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Building a MIDI Thru Box Part 6: Rear Panel Cutouts and Board Mounting

Pete Brown - 12 August 2011

Today I spent a little time at the drill press in my shed, and made a huge mess with aluminum and cutting fluid.

Before you read on, here are the other installments in this series. You may wish to look them over first.

Spotting the Holes

As I mentioned, I went out and made a mess with chips and cutting fluid. My goal was to quickly cut the rear openings for the 11 MIDI jacks and one power jack. The first step was to mark the edge of each jack using a hobby knife. The lines will be the centers of the drill points. I marked using the edges, but offset the whole back first by 1/2 jack, making it much easier to get accurate lines using the jack edges as a guide.

I wanted to use my Sherline mill for this, but it doesn't have quite enough travel to cover the distance from one end to the other in a single setup, and setting up a CNC operation just to drill some holes seemed like a lot of planning for just a little work. I probably will use it for the front panel, if I don't get one commercially made.

I then drilled pilot holes, as I had expected to use spade bits when drilling the aluminum. The pilot holes ended up being unnecessary (and actually in the way) because spade bits are a horrible choice. When I tried to use a spade bit on the first hole, it wandered a bunch and instead of cutting through, simply pushed the metal so it humped up a bit. What I really needed was a 5/8" hole saw, but what I ended up using was a formerly very sharp 5/8" forstner bit and a bunch of tapmatic cutting fluid.


Drilling the 5/8" Holes


After completing the holes, I reamed them with a 3/4" hand-held countersink. This removed the burrs and sharp edges. During the process, I checked the back against the circuit board several times. Despite that, things were still a little off, but will be usable. I really wanted to have larger holes (more like 3/4") to allow a tight fit for the fat plastic cable ends, but couldn't fit that large a hole in the back.


After that was complete, I drilled a hole for the power jack, and reamed it like the others.


Next, I had to cut some clearance holes for the standoffs. You can see the half-circle holes here. You can also see how the workpiece jumped out of the clamps at one point, and the bit skittered across the bottom. It'll be covered by the board, but not a great moment in metalworking history :)


Attaching the Board

The final step was to attach the standoffs to the circuit board, spread epoxy on them, then hold it in place while the epoxy dries. Another option would be to drill holes in the bottom of the case and screw the standoffs in place, but the case top/bottom/sides are made of steel, and frankly, epoxy is good enough for this.


The end result is decent. The 5/8" holes in the back don't really leave any room for error, of which I had a fair bit. It looks like I can get a good connection from each of them, but they're not all quite centered.

Next Steps

Next I want to wire up a Netduino to this guy and try out some of the more advanced ideas I had around routing and monitoring. I'll post about that once I set it up.

posted by Pete Brown on Friday, August 12, 2011
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3 comments for “Building a MIDI Thru Box Part 6: Rear Panel Cutouts and Board Mounting”

  1. jaysays:
    I cringed when I read the part about the forstner bit... That is a pretty expensive bit to be using on aluminum. Although I suppose a 5/8 twist bit is about as expensive, which in my mind would have been a better choice. Along with a slow speed on the drill press. But the bit would have been usable afterwards.

    Lay out the marks, hit the center with a punch(key to an accurate layout), use a twist bit, and hold on tight to the metal so it doesn't spin. I see you used a fence, so that last part is a bit easier.
  2. Petesays:
    Yeah, I agree. That's totally the wrong bit for the job, but I had only a small window of time to work on this and that was all I had that would work.

    On the plus side, I used a fair bit of cutting fluid, so while I surely dulled the bit (especially due to the anodizing), I didn't heat it up and ruin it.

    I also agree on the center punch.


  3. Dansays:
    This project really intrigues me and I'm impressed with the professional job you've made of it!

    I was just wondering how much it has cost in total? I guess the pcd design and rack mount case bumped the price up a fair bit.


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