This is probably one of the most useful yet simple designs I did for the ultimaker, together with the XY bananas blocks (you can get the full list of improvements on http://www.tridimake.com/2014/01/features-and-improvements-for-a-homemade-ultimaker.html) It makes life easier for sure. Fine-tuning the level of the Z end switch is a chronic issue, especially when you switch among different nozzles or bed materials and thicknesses. In this design, the carriage slides tighly in the two concave rails of the outer frame, which in turn is fully secured on the case by four screws: there is NO way the carriage can move otherwise than vertically as it should. Neither the carriage nor the microswitch do touch the plywood for a fluid movement. Most of what I have tried so far proved to be unreliable in this regard, probably because there was always something that (tried to) sled against the internal plywood wall. The only parts here that rub against the plywood are the fixed parts. I designed it for an Ultimaker, but it certainly can be adapted to (some) others. The openscad source code is included as usual.The main screw I used a long M4. Lock it tightly around the top rail frame segment either with pairs of nuts and counter nuts, or glue them where they go after you damage the thread a bit (!). Both solutions are better that using nylocks in my opinion, as they will finally slip anyway in the long term. You may like better an M3 screw (less travel per turn), in which case you'll have to change the value in the openscad source code. Or tell me and I can make it customizable. In my opinion, it is a good trade off when you change your printing bed often, because it's faster to tune with an M4 than with an M3. When you keep the same bed, an M3 may be more appropriate to (extreme) fine-tuning. I used my thumbnut for the main screw, it can be customized at http://www.thingiverse.com/thing:70494 I chose a small diameter and small wings so that I can turn it quickly between the thumb and index fingers. How to mount the rail frame The nut holder (http://www.thingiverse.com/thing:44118) was quite useful to help fixing the Z adjuster rail on the printer frame. Just make sure how you mount the adjuster: the head need to protrude from your frame of course (or use an extra long M4, or a even a threaded rod cut at the appropriate length). MIne goes up a bit too much I guess. FInally, the carriage freeplay can be obliterated thanks to the fixing oblong holes of the frame. Better screw each horizontal pair while the carriage is between them, so that you know how much pinching you need (none was required in my case, it is already tight by design). Now, that's 4 more holes in your frame though! I did not measure the screw length, but they are quite common and you'll guess easily. Hope you like it and that it makes your hobbist life easier :)