3D Printer (Prusa Bear Mod) Build

[Netpackrat]

So, as mentioned in this thread, I have recently gotten into 3d printing. The printer I bought is a Prusa I3 MK3S+, which has been described as "the Apple of 3d printers" mostly because it just works with relatively few issues. I say relatively, because no printer is exactly plug and play, and there is a learning curve with tinkering required for any of them, but the Prusa probably comes the closest out of anything at a consumer level. And they are open source, so you technically don't have to buy one from Prusa, although the clone units (for 1/3 to 1/2 the price) apparently cut a lot of corners, including in places where you don't want corners cut (electronics, hot end components, etc).

https://www.prusa3d.com/category/origin ... a-i3-mk3s/

Anyway, I built mine from a kit (you put literally every component together, took me an entire day) due to a $250 cost difference, plus shipping cost difference, plus it keeps the total cost under the level that triggers customs charges (it's made in Prague). And it's pretty cool, but like most 3d printers it's kind of slow, and about all you can really do about that is find a (somewhat) faster printer and/or add more printers. While I was waiting for the Prusa kit to arrive, I discovered the Voron series of printers, especially the Voron 2.4 which is much faster than the Prusa and offers a bigger build volume. But it's an entirely DIY proposition... There are kits based on the published open source bill of materials, and there is some community support, but it's not like the Prusa where you can always contact the factory (if you bought the genuine article and not a clone) if you run into problems. So I went looking for an intermediate step that will allow me to build some experience with making a printer from scratch with limited support, including custom configuration and editing/compiling my own firmware.

Enter the Prusa Bear Modification... The stock Prusa is great, but one of the areas where it can be improved is in frame stiffness. Most users won't see much difference, but if printing tall objects near the upper end of the Z axis, you might see some print quality reduction. The Bear Mod replaces the stock frame which is made up of flat 6mm aluminum parts plus some extrusions, with a frame that is entirely made up from 20mmx40mm v-slot extrusions, held together with a combination of flat and milled braces at the corners. The printed parts are also beefier and redesigned to fit the extrusions. One beauty of the setup is that if you stay with the standard dimensions on the axes, the printer "thinks" it's a regular Prusa so you can use the standard firmware. You can buy a frame kit with all pre-cut parts and hardware for about $150 to convert a stock Prusa, you supply the printed parts.

That's cool and all and I may eventually convert my standard Prusa, but I want a second printer and I want to build from scratch. So I bought a bunch of aluminum extrusions and the corner braces+joining plates from Amazon, and have been sourcing the other parts elsewhere. I made the printed parts on my Prusa using Starry Blue PETG filament from Overture, following the Bear print settings. The designer is very adamant that you follow his exact print settings for best strength and dimensional accuracy. This included printing some calibration pieces to tune the extruder for the individual spool of filament chosen. The Bear designer specifies that you must do this for each spool used. I don't know how important it actually is, but the process wasn't difficult and I found that it did in fact improve the accuracy of the extruder a little bit.

Anyway that's all I have time to write at the moment. I have some pictures and more to write to bring the thread up to date with where I am on the build currently.

[Netpackrat]

Building my robot army...



So yeah, my buddy was like, you saw The Matrix, right? Terminator? I Robot?

I guess I should have gotten a group shot of all the printed parts, but they are semi-sorted in ziploc bags and I kind of want to keep them organized, so they will be seen as they appear on the frame. Speaking of the frame, I opted not to buy one of the kits mainly because I am going with the extended Z axis, so I was going to be buying plain extrusions and cutting to length no matter what. This was probably a mistake, since the joining plates from Amazon weren't quite right and I had to work around that.



Here is one of the 1m long 2040 extrusions on the bench. One reviewer on Amazon described them as long, hard, and black which appears to have been accurate. From what I gather, a lot of people cut these to length using a chop saw with a carbide blade... I have a chop saw with a carbide blade but it's basically good only for rough carpentry, so I had to find another way to get them square and to length accurately.



I may not have a decent chop saw, but I do have this 2000lb lump of cast iron that I bought a year ago sitting in the garage. No metric tooling for it (everything to do with 3d printing seems to be metric), but I didn't need any for this operation either.... I set up this temporary end stop with a piece of aluminum, took a light cut with a 1/2" end mill, then zeroed the DRO and set it for MM.



