i already have a 50mm? idk how big it is but anyway i wanna put a big 120mm fan in there instead. what should i do to properly cut the plexi and make the fan fit perfect?
I think the only way you're going to get decent results is with a hole saw. If possible, use it with a drill press. If not, a hand drill will do. Either way, go very slow and don't put too much pressure on the plexi. Since you already have a hole, the arbor will need something to drille into. If you have a piece of scrap plywood, that'd be perfect. Then, clamp the plexi to it and you're good to do. If you do it right, you should be able to get rid of the 80mm (I'm guessing) mounting holes too. To mount the 120mm, just drill new holes (again, going slow and applying little pressure). After the cuts, you might want to tape off the window and sand/polish the edge of the hole.
sharpy, 120m fan to trace, razor, plastic grill cover ( allows for 1/4 inch of error).
for a cheap hole saw you are looking at 25$ up to 50-80 for a decent one. that size.
i drop 120's in many cases i bought 20 of them for 4$ each a while back. i use them for everything. i cut my first hole in my case with a extra large flat head screw driver and a hammer on a peice of plywood. you put a plastic grill over it you cant tell a thing.