Don't get those Delta Arm Blues!
Posted: Wed Mar 27, 2013 7:57 pm
This sticky is copied from my from the post called Don't get those Delta Arm Blues! in my build thread. It describes how stiff Delta Arm joints can cause problems with gaps in your prints. There is more information in my build thread along with the Z Calibration Thing STL file. The file can be found here: Z Calibration Thing
Following up on my post yesterday on the controlled experiment to determine why I am getting gaps in the infill on my prints. Here are the 3 test prints laid out to show how they were printed.
[img]http://mhackney.zenfolio.com/img/s8/v77 ... 8494-4.jpg[/img]
Note that I oriented the fill to go from the center of the build plate to the tower. You can read the details in the earlier post.
I know how this story ends so hang in there!
First, look at the part at the Z tower. It actually looks pretty reasonable and is the best infill I've had to date. At this position, there is very little movement along the axis (from the tower to the center of the table) and a lot from left to right.
Now look at the part at X. It may be hard to see but I have gaps at the arrows. This is what I typically have seen and the problem I'm trying to fix.
Then look at the part at Y, it's the worse one of the three. Gaps along multiple sides.
After thinking on this overnight, I hypothesized that one side of the Z delta arm U joints may be stiff or have backlash. I couldn't predict which side but it just seemed like Z was the culprit. I disassembled the delta arms from the Z carriage and immediately discovered that the delta arm on the X axis side was very stiff compared to the others. I took it apart and found 2 problems: 1) I had used a small screwdriver to install the arm on the aluminum bearing - BAD IDEA. I had several gouges in the face of the bearing that had burrs large enough to create a lot of friction. I also discovered that I had not "defuzzed" the delta arm U joint when I rounded the top. The fuzz had caught between the arm and joint, again adding friction. I cleaned all of this up, regreased with lithium grease, reinstalled and prepared for a test print to see if I found the problem. And here is the result - this piece was printed at the Y position so it was in the same spot as the worst of the three prints in the photo above:
[img]http://mhackney.zenfolio.com/img/s8/v85 ... 0018-3.jpg[/img]
This part is very nice! No infill gaps and nicely filled overall.
So I am ready to proclaim that stiff delta rod U joints can lead to odd printing behavior! Given the number of folks with infill gap issues, I think re-investigating the U joint fit will eliminate those Delta Arm Blues!
Now I can go on to more fun things!
Cheers,
Michael
EDIT: some more info on fitting the joints.
I used 400 grit silicon carbide sandpaper on a small metal sanding block to sand the inner sides of the U joints on the arms. This takes off very little material so you have lots of control to minimize the possibility of overdoing it. Once I had the joints fitting well (constantly reassembling and testing) I moved on to cleaning up the holes in the U joints. These were tight and contributed to a lot of the stiffness. I used a new XActo knife blade (a #1) and carefully scraped the interior of the hole with a twisting motion. Then chamfer the edges slightly with the knife. This takes off very little material and is easy to control. It is a SCRAPING motion not a CUTTING. If you scrape you will be able to close in on a great fit in a few attempts. I used a little white lithium grease after fitting too, that helped a lot.
Following up on my post yesterday on the controlled experiment to determine why I am getting gaps in the infill on my prints. Here are the 3 test prints laid out to show how they were printed.
[img]http://mhackney.zenfolio.com/img/s8/v77 ... 8494-4.jpg[/img]
Note that I oriented the fill to go from the center of the build plate to the tower. You can read the details in the earlier post.
I know how this story ends so hang in there!
First, look at the part at the Z tower. It actually looks pretty reasonable and is the best infill I've had to date. At this position, there is very little movement along the axis (from the tower to the center of the table) and a lot from left to right.
Now look at the part at X. It may be hard to see but I have gaps at the arrows. This is what I typically have seen and the problem I'm trying to fix.
Then look at the part at Y, it's the worse one of the three. Gaps along multiple sides.
After thinking on this overnight, I hypothesized that one side of the Z delta arm U joints may be stiff or have backlash. I couldn't predict which side but it just seemed like Z was the culprit. I disassembled the delta arms from the Z carriage and immediately discovered that the delta arm on the X axis side was very stiff compared to the others. I took it apart and found 2 problems: 1) I had used a small screwdriver to install the arm on the aluminum bearing - BAD IDEA. I had several gouges in the face of the bearing that had burrs large enough to create a lot of friction. I also discovered that I had not "defuzzed" the delta arm U joint when I rounded the top. The fuzz had caught between the arm and joint, again adding friction. I cleaned all of this up, regreased with lithium grease, reinstalled and prepared for a test print to see if I found the problem. And here is the result - this piece was printed at the Y position so it was in the same spot as the worst of the three prints in the photo above:
[img]http://mhackney.zenfolio.com/img/s8/v85 ... 0018-3.jpg[/img]
This part is very nice! No infill gaps and nicely filled overall.
So I am ready to proclaim that stiff delta rod U joints can lead to odd printing behavior! Given the number of folks with infill gap issues, I think re-investigating the U joint fit will eliminate those Delta Arm Blues!
Now I can go on to more fun things!
Cheers,
Michael
EDIT: some more info on fitting the joints.
I used 400 grit silicon carbide sandpaper on a small metal sanding block to sand the inner sides of the U joints on the arms. This takes off very little material so you have lots of control to minimize the possibility of overdoing it. Once I had the joints fitting well (constantly reassembling and testing) I moved on to cleaning up the holes in the U joints. These were tight and contributed to a lot of the stiffness. I used a new XActo knife blade (a #1) and carefully scraped the interior of the hole with a twisting motion. Then chamfer the edges slightly with the knife. This takes off very little material and is easy to control. It is a SCRAPING motion not a CUTTING. If you scrape you will be able to close in on a great fit in a few attempts. I used a little white lithium grease after fitting too, that helped a lot.