Y gantry movement is very stiff, even without belts installed. It would make sense if that were related to the occasional skipped steps I’m seeing. I hadn’t seen that since I increased the stepper motor current (though I don’t know what the reference is, so I don’t know to how much) but it happened today about 4 hours into a ~24hour print. Could have been worse, I guess!
The unsupported bolts through 4mm acrylic and a nylon pillar can’t be helping. The wheels are shedding plastic as they rub. Linear rail?
Linear rail and ATP-5 plate are both on order, and SSR and 120V Keenovo heater pad have arrived. Planning a kinematic mount for the new bed. Meanwhile, “thermal runaway detected” two prints in a row so either I have a bad thermistor or I need to PID tune my new merlin build—but if it’s PID tuning, why have I successfully put about a kilo of filament through on that build of Marlin? I also notice that I seem to have a fan mosfet failure on the board, so probably a sign that it’s time to try the replicape conversion again and hope it works better this time. I was kind of hoping to wait until I did the bed conversion so that I could get rid of the 12V power supply altogether, though.
Thinking of getting a bit radical with the linear rail and milling aluminum for gantry brackets and extruder carriage, instead of following the reprap “print everything in plastic” practice. For the gantry, that might let me make a light and simple T bracket that is stiff enough and much easier to disassemble.
Mark Rehorst put his kinematic mount adjustment screws into teflon as a thermal insulator for UMMD,¹ but I don’t believe in the need for that. The contact area is small, and steel has roughly a quarter of the thermal conductivity of aluminum generally. My plan is to use acorn nuts on the end of adjustment screws for the three-axis and two-axis constraints and a flattened screw for the single-axis constraint, rather than using ball-head screws that have to be adjusted through a hole or slot from the top. This way the adjustments can be from below the bed (as now) instead of requiring through-holes in the bed plate.
Mark put his three-axis and two-axis constraint chamfers on tabs across the middle of the plate. I’m planning to mill two chamfer sets on the bottom of the plate; one set using existing holes at the back of the printer for the three-axis and two-axis constraints, and one set at the middle of the plate that would require modifications to the bed structure in case the first ones are a problem. I’d rather do all possibly-needed milling before I attach the heater to the bed. The heated pad is 300x300mm on a ~333x333mm bed plate (I ordered 13.25" x 13.25" to give a little room), so I’ll have 16mm along the edge to add the chamfers. That’s more than enough, since the bed should expand by less than 1mm when it is heated!
Note that this proposed mechanism has no springs to protect in case of a extruder head crash. Don’t do that then? With a really flat surface and single-start Z screws, I should be able to avoid a head crash. I hope. [Edit: Actually, there’s no reason I couldn’t use the existing springs.]
In my build of Marlin, I was using the default Ultimaker PID settings that were also in the source dump provided by GearBest for the printer:
M301 P22.20 I1.08 D114.00
After M303 C7 E0 S240:
…
Classic PID
Kp: 11.19 Ki: 0.57 Kd: 55.23
PID Autotune finished! Put the last Kp, Ki and Kd constants from below into Configuration.h #define DEFAULT_Kp 11.19 #define DEFAULT_Ki 0.57 #define DEFAULT_Kd 55.23
Those PID values are close enough (within a factor of two) that I’d expect the to be OK. So I don’t think this is the key to “thermal runaway detected”, sadly.
Linear rail arrived today. I finally realized, a bit late, that MGN12H is all M3. I have not enough short M3 bolts and no M3 T-nuts, only M4 and M5. I need M3x8 for mounting the rail to the 2020. There is 3.5mm of depth in the block for mounting screws. There’s no perfect combination of standard M3 screw length and US standard aluminum stock thickness for the block mounting screws. I’d like to leave 0.5mm allowance, so 5mm stock and M3x8 would be perfect. I think I’ll mill 1.35mm pockets in 1/4" plate around mounting holes.
My idea is to mount the rail to the top of the 2020 extrusion and replace all the motor and pulley mounts with properly-aligned mounts milled from aluminum stock. I’ll also probably replace the two-z-motor arrangement with a single z motor driving both Z rods with timing belt.
I now realize that the 450mm linear rail and the resulting re-design might mean I could have gone to a larger bed, 14x15" or maybe even 15x15" — but then the heater would have been more expensive and really this is already big enough.