After rough cutting each frame member on the bandsaw, I put them in the milling vise and squared one end, then flipped them around and put the squared end against the temporary stop, and then took each down to length using the DRO. I put a layer of aluminum speed tape on each of the vise jaws to protect the anodized surface. The biggest pain in the ass was moving the stop and repeating the process for each of the 4 different lengths needed. The mill did leave a minor burr but it was not too difficult to remove.

More to follow after I get back from starting another print out in the shop.

[Netpackrat]

Printer running, baby monitor set. Something to keep in mind about printers, is they are fairly dumb machines... If a print fails (i.e. pops off the bed, or there is some other anomaly like layer shift, etc), the printer will happily keep executing gcode even if it destroys itself in the process. So you have to keep an eye on them while they are running, even though once set they don't normally require any intervention on the part of the operator. At the worst extreme, it's possible for it to catch on fire and burn your house down. The Prusa has safety features built in to hopefully prevent this (Chinese machines not so much..), but all the same it's best to keep an eye on it. As an added safety measure, I hung a little fire extinguisher cannister from the bottom of the metal cabinet above where the printer sits on my bench, which is normally intended for kitchen range hoods, but which should hopefully automatically put out a printer fire in the very unlikely event that one occurs.



Anyway here is the bear (bare?) frame assembled. The only decently flat bench surface I could find was here where I usually just have junk piled, so I got it built and squared up here before moving it to the other workbench to continue assembly. One thing that I have seen recommended, is to get an offcut from a place that does granite countertops. You don't need a surface plate but you do need a fairly flat surface to build on. I actually do have a small surface plate but it's tiny.... 9x12 was the biggest one that Amazon would ship to AK for free, so nowhere near big enough for this.

This is the stage where I ran into problems with the quality of the parts from Amazon. I should have either bought the frame kit from one of the Bear approved suppliers, or made my own joining plates from 4mm plate on the mill. The joining plates I bought were enough under the nominal 4mm thickness that I had to shorten all of the 5mm screws slightly to keep them from bottoming against the grooves in the extrusions. A couple of them had the hole locations shifted slightly but I think I have them located such that it won't matter. Otherwise I will have to replace them. All's well that ends well, but I didn't do myself any favors by using these particular components.

In a similar vein, hopefully buying the rest of the parts piecemeal won't bite me in the ass too much. I couldn't see buying a kit from Prusa just to get the parts, and then throwing a bunch of it away, so I have been buying the electronics and mechanicals in various places. I think I have most everything that I need at this point with just a couple of parts that I won't need until later that have yet to straggle in. Could have bought one of the complete clone kits too, but I haven't read anything good about the clone components.



Some of the first printed parts going on for the Y axis. I am at the point where I need to trim the chromed linear rods to length in order to proceed farther. They are hardened steel, so I am going to have to drag out my abrasive saw (which I hate) to cut them. There's a 1mm tolerance on the lengths, so tape measure and chop saw/grinder will be more than accurate enough.

One thing that may not be readily apparent is that the Y rod holders are modified from the standard Bear design. Normally the rods are just held down to the holders with zip ties, but I found a set of files that a user posted that use a printed strap with screws instead of the zip ties. I don't think it will matter structurally, but it should look a lot cleaner.

[HTRN]

The thing about the Voron, is that it's not really a printer or a kit - it's a design app that will spit out a bill of materials for whatever parameters entered - case in point, you can build 750mm3 despite the fact that the "kit" offered is 350mm3.

[Netpackrat]

The thing about the Voron, is that it's not really a printer or a kit - it's a design app that will spit out a bill of materials for whatever parameters entered - case in point, you can build 750mm3 despite the fact that the "kit" offered is 350mm3.

That's interesting; I obviously haven't dug deeply into the Voron world since I have my head wrapped around the current build for now. I did see that a seller on ebay is offering CNC machined aluminum parts for the 2.4 to be used in lieu of the normal printed parts. That might make for a neat build.

[HTRN]

That's interesting;

https://vorondesign.com/voron2.4

Click on the configurator. A pop up window will start asking questions.

This was probably a mistake, since the joining plates from Amazon weren't quite right and I had to work around that.

https://8020.net/ Personally, I'm wondering if you can redesign the Voron to use 30 series extrusion..

[Netpackrat]

That's cool. Would probably go with the LDO kit though. Bought a bunch of LDO stuff for the current build, too.

