For at least the last few months, and possibly since the laser was moved to the current Colab 3.0 location, we’ve seen a slight wiggle in certain lines that should have been straight. The problem often surfaced in laser class. My standard class demonstration job is to cut out a half-inch square from 3mm baltic birch plywood. We’d see a slight wave in the bottom side of the square. Changing the size of the square would change the size of the wiggle, often eliminating the visible effect completely. Occasionally other users would report much more severe wiggle effects in particular jobs.
(photo of a half-inch test square with visible wiggle goes here)
I believed this to be some sort of mechanical problem with the laser. Specifically, it seemed like it had to be something that was loose that should have been tight. The things I had checked before today seemed to be fine. Today I had some free time in the neighborhood of Colab, so I made a concerted effort to check everything that could be involved, and I think I now know what the problem is. I don’t know the exact repair procedure yet, but I’m confident it will be repairable.
The problem is in the table mechanism. As you probably know, our machine focuses the laser beam onto the work material by raising or lowering the table, while the laser tube, mirrors, and focusing lens all stay at their fixed heights. You might not be familiar with the mechanism that raises and lowers the table. There are four vertical threaded rods, one near each corner of the table. A stepper motor drives a toothed belt near the floor on the left side of the machine, and that toothed belt rotates the threaded rods on the left front and left rear corners. Another stepper motor and belt does the same for the threaded rods on the right side. A matching nut is fitted to each rod, and connected to the table, so that rotation of the rods is converted into vertical motion of each corner of the table. Both stepper motors move in sync, so that all four threaded rods rotate the same amount whenever the table is to move up or down. Thus the table stays level while moving up or down.
If any of the rods ever get out of sync with the others, the table will be tilted or even twisted, and manual intervention is required to realign the table. This happens rarely, and only when some mechanical blockage prevents one or more corners of the table from moving up or down. In that case, the stepper motor on the blocked side stalls and/or the toothed belt skips one or more teeth. This failure is noisy and hard to ignore. It has happened a few times, but as far as I know it has not happened since the laser was moved out of the original Colab 1.0 location. I’d remember, because the table realignment procedure is a huge hassle.
None of that has anything directly to do with the wiggle, which is an unwanted motion in the Y axis (front to back) and not obviously related to the Z axis (up and down) motion of the table.
Those threaded rods have to stay vertical to do their job, and they have to be free to rotate. So, there’s a bearing at the bottom end of each rod, connected to the floor of the chassis, and another bearing at the top end of each rod, connected to the underside of a horizontal chassis plate near the midline of the chassis. The weight of the table bears down on the bottom bearing, which is supported by the sturdy bottom of the chassis, and has nowhere to go. The weight also pulls down on the top bearing, which is dangling from the bottom of a panel, held in place by a plastic fitting and two screws that connect the plastic fitting to the panel. I suspect the top bearing is merely friction-fitted into the plastic fitting.
As you can see from these pictures of the left-front and right-front threaded rods, the bearing at the top of the right-front threaded rod has escaped from its plastic fitting. The same is true of the right-rear threaded rod. This is not an immediate catastrophe, because the four nuts connected to the moving table also provide some vertical alignment for the threaded rods. The right rods don’t just fall over. However, they are no longer held rigidly vertical. The tops of the right side rods are free to move a little in the Y axis, moving the table and flexing some other components that are nominally rigid.
But why would they move at all? Shouldn’t they just stay put in their intended vertical orientations? There’s no side force being exerted on the table, after all. Or is there? If you’ve ever put a hand on the laser chassis while it’s running a job, you know that it shakes a little. The substantial mass of the moving gantry, and to a lesser extent the mass of the moving head, creates a reaction force that moves the whole chassis whenever the gantry or head accelerates or decelerates. This is made worse by the fact that we usually leave the chassis sitting on its wheels, instead of deploying the leveling feet to make it sit square on the floor. The chassis (and thus the gantry and the head and the mirrors and the lens) shake relative to the table. Because the moving mass is mainly due to the moving gantry, the shake is mainly in the Y axis, and it only happens for a short time after the gantry has started or stopped suddenly.
This explains what we see with a half-inch test square. I almost always use the left-rear corner as the job origin point. That means the head is initially stationary and positioned over the left-rear corner of the square. When the job starts, the head moves to the right (in the X axis). It accelerates, then (maybe) runs briefly at the designated speed (45 mm/sec in this test) and then decelerates to zero again at the right-rear corner. This is only a motion of the head, though, and in the less-flexible axis of the mechanism, so this doesn’t cause a noticeable shake. Next is a right turn: the heavy gantry starts to move, accelerating forward and then decelerating back to zero. This is relatively straight, but shakes the machine. So after the next right turn, the gantry is stationary relative to the chassis but the table is shaking a little in the Y axis while the head accelerates to the left and decelerates to a stop at the front left corner of the square. That makes the bottom edge of the square a little wiggly! And only the bottom edge.
I tried another test to confirm the theory. I wedged pieces of scrap material between the edge of the honeycomb mesh table and the nearby chassis edges, discouraging the table from moving relative to the chassis. I re-ran the test, and the visible wiggle was gone. We can’t leave it wedged, though, because that would also prevent focusing.
I wanted to know how it was possible that the top bearings had escaped from their plastic fittings. The fittings seem undamaged. It seems like either the threaded rod plus bearing assemblies had gotten shorter by about a quarter inch, or else the distance between the bottom of the chassis and the midline panel of the chassis had grown by the same amount. Perhaps the chassis had gotten bent somehow, maybe during moving between Colab locations. I used a handy tape measure to investigate, and found that the chassis was pretty much the same dimensions on both sides. So the theory has to be that the rod assemblies got shorter.
I’m guessing that the bearing was friction-fitted onto the rod, and that it simply slipped down onto the rod somehow. That matches what the photos above show. However, at that point I was out of time and had to put the laser back together rather than tear it apart to find out how the rod assemblies were built. That part of the investigation, and the formulation of a repair plan, will have to wait for another day.
