Torque Specifications for factory installed bolts?  

RE:

I am also interested in a torque list. My XL may arrive next week. In the meantime I am studying the assembly manual.

You mean the bolt as can be seen in assembly manual "Original Prusa XL Five-Head (Semi-Assembled), part 2-base-side-frame-assembly

Step 3 Extrusion alignment, picture 2 or Step 6 Left rear extrusion assembly, picture 2?

I am a little disappointed, that the intermediate plate seems to be fixed with only 2 diagonal screews instead of possible 4 on all corners. I am considering to unscrew all, cut the 2 other threads, add the screws and retorque all 4 plus the center bolt using loctite blue just to prevent nasty surprises. You can not check them, once the frame is assembled unless you drill holes from below into the profiles. At the rear column that would not weaken the lower profile enough to worry. OTOH, at the middle columns with the z-rails I would rather not weaken the lower profile's area moment of inertia though.

Also, unfortunately, the centering bolt has just 2 spanner flats. I have several quality torque wrenches but no socket for that one. This bolt could be improved by making a proper head, e.g. a hex or torx head, external or internal. Machining an external hex head instead of spanner flats probably does cost Prusa hardly anything more. The exact centering of the bolt in both extrusions neither needed nor desirable.

The frame has to maintain the vertical angle of the profile during assembly and later. It should also keep its resonance frequency high. Especially with the shear forces from the core-xy and input shaping. I will see. Otherwise I might add a diagonal plate to increase stiffness -- especially at the middle columns with the z-rails. The fittings for the sidewalls do not contribute significantly to shear stiffness. I still wonder how the torsional weakness of this rather open shape will be.

BTW: Do the XL or MK4 have any shear compensation? They can not do it as the MK3 does with its initial XY-Calibration of the 4 points.

I agree with you that loose screws must not appear and Prusas QC should fix it. If you buy an assembled printer it should just work ootb. Same with the preassembled parts of the kit. Its like the old story in the UK in the seventies when it was pretty normal to have to fix your new british car every weekend until Toyota and Honda came around. No wonder why British Leyland here in Germany got its nickname "British Elend". That translates into "British misery". The US of A had its own story with GM and Toyota. Search for NUMMI-plant. I assemble the kit myself anyway.

Despite that, I still like the overall design of the XL -- except for its big size. Compared to my MK3S+ and the MK4, I can see many improvements. Yet by still 3d printing selected parts and open source the design they keep the Reprap spirit alive.

Considering the toolchanger which allows for very quick change with very little waste compared to the MMU or the Bambulab, all the hardware that Prusa has put into the XL and the  longterm support, I see the price by no means a bargain but nevertheless reasonable. That is if and only if it will work reliably and produce prints of decent quality.

RE: Torque Specifications for factory installed bolts?

I am a little disappointed, that the intermediate plate seems to be fixed with only 2 diagonal screews instead of possible 4 on all corners.

I think what they've done is adequate and doesn't require a re-engineering. Consider that a Core X-Y printer has substantially less force being imparted due to the work being held essentially still and the motors being fixed in the corners.

I'd suggest that you enjoy your printer as it comes and if you discover an issue later on, then consider a massive rebuild. It's very likely your printer will arrive ready to go (with assembly of the tools of course).

RE: Torque Specifications for factory installed bolts?

@blauzahn Yes in the link you provided, Step 2, the picture with the yellow arrows, those are what I'm calling "end caps".  Now time for a little mea culpa: once we found the gap between "end cap" and the extrusion and disassembled to find the "bolts" were loose we stopped in frustration and because, you know, customers.  With more time on the weekend we (and yes this is a royal we as my coworkers will assume no responsibilities for my misunderstandings...cowards!!) I have now removed those "bolts" rather than just tightening them and discovered they are not bolts at all.  They are set screws basically.  So now I'm guessing those are intended merely to prevent the end cap from rotating around the center stud.  Meaning two are adequate and tightness perhaps doesn't matter.  They DO tighten which must be due to some draft angle? as there is no bottom. 

The issue remains for us and is now maybe worse as I don't know how to eliminate the gap that occurs.  It does mean that perhaps we don't have to disassemble the lower portion of the extrusions for all of the extrusions and can simply remove the offending extrusion which is fortunately one of the back two, and not the z-axis extrusions.

I would still love to have bolt torque specs so I can use a real torque wrench instead of a 3D printed one.  Love the concept of printing whatever you can for assembly and for the machines so parts can be replaced, but I'd rather use a real wrench.  It goes in easier and tightens more accurately.

@EastMemphis I agree in principle and we are huge Prusa fans here.  The MK3s and Minis in our Print Farm have been outstanding.  As I mentioned we have a couple MK3s that have had nothing done in over 9,000 print hours other than maintenance.  We've had to replace x axis rods on a machine that was our fault for overtightening the carriage.  And on one machine we just had to replace the rotary encoder (LCD board).  And that's it.  Simply outstanding.  And love everything Josef Prusa has done for the community and the open source mentality. Met him when Prusa was in Seattle, super nice guy.

