Wooden casing for Logical Engine bit slice

Behold, two sides for the casing of a bit slice, cut using the Trotec laser cutter into 1/2" plywood. I wonder if wood is strong enough? If so, I could save a lot of money, since acrylic is much more expensive than wood. And wood can be stained to look 19th century :)

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The slots are for metal support rods which hold the bit rods in place. If a bit rod needs to be adjusted, the support rods can be slid out, and the bit rods can be removed.

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The support rods will be held in place with sheet metal strips bolted on through the holes. Maybe even brass sheet metal. Decorated etched brass sheet metal strips.

Each pair of sides takes 1h45m to cut.

Some interesting tips I learned while making (and failing to make) several of these: apparently when the laser cuts through wood, it generates a lot of carbon particles. Running the gas constantly helps blow the carbon particles out of the cut, but if the laser doesn't cut all the way through, a second pass doesn't do much. The problem with a partial cut is that the carbon particles tend to stay stuck in the cut even with the gas on. During the second pass, all the laser is doing is heating up the carbonized wood rather than cutting through the rest of the thickness. Therefore, it's important to turn the laser speed down to make a full cut in one pass. Keeping the speed of the laser high and making several passes doesn't work on wood. I had to use a speed of 0.2 (approximately, but not exactly, 0.2 inches per second).

Considering that each side is about 15 inches long, and considering all those cutouts, you can see why it takes so long to cut.

Also, for some reason, the Trotec 500 has a problem with speed settings between 1.0 and 2.0 (not inclusive). Some fine programmer apparently felt that these speeds should also result in the laser power being turned down. So avoid those speeds.

If the ShopBot at NextFab had a drill attachment, I could probably cut this out on the ShopBot a lot faster. I should ask if they can get the drill attachment. The problem is that the standard spindle on the ShopBot rotates too fast for drilling, and cannot be slowed down to drilling speeds.

Noise margins in the Logical Engine

Here's an explanation of how I implemented noise margins for the Logical Engine design. Recall that rods have holes for bumps every 0.3125 inches (5/16"), and that the spacing between rods is 1/2" vertically and 0.625 (5/8") horizontally. Also, I am using 1/4" hex standoffs that have the following measurements:



Here's a setup which will illustrate noise margins. The logic here is two inverters, but that doesn't really matter.


The blue bumps are the bit bumps, while the green bumps are the sense bumps. The hexes are aligned so that the pointy end is along the direction of motion.

Here are the measurements of the bottom pair:


The key measurement is the distance between the flat end of the bit bump and the pointy end of the sense bump: 0.0432". The geometry of the second pair is identical, so the same measurements apply. So if the bit bump on rod #1 were to be slid in front of the sense bump...


Then rod #2 will only be able to move that much, 0.0432":


And now, if we slide rod #3, since the bit bump on rod #2 isn't in the way of the sense bump on rod #3, rod #3 can slide the full 0.3125". Or can it?


Zero clearance! So if the sense bump on rod #2 were just a little thinner, rod #2 will move that much more, and then the sense bump on rod #3 will smack right into the bit bump. You might figure that because the sides are sloped with respect to each other, it's not such a big deal: rod #2 gets slid back by the force of rod #3.

But that's not what noise margin is about. The idea is that the bit on rod #2 should stop short of rod #3's sense bump's path, so that any noise in the movement will not affect the result.

By making the bit bumps square, we solve this problem:


And so the noise margin for a zero bit is 0.0193". The noise margin for a one bit is, of course, 0.125", being half the width of a bit bump.