I’ve stripped the printer back down to a frame with corner braces and empty vertical drag chain. I have started what will be essentially a custom build that shares a few components with the donor X5S.
Belt drive for Z is coming together. I’m so glad I never got around to cutting down the 500mm single-start 2mm lead 2mm pitch trapezoidal lead screws I bought; now I could use an extra few more centimeters conveniently.
I bought trapezoidal pillow blocks for the bottom to go roughly where the Z motors went. I drilled holes in the X rod plate matching the holes in the pillow blocks, then milled spacers out of 1"x½" stock to move the pillow blocks higher. I’m not 100% convinced that having trapezoidal pillow blocks at the top of travel is helpful; it feels like it might be over-constrained, so I’ll just leave them off for now; I can add them later. But the 500mm lead screws reach only to about 13.5mm below the bottom of the top 2020 extrusion, so if I do need pillow blocks at the top I’ll need to mill more spacers.
I’m putting 20 tooth 8mm inside diameter pulleys on the lead screws, and driving with a 40 tooth pulley on the stepper motor. I have a 570 tooth (1140mm) belt loop around it. I’m mounting the motor on a piece of ¼" plate that I’ll attach to the bottom of the lower 2020 extrusion, so the bottom of the motor will be aligned with the bottom of the extrusion. The spacers under the pillow blocks help everything line up so the belt will be planar. I plan to put an extra belt loosely around the base of the lead screw mechanism so if the belt ever breaks I can put a new one on without doing lots of disassembly.
I wrote: “Note that this proposed mechanism has no springs to protect in case of a extruder head crash. Don’t do that then? With a really flat surface and single-start Z screws, I should be able to avoid a head crash. I hope. [Edit: Actually, there’s no reason I couldn’t use the existing springs.]”
Having taken the printer apart, I now see that the six existing bed mounting holes are about 3.6mm wide; more play than I’d like for a kinematic mount with M3 screws and springs holding up the bed, but wider than will give me a good tap for M4 for a hard-fixed bed.
I want to avoid play side to side, and it would be nice to have some protection against damage from head crashes. A potential downside of the single-start trapezoidal lead screw in this instance is that the additional leverage means that I can’t count on stalling the stepper motor before I do damage.
Perhaps I should start by wrapping M3 screws in teflon tape as an ersatz bushing and thermal barrier. This would let me spring-mount the bed. If it works reasonably well, I could perhaps drill the holes out a bit wider and turn some small delrin bushings, or perhaps leave well enough alone. I don’t think I could turn 3mm ID 0.3mm thick bushings very well, though maybe it wouldn’t be so hard? I haven’t turned bushings less than 1mm thick so far, though. Might be a fun challenge?
I assembled the Z stage movement. This is two 20-tooth pulleys on the single-start 500mm lead screws and a 40-tooth pulley on the stepper motor, with an 1140 mm 570 tooth 6mm wide GT2 belt around them. The motor is mounted on the inside of the frame, with the bottom of the motor aligned with the bottom of the frame. The motor is mounted on a flat piece of aluminum stock (1/4") as wide as the motor and as long as the motor plus 20mm for the frame, and drilled appropriately to mount with t-nuts to the bottom of the frame and with the existing M3 screws to the motor (the same screws previously used to connect the motor to the motor plate).
The lead screws are set in trapezoidal pillow blocks mounted on top of 1/2" stock, held down to the frame with M5 screws, going through holes I drilled in the existing motor plates, and tied into the frame with M5 t-nuts.
The trapezoidal nuts were replaced with single-start nuts to match the new lead screw and the whole thing assembled back into the frame, with two belts in place as planned. I aligned the two sides where they bottomed out against the pillow blocks for a reasonable initial reference. I turned the screws quickly by stripping belt over the lead screw pulleys. When the Z carriage was near the bottom, I had substantial wobble at the unconstrained top. Therefore, I expect to machine two more 1/2" spacers and use pillow blocks at the top of the frame, like how it was initially shipped except with the spacers added. Longer lead screws might be a little more convenient but honestly I don’t think it needs to move higher. Apparently the lead screws are sufficiently flexible that this isn’t an overconstraint in practice.
