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RepPresent Build Details, From Start To Bed Install
I figured I should record some of the problems that I've run into while building the RepPresent and how I've solved them, just in case some other RepRapper finds the information helpful. So if you're not interested in the minutiae of building a RepRap machine then you should bail now.
Broken X Motor Mount
When the kit arrived I did an inventory of parts and found that two pieces of the X motor mount had broken off: the upper Z rod guide and the upper half of the X motor mount plate. The paramour used JB Weld KwikPlastik to try to repair the part. As you can see in the image below it seems to be holding on the motor mount plate but the Z rod guide broke off again while I was fitting the IGES bearings:
We tried finding a local RepRapper who could print us off a replacement, but no luck. RepRapPro has offered to ship us a free replacement piece, but they're in Britain and the part wouldn't arrive until after the holidays. So for now we're just going ahead with the build, hoping that the one remaining Z rod guide on that side will be good enough, and the patched mounting plate will hold together long enough, for us to commission the thrinter (our shorthand for "3D printer) and print off our own replacement
Frame Misalignment
When I first built the frame it rocked about the diagonal rod in the base by about a centimeter. I carefully double-checked the lengths of the rods in the side triangles and they were all correct. I checked the lengths of the rods joining the two sides together and they were all correct as well. When I checked the diagonals of the base I found that the diagonal along the rod was a full centimeter longer than the other diagonal, which was weird because to cure rocking about the diagonal rod you're supposed to lengthen it, but mine was already too long. I gave it a try anyway and by the time the rock had subsided I'd screwed the nuts on the diagonal almost to the ends of the rod and the frame was visibly warped. I tried going the other way: shortening the diagonal rod until it was the same length as the other diagonal. That eventually got me to a nice square base but it also increased the rock to about 1.5cm.
Stumped, I took the frame apart and restarted from scratch. This time everything worked out fine. The base diagonals were equal and there was no rock.
I'm not sure what the problem was, but I have a couple of ideas:
Y Belt Drift
After installing the Y axis I had a problem with the belt drifting to the right on both the motor pully and the idler, even though they were properly aligned with the belt clamps on the frog. When I moved the frog to either end of the Y axis, the belt at that end would be nicely centered on the pully/bearing, but at the opposite end the belt would have drifted into the mudguard/motor mount housing and be rubbing against it.
I moved the Y idler to the right a bit and that cured the drift at the idler end, even though the belt was now visibily running at an angle from the clamp on the frog to the idler.
At the motor end I was finding that the stack of five washers used as a spacer was causing the motor mount to twist whenever I tightened its nuts. I just kept at it, rotating some of the washer between attempts, until I finally managed to get the motor mount on straight. The belt still wanders back and forth by about 3mm, but at least it no longer rubs against anything, which was the main concern.
Loose IGES Bearings On The X Motor End
When I mounted the IGES bearings on the X motor end and slid a smooth rod through them, they were too loose and the rod wobbled. The build instructions tell you what to do if the bearings are too tight, but say nothing if they're too loose. I lined the inside of each bearing holder with a piece of Kapton tape. It didn't really want to stick to the plastic but it managed to stay in place long enough for me to push the bearings in. That one layer of tape was enough to snug the bearings up against the rod quite nicely.
X Belt Drift
I ran into more drifting problems with the X belt. Again it was drifting inward and rubbing against the motor housing on one end and the idler housing on the other. I double-checked all my measurements, found a tiny difference in the lengths of the two X rods and corrected that, but the drift persisted.
The primary problem seems to be in the design of the X carriage: the slots for the belt are too close to the body of the carriage. In the image below I've loaded the STL files for the X carriage (red) and the X idler end (blue) into Maya, aligned them along the centers of their X rod guides, and then slid them together so that they overlap.
That tiny gap between the inside edge of the belt notch on the X carriage and the outside face of X idler housing is just 0.5mm wide. Given that there is a 2mm wide washer between the housing and the idler bearing that means that the belt notch is trying to pull the belt onto that washer rather than onto the bearing. The notches should be 1.5mm further out to account for the idler washer.
There is about 1mm of play in the notch so I pushed the belt to the outside of the notch and filed an indentation to keep it there. Alas, it wasn't enough and the drift persisted, although it seemed to happen more slowly. Frustrated, I asked the paramour to take a look. Zie fiddled with most of the same things I'd already tried, but then increased the belt tension beyond that called for in the build instructions. At that point the drift dropped from about 4mm down to a respectable 1mm and the belt stopped rubbing against the housings. Ah, the advantages of having multiple pairs of eyes on a problem.
I tried moving the belt out of the indentation and back to the inside of the notch and the drift reappeared, so both the indentation and the extra tension are required to keep the belt true. I just hope that the tension isn't too much for the motor or its patched mount.
