Ok I'm writing this all for a second time as I accidentally deleted evrything. Noooooooooooo. This mod doubles the X Y axis’s accuracy on Reprap Mendel. It does this by creating a double pulley system. I have noticed that there is significantly less lash on my x axis now and everything seems to move more sharply. Another benefit is this mod allows your machine to run at exactly the same speed as before. http://youtu.be/vVHvbCfxZdU You can see the first test here and get a good idea of how it goes together. The only difference now is the Y belts brackets have all captured knuts and countersunk bolts. Instructions. Hardware. 12 M4 by 20mm countersunk bolts. M4 studding. 4 624ZZ bearings. (The same ones used on all the bets.) Some longer M4 bolts. M4 standard and lock knuts. M4 washers. T5 belting 5m long. M3 knuts. Open T5 belting can be obtained here. http://www.beltingonline.com/t5-standard-breco-open-length-timing-belt-1738 The bearings fit in to the bating brackets with an M4 washer wither side. An M4 bolt is then passed through with a washer either side and the whole lot is clamped. It is best to not tighten the Y axis baring clamp until it is bolted down or it can make it hard to get the base bolts in. You should be able to reuse the old X and Y clamp bolts. The Y axis baring bracket has 4 M3 knuts captured in it. The X axis belt clamps need 4 M4 studs made about 85mm long. They screw in to the captured knuts that are next to the z screw knuts. Don’t over tighten them agist the other knuts or you can break parts. And when winding in lock two M4 knuts against each other. DO NOT USE PLIERS. The blocks then go on so that the belt can pass through them freely. The Y axis belt clamps assembly has 6 M4 capture knuts in each side. These are standard knuts not lock nuts. This is all held together with the countersunk screws so as to keep the profile as low as possible. The belts go round as normal but where they used to be clamped they go round the new baring brackets and back to the new clamps. Pull tight and clamp in line. IMPORTANT. After installing you must exactly double the X and Y steps per mm in the firm where or it will only travel half the required distance.