Building the jig

Before we can build a spar, we must first build a jig. So I made a trip to Home Depot to find the finest-smoothest-straightest-113" long piece of lumber anyone has ever seen - yup, good luck with that one!

I started placing items in the cart and right as I was about to head towards the checkout, I realized the prices were in "per foot"... not by the whole piece. Using their finest lumber would have cost over $300 to make this jig. So I had to come up with a Plan B.

After scouting around, I came across a 3/4" thick laminate board used for closet shelving. A 15.75" x 97" runs about $16 a piece! You can't beat that! So I purchased two of those and I was on my way.

Page 14-6 in the plans gives all dimensions required to build the jig and shows the main "D" backboard panel, along with shelves A, B and C:





Here is one of the 15.75" x 97" x 0.75" panels that I purchased... the plan was to use one to make the "D" backboard panel... and the other to make the A, B and C panels.



Since the shelving is only 97" long, I had to make the "D" panel out of two pieces. I decided to split it such that I would attach it at the jig center line. See sketch below:

 You get the idea... the point being I oriented it such that I used the straightest edge to be the top edge of the jig - I'll explain why later.

So I began to draw all the dimensions per plans...


6.12" down to the "shelf line"...


6.05" down to the shelf line where the spar is canted...


An accurate t-square is a MUST...


I then rotated the panel 180 degrees... and repeated the measurements to draw the other half...


Now to cut the two halves. DO NOT CUT ON THE LINE! The marking is the "shelf line"...  so I doodled a line (hand drawn) between the two halves - since the overhang below the shelf line does not have to be accurate... as long as there is enough space to attach the A, B and C shelves - hence why I wanted the straight edges to be the top part of the jig - since that is the important edge. I then cut along the doodled line.


The canted line however IS important... so this cut had to be perfectly straight.


I then repeated this cut for the other "D" backboard half.

After I had both "D" panel halves cut... I took my other panel board and measured out the following:
(notice that panel C and B are NOT perfect rectangles - they are trapezoids). Cutting a perpendicular line against the edge would not give enough area to make panels B and C.

Here I am measuring all the dimensions to cut panels B and C:




Here I am measuring all the dimensions to cut panel A:


Panel A has a very subtle kink starting 9" away from center line. I used the hand held circular saw to cut along this kink and then sanded it smooth...



So there they were... Panel D (two halves) and Panels A, B and C...



Ok now.. let's get jiggy with it! Get it? Jig-eeee  ::ok ok, cheesy Will Smith joke!:: Any who... moving on...

It was now time to assemble the jig. I started out by laying the right half of panel D flat on my table.. and clamped panel A to it such that it overhung about 2" so I could install screws from underneath (to attach panel A to panel D). I made sure it was square... and that the edge of panel A was exactly on the "shelf line"... I also made sure the center line marking of panel D aligned with panel A's center line marking.





How did I attach panel A to panel D? Watch this...

I got my pre-drill and screw driver charged and ready... and 2" drywall screws...


I measured the distance between the bottom lip of panel D to panel A using the depth gauge of my digital caliper...



In this case it was 0.80"...





The panel boards I'm using are 0.75" thick... so add 0.375" (0.75" / 2) and you get 1.175"... this gets the screw right dab in the middle of panel A... so I marked the spot from underneath the assembly...



Pre-drilled it... very important to pre-drill... else this "wood" splits pretty easily..



And in goes the screw...


I repeated this 2 more times (3 total) on the first half of panel D.

I then turned the assembly upside down on my bench and clamped it as hard as I could. Since the board is 0.75" thick, the clamps hold panel D perfectly at 90 degrees.

I then butted the other half of panel D to the assembly and clamped it in place.


Here's a close up showing the center line where both D panels butt up against each other. Notice how the right side has the screws attached to panel A....


Before I could attach the other half of panel D to panel A, I had to make sure everything was lining up right... notice how panel A is being cantilevered at this point... so its own weight pulled it down about 0.125" at the tip...


Here I am holding panel A up so that it would align perfectly with the "shelf line"...


