Hey guys, In the process of assembling my Z gantry onto the two 7/16 x 7.5" steel rods I found that I could barely push the z gantry down to the bottom, it was extreemly tight. :x Next I popped on the other part that holds the free ends of the two rods and finally got them forced in, and the z gantry would not move by hand at all it was jammed against the outer insides of the plastic bushing tubes. I took everything apart and checked the rod hole location with a digital calipers, they were both off by about 1/32 (1/16") total when compared to the plastic tube measurements in the z gantry. The holes in the upper and lower rod holders seemed to be too far apart, so I dremeled them inward about 1/32 and everything went together much better. However now that i have the ZY gantry complete the Z gantry seems a little loose and sloppy in the plastic bushing tubes, so I am wondering if that will affect my cut accuarcy. I was wondering if I messed up and the rods and z gantry were supposed to be very tight and that the rods would wear into the plastic bushings over time? It seems like it may be worth changing the plastic bushing tubes to brass bushings in the future, but I am not sure.
I believe the holes are supposed to be slightly inward to compensate for the difference between the ID if the PEC tubing and the OD of the guide rods. Are you building a Premium kit, or a Bonez kit? If it's a Bonez kit, my first guess is the tubing you have is too close to the exact diameter of the guide rods. More info from you is needed, I think. As an aside, and a personal comment - Many guys are (in my opinion) mistakenly installing precision guides and rods and bushings, but they are forgetting the vast majority of the structure is wood (MFD actually). That means, if they DO manage to get everything working smoothly today, within a week (as the temperature or humidity changes) their machine is going to start binding up again, because the wood will shrink and grow. Mark went through a lot of prototypes before reaching a compromise between precision and "good enough" which allows an "inexpensive" wooden 3-axis router to work. In other words, be careful what you change. As you may have noticed on my posts, I have changed the drive screws and added a Nylon (i.e. soft) bushing at the top of my Z-drive screw, but those are the only changes to the drive system that I believe will not give me grief in the future. Now that I've changed the z-drive to a 3/8 Acme thread, even the Nylon bushing is gone.
Thanks Dorsal, What you says makes sense, just like wood doors in a house the fits will be different depending on humidity. I have the premium kit, so the tubes came with the kit. I already finished the YZ gantry assembly and sanded the rod holes to make the Z gantry move easily, there is a little slop now because of the gap beween the tubes and the rods, but it seems like it will be fine for cutting foam. For cutting plywood and closer tolerance parts I might have to figure out some way to tighten things up, but I'll finish the rest of the build the way Mark shows it. The only mod I might do is to make the top plate removable so I can get at everything inside a little easier.
Yeah, I made the top plate of the Z-gantry removable, which made things a LOT easier when I changed to the 3/8 drive-rod. And having the top plate of the entire pp box removable has really made all my mods possible - I'm not very good at working on everything from the hole at the bottom.
Back in the Rcgroups build thread, there were other guys including myself that was having the same problem. It's just the tolerances on the pipe and the rod vary. What you did is fine. The slop you created will probably show up a little as wobble when you go to cut. If your just cutting out foam it will probably be fine though.
meistertek, It's not a percision machine as cnc machines go, but you did do the wrong thing. The tubes are oversized, so to make it work the rod pair is designed to work together, pressing outwards. The bad thing about this design is the tight fit is the only thing stopping for/aft play of the z-axis. So it's a compromise between friction and play. Mine was also very tight, to the point where; when it was assembled, it couldn't move. What I did was sand the rods, which is good anyways as it reduces friction. I figured I only reduced the total distance by 0.01" (I think, like reducing the diam of each rod by .005") was enough to have a good fit. But even thought I did a pretty good job on mine, I am now tearing it apart and adding bronze bushings. Making to top removable, also, so I can replace the z-axis if needed. -Kwok
another one with same issue :roll: after one year of your experience, what is the best solution? working on rods or working on holes?
I had the same problem. I used bronze bushings though so I don't know if it will apply here. . When I changed my Z axis to Acme rod, I ended up making an oval shaped hole for the bronze bushing on the bottom of the Z gantry ( that's where it was binding the most). I then made a cap to capture the bushing, but still allow it to float in the oval. Smooth as silk now, and almost no slop. There are pictures in a thread here I can dig up if your interested.
Looks like we will have to revisit the original design and see how we can remedy this issue. This is something along with other things I would like to look at again and see if it can be redesigned without having to raise the cost of the machines. Mark
Thanks for the new replies, I'm under level 1 so I would like to keep things as they are, but going to surf the other relevant threads. The "outwards" of the holes looks like a good idea to gain precision at low cost , currently sanding the rods is giving some improvement so going on sanding or looking for a 11.00 mm rod instead of the current 11.11, maybe easy to find here in Italy
The perfect upgrade i believe Mark would be to cut the holes in line instead of skewed and supply the bronze bushings and linear rods, it will drive the cost up a bit but 80% of the people do this anyways, so it would actually save the buyer cash in the long run.
