Hi Spiritdude, great work on the stiffness of our machine concepts. Looking at this is really overdue, thanks for doing this. Two missing points sprang into my eye:
- The MendelMax is stiffer than the others, due to it's extra triangle at the top, that's right. But it isn't "fully" triangled at the top, the vertical bars can still buckle in X direction. If it were fully triangulated, this triangulation rod whould go down all to the bottom. OK, perhaps that's nitpicking.
- The other one is, less nitpicking, triangulating frames is only one way to get them stiff. Another one is to use stiff bars. Like in single-lever motorcycles for the rear wheel, for example. So far, RepRaps make not really use of stiff bars, but it's thinkable for a Wallace-like design to be stiffer than a MendelMax. Look at ordinary milling machines and drill presses. They don't have triangulated frames, either.
--Traumflug 11:01, 1 August 2012 (UTC)
Thanks for the feedback, Traumflug!
- fully triangulation at the top of MendelMax - let me check, yes, will correct it!
- Can you describe more detailed what a "stiff bar" is, you mean just thicker? The point is, when you use vector-like (in regards of function) rods to stabilize a frame, like threaded rod based RepRaps foremost, the issue of vibration is obvious - the stiffer the bars and its connection to the other bars are, the less "vector-like" (one dimensional transfer of pull/push force) as also triangulation happens (a big footprint of an aluminium extrusion fixated for example). Whereas a threaded rod attached to a printed corner thingy won't function anything but just pull/push (vector-like). I assume, User:Prusajr: Prusa Mendel, and User:Whosawhatsis: Wallace, as well User:Emmanuel: FoldaRap look at where rods or bars do connect, I am not experienced with aluminium extrusions, but I'm very impressed of the photos of printed stuff by FoldaRap (on his flickr stream) and he describes also well how he achieved it (hotend temp recheck + fan to cool print while printing). Also, in one video of Printrbot maker User:Abdrumm states, that he is not pleased with the printing quality of his Printrbot (making pieces for Printrbots) and at that point I realized, just at looking at his videos, the z-rods where shaking/vibrating - which lead me to look at this issue of rigidity and vibration, first from a formal geometrical point of view, and second real world experiences (rigidity of stiff bars and such). I will follow Printrbots further to see if the lack of rigidity at higher Z is measurable:
I was thinking of a form (STL model) to print which reveals:
- skew of frame (e.g. Prusa Mendel frame at building time needs to be measured quite good, but I doubt one achieves 0.3mm exactment or so) - whereas Foldabot seems to do a good job avoiding skew with precise lengths bars (e.g. cut together, making margin very very small).
- vibrations (in particular at higher Z, when the printed item is moved Y)
--Spiritdude 14:19, 1 August 2012 (UTC)
Technically, vibration and skew is about the same. Given constant (extruder-)masses, a weaker machine will always lead to more vibration as well as more skew. As vibration is difficult to measure, skew alone might be sufficient. To get an idea about both, engineers do a FEA analysis to calculate the eigenfrequency/resonance frequency, but this is very complex for assembled structures. The higher the lowest eigenfrequency, the sitffer the machine is.
Regarding stiff beams, here's a picture of a single lever motorcycle. You see, just nothing on the left side of the rear wheel. Looking a the right side, there is some triangulation, but only in the vertical plane. Also no triangulation in the horizontal plane.
Still BMW cycles are well known for their stiffness. Why? Because this single beam is really hard to bend. Taking a big hose/tube is usually the most simple way, but U-like shapes with stiffeners inside are also possible. Unfortunately I can't find a picture which shows these stiffeners.
--Traumflug 17:53, 1 August 2012 (UTC)
I really mean static skew and not dynamic vibration where skew appears as result - the static "skew" from the correct angles, e.g. 90 degree angles of the z-rods - the rod-based models with nuts/bolts to position or define the length of a side is subject of skew at the moment of building it (Prusa Mendel and other rod-based RepRaps), and just with a manual measurement tool you might achieve some precision. As long you print low Z stuff, it doesn't show up, the moment you go higher your skew from the frame will show in the print plus the vibrations from z rods increase the position/printing error even further.
Stiffness - it's a big topic, are there any ideas to improve stability of RepRap with more "stiff" rods you have in mind, that maybe simplifies our exchange discussing some actual design ideas.
--Spiritdude 18:19, 1 August 2012 (UTC)
Cost of Printing----
You mentioned two parts, I think these are easy to calculate for material use. Electricity will differ per machine, a heated bed has a 300% increase in wattage. cost of electricity is different for people all over the world.
Perhaps you want to add a thrid: maintainance
- Replacements due to wear
- Upgrades to your printer (i.e. keep up with Reprap trends like LM*UU barings, smaller nozzle sizes)
- Time impacts of maintanance (time needed for maintanance, downtime of the machine)
--Bitflusher (August 16, 2012) (you can sign with 4x ~)
--- Thanks for the input, sure the cost of electricity varies, but I expect little variance (max +/- 15%), e.g to to print say 1cm3 (with x% fill) = x mm filament + e mins electrity (at x kW/hr cost with/without heatbed) = costs
The maintenance cost are harder to calculate, but surely to be considered for larger time e.g. 2-3 weeks non-stop operation I guess. My focus is to give an impression how much a RepRap costs for printing, as many people seem to assume it's free of cost, in particular it is interesting to say, it is better to print a RepRap or create molds and make pieces with injecting plastic - compare speed but also use of resources. Spiritdude 17:33, 16 August 2012 (UTC)