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[Résolu] How much weight can the MK3 bed safely support?  

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bviz2
(@bviz2)
Active Member
How much weight can the MK3 bed safely support?

I have a couple of the flexible arm desk lamps laying around where the clamps have broken.  I decided to design a weighted base that will hold 2 of them.  The design is a 150mm hemisphere.  Being mildly lazy I plan is to stop the print towards the end and fill the sphere with weighed pellets (BBs).  

That got me thinking:  I don't want to break my printer by adding too much weight.  What is a safe weight that the MK3's bed can handle during a print?  

Ben

Best Answer by Vojtěch:

I have done some math:

  • at 750g load, the Y rods will bend by 0.05mm, which is a whole layer at high resolution
  • at 500g load, the Y motor will start skipping doing infill at 200mm/s and 1250 mm/s² acceleration

I was too lazy to calculate belt stretch due to acceleration, but it'll be very similar, showing significant artifacts also around 500-700g. And that is including the heat bed assembly. The printer certainly isn't designed to handle a PLA print with 100% infill, full build-volume cube, which would weigh about 13kg.

Publié : 24/05/2019 2:02 pm
bobstro
(@bobstro)
Illustrious Member
RE: How much weight can the MK3 bed safely support?

You'll start to suffer poor print quality due to the amount of weight that has to shift directions on Y moves. I'd expect to get skips and catastrophic layer shifts at some point. It's possible you might screw up your printer calibration. Why not just design a screw-on cap to allow easy filling and emptying?

My notes and disclaimers on 3D printing

and miscellaneous other tech projects
He is intelligent, but not experienced. His pattern indicates two dimensional thinking. -- Spock in Star Trek: The Wrath of Khan

Publié : 24/05/2019 2:08 pm
bviz2
(@bviz2)
Active Member
Topic starter answered:
RE: How much weight can the MK3 bed safely support?

Those concerns are why I am asking.

A plug is an obvious solution.  My concern there is simply one of not trusting my design to ensure the plug is robust.  I'd prefer it to be on the bottom for esthetic reasons. 😉 Depending on the answer I get here I probably will design a plug in.  This design just had me thinking about the problem.

Another is to allow it to be filled through the tubes that hold the lamps.  I'd prefer for the interior to be sealed. 

 

Publié : 24/05/2019 2:29 pm
bobstro
(@bobstro)
Illustrious Member
RE: How much weight can the MK3 bed safely support?

The drawback to the "bed slinger" designs is that very weight that has to be moved around for Y movements. You usually want to lessen the weight. If you add a considerable amount of weight, your acceleration and jerk values are going to be thrown out the window as the hardware can't deliver what your settings promise. To answer your question, I'd say "as little as possible".

What are you designing your part in? Fusion 360 makes threads easy to work with. A twist-in plug is another possibility.

My notes and disclaimers on 3D printing

and miscellaneous other tech projects
He is intelligent, but not experienced. His pattern indicates two dimensional thinking. -- Spock in Star Trek: The Wrath of Khan

Publié : 24/05/2019 2:57 pm
Vojtěch
(@vojtech)
Honorable Member
RE: How much weight can the MK3 bed safely support?

Even in OpenSCAD, with the threads library, a thread is a one-liner.

Publié : 24/05/2019 3:11 pm
bobstro
(@bobstro)
Illustrious Member
RE: How much weight can the MK3 bed safely support?
Posted by: Vojtěch

Even in OpenSCAD, with the threads library, a thread is a one-liner.

Had not seen that before. Have you used it? That opens up some of my test part horizons!

My notes and disclaimers on 3D printing

and miscellaneous other tech projects
He is intelligent, but not experienced. His pattern indicates two dimensional thinking. -- Spock in Star Trek: The Wrath of Khan

Publié : 24/05/2019 3:33 pm
Vojtěch
(@vojtech)
Honorable Member
RE: How much weight can the MK3 bed safely support?
Posted by: bobstro

Had not seen that before. Have you used it? That opens up some of my test part horizons!

Yes, it really is as easy as it looks.

