If you've ever thought, "Gee, these towers would be a lot easier to align if I had several more arms," this is for you! Once installed, it will pull the towers as close to the center of the bed as possible. Friction keeps the towers from moving up or down, and will greatly reduce the tendency of each tower to "walk" as you torque down the T-slot nuts.
NOTE: I've since switched to using band clamps. They're more expensive, but also a lot easier to use, and you can have them ready before your printer is assembled: http://www.amazon.com/gp/product/B0000DD1P8/ref=wms_ohs_product?ie=UTF8&psc=1
Otherwise, read on!
I got the original idea from someone else on the SeeMeCNC forums. I forgot his name. (If that was you, let me know and I'll credit you here.)
I have a backburner project to adapt these clamps so that the threaded rods sit completely outside the build volume, so they can be left mounted permanently. This is not just good for keeping it aligned - it will also make the structure MUCH stiffer, which is good if you want to do some serious high-speed printing. Other projects are a higher priority for me right now, but if some people ask I'll set aside some time to finish the permanent clamps.
You will need six footlong 1/4"x20 threaded rods, twelve 1/4"x20 wingnuts, or similar. You can get them from http://mcmaster.com or from most hardware stores. Finally, get yardstick/meter stick to set the height of the top plate correctly. Don't leave this out!
The tower clamp has two features you will need to pay a little attention to, so print one and test it before you print the rest.
First, there are a couple of "bent fingers" at the centerline, roughly between the thumb grips. When the wing nuts are tightened down on the threaded rods, those fingers are what keep the clamp from bending and breaking. The tolerance is fairly tight. Make sure that they don't meld together while they're still hot. Adjust your filament flow multiplier as necessary.
Second, there is a little clip that has to be fitted onto the end of the clamp after it's installed. There is a little shelf at the bottom of where the clip sits on the clamp, which will keep it from falling off. Look for that and make sure it isn't upside down. Make sure that it's loose enough that you can install and remove it without undue force, and tight enough that there is no play. If you install the clamp and it freely slides up and down the tower, it's too loose. If the fit isn't perfect, adjust your filament flow multiplier. (If you've got it printing the "bent fingers" correctly, it's probably already close, if not perfect.)
Oh yeah - print at least one extra in case something breaks. Also, I would use a minimum of 20% infill for PLA and 30% for ABS. You will be putting considerable force on these clamps, so more is better.
The heated build plate and EZStruder should NOT be mounted - they will get in the way. Make sure all the T-slot nuts (top and bottom) as well as the vertical screws on the top idler mounts are loosened. You can let the print head sit on the bottom plate - there is no reason to dismount the carriages or anything else.
Once you have followed the above steps to make sure you are pritning these right, take three of them and install the threaded rods and wingnuts as shown in the photo. Fit one of them in the middle of one of the towers. It will wind up at the bottom - just not yet - so DON'T install the clip just yet. The holes the threaded rods go through are bigger on one side than the other so that you can move them up and down. You will have to do just that to get the second and third clamps installed. It will take a little messing around to figure out how to do it.
Once you have all three clamps installed, slide them to the bottom of the build area, install the clips, and then tighten down the wingnuts. (If you try to tighten down the wingnuts first, they will pull the jaws of the clamp open and you won't be able to fit the clip.)
Then, repeat the process and secure the clamps as high up as they will go.
Once the wingnuts are tightened down, put a screwdriver between the bottom of a tower and the idler directly below it and (being careful not to crush any endstop wires) use it to lever the tower up until it's level with the little line engraved on the motor mount plate. Repeat for the other two towers. Tighten down the T-slot nuts around the bottom. The same process will work for the top.
Your Cheapskate carriages will be about halfway up the towers. This turns out to be really useful because of the cone-shaped spacers on one side. (The eccentric cams are on the other side - don't use these, they won't be even enough.)
Sit a yardstick/meter stick on the bottom plate next to one of the towers, with the wide edge of the ruler resting against the screw that holds the cover bracket in place. The narrow part of the middle of the ruler will be pushed up against the two cone-shaped spacers on the carriage. Since there are two of them, one directly over the other, the ruler will be perfectly vertical as long as it's touching both of them. The top of the ruler can be allowed to bend a little to clear the top plate. (The bending is OK because you're going to make sure to seat the ruler against the screw on the bottom and the spacers halfway up in exactly the same way for each tower.)
Look at where the bottom of the top plate touches the ruler. Use a screwdriver to lever up the top plate until it's even with one of the fractional marks on the ruler and then make note of that mark. (Or, push it down with your hands and then lever it up to just the right height. You will need to lever it with something once the wing nuts have been tightened.) Tighten down the idler mounts on the top of that tower, but not the vertical screw.
Move the ruler to the next tower, being sure to seat it against the screw and the two spacers as before. Then, do the final tower. That done, you can tighten down the vertical screws on the top idler mounts.
When you're all done, remove the clips to free the clamps and squeeze the thumb grips to release them. Leave the clips and threaded rods assembled - you might need them again, especially if you move your Rostock around.