I'm building an RV-3B, the wing is in a stiff jig and the jig itself could be used.
For the horizontal stabilizer, I taped pieces of mylar with extended centerlines to the substructure, and was able to use these to line up the drilling line. I made a mark one inch outboard of where I thought the skin would be, and knowing the distance from that to the outboard-most hole, could locate that one.
2. Sometimes it's necessary to lay out the drilling pattern on the substructure and transfer the pattern to the skin. Using the references, I made a sketch or a table of dimensions, and then after the skin was in place I was able to mark the skin to these.
For the tanks, I had an aft reference (the forward edge of the main skin) and a lateral reference (two of them, the outboard skin's inboard edge and the jig). With the tank skin clamped down in exact position, I marked the rib centerlines that were extended to the main skin and the lateral position of the centerlines. That let me draw the centerlines with some confidence, and I simply measured forward from the aft edge for the rivet holes.
3. You may have heard that sometimes, if you're careful, you can stop drilling as soon as you've broken through the skin and see the substructure underneath. Here's an example:
You can only do this if there's a slight gap between the skin and the structure. You can feel that. If the skin is tight to the substructure you can't feel the difference. Still, it's worth stopping and checking if you can, because it gives you a chance to move the centerline just a bit. That won't help the hole you're drilling but it'll help the next ones, and might save some parts.
Here is my right tank substructure showing the holes. Every one of the rib holes was drilled blind, and you can see that I didn't hit every location - but I was close enough.
Now that the skin can be clecoed to the ribs, I don't need the wooden pieces. They were in place to prevent the front of the ribs getting crushed.
Dave
