- Published: Monday, 22 August 2016
- Written by Jon Chandler
- Hits: 1613
The FAA recently asked me to come up with a solution for mounting one of my Wake Turbulence Timers at a location with a metal console panel; they didn't want to cut a hole through the panel but preferred a surface-mount solution instead. One hard requirement was that it use a standard timer with no modifications in case the timer had to be replaced at some time in the future.
T-Slots and Tabs
I have used t-slots in laser-cut acrylic before to make a stand-alone enclosure for use in a training center but I'm not sure that approach will stand up to day-to-day use and abuse past the point where I no longer care. The technique is handy for some applications but perhaps not a heavy-duty industrial application.
Apparently I haven't posted about the t-slot method here before, so I'll touch on it lightly. In a t-slot joint, one panel has tabs that fit into slots on the mating panel, and a t-slot that's (typically) sized to fit a 3mm nut across the flats. A screw goes through the mating panel into the nut and is tightened to hold the joint secure.
The results can be stylish, but there are a few shortcomings. The first is the amount of effort it takes to lay out the pieces to do this. The second is that the panel with the slots must overhang the mating panel to provide a strong joint. Finally, acrylic is brittle, so it's easy to break the t-slot if care isn't used in tightening the screws.
The results can look good, but as I said, I'm concerned about durability in the current application.
Elecrow Hexalithus Fixed Block Acrylic Link Block
That's a mouthful! Elecrow has some 10mm cubes with 3mm threaded holes in each face, designed to hold acrylic enclosures together. These would work well for my enclosure but I only have a few of them on hand for another project I'm working on - I didn't want to take the time to order more.
Open Beam Aluminum Extrusion
I did have some Open Beam aluminum extrusion on hand. This is a smaller version of the large material that's been used commercially for years. The cross section is 15mm x 15mm and the slots are sized to hold 3mm nuts; the ends may also be tapped for 3mm screws.
This is the ideal solution to make a custom enclosure for this application, using laser-cut acrylic panels.
This was my starting point. I knew I would have an Open Beam column in each corner to hold the enclosure together and the Wake Turbulence Timer panel had to fit on top of the enclosure. The simplest solution would be to build a custom timer with an over-size panel, but the customer insisted on using a stock timer. The support columns could not move in far enough to use the existing panel mounting holes because of the circuit board location. In fact, the columns need to be even farther out so that the nuts that will hold the timer clear the column.
Now the design is starting to take final form. The columns were moved out to nice locations (even numbers to work with) - the locations in inches are shown on the top of each column. A column was added at the center of the long panels for additional support.
The grey pieces are the sides, made out of 1/4" material my vendor supplied Note how the ends overlap. Acrylic comes in 1/8", 1/4", etc. and 3mm, 6mm, etc. thickness. It's important to know the actual thickness of the material used to get the overlaps right.
The blue piece is the inner top piece that will support the timer. It will be inside the side panels so it won't be visible.
This drawing provided the lengths for the side and end panels, and the overall size for the top and bottom panels.
The next consideration was the overall depth of the enclosure. The timers use a stack of 2 printed circuit boards and a standard USB B connector for the power connection. It's desirable that the enclosure be as thin as possible, which is limited by the length of the USB plug. I'll use a USB plug body without the shell to keep this dimension as small as possible.
The enclosure will be held to the console panel using double-faced foam tape. The bottom panel gives the tape somewhere to adhere, the provision must be made for the button-head screws that hold the panel in place. I set the length of the Open Beam columns to a nice number - working in our archaic imperial system, the length is 2 3/8" - and adjusted the length of the side panels to accommodate the screw heads.
The top panel will consist of a frame around the timer cut from the same Rowmark ColorCast material as the timer panel. To converse material, I actually made a timer panel in the center of the frame when the frame was cut. The black lines in the below drawing are cut through, the red is engraved through the color layer on the back of the ColorCast material and later filed in with black acrylic paint.
The shaded area in the below drawing is the inner top panel. It sits inside the side panels and provides a shelf for the timer to rest on and be bolted to.
As a final check before laser cutting, I like to arrange all the panels on the same drawing and make a quick reality check. The overlaps all work out, the necessary holes are in the right positions and so on.
The laser cutting is done by a nearby vendor; I send the files and a few days later my pieces are ready to pick up. It is a joy to have the pieces exactly as designed, the holes in the right locations, etc. - assuming my drawings were correct!
I cut the Open Beam material with an inexpensive cutoff saw from Harbor Freight with an abrasive wheel. I have since learned that abrasive wheels are not recommended for cutting aluminum; aluminum chips can stick to the wheel, expand when they heat up and cause the wheel to explode! A diamond blade or toothed blade designed to cut aluminum is the better choice.
I rigged up a stop to get all of my Open Beam pieces the same length. You might guess from the short pieces being cut off that I made them too long initially.
Here's a view of the abrasive wheel I used in the saw. Abrasive wheels are not recommended for cutting aluminum.
It should go without saying that the Open Beam will be hot after it's cut. Should go without saving...
I cleaned up the cut ends of the Open Beam with a Dremel tool and a rubber abrasive wheel after cutting. Make sure no rough edges protrude from the Open Beam and that the channels for the nuts are clear.
I tapped each end of the Open Beam with a 3mm tap which will be needed to secure the top and bottom panels. Finding a 3mm tap at the local hardware store is unlikely. I got mine from McMasterCarr.
One final item is needed before starting assembly - the 3mm screws and nuts. If you look at the Open Beam picture above, there is not much depth in the channel past the nut. In my case with 1/4", an 8mm long screw barely engaged the threads and a 12mm screw bottomed out before the panels were tightened down. A 10mm long screw worked perfectly. 3mm screws typically come in 2mm length increments which should allow a good fit to be had.
Here's a look at all the pieces except the top and bottom panels.
The side panels screw into the Open Beam. I started by putting screws in all the holes of the front and back pieces and putting nuts on all of them. The nuts can't be cranked down too much or they won't be able to slide into the Open Beam, but you don't want them too lose or the screws may back out as you're sliding the extrusion in place.
Slide the extrusion over the nuts; the flats of the nuts must be parallel to the extrusion but with a little jiggling, it wasn't too difficult to slide each piece of extrusion into approximate position. Tighten the screws gently when the extrusion is in its approximate location. Install the extrusions on the front and back pieces, then install the end panels.
Next, put the inner top in position and loosen the screws for each piece of extrusion and adjust until the top is flush with the sides. There's a little side-to-side play in the extrusion; adjust the side position to get the best fit to the front and back panels. You may have to adjust the extrusions several times to get the perfect position.
The above picture shows the green translucent bottom panel. Since the enclosure will be taped to the console panel with this surface, I don't care about the color. I told my vendor to use whatever was convenient.
Once the inner top panel was correctly adjusted, the top panel frame was put into position. You can see the ledge when the timer panel will sit.
And finally, the assembled unit. The timer will be secured using bolts and self-locking nuts.
This enclosure came out pretty much as expected. If I was doing it again, there are some slight changes I'd make:
Use 2 screws per support instead of 3. Over the span involved, 2 screws will be adequate.
Use matt material for the side panels. Shiney isn't the best from a glare potential and it shows fingerprints.
Use black screws instead of bright screws. This has been my plan but when I changed to high-strength screws, I didn't realize they weren't black.
Make the front panel frame slightly oversize (overhanging the sides) and slightly round the corners.
Designing a custom enclosure with Open Beam was pretty simple and far less tedious then using the t-slot and tabs method. The enclosure is quite strong and rigid and will survive the rigors of Air Traffic Controller use for a long period.