Publié : 24/05/2019 4:28 pm
bviz2
(@bviz2)
Active Member
Topic starter answered:
RE: How much weight can the MK3 bed safely support?

I use OpenSCAD and I've had the threads library for a while, I just haven't experimented with it [threads library] yet.  This might be the time.

Design aside, my initial question is still a reasonable one.  Nothing wrong with understanding the limitations of the tools you are using.

The engineers should have this as a basic design requirement.  From a design perspective a "reasonable" high value for this is the weight of a full print volume cube at 100% infill with ?nylon? filament.  I doubt they would design it for a metal impregnated filament but an extremely conservative engineer might.  Personally I think the 100% infill cube is overkill but a value I would seriously look at if I was part of the design team.

The quality of the print due to momentum issues is an interesting aspect of this question I hadn't thought about.  I had thought about it from the perspective of can the motor, belts, and bearings support the weight.  There is a fun homework or test question in here for a first year physics or mechanical engineering course.

Ce message a été modifié il y a 5 years par bviz2
Publié : 24/05/2019 5:33 pm
Vojtěch
(@vojtech)
Honorable Member
RE: How much weight can the MK3 bed safely support?

I have done some math:

  • at 750g load, the Y rods will bend by 0.05mm, which is a whole layer at high resolution
  • at 500g load, the Y motor will start skipping doing infill at 200mm/s and 1250 mm/s² acceleration

I was too lazy to calculate belt stretch due to acceleration, but it'll be very similar, showing significant artifacts also around 500-700g. And that is including the heat bed assembly. The printer certainly isn't designed to handle a PLA print with 100% infill, full build-volume cube, which would weigh about 13kg.

Publié : 24/05/2019 6:34 pm
bobstro
(@bobstro)
Illustrious Member
RE: How much weight can the MK3 bed safely support?

I just don't know that it would result in anything more than a thought exercise. You could come up with a theoretical worst-case print, consisting of a solid block printed at maximum dimensions (250x210x210mm) using the density of your favorite filament (1.24g/cm^2 for PLA... or 2.46g/cm^2 for copper fill), but there's no performance spec that says the printer was actually designed for this. I doubt those $400 Chinese knock-off printers would handle a 300x300x300 print. And what constitutes failure? Inability to move, or simply degraded print quality?

You could always print XYZ cubes with progressively heavier loads on the the bed yourself I suppose. Even then, the distribution of the weight is going to be important, and a low-flat lead weight probably wouldn't be a good indicator of an actual print with weight distributed up high.

In any case, why bother? There are simple enough ways of avoiding stressing the printer to find out. I mean, if it really interests you that much, feel free to do destructive testing on your own equipment, but I haven't seen anything officially stating such a value. I think an engineering student would have to be pretty bored to find out!

My notes and disclaimers on 3D printing

and miscellaneous other tech projects
He is intelligent, but not experienced. His pattern indicates two dimensional thinking. -- Spock in Star Trek: The Wrath of Khan

Publié : 24/05/2019 7:19 pm
bviz2
(@bviz2)
Active Member
Topic starter answered:
RE: How much weight can the MK3 bed safely support?
Posted by: Vojtěch

I have done some math:

  • at 750g load, the Y rods will bend by 0.05mm, which is a whole layer at high resolution
  • at 500g load, the Y motor will start skipping doing infill at 200mm/s and 1250 mm/s² acceleration

I was too lazy to calculate belt stretch due to acceleration, but it'll be very similar, showing significant artifacts also around 500-700g. And that is including the heat bed assembly. The printer certainly isn't designed to handle a PLA print with 100% infill, full build-volume cube, which would weigh about 13kg.

@vojtech-p6: Cool.  Thanks. That definitely gives me some perspective on the limits. 

@Bobstro - I want to avoid destructive testing which is the basis of the question. :-). Note that in my original question I worded it as "safely support."  There is quite a bit of wiggle room in that terminology but I would expect a reasonable quality of print with no damage to the printer other than the normal wear that occurs with every print. 

