time to use the fly cutter.... the what?

Got to the shop this morning for a couple of hours.  Trying to squeeze in a bit of time here and there before going to sleep before nightshifts.

First thing I finished off was the fuel tank bay inboard wing rib cap repair.  Finished deburring the holes, both the old bad ones and my new ones to prevent any further cracking.  The new cap repair is really stout and should give a great surface to attach the wing skin back on compared to the original rib flange which was mangled by the previous builder:

The missing wing root doubler has been drilled according to the plans and is clecoed in place.  I've yet to decide which order to rivet this in place, but I'm going to wait until we have all the other items (nose rib, root nose rib, root rib, wing attach bracket, etc) gathered so I can test assemble and measure everything.  This took a lot of careful back drilling through the spar web to get it right, and I'm happy with it so far:

In a lot of places of any given airframe, the designers of light aircraft take advantage of the inherent structural strength of aluminum to lighten the overall structure of the aircraft (lighter is better).  A common method is the use of lightening holes.  You can see them (the large circles) in the spar web of the above picture.

Lightening holes serve a number of purposes besides weight reduction (obvious).  Wiring, hydraulics and or fuel lines can be passed through these easily.  It also provides an opening for inspection of control linkages that might be inside otherwise closed cavities without having to remove the skins.  The most common place they are used is in wing ribs, including the replacement root wing rib I'm currently making for the 701.

Lightening holes are also flanged which provides even more rigidity to the part (more on this in a future post).

Making lightening holes is where the fly cutter tool really shines.  Cutting perfectly round holes by hand is near impossible.

So what is a fly cutter?

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From Wikipedia:

A fly cutter is composed of a body into which one or two tool bits are inserted. As the entire unit rotates, the tool bits take broad, shallow facing cuts. Fly cutters are analogous to face mills in that their purpose is face milling and their individual cutters are replaceable. Face mills are more ideal in various respects (e.g., rigidity, indexability of inserts without disturbing effective cutter diameter or tool length offset, depth-of-cut capability), but tend to be expensive, whereas fly cutters are very inexpensive.
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Most fly cutters simply have a cylindrical center body that holds one tool bit. It is usually a standard left-hand turning tool that is held at an angle of 30 to 60 degrees. Fly cutters with two tool bits have no "official" name but are often called double fly cutters, double-end fly cutters, or fly bars. The latter name reflects that they often take the form of a bar of steel with a tool bit fastened on each end. Often these bits will be mounted at right angles to the bar's main axis, and the cutting geometry is supplied by using a standard right-hand turning tool.
Regular fly cutters (one tool bit, swept diameter usually less than 100 mm) are widely sold in machinists' tooling catalogs. Fly bars are rarely sold commercially; they are usually made by the user. Fly bars are perhaps a bit more dangerous to use than endmills and regular fly cutters because of their larger swing. As one machinist put it, running a fly bar is like "running a lawn mower without the deck",[2] that is, the exposed swinging cutter is a rather large opportunity to take in nearby hand tools, rags, fingers, and so on. However, given that a machinist can never be careless with impunity around rotating cutters or workpieces, this just means using the same care as always except with slightly higher stakes. Well-made fly bars in conscientious hands give years of trouble-free, cost-effective service for the facing off of large polygonal workpieces such as die/mold blocks.

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I particularly like the quote from the machinist that it's like "running a lawn mower without the deck".  After drilling my into my hand a week ago (I'm fine by the way), I'll take this as a warning!

The fly cutter I used is a single arm one.  It's like a high speed compass that cuts metal:

 
​Adjusted to the right radius/diameter required using the set screws and placed into the drill chuck.  When spinning it clearly will hurt you if it's not respected!  Here it is with the root rib:

The only safe way to use the fly cutter is to clamp the part being cut down to the drill press table and keep you hands well clear.  Trying to stop a piece of spinning aluminum should the cutter jam would be foolhardy.  I used wooden blocks to prevent marring the aluminum with the C-clamps.  It was also important to make sure the clamps were clear of the rotating fly cutter arm (that also would be very bad):

The secret with any machine tool cutting is to go slow and steady and use lots of lubrication.  Once lined up correctly, I began to make the cut, adding a little WD40 as I went.  Here is the cut well into the process (yes, the drill was stopped for the picture):

As soon as the cutter breaks through, pressure is lifted to prevent a chance that the cutter tearing the remaining aluminum.

Re-position and cut two more holes without issue.  It takes a bit of time and I'll have a ton to do for my 750 parts, but it's worth it to have nice clean holes.  Here is the root rib with all three holes cut and the cut out discs.  They are sharp, I wonder if that's how they make pizza cutting wheels for the kitchen!:​

All cleaned up and waiting for deburring and flanges.  Looks great!

Next up, flanges!  Stay tuned and thanks for reading :)