Adjusting Gantry Roller Spacing

There needs to be a way to adjust how tightly the gantry is held to the rail. I find that objects that have geometrically complex outer surfaces exhibit very significant ringing, which mares the surface quality in those areas.

The photo I've enclosed below is a 2 X 1 inch closeup of just such an object. (Sorry, this object is printed in black. So the contrast is very high.) The section shown shows a cross-hatch pattern that involves many tight curves. While laying down a bead in these zones so much bounce builds up as the gantry makes the turn that it takes almost an inch of relatively straight travel to settle back down. This results in the entire surface in this section of the print to be completely covered in waves that aren't in keeping with the macro design intent.

The gantry is held in place along the length of the rail by 4 roller bearings. Two of the rollers ride along the slot on the top of the rail, while the other two fit into the slot opening on the rail bottom. The rubber rollers have a 45 degree bevel that matches the bevel of the rail's slot. The contact profile of the bearings seems to be well matched.

Yet the current design doesn't appear to include a means to adjust the spacing between the top and bottom rollers. At least on my rig, the rollers don't pinch or squeeze the rail tightly enough. So there's three types of slop in the movement of the gantry. The first type of slop allows the gantry to move ever so slightly forward (away from the rail) and back (toward the rail). The second type results in the gantry tilting forward along the X axis, like a braking car. Finally the third type is rocking on the X axis of rotation as it moves along the rail. All three forms of slop are most evident when the vector of movement changes (the gantry changes speed or and even more so when it changes direction) and when the vector of extrusion changes (the bed changes speed / direction).

A balance needs to be struck in finding the optimal distance between the top and bottom rollers. Too loose and you end up with the slop that causes ringing. Setting the rollers to pinch the rail too tightly will cause drag, which could show itself in a number of ways including accentuating belt slop. But not being able to adjust roller tightness leaves the current design without a means to manage the inordinate amount of ringing that I'm seeing in my prints.

- Bob
On the 1.5+ model, the lower two rollers are mounted on eccentric spacers. You can turn those with a wrench to tighten the rollers on the rail.
@nopick: Thanks for the tip about the eccentric spacers. I bought my unit ready built by gCreate. So I'm probably not as familiar with the mechanics as you are. I've inspected the gantry but haven't been able to locate these spacers. Can you tell me where I'd access them and how I'd adjust them?

And the plot thickens...

While looking around for the the spacers I found another condition that's probably contributing to the ringing. I inspected the rollers that the X axis rail rides along as it's moves vertically along the Z axis rails. There are 3 rollers on each Z axis rail. What I'm seeing is that the top outside rollers aren't even touching the Z axis rail bevel. The 3 rollers bearings on each Z rail form a triangle that's meant to lock the X rail "pitch". However, with the top roller not even making contact with the Z rail, the X rail is rocking (rotating) back and forth during printing. Even though the bottom outside roller and single inside roller appear to be making firm contact with the Z rail, it appears that this loose connection between the top roller and outside Z rail slot allows for even more movement than I'm seeing with the gantry rollers. This could obviously be another possible cause of the inordinate ringing that I'm seeing. To demonstrate, I have a video snippet of this movement as I manual grip the X rail with my hand here.

Are there mechanisms I can use to adjust the alignment of this assembly both to rotate it to bring all 3 rollers into vertical alignment with the Z rail and to tighten the spacing between the inside and outside rollers so that they all make solid contact with the Z rail slot bevel?


- Bob
Same thing on the Z rollers. There is a bolt holding the wheel onto the structure. At the end of the bolt, on one side of the wheel, there is a nut. On the other side of the wheel, there will be a place to use an open end wrench. That is the eccentric spacer. When you turn it, you will notice the wheel moving toward or away from the rail. Snug the wheels to the rail but don't over tighten them. On axis that have two wheels on one side on one on the other, try adjusting the single wheel side first to see if that corrects the problem for the other two wheels. If it does not, snug the other two against the rail.