Full crosbraces for Prusa Mendel

Finally:

Full crossbracing for Prusa simplified Mendel by Lanthan

[www.thingiverse.com]

Oh, somebody finally taking care of one of the basic weaknesses of the Mendel design? Great!

---

Generation 7 Electronics	Teacup Firmware	RepRap DIY

heh:

Brace


/tom

Edited 1 time(s). Last edit at 01/09/2012 05:25AM by TomCeeMe.

---

regards,
Tom

Diagonal cross bracing only works well when it is corner to corner, like Darwin. Then you can't skew a rectangle of the frame without stretching a threaded bar and this is very hard because they are strong in extension.

With the Mendel flat sheet fitted it is a bit stiffer than a Prusa, but not a lot and still wobbles. The bracing is only halfway down the rods, so to skew it you have to bend a rod in the middle instead of the at the end. It is stiffer but not solid like a full diagonal is. I would guess about 4 times stiffer. If you push on the top corner can you displace it noticeably? I certainly can on my Mendel.

You can't fit full diagonals on the triangular design, which is why I dropped it on my Mendel90. I will release a version of the design compatible with the Mendel / Prusa rods and LM8UU bearings, so people will be able to print there own upgrades and reuse the parts.

There are lots of attempts to make stiffer Mendel by using stiffer materials like aluminium extrusions, but the odd thing is they all keep the same flawed shape.

---

[www.hydraraptor.blogspot.com]

@Traumflug: yes, no octopuses nor lustful segments of curved bodies (nor Lovecraftian mashups of those two mass print categories), only tech! On the other hand, doesn't it augment the machine with some sort of four-eyed awed, almost smiling face?

@TomCeeMe: Yes, certainly! But I cut the lower rods short...

@nophead: Yes, E PUR SI MUOVE! it still moves, by firmly pushing I can bend the top by 1 or 2 mm along x... but this is much less than before! Crossbracing the bottom rectangle will incrementally help further reduce this.

To fully stiffen the pyramid one would need another, inverted, 90-degrees-rotated, pyramid over it (not that it wouldn't look kewl!) taking even more space than a cube... or top and lower X rods more than twice as long to crossbrace 'em externally... that adds a lot to the shopping list (BTW there's a huge difference between stainless and zinc-plated rod hardness that must further impact on flexion, I do not know how much people are aware of this)

Yes, me too am convinced that the full answer is something in the line of the Mendel90, we are all waiting for your published parts

Moreover I am still in the process of wiring electrical cable to the tool shed and refurbishing it, so I can install the router there, etc. (have quite some old MDF/formica/similiwood old panels from ugly thrashed furniture to recycle, so I won't be buying pre-cut mdf sheet from the shop).

I think I'll recycle the cupcake first (acme Z rods, power supply, 608zz bearings, hot end, the ramps electronics...) so I decided to crossbrace the mendel.

We abandoned round housing about 10'000 years ago, as well as tipis some hundreds of years ago, there must be some structural and space use reasons ;-)

Lanthan Wrote:
-------------------------------------------------------
> (BTW there's a huge difference between stainless
> and zinc-plated rod hardness that must further
> impact on flexion, I do not know how much people
> are aware of this)

I used to assume that hardened rods would stiffer, but somebody who knows more about mechanics than myself stated in these forums that modulus and hardness are not related and that all steel rods had similar stiffness.

>
> Yes, me too am convinced that the full answer is
> something in the line of the Mendel90, we are all
> waiting for your published parts

I am working on it nearly all my spare time.

Edited 1 time(s). Last edit at 01/09/2012 04:09PM by nophead.

---

[www.hydraraptor.blogspot.com]

Yes, almost all steels have the same modulus of elasticity, which doesn't correlate much at all with with hardness. Stainless might have slightly different modulus, but I don't have that data handy.

Mendel frame stiffness is something I've been thinking about. Stiffness of a beam (say, a rod in the frame) is proportional to the section modulus, which is a combination of the material's modulus of elasticity and the first moment of area of the beam's cross section (about the neutral axis). Lots of beam formulae and section data on the web if you ever want to look them up.

