As the next step in this process, with all the center holes drilled in the brackets, it was now time to back drill all the remaining holes. Some build logs suggest clamping the bracket down, while others say to bolt it down tight so it does not move while back drilling the two remaining holes. I decided to apply both techniques, because things have a way of moving on me whenever I try to drill accurate holes through solid material.
I knew which clamp I wanted to use, but the problem was that the clamping surface was wider than the small half inch wide flange of the Z bracket, and had a tendency to want to tip sideways when I applied the clamping force to it. So I came up with this ingenious idea of leveraging the use of one other Z bracket to essentially increase the surface area of the flange so that the clamp could sit properly. I started on the outboard end and worked my way to the inboard end of the brackets.This method of clamping worked really well, up to a point:
I used a locking clamp for metal with the wide pads as shown in the pic. I did not want to use the one with the stub in the end as I found these to have a slight rise in the metal like a bit of slag from the forging process, and this has a tendency to leave an impression in the metal that you are clamping against. So the flat pads are better for this operation to prevent damage to the bracket flange and the spar. The other side of the clamp is on the other side of the wing spar:
Note the clearance from the waffle plate on the back side of the wing spar - it is close so you have to be mindful of that. Also note that the clamp only reaches one side of the bracket,since the top wing skins are still attached to the other side.The goal then is to ensure that the bracket is truly square to the wing spar,then clamp in place using the procedure above, then you can back drill the hole farthest from the clamp (wing skin side):
I used the #12 drill bit for this as shown, WITH the drill stop to avoid damaging the wing spar. You can use the reflection of the drill bit against the wing spar web to verify that the drill bit is straight. i was very worried that I might cant the bit off to one side or the other causing a warbled hole in the flange, but everything actually went very well:
After the first hole is drilled you attach the bolt and nut assembly to that hole. This locks the bracket in place so that the clamp can now be removed and the final hole back drilled into the flange:
This process was repeated until I got to the last bracket - the inboard one,which, as you may recall, has the holes further outboard from the web of the bracket than all the others. This is when I realized a small problem. How do I "backdrill" a hole in this flange when the nut plates are already installed. OOPS! Didn't see that one coming. Add to this the fact that this bracket resides in the area of the 4 very closely spaced ribs for the wing walk area, and you don't have much room to do anything, much less drill holes.
So here is how I resolved the problem (almost):
1. I used the nut plates rivets on the top side to draw a center line with a sharpee all the way to each side of the wing spar web. Basically I am going to extend the center lines of each hole so that I can transfer the lines to the bracket flange.
2. Admittedly I just kind of eyeballed this and had to use some odd-sized straight edges to draw the lines. One of them was the edge of a jig saw blade. It turns out that the holes are all on1.75 inch centers. I marked the lines, attached and squared up the bracket with the center bolt, and then transferred the lines to the bracket. Then I removed the bracket, verified the lines were in the correct place,and took it down to mydrill press.
The first hole was drilled spot on as shown below:
Then,as I mentioned at the beginning of this post, I managed to commit the ultimate sin. I went to drill the final hole of the final bracket for the left wing, and was looking forward to being able to move on to the next step of fitting and drilling the other flanges of each bracket to the baffle plate. Unfortunately, my drill pres clamping arrangement failed and the part slipped and the hole got drilled in the wrong place. This means yet another order placed with Vans. Man am I pissed. It got worse,but I'll save that for the next post.
I need to order the Proseal anyway, so its just as well. I still have to backdrill the right wing brackets, so there is still plenty to do. FYI, a new T712 bracket costs 2.50 plus shipping. I am going to order a full set of 14 more of them - just in case :)
No comments:
Post a Comment