In this picture you can see the finished product. A USB Interface Board is hot glued to the outside of the typewriter, where the USB cord can be plugged in easily. The Interface Board is connected via 4 twisted brown wires to the Sensor Board, which is mounted onto the crossbar with hot glue. Flat metal contacts reaching from the Sensor Board wrap tightly around the crossbar, and are positioned underneath each typewriter key. 

There are also three reed switches (a type of magnetic sensor) on this machine, used to detect space, shift, and carriage return. Each of these reed switches has two black wires connecting the switch to the Interface Board. Lastly, there is a single black wire screwed into the chassis of the typewriter (the Sensor Board and Interface board communicate with each other through the chassis of the typewriter -- see the How it Works section). So, that is a total of 11 individual wires, which travels in 4 separate bundles. That is not a lot of wiring, considering the amount of signals we are trying to sense!

The wiring for the usb typewriter is not that complicated -- between 9 and 11 individual pieces of wire will be used - but they travel in pairs, so its more like 3 or 4 bundles of wire.

UPDATE:  I have seen a few newer typewriters that have the crossbar on top of the machine, "under the hood" so to speak.  Check underneath the cover that covers up the ribbons.

Underside of a Smith Corona portable. The green arrow points to the crossbar.

After this step, the keys will be covered in metal and paint shavings, not to mention all the gunk from your typewriters long life.  You don't want that stuff to gum up your contacts, so after this step you should clean the area thoroughly with a toothbrush, a qtip, and a cleaner such as Simple Green.  Clean off the cross-bar while you are at it.  

using a metal file to shine up the keys in the area where they meet the crossbar.

Using the cutting disk on a Dremel to shiny up the keys underneath the crossbar.

The crossbar is generally made of metal, which will interfere with the circuitry of the sensor board if left exposed.  Therefore, we must cover the crossbar with insulating tape.  A single or double layer of Gaffers tape is sufficient for this job -- it is durable, it repositions well, and it doesn't leave a residue.  In a pinch, you could use heavy duty duct tape.

NOTE: on some typewriters, the crossbar has a jagged "cityscape" profile, instead of just being a smooth bar. In this case, you may want to file or dremel the cityscape so that it is more like a gently hilly countryside. This is just so the tape has a smooth, not jagged, surface to stick to.

Wrapping tape around the crossbar

tape is tightly wrapped around the crossbar.

The "feather" contacts of the Sensor Board must be wrapped around the Crossbar using tweezers or pliers. The goal is to have one feather contact underneath each key bar, and to have the feather contacts wrapped as tightly as possible.  I find the tweezers work well for initial positioning of the contacts, and the pliers work well for pulling them snug around the crossbar.  Its ok if the contacts are not super-flush against the crossbar -- you just don't want them so loose they are floating around or sticking up towards the keys.  The only part of the contact you are concerned with is the part that comes near the keys -- that part should be as close to the crossbar (and far away from the keys) as possible.     If you find it absolutely necessary,  very small amounts of superglue can be used to pin the ends of the contacts in place -- just keep the glue away from the part of the contact that will touch the keys.

For this step, its also a great idea to use small dabs of hot glue to fix the board's position. You don't need much glue at all, since the contacts themselves should grip the crossbar fairly tightly.  

I usually cut the length of the contacts down with clippers a little bit after this step -- its not really necessary, but it makes everything look a bit neater, and makes it less likely the contacts will short together.  The technique is just like cutting someone's bangs. (sorry, I don't have any good pics of this step)

Example 1:

wrapping contacts around crossbar

wrapping contacts around the crossbar

 

Using pliers to twerk the contacts into position. Bend/pull/kink them until the part of the contact nearest to the keys does not stick out much.

Example 2:

This device followed the same installation steps as the previous example, except you can see that the contacts are wrapping in the opposite direction (away from the installer.) In this case, instead of being pulled taut, the contacts were fed over the crossbar, then hooked into place with a curved implement, such as an allen wrench or bent-nosed pliers. This method is not preferred, but sometimes it is the only way the board will fit.

UPDATE: I have found that pcb material makes an excellent spacer -- a lot better than wadded up tape. It is just the right thickness, and adheres well with superglue. If I can't find a convenient, stationary typewriter part to adhere this spacer to, I will intentionally jam one of the less useful keys (like the @ key), so it cannot move, and attach the spacer to that stuck key.

using a wad of tape as a spacer to push the crossbar away from the keys.