Broken X Motor Mount
When the kit arrived I did an inventory of parts and found that two pieces of the X motor mount had broken off: the upper Z rod guide and the upper half of the X motor mount plate. The paramour used JB Weld KwikPlastik to try to repair the part. As you can see in the image below it seems to be holding on the motor mount plate but the Z rod guide broke off again while I was fitting the IGES bearings:
We tried finding a local RepRapper who could print us off a replacement, but no luck. RepRapPro has offered to ship us a free replacement piece, but they're in Britain and the part wouldn't arrive until after the holidays. So for now we're just going ahead with the build, hoping that the one remaining Z rod guide on that side will be good enough, and the patched mounting plate will hold together long enough, for us to commission the thrinter (our shorthand for "3D printer) and print off our own replacement
Frame Misalignment
When I first built the frame it rocked about the diagonal rod in the base by about a centimeter. I carefully double-checked the lengths of the rods in the side triangles and they were all correct. I checked the lengths of the rods joining the two sides together and they were all correct as well. When I checked the diagonals of the base I found that the diagonal along the rod was a full centimeter longer than the other diagonal, which was weird because to cure rocking about the diagonal rod you're supposed to lengthen it, but mine was already too long. I gave it a try anyway and by the time the rock had subsided I'd screwed the nuts on the diagonal almost to the ends of the rod and the frame was visibly warped. I tried going the other way: shortening the diagonal rod until it was the same length as the other diagonal. That eventually got me to a nice square base but it also increased the rock to about 1.5cm.
Stumped, I took the frame apart and restarted from scratch. This time everything worked out fine. The base diagonals were equal and there was no rock.
I'm not sure what the problem was, but I have a couple of ideas:
- The first time around I was over-tightening the nuts. So much so that some of the lock washers pushed down into the rod holes on one side, skewing the rods a bit. I'd guess that about half the rocking was due to that. The second time around I was gentler, tightening the nuts by hand and then just adding one or two turns with the wrench.
- The rod holes in the printed parts were too big. Even before fettling the threaded rods were quite loose in most of their holes. While that only meant a misalignment of perhaps half a millimeter, that got amplified across the length of the machine. I figure that accounted for maybe a quarter of the rocking. To work around that problem I held the loosest of the rods firmly against one side of their holes while tightening their nuts.
- I was using the kitchen counter to level the machine since that was the flattest surface I could find in the house. But even the counter had a dip of a couple of millimeters, which accounted for the remaining quarter of the rock. I got around that problem by measuring the rock about one diagonal, then rotating the machine 90 degrees on the counter and measuring the rock about the other diagonal. When the two were equal the frame was properly aligned.
Y Belt Drift
After installing the Y axis I had a problem with the belt drifting to the right on both the motor pully and the idler, even though they were properly aligned with the belt clamps on the frog. When I moved the frog to either end of the Y axis, the belt at that end would be nicely centered on the pully/bearing, but at the opposite end the belt would have drifted into the mudguard/motor mount housing and be rubbing against it.
I moved the Y idler to the right a bit and that cured the drift at the idler end, even though the belt was now visibily running at an angle from the clamp on the frog to the idler.
At the motor end I was finding that the stack of five washers used as a spacer was causing the motor mount to twist whenever I tightened its nuts. I just kept at it, rotating some of the washer between attempts, until I finally managed to get the motor mount on straight. The belt still wanders back and forth by about 3mm, but at least it no longer rubs against anything, which was the main concern.
Loose IGES Bearings On The X Motor End
When I mounted the IGES bearings on the X motor end and slid a smooth rod through them, they were too loose and the rod wobbled. The build instructions tell you what to do if the bearings are too tight, but say nothing if they're too loose. I lined the inside of each bearing holder with a piece of Kapton tape. It didn't really want to stick to the plastic but it managed to stay in place long enough for me to push the bearings in. That one layer of tape was enough to snug the bearings up against the rod quite nicely.
X Belt Drift
I ran into more drifting problems with the X belt. Again it was drifting inward and rubbing against the motor housing on one end and the idler housing on the other. I double-checked all my measurements, found a tiny difference in the lengths of the two X rods and corrected that, but the drift persisted.
The primary problem seems to be in the design of the X carriage: the slots for the belt are too close to the body of the carriage. In the image below I've loaded the STL files for the X carriage (red) and the X idler end (blue) into Maya, aligned them along the centers of their X rod guides, and then slid them together so that they overlap.
That tiny gap between the inside edge of the belt notch on the X carriage and the outside face of X idler housing is just 0.5mm wide. Given that there is a 2mm wide washer between the housing and the idler bearing that means that the belt notch is trying to pull the belt onto that washer rather than onto the bearing. The notches should be 1.5mm further out to account for the idler washer.
There is about 1mm of play in the notch so I pushed the belt to the outside of the notch and filed an indentation to keep it there. Alas, it wasn't enough and the drift persisted, although it seemed to happen more slowly. Frustrated, I asked the paramour to take a look. Zie fiddled with most of the same things I'd already tried, but then increased the belt tension beyond that called for in the build instructions. At that point the drift dropped from about 4mm down to a respectable 1mm and the belt stopped rubbing against the housings. Ah, the advantages of having multiple pairs of eyes on a problem.
I tried moving the belt out of the indentation and back to the inside of the notch and the drift reappeared, so both the indentation and the extra tension are required to keep the belt true. I just hope that the tension isn't too much for the motor or its patched mount.