CubeXY X-axis Re-redesign

One of the great things about hobbies is the lack of deadlines. Unlike in the real engineering-world, I can keep tweaking and perfecting designs as much as I want. This printer is turning out to be a great outlet for that.

I mentioned in an earlier post that the "12"mm SS rods that come with the CubeX are undersized. I finally measured them with a micrometer. They're 80um under, which for linear bearings, is WAY undersized. The THK linear bearing spec for 12mm LM shafts calls for -6 to -17 um. That probably contributed to the bad bearing wear I saw on the stock bearings. I purchased 4 new 12mm rods from PDTech on eBay: they actually listed a diameter tolerance spec, and they were case hardened, both of which are important for linear shafts (and neither are common for cheap chinese linear shafts). The only downside is that they're chrome plated steel, not stainless steel, but case hardened stainless steel shafts are very expensive, so I didn't buy those. I only purchased four since only four of them will have linear bearings on them. They ended up being about 11um undersized, which is right in the middle of the THK tolerance range. They result in noticeably smoother linear bearing movement and less (almost no) slop.

I purchased them long, partially because some of PDTech's non-standard lengths were actually cheaper than their standard lengths (455mm was cheaper than 450mm, not sure why), but mainly because I plan on making the printer taller. I mentioned this in passing in a previous post, but there is a lot of room between the top of the top aluminum cylinder shaft holders and the top of the acrylic shell. I can safely increase the length of the vertical shaft/rods by 40mm and still leave plenty of clearance between the hot end assembly and the lid. 40mm is convenient because the stock vertical shafts are 400mm long, and the stock X-axis shafts are 440mm long, so I can replace the 4x 400mm stock shafts with the two 440mm long old x-axis shafts (rear of printer) and two of the new tighter tolerance shafts (front of printer). I'll have to redrill 4 screw holes in the acrylic shell, and cut some acrylic out around the XY motors in the wire-mounting back plate, but that's pretty much it. Totally worth getting another 40mm of Z travel, for a total of about 325mm.

Taller CubeXY

While trying to find actual tension force measurements of belts in corexy printers (never found any), I stumbled across this interesting post on belt compliance. This person measured belt compliance at 10N preload (tension) + 10N. Basic 6mm GT2 fiberglass reinforced belt (like the kind I bought), has  modulus of 0.0067 %/N, while 6mm GT2 steel core belt has a modulus of 0.0024%/N. The given example is a 1kg carriage being accelerated at 3000 mm/s2 with a 1000mm belt. 1kg*3m/s2=3N. 0.0067%/N*3N = 0.02%. 0.0002*1000mm = 0.2mm, which is about half a nozzle width, which would probably result in very bad ringing. I had been planning to drive my corexy, which has approximately 2m long belts, at 6000+ mm/s2, with an approximately 1kg carriage. Doing this math for my printer yields 0.8mm of stretch, which is terrible. A steel core belt would be about 0.29mm of stretch. I bought some to try. The disadvantage of steel core belts is that they're heavier and stiffer, which means less power ends up in the motion of the carriage. Steel core belt minimum radius is also larger, mainly to prevent kinking and fatigue, so I may not be able to use them...we'll see. I'll probably start with fiberglass core belts. This post has a lot of details on belts.

X-axis Re-redesign

Due to the FEA results from the last post, I decided to completely redesign the X-axis to make it stiffer. 

The X-axis plate is flat now, and the pulley blocks also hold the linear bearings. The Hemera exttruder has been rotated 90 deg. The x-axis plate is 149g and made of aluminum. It's about 25% lighter than the previous steel design, and 45-75% stiffer, depending on the bending mode. It'll be machined out of scrap 1/4" ground plate. The pulley blocks will be machined from T6-6061 bar stock. The fan mount plate will be band sawed out of 1/8" aluminum plate, and I'm going to use flush press-in threaded inserts in it instead of threading it. The motor-carriage mount will be milled from 2x2" x 1/8" wall square aluminum tube, which holds the extruder motor on the inside almost perfectly. Unfortunately, almost all of the previous machining I did won't be used. Luckily, the nozzle ended up in very close to the same place, so the same bed plate can be used. I will have to print new bed mounts, though, because the bed has to be shifted in +Y 5mm. This system should result in  much stiffer X and Y axes. Here's a close up of the new extruder assembly:

This version is far less elegant than the previous one. The belt tensioner on the previous version was very simple and compact. This one uses the two stock X-axis belt tensioner cylinder things mounted inside of a 3D printed ratcheting assembly. I'll use a wrench on the printed hexes to tighten the belts, then tighten the button head screws down to hold the tensioner in position. Mounted off of that assembly is the inductive probe, which is conveniently closer to the nozzle now. The other side of the extruder has the fixed belt mount which takes each belt in and turns them 180 deg for plenty of belt engagement. Both the belt mount and radial fan are screwed to the 1/8" aluminum plate, which in turn is screwed to the side of the Hemera extruder motor. The fan location and duct are also less elegant. The previous duct completely encircled the nozzle. This one only has room to blow from one side. Because the belt heights are fixed by where the motors are mounted, and the requirement that belt segments be straight, the belt tensioner and fixed belt mount could only be located in the locations shown. Also, since Hemera was rotated so the length was along Y, the only place with room for the fan was on one of the X faces. Because the fan has its inlet on one side, that restricted the fan to the -X side. The fan also couldn't be located too far in +Y or it would impede the cooling flow from the hot end cooling fan. All of these things meant that the fan had to be mounted kind of high and in the location as shown. The LED light will be taped under the motor. I tried about every way I could think of to come up with a better way to configure the extruder assembly, but this was the best. While it's not as pretty or elegant, this should be a far stiffer and better performing design.

I've put in new material and screw orders. I should be able to finish machining the new X-axis plate next week. I'll start 3D printing the new plastic parts this weekend.