Drillled and Tapped Pitot Tube and Mast

Man the last two days have been nerve racking. Most people write about these steps in their builders logs as though it is no big deal, but for me this task has to rank up there with all the tasks that I absolutely cannot stand  about this project - which includes the task of drilling raw holes in round or awkward shaped materials. I absolutely hate having to figure out how to jig, clamp, align, and whatever else that has to be done to do this right. You know, all the gory details that nobody ever seems to talk about in their build logs.

For starters, the shortest screws I could find locally were 3/8 inches long, and I really needed 1/4 long mounting screws. The solution - A dremel tool with a cutoff wheel, and a clamp. I started out by using one of my plastic squeeze clamps, holding it in one hand with the dremel tool in the other. This worked great - right up to the point that the heat from the screw started melting the plastic holder. So I stopped that foolishness to think of another way to hold these screws so I could cut off the last 1/8 inch. I have done this numerous times when building RC airplanes over the years, so this technique was nothing new to me - just that the length of the screw was  a bit shorter than I was used to working with. What I finally came up with was using 2 thcik wood blocks, adn inserting the screw head in between each block and clamping it tight in my table vise so that the screw head would mash down slightly into the wood. This made a stable way to use the dremel to cut off the ends of each screw. I then got out an 1/8 inch thick piece of aluminum from my trim bundle, and drilled and tapped the same hole that I was about to drill and tap in the real thing. At least I remembered to do something right. That all went as expected, and after a small amount of filing on some of the freshly cut screw ends, each of the 4 screws inserted cleanly into the threaded hole.

Now to drill the holes in the assembly. Silly me thought that I had it all figured out. TO do this right, it requires drilling somewhat precise holes, so that meant it was time to use the drill press for this. UNfortunately that means clamping and positioning the table in just the right spot so that the hole is drilled correctly. Here is a shot of my arrangement, complete with wood blocks, a drill clamp, and my #40 drill bit with a drill stop positioned so that it drill no larger than a 1/4 inch hole. It all looks pretty lined up too.....

By the time I had taken this pic, I had already drilled the two rear holes. That worked out ok, except for one thing that I will show in a bit. For the rear holes, the assembly was able to be clamped solidly into position without moving. To amke things a bit easier, the rear holes are drilled in a location where there is very little curvature to the assembly. However, from the above pic, you can see that when you drill the forward holes, this becomes a bit of a challenge. What I found was that the assembly would not clamp down tight in this position duw to the extreme curvature of the mast. Instead, the forward face of the assembly kept riding up on the wood block with each turn of the clamp handle. The one thing that it was doing was placing the assembly at the correct angle for the drill bit. It worked great for that. I thought I could just hold the part steady with my left hand while I ran the lever on the press to drill the hole.

I should also mention that I set the stops on the drill press in addition to using a drill stop on the drill bit to ensure that I would not drill the hole too deeply. I also secured the mast and the tube with gorilla tape as shown in the above pic. SO, everything was in good shape, right? WRONG. As soon as I turned on the drill press the clamp vibrated lose and the assembly would shift out of position. So much for that idea. I was not about to spend frickin hours making a jig that holds the pitot in just the right position to drill these stupid holes.

Too bad, because it sure looked like it would work great - here is another pic - everything looks spot on lined up. Note the angle of the pitot tube that is required to drill a straight hole through the mounting material in the inside of the pitot tube.

So at least I had the good sense NOT to try to drill these holes using this method. I stopped for the night so I could rethink this and figure out what to do.

So shift forward to today. The next pic shows what I basically came up with - take the assembly outside to the vise, clamp it upright against the two wood blocks, put the #40 drill bit in my cordless drill with adjustable speed trigger, lined up the angle as best I could, and drilled the first pilot hole. Then I turned it around and did the same thing on the other side. I did NOT use my air drill for this - the trigger is too unpredictable and hard to control. I needed to start this slow and keep the drill steady all the way through.

After that was done I changed drill bits to a #36, and drilled through each hole again to get to the correct size for tapping the 6/32 threads for the mounting screws. Then I ran the tap through each hole. In short, I free-handed the stupid holes. Other build sites I have visited where folsk are using their drill press actually don't seem to be doing much better than me. Several of them show holes that are drilled that are not really at the correct angle as determined by the aluminum flanges on the inside of the pitot tube. WHne you see the screws being inserted in the pics it clearly shows that the holes are not aligned correctly with the face of the mast.

For the #36 drill bit as well as the 6/32 tap, I used some electrical tape wrapped around the shaft to serve as a drill stop of sorts so that I would not risk drilling into the stupid air lines in the pitot tube. The rear holes are no problem, but the forward holes will get you into trouble if you run the drill bit too deep.

The above pic hints at my next problem. A couple of the holes were bottoming out on the tap - the reason - the hole was not drilled quite deep enough for the tap. A quick look at the tap revealed the problem. The tip of the tap barely has any cutting threads on it, and tapers off quite sharply at the tip. This basically means that the tap probably requires at least 1/16" more distance to allow the cutting threads to properly seat in the material all the way through the hole. I thought I had this all worked out by just continuing to run the tap alittle bit further, that is until I got to the forward hole on the left side.

The other holes had been drilled all the way through the aluminum flanges on the inside of the pitot tube - all of them except the one on the left front hole. When I tried to torque the tap a bit further in that hole, all of sudden it just let go - no threads, no resistance. Yup, I managed to strip the #$%&;^&;*^% hole! What an ass I am sometimes. That was about as stupid as it gets.

Out came the #36 drill bit to finish drilling through the material, and then I reran the tap through the hole. Bottom line is that this hole only has about 1/8 inch or less of viable threads that are actually grabbing and securing this screw - not good. But at least it does grab. Something tells me that before this is all done I will probably be resizing that hole for a #8/32 screw.

And now for the finished masterpiece. The left side with mounting screws inserted..... and NO I am NOT going to go through the trouble of countersinking these stupid things. Enough is enough already. I'll take the drag penalty.

...and then the crappy looking right side.

Note the mis-aligned screws. The sucky part is that this actually happened on the drill press. I must not have had the first rear hole lined up in quite the right spot, and so it got drilled slightly lower than where it should have. The other 3 screws are relatively lined up with each other. This is purely cosmetic, but pisses me off jsut the same.

SO there you have it. IF anyone can royally screw up such a simple task, it is surely me. At least the damn thing is drilled and mounted. I can't wait to see how bad I can screw up the mounting the mast to the wing spar - stay tuned for that episode of dumb and dumber.

OK, I need to go cool off now, this has been an exhausting and disappointing couple of days. I bought a "kit" for a reason. If I would have wanted to spend all my tine laying up undrilled holes in all sorts of wildy contorted non-linear objects, I would have built it from scratch. For me, this process ranks up there in the top 5 things about this project that I hope I never have to do again. I sure hope that I did not drill too far into the pitot and either scar the tube or puncture it outright. Initial "blow" tests through the line seem to indicate everything is working correctly, but only time and repeated wear and tear will tell, since you can't see any of the tubing buried underneath the silicone deep inside the pitot tube....