Adding 3D Printed Z-Axis Rod Stabilizers

A project log for Monoprice Select Mini Maximum 3D Printer Mods

Low/zero cost upgrades for the Monoprice Select Mini 3D Printer.

U.S. Water Rockets 09/16/2016 at 19:041 Comment

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While disassembling the printer to add some accessory wiring (to be explained in a future project log) we observed that the Z-Axis linear rods were somewhat loose. After inspecting the assembly, it was apparent that the 4 brackets which hold the rods in place at the top and bottom of the printer have a hole which each rod seats in which is slightly oversize. This is likely to insure that variations in part tolerances will not prevent the parts from fitting together.

The downside to the oversize bracket holes is that it allows for some play in the rods, which is magnified the farther along the X axis the hot end moves out. The gantry acting as a lever multiples any play in the rods. This issue can lead to defects in prints. Further compromising the design is the discovery that the rails are cut slightly shorter than the distance between the brackets, This is also obviously done so that the rails could be cut with loose tolerances and would always fit in the printer, but the downside of this approach is the rails can also move up and down in their brackets, as they float freely in the brackets with a small gap on the top and bottom. The combination of loose tolerances in the brackets and rods also contributes to making the printer noisy, as these loose rails tend to rattle under certain conditions.

Our solution was to print some stabilizers which would fit under the existing brackets and through the inclusion of a tapered hole for the rods, the rods would be centered in the stabilizers and held firmly in place. These stabilizers were designed and printed but upon testing we discovered that variations in the lengths of the rods and possibly in the bends of the sheet metal enclosure would lead to different length of rod protruding into the tapered seat, so longer rods would push farther in than shorter rods. This causes the shorter rods to be held less firmly than the longer rods, so the stabilizing was not uniform depending on rod length.

Our second design solved the issues we discovered. The improved stabilizers eliminate the tapered seat for the rods and have a feature on the outboard side of the stabilizer which is designed to flex outward when the rod is inserted. This feature is tapered in a way that causes it to compress very tightly around the rod when the clamp is installed.

To install the rod stabilizers, we had to remove the covers from the bottom of the printer and rear of the tower. Then we removed the M3 Cap screws which hold the rail brackets to the top and bottom of the tower. There are two brackets with 3 screws and two brackets with 2 screws.

We also removed the Z-Axis stepper motor by loosening the grub screws holding it to the threaded rod for the Z-Axis, and the two screws holding the stepper motor in place. This let us slide the gantry assembly out of the rear of the printer and slide the rails off the brackets.

Installing the printed stabilizers was very straightforward. The first step is to slide one of the brackets over the rod. The next step is to push the rod into the stabilizer through one of the rod openings. The rod should be hard to press into the hole, as it has to spread the opening to pass through. This is normal. The stabilizers only go on one way, so make sure you are pushing them on with the clamping feature facing the bracket. The third step is to realize you forgot to put the bracket on and so you have to remove the stabilizer and then start over with the bracket.

With the stabilizers in place, you can slide the brackets down and in place over them. The brackets have tight tolerances with the holes in the stabilizers, so make sure toy press them straight on. They should snap in place against the brackets with a little effort.

Now, you can reinstall the assembly in the printer tower. When you tighten the cap screws on the brackets, the clamp features on the stabilizers under the brackets will constrict tightly around the rods and secure them from moving.

You can now reassemble the rest of the printer and you should see less play in the tower!

Congratulations! You've just made a huge improvement to the Z-Axis stability for a few pennies of filament and a little of your time!


Heroic.Doses wrote 09/17/2016 at 19:37 point

This is great! Will print this soon as I get home!

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