That said, we've always assembled from kit except for the MK4 and XL.  Both of those we're finding factory assembly issues.  As a hobby we'd have more time to play with it.  As a business it cuts severely into productivity.  I still am very hopeful that with tuning both the MK4 and XL will end up outstanding.  But at the moment the MK4 print quality lags far beyond the MK3s and we can't use it for certain designs for customers.

If we figure out the cause of the gap we will update the post.

RE:

As a hobby we'd have more time to play with it.  As a business it cuts severely into productivity.

At this point, I think both the MK4 and the XL should be considered "experimental".

I have a factory assembled MK3S+ and the only thing I've done is fix my own generated problems. The machine itself has been absolutely flawless and production worthy. Maybe in five years the XL will be up to that level. I'm hopeful anyway.

RE: Torque Specifications for factory installed bolts?

Yes, I am still confident that I can enjoy it as is. I am just a tiny little bit disappointed -- a minor thing. I have no intention of a massive rebuild/disassembly unless necessary or really beneficial. I did that more than a year ago with my MK3S kit after a while in order to improve the geometry and maintainability. I took my time to thoroughly reassemble the whole frame and all axes until I was satisfied with the results.

So I might perhaps add the 2 screws right away. Once assembled, you cannot add them without at noticeable disassembly. Otherwise I try to keep everything else original, especially at the beginning, when I have the least experience with the XL and may have to contact support or this forum.

I deliberate decided to finish the order of the core-xy XL (and did not change it into an MK4) for the reasons you mentioned.

I ordered the kit, not the assembled version. To me, that will be fun and saves a lot of money.

RE:

I just finished assembly and my advice to them would be:
a) suggest some tools to buy (torque wrench + long bit?)
b) specify the torques

Nobody expects them to supply a proper torque wrench, but many customers would have one around or wait for an excuse to get one. Mine was sitting right in front of me the whole time but without the specs it's pointless and there's nothing among the comments. Ok for one build, but doing this again would drive me crazy. I was scared shitless about stripping a thread or breaking a screw and having to redo it all from scratch.
No idea why they're using TORX almost everywhere except where torque is relevant? Maybe the actual torque is so high that the tool tip would break? But then the torque would also be above spec for the screw...

Without this (and the LCD cable routing) the build would actually be really pleasant. Can't even imagine how many customer support calls this will produce in the long run, already the comments are full with people breaking the Allen key when torquing the docks.

The correct torque appears to be just below the point at which the key would strip the screwhead out.  I wonder if the medium quality screws themselves are selected because they will fail before any significant damage is caused.

Cheerio,

RE:

Isn't the maximum damage when you have to redo from scratch or the screw breaks in a machined part? Like the strip that holds the docks? Or do you mean damage to the person doing the assembly 😆 
The bits of the assembly using that printed torque indicator just felt terrible.

RE: Torque Specifications for factory installed bolts?

I too like working to the precision of using torque specifications and wish specs were published. I doubt that even Prusa has them.

However, I don't remember any fasteners in my MK3S+ or MK4 that need to be tightened anywhere close to stripping or breaking. Numerous instructions caution against over-tightening. The normal tendency for most is to over tighten. So the use of a torque wrench and specs would cure that.

RE:

I found the tiny torque wrench to work just fine. I have a drawer full of torque wrenches and didn't even think about using one. The little gadget is clever and works. They should have supplied torques though. Personally, I rarely use a torque wrench as I have one built into my hands. I can tell exactly when a screw, bolt, or fastener is at the limit.

As for the screws, I had one strip out, the screw failed at max torque and that gave me a pause. It's a good thing they supply spares of each fastener or I would have been stuck.

I like torx fasteners. I think they've got excellent grip, far better than simple allen key screws. Why did they go that route? I'm guessing it had to do with availability more than anything. 

If you want to know the torque setting that the supplied "wrench" measures, just put it in a torque wrench and measure it. 

RE:

For approximate orientation only:

For a steel-steel connection i would prefer fol. table:

Anzugsmoment_Stahl_Stahl

for a steel-plastic (PA 6.6) connection i would reduce to a fifth or a tenth (see steel-steel table 4.6)

(PA 6.6)

wbr,

Karl

RE: Torque Specifications for factory installed bolts?

@eastmemphis - probably good advice from a mental health standpoint to treat as "experimental" !!  And I wish I had a better feel for tightening bolts like you do, I tend to overtighten worrying about vibration so torque specs protect me from myself.

@blauzahn - the threads are already in the extrusions for the other two diagonal corners, so you'll just need the set screws and you'll be all ... set. 🙂

@ntdesign - Agreed.  The cable routing was annoying, but I get it, plenty of people want it to look pretty when done.  We don't care how it looks so would love an option to just strap cables to the bottom of the frame.  Guess we could have just designed something.  And agreed that the whole torque issue using the supplied torque "wrench" was somewhat terrifying, lol.  Ruined one bolt head and haven't solved our original problem yet. 

I do think we (meaning my company) forget on occasion that Prusa machines ARE hobby level machines simply because they are so good and have been so reliable for us.  Sometimes have to sit back, eat a cheat-Haribo and just figure another way.