I then installed 3 screws same way I did with the other half of panel D. When all was said and done.. Panel A was 100% flush with the shelf line...





Now to attach panels B and C. Since they droop down, their mating edges don't align. No problem.. that's what beveling is for! The picture below shows panel B on the left... and panel A on the right. Notice the pencil mark on panel B to show me how much to bevel out...



I used my band saw to bevel panels B and C... here I am test fitting panel C...


Oooooo... much better fit :)



I then attached panels B and C the same way I attached A to D... clamping the panel into place, pre-drilling from the backside and installing 3 screws per panel.


At this point, nothing was really holding panel C aligned with panel A. So to make sure it would stay aligned, I installed a nail between the two panels. Here is panel A on the left and panel C on the right. I also did this where panel B mates with A.



While this helped keep the panels aligned, it didn't help keeping them together. Holding the assembly up from where the two panels mate would cause the panels to separate. I thought about attaching a 2x4 along the entire back of panel D (like the plans suggests) - but good luck finding a straight 2x4! There had to be a better way!



When pressing down on the assembly... it forces the two panels together. So then I came up with a plan to use a metal plate to pull both panels together.


First I installed the bracket onto panel A...


And while pressing down on the assembly (and hence bringing the two panels together), I attached the bracket to panel C. Notice how I purposely tilted the screw so that it would pull the two panels together.


I then added a few extra screws to make sure it was secured.


Oooooo... looking goooood! No way these panels are coming apart now...



But not quite there... a few more things to tidy up...


First, I wanted to make sure the two D panel halves would remain flush. At this point, the only thing keeping them flush was panel A. I needed to attach a "bracket" on the back side of panel D... so I used a 2x4 puck as a "bracket"... I suppose I could have used another metal bracket, but I had only purchased 2.




Here is a front view showing how I attached them together using 4 screws (2 on the left side... and 2 on the right).



Using an L angle as a straight edge, I placed it on all top edges of the jig to verify straightness. All was well except for a 4" run along the right top edge...


It was difficult to get a good picture of it, but there was a 0.125" gap just in this area - poor use of the circular saw I suppose...



Any who, I knew this edge will be used later on to trim the foam panels down to size - so it's important to have this edge straight as an arrow! So I mixed up some Bondo and slabbed it on...



Waited an hour or so for it to dry and sanded the edge level.



Taaaa-daaaa! Perfectly straight - gap filled!


Looking goooood!




The final step was to make the support panels that hold the entire jig level. As I was about to start cutting the E, F and G panels... I realized it wasn't necessary. The jig is already being supported in the back by the panel D overhang.

I added support to the center... using shims to fine tune the final 0.25"...



And added support feet to the front sides... here's the left...



Here's the right support foot...


Checking to make sure the entire jig is perfectly square...


And there you have it.. a perfectly square jig ready to build a spar!




5 comments:

Sundog said...

Hii Ary,
Fuel tanks - any further ideas on the material breakdown, or using bladders, etc??

Ary Glantz said...

Haven't focused attention to that topic yet. After center section spar will come canopy work.. then canard... then strakes... so I still have a while to go before I tackle that subject.

-Ary

Tango Charlie said...

Can't beat the finish of those melamine panels. Sure is hard to work with that press board, though. I wonder if a sheet of decent plywood would be a good compromise.

Capt Meatballs said...

Nice jig Ary.

Do you think wood glue and screws would be enough to hold the jig together, or are the metal brackets necessary?

Marco

Ary Glantz said...

I suppose you could do it that way, but wood glue is messy and you have to wait for it to dry - and what do you do if it dries unevenly? Sounds like a pain!

The metal brackets assures the boards are perfectly even and you can verify it right away.

Before I came up with the metal bracket idea, I tried to drive a long screw from one board to another, but those melamine panels aren't dense enough to get a good grip from the threads... so the screw just pulled right out. The metal brackets let you use multiple screws so it distributes the load similarly to how rivets distribute the load from an aircraft wing skin to the ribs.