I've been looking for a Y-Belt tensioner for the AnyCubic i3 Mega for a long time so I finally designed my own.
The base mounts with two of the bolts for the Y-rods to avoid having to drill holes in the case to mount the base assembly. (the original M3x8 might work, but I used longer M3x12)
The idler pulley should be mountable with original bolt (M5x16) and nut but it is very close. M5x18 works better but is not a common length. M5x20 is just a little too long and scrapes the case. I plan to make another carrier without the indent for the bolt head to see if I can get a common bolt to fit perfect, but there have also been reports that the original bold has worked fine for some prints. I am actually using the original bolt and no nut, and it has been fine, but securing it with a nut would definitely be better.
The tension bolt should be an M5 hex head cap screw, with the head going inside the carriage and the nut in the knob. I used an M5x35, and previously recommended a 30mm instead, however 30mm is a bit tight if your belt is reasonably taught with the stock mount, so it's better to use with a 35 unless you mind the end of the bolt poking out the nut head in front of the printer.
The pulley carrier originated from https://www.thingiverse.com/thing:2482051 - "Anet E10: X belt tensioner by MrCrankyface". I have added the hole for the nut and enlarged the hole for the tensioner bolt to fit M5 instead of M4. When inserting the pulley, it is very tight, but as long as you don't tighten the bolt too much it will loosen up.
V1 - 20180210
- The bolt hole doesn't go all the way through the front pate, only half way.
- Printing the knob in the default orientation without supports caused some in-the-air printing for layers 38 and 39 as it transitions to the shaft.
V2 - 20180217
- Fixed the bolt hole in the base.
- Modified the bottom of the nut hole in the knob to make printing easier without supports
- Chamfered the outer edge of the nut hole in the knob to make it easier to fit the nut.
V3 - 20180313
- Replaced original 123d file with correct one to match v2 stl files.
V4 - 20180403
- Model is unchanged, but updated notes about the tension and pulley bolts.
v5 - 20190217
- After a year of use, mine is starting to show cracks in the cross braces of the open base.
Mine was printed in PLA @0.2mm layer hieght with 25% rectilinear infill. Using a denser infill or stronger material would help, but I have also uploaded a solid version, in case anyone wants to use it instead.