Because I’m using 20 teeth on the lead screw and 40 on the motor, the move from 4-start to 1-start lead screw will only double instead of quadruple my steps per Z mm. Downward pressure on Z won’t make the bed creep. And a single stepper with a belt around the two lead screws will keep the two sides in sync. I like this better than the dual motor design. missing/deleted image from Google+
Side bottom view of motor mount plate, also showing 1/2" spacer below the lead screw. It’s also a better shot for seeing the extra belt, which is relatively snug around the trapezoidal pillow block mounting screws.
I did these parts in aluminum. The spacer could be plastic, but I would expect the motor mount eventually to creep under load if it were made from plastic. missing/deleted image from Google+
I noticed when I was test-fitting that the X 2020 gantry (484 mm long) seems to just barely fit between the MGN12 trucks on the linear rail on top of the Y gantry 2020 top of the frame. But I haven’t mounted them, they were just sitting there loose, so I did the math…
Total X width is 530mm, so 510mm between centers of the Y gantry 2020. The MGN12 trucks are 27mm wide. There is half of each of the two trucks on either side of the centers, so 510mm between centers minus 27mm (two halves of the trucks) is 483 mm to fit a 484 mm extrusion.
Maybe I can mill the alignment brackets off by 0.5mm and make it fit? Alternatively, I can mill 0.5mm off the top 10mm of both top sides of the X gantry 2020 to make it just perfectly fit. I’ll have to think about that!
I milled 0.5 mm off the top 10mm of both sides of the x gantry 2020. I milled alignment jigs for mounting the linear rail, and mounted it with all the M3x8 screws and t-nuts that didn’t interfere with existing screw heads in the 2020.
I mounted the X linear rail on the side of the x gantry extrusion. I haven’t yet decided what print head to hang off it first, but the nice thing is that I can do anything that matches the screw pattern of the MGN12 trucks, which should make it easy to change up.
I will have to mill spacers to add trapezoidal pillow blocks to the tops of the z screws because they wobble. Sadly 1/2" stock is about 1.5mm too thick, so I’ll have to face them.
Then I’ll have to mill T shapes to attach the Y trucks to the X gantry 2020. That should be easy enough. But if they aren’t perfectly square, my prints will be skewed.
I milled the spacers for the z lead screw tops, for mounting the pillow blocks. I made them just like the bottom blocks, then realized I couldn’t get the t-nuts into the slot in the 2020 without being able to drop them at least part way down over the lead screws. Rather than worry about how deep a pocket to mill, I just drilled all the way through the middle. A pocket would have made installation a tiny bit easier, maybe.
This is not a precision part, and it is not under significant stress. The hole and half holes are intentionally oversized for easy fit. If I hadn’t already been set up to mill these pieces, printing them would have been appropriate. The half holes could easily be normal through holes in a slightly longer part.
Due to the flexibility of the lead screw, these are not actually an over-constraint. They solve the problem of free end whipping when the bed is at the lower travel extent, and reduce vibration/chatter throughout travel range.
Because I removed 1.5mm from the face, M5x25 screws were about 1.5mm too long. So I put three washers under the screw heads. Good thing this isn’t a precision part!missing/deleted image from Google+
No pictures tonight, but… I milled some prototype X gantry holders. I used .25" aluminum, 26mm x 100mm, which was just enough to try out the movement and not enough to mount pulleys on for driving the movement, but it let me experiment with a design to keep everything square.
I milled a pocket in the bottom 1mm deep. It was 2mm from one long edge, and 20mm wide, leaving 4mm on the other edge. The pocket went all the way to the edge at one side and to 26mm from the other edge. The pocket fits the 2020 extrusion to hold in straight. In the last 27mm, I drilled 4 3mm holes, 20mm on centers, to fit the M3 screws needed for the MGN12H carriages, and I pocketed 1.25mm deep around each of those 4 mounting screw locations so that M3x8 screws would be the right depth to mount to the carriage trucks. I drilled 3mm holes centered 10mm in from each edge of the pocket and 8-10mm from the ends of the pockets. I used M3x8 screws and M3 t-nuts to mount to the X 2020. I made two of these, in mirror image, for left and right mounts. The 2mm edge of the pocket is toward the front, and is small to avoid interference with the front-mounted linear carriage on the X 2020. There is more room on the back, thus 4mm. The 2mm and 4mm edges hold the 2020 extrusion square.
