Thursday, September 7, 2023

A few updates to Arrakis 2.0

Arrakis 2.0 recently developed a minor problem- the ball started shifting while it was drawing patterns and it started making unusual noises. When I opened it up I found that the pulley on the left side motor had come loose, but wasn't completely rotating on the motor shaft. It would rotate a little with some reversals of direction, galling the motor shaft in the process. 

I had used some locktite on the set screws for the pulley and couldn't get them to tighten, and could barely get them loose enough to get the pulley off the motor shaft. I installed a new pulley which quieted the mechanism back down and I decided to address a couple things I had been thinking about for a while.

The homing sensors were mounted high on the motor mounts and were easy to break if I was taking the table apart to transport it. I wanted to redesign that part of the mechanism so they sensors and flags could be positioned where they would be less likely to get broken.

There were three parts that needed redesign/print. The magnet carriage, the left side Y axis bearing/pulley block, and right motor mount. 

Magnet carriage

The original magnet carriage design had a flag on the top piece which is why the X axis home position sensor was mounted in a precarious position on the right side motor mount. I wanted the sensor to be mounted lower, so I had to redesign the magnet carriage to put the flag on the bottom piece instead of the top piece. I also wanted to make the X home position somewhat adjustable and decided it would be easier to adjust the flag than to adjust the sensor position, so the flag is now a piece of plastic that attaches to the magnet carriage with a screw. The new magnet carriage consists of the old bottom piece that has become the top piece, and a newly designed and printed bottom piece that includes a tab to mount the homing flag.

Original magnet carriage. The projection to the left is the flag for the X axis homing sensor.

The new magnet carriage design. The old bottom piece has become the top piece and the flag (black) mounts with a screw so it can be repositioned if needed. 

Right side motor mount

Now that the flag was moved lower, the sensor could be mounted lower, so the newly designed motor mount includes an extra wall to mount the X axis homing sensor. I mounted the sensor using some industrial double-sided tape. If the adhesive lets go, I'll drill some holes and screw the sensor to its mount.

The original right side motor and mount with the dangerously positioned X axis homing sensor.

The new right side motor mount with the X axis homing sensor moved to a much safer position.

Left side Y axis bearing/pulley block

The original design had the Y homing flag glued to the top of the pulley block and the sensor on the top of the left side motor mount, both in easily bumped positions. The new design has the flag printed as part of the bottom piece of the pulley block, and the sensor mounts on the side of the motor using the same tape I used for the X axis sensor.

The original left side Y axis bearing/pulley block with the Y axis homing flag glued to the top of the block.

The new left side Y axis bearing/pulley block with the homing flag printed as part of the bottom piece.

The original left side motor with the Y axis homing sensor mounted on top of the motor mount where it could be easily broken.

The new position for the Y axis homing sensor on the left side motor. It is taped to the motor using some double sided adhesive tape.

Now everything is secure and in positions that are not likely to get bumped when taking the table apart or putting it back together.

Saturday, July 22, 2023

CoreXY X-Axis Wobble Revisited

A while back I noticed that the X axis would wobble a bit when the magnet in the Arrakis sand table was moving only in the X direction. This got me thinking the same could be happening in UMMD, my corexy 3D printer. I mounted a gauge on the Y axis rails and measured the movement of the ends of the X axis and found slight wobble with a period of 40 mm which I attributed to a a poorly drilled 20 tooth pulley on one of the motors:

One of the results when the cheapo pulleys were used in the printer. Note the 40mm period of the "waves" corresponding to one full rotation of the 20 tooth drive pulleys.

I could see the belt wobble slightly as it went on/came off the A motor (left side) pulley, implying a poorly drilled hole in the pulley. Poorly drilled means either not centered, not aligned with the axis of the pulley, or too large. Any of those conditions could cause some wobble. A poorly drilled pulley acts like a cam and as it turns, it slightly modulates the belt tension. The belt tension modulation causes the X axis to wobble relative to the Y axis.

At the time I built UMMD, Gates belts and pulleys were only sold through industrial distributers who had large minimum purchase requirements. IRIC, I had to buy 50' of belt to meet the minimum sale requirement. I ordered pulleys via Ali-Express or bought them from other sources, definitely not Gates-made parts. 

Recently, someone on a forum mentioned that Filastruder sells genuine Gates pulleys. I decided to order some to see if they were better than the cheapo stuff I used. The pulleys I ordered arrived in about a week. The packaging is unmarked and there are no Gates logos on anything, so I was a little skeptical and a little disappointed. I decided to install the pulleys on the printer and see if they were any better than the cheapo Ali-Express pulleys that I've been using.

I pulled the old pulleys off the motors and compared them to the new Gates pulleys- the new pulleys had thicker flanges, and the finish on the teeth was visibly smoother. Then I tried them on the motor shafts. Wow! Big difference- they fit very closely on the shafts, unlike the originals that fit loosely. I had to lightly sand one motor shaft to get the new pulley to fit (old pulley set screw galled the shaft, or maybe a little hardened lock-tite caused interference).

Chinese pulley on the left, Gates pulley on the right (I didn't take a picture of the 9mm long Gates pulley before I mounted it on the printer). Note the thicker flange at the top of the Gates pulley and the thicker base where the set-screws go in. It's hard to tell from the picture but the Gates pulley tooth finish is better than the Chinese pulley (though the Chinese pulley has been in use for several years, so maybe not a valid comparison). My caliper says the Chinese pulley has a 4.97mm hole diameter, the Gates pulley reads 4.95mm.

Next I installed the digital gauge and reran the test file with one modification- I ran it at Y=5 and Y=-3 instead of Y=5 and Y=-35. With the old pulley, I could see visible run-out as the pulley rotated. There wasn't any visible runout with the Gates pulleys. 

Gauge mounted on the right side Y axis rail to measure deflection of the end of the X axis as the extruder carriage moves along the X axis. After running tests on the right end, I ran the same tests on the left end of the X axis.

Here are the results of the test with the new pulleys mounted on the motors:

Interesting results here. There is relatively large wobble of the left end of the X axis (attached to the left side Y axis bearing block), but it no longer has the 40mm periodicity that was evident using the original pulleys. 

The right side looks a lot better, with almost no deflection except for when the extruder carriage is at the far right side of the bed (at X=140 and X=145mm). I would attribute that deflection to an error in the position of the pulleys on the right side Y axis bearing block or the belt clamp on the right side of the extruder carriage that is causing the belt to go out of parallel with the X axis rail.

I don't know how to account for the greater variability measured on the left side of the X axis compared to the right side. I'm going to have to think about that for a while. If you have any ideas, post them in the comments below.

Does it matter? It's hard to say. I never saw any print quality problems I could attribute to the old pulleys, but maybe it's because I never looked for it. There are many potential sources of error in the mechanism that could lead to print quality problems. Using quality pulleys eliminates one of them. The Gates pulleys from Filastruder cost about $5 each, so using them isn't going to break anyone's budget. In future projects I'll use Gates pulleys. Filastruder also sells Gates belts in 100mm increments.

Note: I bought the pulleys myself and received no compensation for this test/post from Gates, Filastruder, or anyone else.

The data and graphs are here.