[HTRN]

One of the things I really like about RatRig Vcore3, besides the way the table works, and the larger work envelope vs the Voron 2.4 kits, is the fact that they use a larger(and thus more rigid) extrusion series (30 vs 20). I just wished they used a larger linear rail (both apparently use a 15 series linear rail)

Just found this: https://3ddistributed.com/corexy-3d-printer/

Basically, they're selling the core components, leaving the end user to source the commodity components like rails, motors, lead screws, platen, etc. What this means that's it's really scalable - 610mm3(approx 2 feet on the side) should be doable. Plus they'res no real plastic components inside, so doing a heated chamber is also possible.

[Netpackrat]

I made some modest progress last night:



Ended up using an angle grinder to trim the Y rods, then shortened/squared them to final size on a disc sander. Located the LH rod holders using the printed reference tool (the angled part without any holes that's visible in the last couple pictures). Got the bearings installed to "squashed frog" (Y carriage) and on the rods, and the whole thing installed and the RH holders tightened in place. These bearings are a little noisier than the ones in my Prusa. They are the same bearing, but it may be due to not having gone for the genuine rods, or it may just be one of those things.

I used an alternate part/model for the Y rod holders from standard... The rods snap into the holders and then zip ties are added for security, but for a cleaner look I found these that somebody drew, which use a strap that bolts across the top. I now think they are not necessarily better than the zip tie version. They look better, but it's possible to induce a bow into the rods if you tighten them too much, so these are just barely snugged down. The designer of the alternate part recommended printing them out of some high zoot material at 100% infill but I used PETG like the rest of the parts but at 100% and even that seems like overkill given that the standard setup is a zip tie.

Wasn't able to get the whole Y axis completed due to ordering the wrong parts... The bag that I thought contained the drive pulleys for the Y and X motors only contained plain pulleys, so I had to make another order which is super annoying. I went ahead and hung the motor anyway since it looks like I should have room to get the pulley mounted OK after the fact. I should be able to press forward with the other axes while I await the pulleys. I also thought I had bought m3 flat washers with the rest of my hardware, but I can get those at Ace Hardware today.

I think I am going to start putting together the extruder next. The Bear Mod can be built using the standard Prusa X axis and extruder, but there is also a dedicated Bear version ("Bear Exxa") with better cooling and X belt path/adjustment. I also found a set of alternate parts for the Bear Exxa that uses 10mm Z rods, which will help maintain rigidity with the taller than stock Z axis, so I am going to use the Bear Exxa instead of the regular Prusa setup. It means using custom firmware, but I was already needing that for the extended height anyway. Mods on top of mods... What's the worst thing that could happen?

[Netpackrat]

No progress here, but I thought I would show a picture and talk about the linear bearings a little bit. This is one of the 10mm bearings for the Z axis, and there are 4 rows of little ball bearings inside, each spaced 90 degrees from its neighbors. Orientation in the Z probably doesn't matter much but the build instructions of both the Prusa and the Bear Mod are pretty specific about how they should all be oriented for best wear characteristics.

The 8mm bearings are about the same but half the length. The standard design has these doubled up on each Z rod, but the guy who re-drew the axis for the 10mm rods reasoned that it made just as much sense to use the longer versions since 2 of the long bearings are generally cheaper than 4 of the shorter ones. They were a bit of a pain in the ass to find though; I eventually found an ebay seller located in Germany who carried them.

There's some minor controversy surrounding proper lubrication of the bearings... Prusa maintains that the bearings come pre-lubricated from the manufacturer, and there is no need to add any additional lubrication until it is time to perform routine maintenance. The other camp is adamant that the pre-existing oil is only a preservative, and does not adequately lubricate the bearings in use. This faction further holds, that prior to use, one must remove this "shipping" oil by soaking all the bearings in isopropyl for a specified number of hours, and then re-lube them using a proper silicone grease. The Bear manual thus far seems to be silent on the issue...

Both with the Prusa assembly and this build, I haven't done anything to remove the existing lubricant beyond wiping the excess from the exterior. I do pack them with a silicone grease prior to installation, but most of it seems to end up coating the surface of the smooth rods despite the presence of seals at the end of each bearing. You can 3d print a special nozzle that screws onto the end of a tube of Super Lube grease for purpose of packing the bearings without leaving a lot of excess inside.