Anyhow, this results in the smooth rods being much stiffer than the threaded rod. The load-bearing section of an M8 rod is roughly Ø6mm, and the smooth rods are Ø8. That works out to a big difference in the moment of area. If you hold the two in your hands, you can easily get a visible deflection of the threaded rod, but not the smooth rod. So, if the bottom portion of the Z axis smooth rods was rigidly clamped (like Printerbot), the upper portion of the frame might not be necessary at all. Such an arrangement might even be more rigid than a Mendel triangular frame. Intend to try this idea when I get some time.

@Nophead & Dale Dunn: Thank you for the clarifications!
Found the young modules of several steels [www.engineeringtoolbox.com]
indeed they are very close.

Further searches: hardness shows some relation to young modulus and the speed of sound in the material according to this paper
[www.ndt.net]
we are talking of changes of a magnitude of 4%

So indeed cross-section will play a bigger role.

I do not believe a cantilevered (single ended clamped) beam structure like the printrbot will improve the situation in terms of vibrations, unless it features quite fat smooth rods. According to textbooks, the vibration frequency of a double clamped beam is higher than that of a cantilevered beam by a factor of about 6.4.
It is the lower frequencies that we want the less in our frames.

Plus, you definitely lose the possibility of dampening by local energy absorption across heterogeneous materials (wood/plastic/steel...) "a combination of stiffness and loss"
damping by polymers: [silver.neep.wisc.edu]

Quote

According to textbooks, the vibration frequency of a double clamped beam is higher than that of a cantilevered beam by a factor of about 6.4.

That's true. However, I think it plays a major role how this beam is connected to the rest of the machine. Screwing a rod into an oversized hole of a small plastics part produces pretty much a hinge. So, rods either have to have very solid support on one side or need a design which would be stable with all corners being a hinge, too. Triangles are well known to be stable, even when their corners are flexible

---

Generation 7 Electronics	Teacup Firmware	RepRap DIY

I'd be curious to see how rigid a cross-braced Mendel is if the frame is constructed of smooth rod rather than threaded rod. Even short cross-bracing would make the vertices much less like a hinge and more like a rigid mount for the beams. Construction methods with a jig might even be favorable when compared to using threaded rods.

Dale Dunn Wrote:
-------------------------------------------------------
> I'd be curious to see how rigid a cross-braced
> Mendel is if the frame is constructed of smooth
> rod rather than threaded rod. Even short
> cross-bracing would make the vertices much less
> like a hinge and more like a rigid mount for the
> beams. Construction methods with a jig might even
> be favorable when compared to using threaded rods.

Extruded aluminum is much easier to work with and you can attach all sorts of things to it securely. It also has an even larger cross section. If you are going to change materials but stick with the mendel design, it is THE material to use.

**Cough**Prism**Cough***

There's also mendelmax, but i wish they'd bring their design back home and put more info up on the wiki.

Nophead is right, though, that the design lacks X-Z rigidity. However, like all engineering, it is a balancing act of design compromises, and the Mendel's unique shape was what roped me into the reprap project in the first place. These printers are going to end up where the computer is, possibly a den or living room, so aesthetics can't be totally discounted.

Edited 1 time(s). Last edit at 01/10/2012 11:09AM by Buback.

I've always assumed that the aluminum extrusion is significantly more expensive than threaded or smooth rod. I don't actually know. How does it compare?

For the prism, each 400mm piece of 2020 from misumi was $2.28, and you need 9, so $20.52. It doesn't add significantly to the cost of the printer, really.

You have to generate a quote on their website in order to get those prices, though. the prices they list are higher.

What were expensive are the steel t-nuts: 1 bag(100 nuts) was $19.32!!

the freight for the order was $6.94. cheaper than the threaded rod i ordered for my mendel, since it came in a long tube instead of a small box like with the extrusion. Plus, i had to cut the threaded rod myself. Generally speaking, it's probably the same price, unless you live next to a hardware store that carries metric threaded rod.

