I Ruined a Spar

 The bottom of the holes, where the drill bit comes out, has some wood fiber burrs, even with a sharp drill bit.  The factory ignored it.  I'm to nutty to ignore it.  A 30 degree break-away knife works great to clean up the holes.  I normally just use a backup board to prevent this but I couldn't get it as flat and square as just using the table.

 The first spar I drilled got ruined because I started drilling a hole in the wrong place for the strut/brace wire fitting.  I had the template upside down, tall wire end pointing at the tip instead of the root.  It was the result of how I made the templates for the fittings and not checking the fitting outline I drew on the spar.  Fortunately Aircraft Spruce hadn't shipped the other spar material so they were good about adding another spar blank to that order.

This all started because of how I made the templates.  I made the holes for the 1/4" hole and the .377" hole using the Whitney punch.  It doesn't make holes as big as 3/8".  I could have made the holes actual size because I have 3/8" and 1/4" duplicating punches.  I like the shorter centering nib on the Whitney punches, rather than the duplicating punches, because you don't need as thick a spacer under the template to get the punch to set completely in the template hole.  This all worked perfect for making the fittings.

 I could have used the actual fittings to locate the holes in the spars but they're all welded and painted and a tight fit to the spars.  So I used the original templates.  After ruining the first spar I decided I needed a better process so this didn't happen again.  I also wanted to use the original templates for better accuracy.

I start by drilling the 3/8" hole first.  I bought a bag of small bushings at Oshkosh some years back and regularly find uses for them.  This one is 3/8" O.D and 1/4" I.D.  It allows me to use a 1/4" bolt in the original template hole to position the template for the next hole.

 For the 1/4" hole I aligned the 3/16" drill bit in the template hole.  While holding the spar so it didn't move, I changed to the 1/4" bit and drilled the hole.

I made a bushing for the 1/4" hole from a piece of 1/4" aluminum tubing.  It allowed me to use a 3/16" bolt for this hole in the template.

 With the first 2 bolts in the 3/16" bit was aligned to one of the holes, the template removed and the hole drilled.

The same was done for the 4th hole.

 A truly tortured process, but it worked very well.  All 4 bolts fit perfectly.  Because the table is so square, they also fit perfectly on the backside of the 1 1/4" thick spar.  Not bad.

Drilling Spars and the Optical Center Punch

 With all the layout work done, I'm ready to start drilling bolt holes in the spars.
The first task is to make sure the table is perpendicular (square) to the drill bit.  I'm using the ShopSmith to do this because I can adjust squareness and drill speed better.  I start by getting a long drill to use as a reference to measure how square the table is.  Rotate it by hand to make sure it's not bent.  A piece of rod would work if it's straight.
I used the drafting triangle to check squareness in several directions around the drill.  Because of it's size, using it also saved re-positioning the table when I was done.

 I set up the rollers to hold the spar flat while drilling.

 The ShopSmith drill bits have a unique shape which cuts wood well.  The little skirts on the side also help me later when aligning the drill in a template or part to assure multiple holes match between the part and the spar.

 Working with Wood always makes me crazy when center punching holes.  The punch wants to move over to be in the soft wood between growth rings.  The solution is an Optical Center Punch.  The one I'm using is the H5781 by Grizzly Industrial.

 It has a holder base for the center punch with cork on the bottom so you can keep the punch in position without sliding into the soft wood.
To locate it you use an Eyepiece with a Reticle on the bottom end.  This one comes with a Cross Hair reticle and a Point reticle.  I generally prefer the cross hairs because I normally layout crossed center lines, which I find it easier to align with.
It also has a prick punch, the pointy one, and a punch for standard twist drills.  I'm using the prick punch with the ShopSmith drill bits.

 You put the eyepiece in the holder and then align the reticle with your hole location marks.  I find it real easy to see getting it aligned.

 Hold the base tight to the spar.  Remove the eyepiece and insert the punch in the same hole.  An appropriately light tap with a hammer and you've got your mark.

 When the part I'm going to install has multiple holes I prefer to drill the first hole using the center punch and then use the part or template to locate the added holes.

A bolt in the first hole allows the template to be held Accurately in position.  The skirts on the ShopSmith drill bit make it real easy to align the added hole to the drill.  With a regular twist drill I just raise the part or template with some washers under it at each hole.  With the ShopSmith drill bit I put a center punch mark a little deeper for clearance of the tip when I'm using a thin template.  It's easier than fooling with washers.

 I always remove the template before drilling.  Bad things can happen otherwise.

The holes drilled and the part fits perfect.  I this case I wouldn't need the table perfectly square since the part goes on the side we're drilling from.  For the welded strut fittings the holes have to match on both sides so they really need to be square.

Time to keep drilling.

