So as anyone who owns the D9 knows, the Y axis setup is a bit of a mess. Just hold it and put a little rotational pressure on it - no stiffness at all. I saw a lot of people upgrading to hiwin style linear rails, but that seemed overkill to me. They are designed for high force CNC environments, and while I am sure they are better, the existing dual aluminium extrusions should be able to do the job. With extra gantry plates built in, anyway. I had a bunch of parts for the openbuilds style mini v gantries hanging around, so I decided to mod them in. You can buy from openbuilds themselves, or go with cheap chinese clones from aliexpress etc, but bear in mind the bearings on the latter are much lower quality. Still, they do the job, and are the same bearing quality as Wanhao used for their own wheels...
Huge improvement. I havent done a great deal of printing yet since installing these, but I thought I'd upload now and let people play with it.
OK so details: You'll need to drill four holes (each one through the two layers of the base at this point), suggest 5.5mm, in line with the middle of the rails. 20mm between holes, and I put the first about 66mm from the end of the plate. I think perhaps I should have put them a bit further in, as I might have lost a few mm of travel as a result. Still, not a big deal, I'll probably just end up moving the two aluminium rails away from the LCD screen a little to give me the extra travel. However, I strongly recommend you take your printer and note the exact location of the plate when it is homed. Mark and drill with that distance in mind, so that the gantry wheels dont exit the aluminium V extrusion or you lose some off your 300mm built distance. Oh and while I did two holes per gantry, a single hole in the middle would also work, albeit not quite as stiff. Perhaps easier though. And do I need to say ... measure, mark, use a sharp point bradawl etc to make a indentation, drill with smaller bit first, then drill 5.5mm? I'm sure i dont need to say that do I :).
Because of the location of the limit switch, it will also need to be moved. I've added a little holder that sits on top of the plastic stepper holder. Getting it in place with a t style nut underneath is quite the fiddle if you dont want to take everything apart, but is possible (curved tweezers or just a bent piece of plastic, tack nut in place with superglue or whatever, offered up). Then there is the prod bit that needs to be added to push against the limit switch. I had cheap nuts hanging around that I designed to slot these in, you probably wont have them so I cut the slot out and uploaded as a second design. Finally, I have to be honest, for the switch prod bit on my own version, I had to cut a section off one end to make it fit properly. I've ballparked it for this model, so hopefully it will work for you, but let me know if you need it modded. Not a complex part though, lots of simple ways to do this!! Addition for mine, I took the end that pushed against the switch and added a layer of superglue, let set, to produce a harder end. Probably unnecessary but hey.
Lets see what else.... 20mm screws to hold the gantry plates should work. I have to cut down some 25mm as I had nothing available. 35mm I think for the limit switch? Not sure the limit poking bar thing though, maybe I used 25mm? Easy to measure them all though. Thin cuts you can see on the base of the gantry housing/spaces are just to release stress from printing in ABS or whatever, reduces tendency to warp and peel up at the edges. Goes against the bed when printing, obviously. Ignore thingiverse orientations. Also, cheaper Chinese gantry assemblies can use screws that extend just a tiny bit too far, so they can rub on the extrusion underneath. Will be pretty obvious if this is taking place, in which case you'll want to grind/file a tiny bit off the end. Generally doesnt need to be too much. Also a problem on some of the screws Wanhao used on their wheels, one they didnt fix...
Final comment, if you think the rails are not stiff enough then there are two other things you can do. Either get hold of some epoxy resin, as thin (low viscosity) as possible, and pour it into the unused grooves of the rail (top and bottom, although you could probably add a small amount into the sides the wheels roll on). Dont let it drip out the ends, and think of some way to keep the screw points clear, of course. Add chopped carbon fibre, glass fibres, sand etc to make it even better / stiffer (look up "epoxy granite" if you are unfamiliar with this technique). This will also produce a nice damping effect on the rail. Alternatively, and I might do this at some point, consider supporting the rail underneath with struts/rails extending out from the outer frame. Or second rails directly underneath the entire length (epoxy fill the void between both?). Not too difficult, just need brackets and something to use. The alu extrusions are super cheap.
Other D9 upgrades of mine:
Link to OpenBuilds mini v gantry:
Much cheaper clones available from China, but if you have the $$ then support US business!