3030 is much, much stronger. I think a future redesign for the prism would incorporate some 3030 to beef up the x-z plane.

edit: Oddly 8020's website lists a moment of inertia for their 2020 that is double that of misumi (1.6826 cm^4 vs. 0.742 cm^4).

edit2: it seems that m4 thin square nuts (din 564) will probably work adequately, and are much cheaper at ~$5/100

Edited 2 time(s). Last edit at 01/10/2012 04:42PM by Buback.

Not so easy to source, those profiles.
Alas Misumi won't sell to ordinariy humans (non eneterprise customers) in many countries. When I asked why, "it is enterprise policy" was the aswer. (!)
on eBay in France, 30x30 profile is being sold at 5 Euros each 40 cm piece, plus 5 Euros transport. Sources in Germany are sort of less expensive, but need to check the transport costs. Anyway, significantly more than threaded rod (zinc-plated is 1.5 euros per meter at local store)

nuts for t-slots: there isa kind of nuts being used across MDF panels that will fit the t-slots (M5 or M6) and be reasonably stable (used that for a CNC bed until I got the t-nuts). They cost is lower. Also, if you have a column drill, it should be possible to tap and cut Al or brass profile into nuts

Personally I would not want to thread Al for any thread diameter greater than the thickness of the nut. My experience making a test rig out of 40x40 Bosch Rexroth showed that any home made t-nuts should really be made out of steel if there is any expectation to be able to tighten it satisfactory. This was mainly due to the fact that there is not that much depth to the slot. The wall thickness of the extrusion is also too thin to tap more than M3 (assuming that the M3 bolt is under low load).

On the note of suppliers, the Bosch Rexroth is fairly expensive (compared to the Misumi) but very good, extremely stiff and also has published Ixx, Iyy, surface area and modulus of elasticity making it very easy to make calculations to predict performance. It cost about £10/m from rs-online which i know has EU distribution.

Personally when I make my next printer/CNC mill I currently intend on using the Rexroth as its very easy to work with and I know what its like to work with. As an aside I have access to FEA software should anyone want me to model any designs/theories (although I'm currently finishing my MEng so will have limited time). I would be more than happy to do deflection predictions as well as natural frequency determination, although due to time constrains they will be simple beam models. Over summer I plan of developing a new frame using FEA and the optimisation techniques I have learnt throughout this year.

Lanthan, I'd appreciate if you could post any links you have to misumi profiles distributors in europe - having a hard times finding one currently, and contemplating building a prism.

[Sorry for thread hijacking]

Edited 1 time(s). Last edit at 01/11/2012 08:45AM by DeuxVis.

Wow. Much cheaper than I expected. Cheap enough to order a bunch just to have it on hand.

hmm well Misumi required me to put in my business name and ship to a business address, but i just put in my work company name and email and address. They even had a sales rep call me up after my order, so they think i'm a business. I don't know if your employer would have any problems with having things shipped to work, but if they do, maybe a relative or friend would let you use his/her employer?

But i totally agree that misumi doesn't seem to want to do retail work and so seems to have an ordering system designed to disuade normal people.

yes Sorry Lathan w all the thread-jacking. great job with the crossbraces.

@Buback: Certainly! I could order with the university address or something like that, arrangements are always possible I could see with ppl from IT dept for example, but I tend to not to buy from companies that practice this sort of discriminatory policies, namely refusing to sell to individuals etc. >< That's too much "the old system" the Internet has helped us beat back up to some point... there is always a risk of seeing it come back full blown. It is still too much in the mindset, alive and lurking. (sorry for the fear & loathing rant)

@DeuxVis:
En France
[cgi.ebay.fr]

In Germany
[cgi.ebay.fr]

@yellow-fish: THX for the RS-online link!