Selecting Spar Quality Wood

I was looking up references to give a talk on this at our next EAA Chapter meeting and thought I'd share this.  There are more things to look for, any defect should be questioned, but these are the more common things to consider.  The critical area for spars is the upper and lower portions where most of the load is carried.  For the WACO NINE the spars are routed to an I-beam shape so the flanges are critical.

Most of this come from the EAA Video "Basic Aircraft Woodworking" and AC65-15A.  There are plenty of other references to use if your are unsure about a specific defect.

 Sitka Spruce is the preferred wood because it is lighter for the same strength, but Douglas for is an acceptable wood for most plane I care about.  Spruce can have less Annular Growth Rings then Fir and be acceptable, part of why it weighs less.

When looking at the end of the board, the rings need to be at a 45 degree angle or steeper for at least 2/3 of the width of the board.

 Trees grow from the bottom up so the bottom is a larger diameter than the top.  As a result when we saw boards from the tree the growth rings usually run at some slight angle across the board face.  the board is stronger along the growth rings than across them.  Ever split some wood? Because the top and bottom of the spar carry the load, the slope of the growth rings on the outer 1/8 needs to less (15:1) than the interior (10:1) of the spar. 

Not Acceptable for a spar, or much of anything else.

 Once you have good straight strong wood you need to look for small defects. 
Small Hard Sound Knots may be acceptable if they are:
- Not loose
- Not running through the depth of the beam
- No bigger than 3/8"
- Not into the I-beam flange (high stress areas)
- Do not cause the edge to diverge
- Not closer than 20" for 3/8" knots

There is also something called Pin Knot Clusters which might be acceptable.

 Pitch Pockets, little pockets of sap, may be acceptable but there are a lot of limits.

AC65-15A has some tighter limits than the EAA video.  As an example it limits pitch pocket depth and width to 1/8" and length to 1 1/2".

Where the there are multiple criteria, use the the more restrictive limit

 Pitch pockets need to be further apart if they are in the same growth ring.  AC65-15A limits them to 14" in the same ring.

Remember Advisory Circulars and the EAA video are advice.  The person doing the work needs to use their best judgement from all they can learn to make safe decisions.

All Upper Spars Ready To Drill

 The aileron hinges are bolted directly to the rear face of the 1/2" thick web in the center of the rear spars, on the upper wings.  The hinges are an eye bolt and fork arrangement with a 5/16" bolt end.  The hinge sets in the carved out area and the rear face but the forward face doesn't need to be routed.  The areas where other things are attached are not routed, to provide extra strength.  The forward face of one of the old spars is not routed for the hinge at the tip of the spar.  The spar web is cracked at one of the inboard hinges.  I think the extra wood is needed at each hinge.  It means making the fork half of the hinges 1/4" longer to allow for the extra wood.  That's my plan right now.  Until I assemble the wings I can easily route these areas.

The layout is done on both of the upper rear spars.

I've started on the lower wing front spars.  When I get the lay out done on all the spars I'll start drill, then routing.

First 2 Spars Ready to Drill

 The 2 Upper Front spars are ready to start drilling and routing.

I set an original spar between them for comparison.  Everything matches exactly.  When they routed the spars, they used a shaper cutter of about 6" in diameter.  I have a plan to duplicate that shape at the ends of the cuts for the pads where the various fittings get installed.

The tip on the original spar must have cracked because they glued on a piece of spruce, 1/4" thick, on each side.  It gave me the idea to leave an unrouted area at he tip to have more wood for attaching the wing tip bow.  When I looked at he rear spar I found they had done this to strengthen the area of the outboard aileron hinge.  I'm working on laying out the routed areas on the rear spars now.

Holes For Fittings for Upper Spars

There is only one Airworthiness Directive on the WACO NINE.  It was the result of wing spars on the upper wings twisting and cracking at the root ends.  At the time the belief was that it was the result of poor quality spruce used in the spars, but no one knew for sure.  I have the factory drawings for the parts and how they were to be installed.  The installation drawing gave the idea of how to do the installation but no actual dimensions for where to locate bolt holes.

 I spent a few hours bringing up the old drawings in TurboCAD, scaling them to actual size and the redrawing the parts to work out what they planned for hole locations

With these dimensions I've located the holes on the first 2 spars.  I've drawn the parts on the spars to help make installation more obvious later.  The upper and lower straps go on both face of the spars with 1/4" bolts.

There are also clamping plates on the top and bottom of the spars .  I'm not sure this makes a big difference but we will comply.

 The fittings for the wing struts and brace cable are also drawn on the struts to help make it obvious later.

This last bit is the taper down to the wing tip bow on the last 12 inches of the spars.

I'm going to lay out the areas to be routed as well, and I have the other 2 upper spars to finish marking up.