I am currently working toward my airframe & powerplant certification, and as such, I wanted to make a useful project that would improve my airframe skills, improve my ability to produce airworthy rivets, all while making an end product that was both useful and visually appealing.
I decided on making a simple toolbox. Using 0.040" 2024 T3 Alclad alumminum plate and AN470 2117 T3 solid universal head rivets, some basic hardware store grade catches and a handle, I produced the simple aluminum toolbox.
Now, of course, I didn't just grab some metal, makes some bends and start throwing the toolbox together.
The Design Phase - The Math
I started off with some very rough design parameters. The toolbox was to be approximately 18" wide, 7" deep and 6" tall. These are known as the mold lines of the project, from which I found my setbacks, bend allowances and leg lengths.
To calculate the layout, I started with the mold dimensions, which are the exterior dimension of the toolbox, or really any project you wish to layout.
I then calculated my setback and bend allowances, which are as follows.
For 90 degree bends
Setback is calculated using the formula
SB = K(T+R) where K is your K-factor, T is the thickness of your material and R is the radius of your bend, which is also the radius of your brake. Because the K-Factor for a 90 degree bend is 1, we can simply the formula to be SB=T+R,
Bend allowance is calculated using the formula
BA =(2π (R + 1/2 T)) /4
BA =(2π (R + 1/2 T)) /4 where T is the thickness of your material and R is the radius of your bend, which is also the radius of your brake.
Once you know the setbacks and bend allowances, you can calculate your leg distances for layout.
On a piece of sheetmetal that you are making a single 90 degree bend, you would calculate the distance of the legs by taking the first leg of the bend and subtracting a setback, then measure your bend allowance distance and then add the second leg distance minus another setback.
For bends other than 90 degrees
Once you know the K-Factor, you can calculate your setbacks and bend allowances. It is important to remember that you need to calculate setback and bend allowance for EVERY angle you bend and to make sure to use the correct values for the correct angles when doing your layout.
To calculate setback for bends other than 90 degrees
SB = K(T+R) where K is your K-factor, T is the thickness of your material and R is the radius of your bend, which is also the radius of your brake.
To calculate bend allowance for bends other than 90 degrees
BA = (0.01743R + 0.0078T)N
Bend allowance = (0.01743R + 0.0078T)N where R is the radius of the bend, T is the thickness of the metal and N is the number of degrees in the bend which you wish to make.
On a piece of sheetmetal that you are making a single bend, you would calculate the distance of the legs by taking the first leg of the bend and subtracting a setback, then measure your bend allowance distance and then add the second leg distance minus another setback.
Once you have your layout, much like you see here. For reference, the toolbox's mold lines are 18" x 7" with a 4" front and a 5" back. Using a 1/8" radius brake, my bend allowance for .040" alclad is 0.2280"
Now the easiest thing to do is take a micrometer and a sharpie and begin laying out everything on a piece of sheetmetal. Make all of your cuts and relief holes first and then you are ready to start bending everything up.
To layout your bends as they will be positioned in the brake, measure the distance of the radius of the bend, in my case, 1/8" or 0.125" from the intersection of the leg line / bend allowance line which is sitting under the brake. This measurement is your sight line, which you will align to the front of the brake edge. You should just see your sight line and it should be tight to the front of the radius. Lock in your brake and make your bend.
Its important to remember that if you want precision for your final product, you need to keep every aspect of your precision within the tolerances you set for the project. I kept all of my measurements on layout to +/- 0.0005", however, I know that there was error from using a ultra fine tip sharpie for layout, in that even though they do have a pretty precise tip, its line thickness can vary dependent upon pressure, ink flow, etc.
To prevent cumulative error, remember that its best to set a reference edge to make all of your measurements off of and make all of your measurements from that edge when possible. This will prevent gain in length because you are not having to estimate exactly where the center of your sharpie line is from which to base your next measurement.
The Design Phase - Layout
Once I calculated the length of the legs and the bend allowances for the toolbox, I began laying it all out in AutoCAD. I originally attempted to do a quick hand drafted sketch of the project, but after being unhappy with the appearance, I decided to spend a few minutes drafting it up, which really helped when it came to procuring quick accurate dimensions of unknowns.
Panel A dimensions
'Panel A' was the base pan of the project and consisted of the front, bottom and back of the toolbox, as well as the side lips which would serve as attachment points between panels.
Panel B dimensions
'Panel B' was the lid strike plate which would hold the lid tight in place and prevent it from distorting / moving when closed.
Panel C & D dimensions
'Panel C/D' are the side panels of the toolbox. I kept the dimensions pretty tight, but ended up modifying them slightly and shortening the width / height dimensions so that the panel would sit flush and out of the radius of the bends.
Panel E dimensions
'Panel E' was the lid of the toolbox. To follow the dimension of the side panels, I designed the lid to make two 20 degree bends and a 70 degree bend on the front.
Once I had everything drafted up. printed out and ready to go, I started laying everything out on the aluminum with a sharpie. To protect Alclad, it is faced with a thin plastic film, which is of great benefit, because you can write all over it with a sharpie and its quick and easy cleanup once you are done.
I decided on #4 or 1/8" rivets for the project. Based on this, I found my minimum edge spacing for the rivets, which is 2D or 2 times the diameter of the shank of the rivet, which in this case would be 1/4".
I drew out all of my rivet locations, used an automatic center punch to mark centers for easy drilling and then drilled my holes with a #30 drill bit. I used clecos as I went along to hold the sheetmetal in place so that my holes would not become out of alignment, as the tolerances for fit of solid rivets are pretty tight.
For those not familiar, clecos are spring loaded devices which are color coded based upon hole size, and are designed to maintain a friction fit upon the holes in which they are placed to maintain proper hole alignment during the drilling and riveting.
- Silver - 3/32"
- Copper - 1/8"
- Black - 5/32"
- Gold - 3/16"
Once I had all of my holes drilled and everything cleco'ed together, I began riveting everything together.
To rivet solid rivets, you use a rivet gun, in my case a 3X pneumatic rivet gun with a correctly sized universal head rivet set and a bucking bar.
The Rivet Gun
The Bucking Bar
Bucking bars come in either tool steel or tungsten. Tungsten is preferable as it is significantly more dense than tool steel, meaning that more of the energy from the rivet gun is transferred back into the rivet and less is absorbed into the bucking bar. The only downfall is the price.
The Final Product
I used the NACA method to flush rivet the doubler onto the lid of the toolbox and countersunk the rivets at 82 degrees to half of the depth of the .070 doubler plate, riveted them and then attempted to mill off the shop heads, however, I was unable to get any of the microshavers into the tight space, so, since its only a toolbox, I used a dremel with a cutting wheel to get them close, then used a right angle die grinder with a gray scotchbrite wheel to try to clean up the edges as best as possible.