@all: you guys are always welcome to my threads

Yeah I love the Prism, but I think I'll build some Nophead-inspired contraption first. As for the aesthetics, I currently have the cnc router (provisorily) stationed in the living room, so I guess anything will go ;-)
Meanwhile, been re-designing clampable x-ends for flanged bushings (sigh)

Buback Wrote:
-------------------------------------------------------
...
> But i totally agree that misumi doesn't seem to
> want to do retail work and so seems to have an
> ordering system designed to disuade normal people.

Definitely not a retail outfit, so they're geared to be business friendly. I've had them ship to my house without any mention of who I work for. I got the follow-up call too. I don't remember what I put in the field for business name. I might have just said "hobby"

bit more digging on rs and found that the 20x20 is only £15 for 3m, £5/m seems pretty decent. The fixings are very expensive but brackets are not hard to make, especially if you have a CNC mill! the groove for the 20x20 is 6mm, M3 square nuts a 5.5mm and m4 are 7mm. The M3 wouldn't be a perfect fit but it's diagonal length is 7mm so would lock into the groove without rotating. Personally I don't think this would give a strong enough joint although that is defined more by the bracket and how many screws than the size of screw itself.

Do bare in mind though that the 40x40 is >10x stronger than the 20x20 and only 3x times heavier. plus the 40x40 has 10mm grooves which seem to be the standard from the way things are going.

Also just remembered about a product call helicoil. Not sure of a supplier but they can give a very good thread into aluminium thats much more durable that just tapped Al. Essentially they give a steel thread set into the aluminium. Not sure if its a good idea but this could be used for threading plastic, it would be interesting if someone got any for threading Al, for them to try threading extruded plastic with it.

Edited 1 time(s). Last edit at 01/11/2012 12:34PM by yellow_fish.

m3 plus fender washer (9mm diameter) fits perfectly, except for the spinning nut, but i've made some m3 tnuts that print nicely and prevent spin (up to some unknown torque. i haven't investigated at all). I would only use m3 in situations where weight (edit: or space) is an issue, though. I used mostly m3 when designing the Printonian truss, but used m4 with the Prism. I think the mendelmax uses m5.

Edited 1 time(s). Last edit at 01/11/2012 12:48PM by Buback.

The Rexroth only sell M6 and M8 t nuts for their 10mm groves. This reflects how strong the 10mm grooves are and the pressure they can take. Using smaller screws seems a little pointless to me as you can't fully exploit the strength of the construction system.

No offence meant by this but to me is seems a bit stupid to print T nuts which are not as strong as steel (or even Al) for use in a structure where your trying to maximise the rigidity. Its fine to use M4 for the 20x20 but personally I would want a stiffer frame than that and actually ensure that any issues due to lack of rigidity of the frame during high speed printing is removed. To achieve that I would optimise the brackets for uses with extruded plastic (which is an orthotropic material, like CF, where the strength is in shear and compression but not so much between layers) and then use steel T-Nuts.

If, on the other hand I just wanted to simplify the build and reduce build time by using extrusion, as opposed to increasing rigidity, then I would be quite happy to use printed T nuts. It does appear that we are reaching the limits of the development of frames using printed parts if we want to print with great accuracy at high speed, whilst having a small and light machine compared to the build area.

One more thing, nut and washer is not a good combination for T-Nuts which may be placed in the middle of a length of extrusion as you cannot tighten them. Any T-nut cannot need access from the underside.

A potential improvement would be to use the captive nut printed version and then add recess on the top and bottom for washers.these washers spread the load across a larger area of plastic making it take much higher loads and stop the plastic making direct contact with the Al. I hope that made sense, but if not I'll knock up a quick solidworks for you to see what I mean. The only advantage this method has over brought ones is a very significantly reduced cost.

This seems like a very negative post, and I guess that's because I personally feel that I would prefer better print quality over excessive use of printed parts in the frame. I also feel that the quality of printing is not really there yet on my machine to print structural elements that can compare to any isotropic materials such as moulded ABS. I find the that interlayer bonding is significantly weaker than the in plane strength and when I have some time I plan on scientifically measuring the structural properties different infill patterns provide and attempting to improve the interlayer bonding to create an isotropic material.

