Preparing for drilling the holes for the Center Elevator Hinge Assembly

Sooner or later in this process you have to become intimately familiar with Aviation quality hardware and tools. Once the elevators are finished, the next step is to temporarily hinge them in place on the HS, one at a time, and center them up with the HS so that mounting holes can be drilled in the control horns. These holes must closely fit the 1/4 inch bolt that secures each control horn to the center bearing assembly, which was the very first  assembly of the tail kit that was riveted together many many many months ago.

You also learn the difference between close-tolerance and other types of hardware. The holes and bolts that that are used in close tolerance applications are designed to prevent excessive play in the components being attached by providing a very exacting fit. Abnormal wear on the hinges and bearings, control flutter, and control binding are just a few of the potential problems that can result if both the holes and the bolts are not properly sized and attached.

The assembly of the center bearing/hinge on the HS, and the drilling of the mounting holes in the elevator control horns that accept the bolt that is inserted through this bearing, must be done exactly right. Vans tells you to "make" a center bearing insert or drill guide of sorts that has a 1/4 inch OD and a 3/32 inside diameter (ID). A pilot hole is then drilled through the exact center point of the hole in the center of the bearing. After reading several posts on VAF and comments on various builders logs to determine what to use for a drill guide, I discovered a couple of solutions, either of which should work quite nicely.

I am very fortunate in my part of the world in Denver Colorado to have access to a hardware store known as the A&A Tradin' Post. It is an independently owned and operated hardware store in this area that is the "go to" place for hardware items that cannot be located anywhere else. It is the only place of it's kind within a 4-6 state region as far as I am aware. They do not supply aviation-grade hardware, but they do stock tools and other hardware items used for building the airplane that may be hard to come by elsewhere, and this can come in handy.

The pic above shows 3 drill bits, a set of aluminum bearings, and a set of nylon bearings. OD dimensions for both bearings are about 1/4 inch OD, and have slightly different inside diameters that will accept very specific sizes of drill bits. Each one is also 1 inch in length, which works well for drilling the pilot holes in the control horns on either side of the bearing. While neither of them has a 3/32 inch ID hole as called for in Vans plans, they will still work well for drilling the pilot hole in each control horn. The final hole size is supposed to be 1/4 inch, so any smaller sized drill bit should be just fine for drilling the pilot holes.

The nylon one can be found at Lowes stores,and several builders use this one for the drill guide. Not having  the best success with drilling out holes in the lead counterweights, I was just a bit concerned about using a lense dense material as a drill guide. Several others on VAF said that the aluminum one gave a more precise and sturdy platform for the drill bit, and the end result is a more precisly drilled hole in each control horn. Problem is that most builders that opt for the aluminum bearing find one that is supplied by an aviation hardware supplier, and the postage alone for shipping it is ridiculously expensive. This is a waste. I was glad that I was able to find an aluminum bearing locally at A&A instead having to order it from the supplier. As I said earlier, if you have ever had to locate any oddball type hardware in this region, you already know that the chances that A&A will have it are very high, so I expected nothing less.

Now back to the hardware lesson. Again with thanks to the experts on VAF, I have learned that drill bits are sized in about 3 different ways:
by wire size (numbered)
letter size (A-Z)
fractionally, which is what you typically see at the home improvement stores and most hardware stores.

Each of these methods has a range of different tolerances under which each bit is milled. Drill bits that are milled using one method may be slightly smaller or larger than similar sized bits that are created using a different sizing method. Then there is the added variable of the person, method, and tool used to do the drilling. Using a hand drill, for example, can be less accurate than drilling with a drill press, and sometimes the opposite can be true.

The best procedure for drilling these holes as accurately as possible is to find a drill guide that will fit snuggly in the inner race of the hinge bearing with a truly straight center shaft, and then find a drill bit that fits as closely as possible inside that center shaft without going over. Once the drill guide is in the hinge bearing, you place the drill bit throught the hole in the center of the guide, and drill the pilot hole in the control horn. Then you remove that elevator from the HS, mount the other elevator, and drill  the hole in the other control horn in the same manner. It is absolutely imperative that the elevators be properly aligned with the HS, and that they are secured tightly in place, with no possibility of moving while the control horn is being drilled.

To make matters even more challenging, several builders have reported that they ended up with a hole in the control horn that is too close to the weld that holds the parts of the control horn together. When this happens the head of the bolt and the nut on the other side do not have enough turning or tool clearance, and the hole has to be filled by welding it closed again, or by applying a patch and redrilling it again.I do not want to go through any of that crap, so I will take my time here, just as I did with the trim tab.

Anyway, the aluminum bearing from A&A Tradin Post accepts a number 27 drill bit quite nicely, and the nylon bearing accepts the slightly smaller number 28 drill bit nicely. I will use the aluminum bearing and the number 27 drill bit that I purchased from A&A when the time comes to drill the pilot holes through the control horn, and I will make careful measurements ahead of time to verify that there will be sufficient clearance for the bolt head and the nut to turn freely.

The last drill bit in the above pic is a Letter D size drill bit, which turns out to be just a few thousandths smaller than a 1/4 inch drill bit. This is the bit that I will use for the final step drill of the holes in the control horn before I switch to a # 12 fluted reamer. Why use the reamer instead of the 1/4 inch drill bit? The Reamer leaves a truly round hole that is much cleaner than one created with a drill bit. I found out that not only did A&A have the aluminum bearing I wanted use for my drill guide, but they also have full sets of drill bits in either of the three sizing methods that can be purchased individually on an as needed basis. Nice to know I can get whatever size bit I need for future applications.

Before I get to that point, however, I still need to wrap up rolling the leading edges..... hopefully tomorrow.