X, Y, Z Axis details - Part 2

A project log for Monoprice Select Mini Electro-Mechanical Upgrades

Endeavoring to build upon the existing fanbase work of this $200-ish printer.

Michael O'BrienMichael O'Brien 08/27/2016 at 00:594 Comments

Note 1: I capitalize "X-axis", "Z-height" and the like for legibility when scanning/skimming; pedants can hate me later.

Note 2: This is where idealistic, that is mimal change for best outcome, modifications are. For measurements and construction details of the mechanical structure see Part 1.

Disclaimer: Due mostly to human error but some manufacturing tolerances, the following apply.

Let's get crackin'! the simplest two axis to modify are the X-axis and then the Z-axis. Before we start, keep in mind that increasing the travel distance of any axis creates the ability to have the current wiring suddenly become too short. Effectively, you're going to have to re-work a good portion of the machine in order to have these modification 100% usable.

Either way, it is possible to stiffen the Z-axis a bit by boring out the carrier's rear bearing mount, the two mounts for the shaft, and dropping in an 8 mm shaft. Though the most flexion will take place on the front two shafts, the rear shaft can be upgraded to an 8 mm shaft and linear bearing to aid in rigidity.


The graphite injection molded Z-axis-bearing-and-A-axis-NEMA-17-holder is 45 mm tall. It has the sheet steel cover that is a hair over 58 mm tall that bolts to it. If you remove this, you technically gain 17 mm of z-height travel. At this point, there is 25 mm of threaded rod left before you hit the coupler. Cut, file, or grind off another 8-9 mm of the back chassis's frame, re-position the z-height limit switch by 1" or ~25 mm, put a 1" spacer under the tower, and you just increased vertical build height by ~20%. In you want more and have a greater mechanical inclination, you can buy replacement Z linear rods and a threaded rod for the Z-axis drive for less than $25.


The X-axis is a different story. Both X-axis rods are friction fit into the graphite composite pieces and have a little bit of glue/sleeve lock/retention on two ends. With the help of some elbow grease and vice grips, I can matter-of-factly tell you you that the rods are 209 mm and 287 mm long. The top rod slides through 2 holes thus provides alignment for the bottom one to a degree. Their total is just under a 500 mm stock length.

McMaster's next size is 2x the length, 1.2 meters and though it's expensive compared to the printer at ~$32, it's not that bad. Now and 8 mm rod will be stiffer, but also heavier and the Z-axis carrier doesn't have enough meat to bore out the holes. Given the machine we have, I'd say that if you want to expand the X-axis too much beyond 200 mm, you'll need the rigidity of 8 mm rods for this gantry. The added weight will then necessitate at least 1 of the 3 Z-axis rods being switched out to 8 mm as well to counter the flexion.

Z-axis Carrier

There is one problem though. The X-axis carrier is only 17 mm OD for the bearing retention portions of the composite. On the Z-axis composite, the front two are ~17-17.75 mm OD, but the rear is 20 mm OD. You'd get a 1-1.375 mm wall on the front two if you wanted to put an LM8UU/LM8LUU in there but the rear bearing retention portion would give you about a 2.5 mm wall. BUT, the rear Z-axis linear bearing mounts are 11 mm thick, not 9 mm and the rear A-axis bearing retention portion of the composite is 20 mm. In other words, swapping out and drilling out the single Z-axis linear rod from 6 mm to 8 mm is feasible without facing integrity.


As soon as you start talking the extension of the X-axis or Y-axis though, you have to address the drive problem; the timing belts. I'm going to default to McMaster again on this. It appears that "MXL" is a designation for miniature extra light which is a designation for tooth pitch and that the number is supposed to reflect the number of teeth thus 236MXL means 236 teeth at 0.08" tooth spacing. McMaster's neoprene belts have more teeth that this, but their urethane belts are just fine. So as such the 235MXL should be a rather suitable replacement for the stock belt(s), which 2 small catches.

It appears that the stock belts are 5 mm tall or about 3/16ths. McMaster only carries 1/8" and 1/4". If you're creative and crafty enough with a razor blade/knife you can chip away the the X-axis carriage to accommodate the larger belt. BTW, the Y-axis requires no such accommodation.

The second one concerns the Y-axis. The urethane belts are "dust free" and due to the advertisement of Kevlar reinforcement, I picked up the urethane belts. the fun part is the the urethane has a very low surface friction and is also a softer hardness compared to the stock belt. This is not an issue for the X-axis, but its a problem with the Y-axis being a cut and zip tied. The teeth like to slip past each other despite a strong zip tie clamping force once the belt is tensioned. It is disconcerting to have dust from the neoprene belt, but it's something I'd suggest we all live with for the time being.

X-axis Modification

I'm going to start small and consider going to ~200 mm for the X-axis. With crude arithmetic-backed guessing the 320MXL025 will give me a travel distance of ~211 mm. After additional guessing, I get 373 mm & 295 mm lengths of rod required, or 2x 400 mm lengths and trim to fit. Here is a quick video if you want to see the completed assembly after I tweaked things.

Final travel distance is ~221 mm, after assembly with the Ultimaker Belt Tensioner and adding some washers in the end cap to get the tension adjusted better. I advise 297-298 mm for the short rod and 365-366 mm for the long rod for final measurements. I'm using blue thread locker, Loctite 243 specifically, on the rods since my sleeve retainer seems to have gone MIA. After letting things cure for a few days, I don't expect to get the rods out without destruction of the carrier as the composite was partially plasticized by the Loctite, as expected.

With longer rods I will need a new bed else I don't have a larger build space ;). This is something I'm looking into for a simple solution so we'll see what happens.


U.S. Water Rockets wrote 09/23/2016 at 16:55 point

Have you done any investigation into the Z-Axis wobble that some people report? Some of the images show a very bad wave. Mine doesn't exhibit this, so I can't troubleshoot the issue.

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Michael O'Brien wrote 09/23/2016 at 20:06 point

In part one I mentioned the mount brackets. Yes, the rods wiggle, but that is axial play. There is almost no radial play whatsoever, which would cause that waviness people are seeing.

I think it's coming from a combination of print speed and belt tension, but the actual problem is the stock pulleys. Out of the 5 on mine, only 2 have a centered center hole and spin without a wobble.

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U.S. Water Rockets wrote 09/23/2016 at 21:08 point

Are there any good and cheap replacement pulleys out there?

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Michael O'Brien wrote 09/24/2016 at 18:13 point

don't know yet

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