My basic point ,I guess, is that the prusa is weak mainly in the x axis and wobbles. This is not so much the rods deflecting as the plastic deflecting. Since we know that he plastic is the weak point of the Prusa, surely we want to ensure that it is not the weak point of the next gen, and therefore should use it very carefully in critical points like joints, where it kinda failed before.

The printed tnut is really just to prevent the nut from spinning. I also recommend using a washer with the tnut, but don't have a depressed cavity because it won't print. You would certainly have a problem if you removed the screw, because the washer would slide out, and getting them back into place would be a major chore. I haven't assembled all of the printonian yet because i got sidetracked with the prism.

I wouldn't use m3 with extrusions in a printer, though, except to attach accessories or sensors.

Perhaps M5 are the way to go, but as you say, the plastic is the weak element, so why bother. I'd rather use plastic instead of manufactured parts, though, since extrusion already feels like cheating a bit already, as do threaded rods.

The Sells Mendel has much thicker vertex brackets but it still isn't stiff enough, so I don't think you can blame the plastic brackets. It is simply the geometry that is wrong.

As stated before triangles are strong even when made with soft brackets, they are just not strong in the direction they need to be as deployed in Mendel.

I use plastic brackets in my Mendel90 and they seem plenty strong enough even though they only have 3mm walls or are 4mm thick. They are strong strong in the direction they need to be. I could make them a lot stronger, but having built the prototype I am happy with its stiffness.

As far as aesthetics are concerned I agree MDF does not look pretty, but I think acrylic sheets with big radii on the corners will look nice in a domestic setting.



Edited 1 time(s). Last edit at 01/11/2012 03:42PM by nophead.

---

[www.hydraraptor.blogspot.com]

yellow_fish Wrote:

> It
> does appear that we are reaching the limits of the
> development of frames using printed parts if we
> want to print with great accuracy at high speed,
> whilst having a small and light machine compared
> to the build area.

Printed plastic might still have some role in a frame if we value it specifically for dampening purposes.
More data and modelization would be useful for those purposes.


At the other extreme: would you consider a frame made with arc-welded iron profiles? the inverter/welder comes with an initial cost, but the base material is quite inxpensive... needs a good rig to pre-position the profiles with enough precision...

now where are those .scad / .stl files Nophead??

just kidding, I know a lot of time is involved with it!

/tom

---

regards,
Tom

Fair enough, I retract the comment about limits of plastic. Although I still feel that interlayer bonding can be improved.

If anyone has anything they want modelling, give me the data to model and I will do my best to model it, for example frames ect, especially for vibration as that appears to be more of an issue than deflection due to static loads.

As an aside, if anyone can give me a rough set of loads that are produced during printing I will try and do a topology optimisation and find an optimal structure. Obviously the more complete and accurate the data the better I can perform the optimisation. I would calculate the load myself but I simply do not have the time right now, but I need to work out how to run optimisations in ABAQUS from script and this would be a relatively simple task to attempt it on.

@ Nophead:

I've been following your progress on the mendel90 and the design does seem very good, a vast improvement over the mendel. The orientation of the material (MDF/acrylic) does have a huge effect on its strength and your design exploits that, meaning you can get away with using minimal material. Time will tell how acrylic stand up to the vibration of a fast printing reprap, Although MDF is another alternative I think acrylic is the better option as it is stiffer, although due to that stiffness it is also less forgiving, especially if its badly machined and has jagged edged to propagate fractures. I have no idea if this will be an issue in practice and look forward to seeing the fruits of all your hard work! If it does work as well as expected then it will be a vastly cheaper and simpler machine. I say simpler because putting all the nuts on a mendal, although easy, is tedious and time consuming. It's also not that easy to adjust when you clamping something which deforms as you clamp therefore changing its position, like the smooth rod for the y axis, or the z axis for that matter.

Yes the Prusa bar clamps are a pain to adjust. I replaced them with Mendel bar clamps on mine and just fitted two of the four bolts.

---

[www.hydraraptor.blogspot.com]