Mistakes I made included using a chuck instead of a collet to hold the center drill for starting holes in the first of the two holders. This made the holes insufficiently precise and one of the four just doesn’t match up, even after drilling out with a 3.2mm oversize bit. Using a 3/16 collet and a #2 center drill to start holes for the second holder was more precise. (I know better than to have tried the chuck…)
It runs square. It’s a little bit stiff, and it’s stiffer toward the back, which indicates that the linear rail isn’t quite straight, even installed with the jigs I made. Or maybe that the frame isn’t quite as square as I think. That will take some investigation.
The real holders will have to be larger and have mounting built in for pulleys. I’ll probably use two pulley stacks with vertically-aligned belts aligned with the front of the extruder carriage truck. Alternatively, I could just use the existing holders and screw some square tube on top to hold the pulley stacks. I’ll see.
I milled some 1.5" square tube (more scrapyard special!) with 1/8" walls, lining up 5mm holes for M5x45 bolts through into the M5-tapped holes in the gantry plates. I did two in mirror image, one for each side. They sit on top of the screw heads for the gantry plates, which isn’t obviously optimal; maybe I should instead use some spacers or bushings. But since they contact only three screw heads and three points determine a plane, maybe this is actually the best arrangement. Don’t fix what ain’t broke?
I noticed, finally, that I had lost track of where I should drill the tapped holes in the gantry plates, and ended up with the belts lined up with the bottom of the front-mounted linear rail carriage instead of with the ball bearing slot in the linear rail as I had originally intended to do. Doesn’t really matter, probably. It will just change my carriage design a little bit, and since I haven’t started that design it’s a distinction without a difference.
I haven’t yet added bushings/spacers to the M5 bolts to hold the idlers in place. The bottom right 20T idler in this picture is just there in place of a bushing I will add later. I just needed to test out the height for now.
The way I’ve stacked this up moves the belt path much higher than it was on the printer as shipped. I’m thinking of mounting the front corner idlers inside 2" square tube, which would require buying 55mm M5 bolts. An alternative would be just using 1/2" aluminum pieces which would not require support at the top, perhaps. I also need to think about machining new motor mounts.
To drill the bolt holes more precisely, while I did center-drill the holes with a collet-mounted center bit to start the holes, I also used a 5mm collet to hold the 5mm drill bit instead of a chuck. The holes lined up very nicely! missing/deleted image from Google+
I occasionally make progress on this project. A month or more ago, I added bushings where they are missing in the picture above. I reused some of the bushings that shipped with the printer, then added a few more in custom lengths. A few weeks ago I milled out some thicker aluminum blocks as spacers for mounting the motors and some inch-thick blocks off the front corners for mounting corner pulleys. The thick blocks mean, I hope, that I don’t need to support the pulley bolts top and bottom; I’ll see how well that works. Because of the square tube holding the pulleys on the Y carriage, I had to put the front outside pulleys well outside the original printer envelope to keep the belt out of the way event when the carriage is at full back Y travel. The alternative would have been yet another idler.
Yesterday and today I milled a single-extruder bowden extruder carriage with belt clamps. I used a 45⁰ chamfer bit to score the belt-holding faces with .2mm lines 1mm apart all across the face. After loosely installing the belt, I tightened the screw part way, grabbed the belts with pliers, and pulled tight. They stayed tight while I screwed down the clamp. What I have now is still the stiff steel-core cable that shipped with the printer, but I plan to replace with glass-core because it’s pretty stiff going around all those pulleys… But both belts pluck about the same note so they are approximately equivalently tight, and the XCR3D XCR-BP6 mounts tight; it’s not going anywhere.
The belts are a bit too far apart in height because I used an M5 screw for the clamp. In retrospect, an M4 might have been a better idea, and I have more of those handy anyway. The larger screw will create a bit of angular deviation at the extremes, but probably not enough to measure.
I’m also planning to make a carriage for direct feed with the aluminum titan clone. There are three known problems with those: the hobbed gear is installed in the drive gear upside down so it doesn’t align with the filament correctly (fixable by buying the original), it doesn’t have a filament constraint (fixable by making and installing one) and it doesn’t use a bearing around the motor shaft used as a pivot for the arm (I hope fixable by milling it out and installing MR58 bearings).
Sorry for the ugly picture. missing/deleted image from Google+