Thanks to both of you for your help and thoughts.  Now to experiment with the threads library. 😀 

Publié : 24/05/2019 8:12 pm
meroped33
(@meroped33)
New Member
RE: How much weight can the MK3 bed safely support?
Posted by: ben.v3
Posted by: Vojtěch

I have done some math:

  • at 750g load, the Y rods will bend by 0.05mm, which is a whole layer at high resolution
  • at 500g load, the Y motor will start skipping doing infill at 200mm/s and 1250 mm/s² acceleration

I was too lazy to calculate belt stretch due to acceleration, but it'll be very similar, showing significant artifacts also around 500-700g. And that is including the heat bed assembly. The printer certainly isn't designed to handle a PLA print with 100% infill, full build-volume cube, which would weigh about 13kg.

@vojtech-p6: Cool.  Thanks. That definitely gives me some perspective on the limits. 

@Bobstro - I want to avoid destructive testing which is the basis of the question. :-). Note that in my original question I worded it as "safely support."  There is quite a bit of wiggle room in that terminology but I would expect a reasonable quality of print with no damage to the printer other than the normal wear that occurs with every print. 

Thanks to both of you for your help and thoughts.  Now to experiment with the threads library. 😀 

It's possible you might screw up your printer calibration. Why not just design a screw-on cap to allow easy filling and emptying?

Publié : 26/05/2019 5:47 pm
bviz2
(@bviz2)
Active Member
Topic starter answered:
RE: How much weight can the MK3 bed safely support?

@vojtech-p6, @Bobstro Thanks again for your help.  After a few experiments with the SCAD thread's library I ended up with a design with a plug that seems to work.  I have yet to fill it with weights (BBs) but it all fits.  I'll post it to PrusaPrinters once I've added the weights and see it work.  While I was at it, I went ahead and added a tube so a clamp could be used.

Ce message a été modifié il y a 5 years par bviz2
Publié : 27/05/2019 2:48 am
bviz2
(@bviz2)
Active Member
Topic starter answered:
RE: How much weight can the MK3 bed safely support?

Here is a cross section of the design...

Publié : 27/05/2019 2:59 am
meroped33
(@meroped33)
New Member
RE: How much weight can the MK3 bed safely support?
Posted by: meroped33
Posted by: ben.v3
Posted by: Vojtěch

I have done some math:

  • at 750g load, the Y rods will bend by 0.05mm, which is a whole layer at high resolution
  • at 500g load, the Y motor will start skipping doing infill at 200mm/s and 1250 mm/s² acceleration [url= https://youjizz.one/ ][color=#000000]Youjizz[/color][/url] [url= https://123porn.vip/pornhub/ ][color=#000000]Pornhub[/color][/url] [url= https://tubegalore.vip/ ][color=#000000]Tubegalore[/color][/url]

I was too lazy to calculate belt stretch due to acceleration, but it'll be very similar, showing significant artifacts also around 500-700g. And that is including the heat bed assembly. The printer certainly isn't designed to handle a PLA print with 100% infill, full build-volume cube, which would weigh about 13kg.

@vojtech-p6: Cool.  Thanks. That definitely gives me some perspective on the limits. 

@Bobstro - I want to avoid destructive testing which is the basis of the question. :-). Note that in my original question I worded it as "safely support."  There is quite a bit of wiggle room in that terminology but I would expect a reasonable quality of print with no damage to the printer other than the normal wear that occurs with every print. 

Thanks to both of you for your help and thoughts.  Now to experiment with the threads library. 😀 

It's possible you might screw up your printer calibration. Why not just design a screw-on cap to allow easy filling and emptying?

Those concerns are why I am asking.

A plug is an obvious solution.  My concern there is simply one of not trusting my design to ensure the plug is robust.  I'd prefer it to be on the bottom for esthetic reasons. 😉 Depending on the answer I get here I probably will design a plug in.  This design just had me thinking about the problem.

Another is to allow it to be filled through the tubes that hold the lamps.  I'd prefer for the interior to be sealed. 

Publié : 28/05/2019 6:38 pm
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