Introducing the Prism Mendel

I just wanted to announce my Mendel update, the Prism Mendel.

I've been working on this idea for about 6 months. It was hard waiting to announce this, but I wanted to get everything up and running before I posted anything. It all works and the parts print well, so it's time.

Parts were designed in sketchup, and the files and .stls will be posted to the wiki very soon (tonight). The build instructions are relatively straightforward, but that's easy for me to say. If you have any questions or need help assembling, you can PM me or ask here.

If you like the design and can use OpenSCAD, I'd appreciate help translating the files into scad format.

[reprap.org]

Forgive the crosspost, but the Extruded Aluminum forum doesn't see as much traffic as this one.

Nice job.

It's a little like mendel Max then.

Happy Christmas.

RICH

---

[richrap.blogspot.com]

yeah very much like it, except the prism is more like an updated Sells, instead of an updated Prusa. The z axis belt runs in a simple circle (actually more like a square, really) so is much more reliable and it's simple to adjust the tension. The prism also doesn't need feet, like the old mendel did. I don't think there's anything wrong with the mendelmax, but i just wanted to go in a different direction.

The frame, as you can see, lacks the 'ears' of the standard mendel frame. not only is it a cleaner look, but moving out the triangles opens up the build area, making it much easier to reach in and clean off the nozzle or remove the bed. All the extruded aluminum is one length (400mm) which makes assembly sourcing and assembly pretty simple. I'd say the mechanical assembly with extruded aluminum is almost as easy as assembeling a complicated lego model.

The parts are also easy to print on even a badly calibrated machine. The only part with an overhang is the Ztop, which is a truncated V shape with 60 deg overhangs. I think even that part can be made easier to print. Granted, the prism design currently is only for the frame, and i guess you could say parts of the z and y axis. The carriages/extruder are up to the user, though i think i'll add some standards in case people want to build kits, and so i can standardize instructions for a complete build manual.

Your design appears simple and rigid. I like the feature were you can align stuff in along the length of the extrusions. printed parts seems minimal. Design is expandable (can scale it up)

Possible Issues:
from looking at your photos I would be concerned about Y axis alignment in the Z direction... possibly corrected by adjusing the corner joints?
Also the alignment of the upper Z axis guide rod in the X-Y also appears tricky.... mendel can also be tricky because its adjustable in one axis only.
Are the extrusions expensive?
I am also wondering about maximum part height. Mendel streches the frame up a bit by the corner bracket design.

Possible enhancements:
I like the single z axis stepper driving two screws with one belt. I would move the belt idler from the current location to between the motor and the timing belt pulley on the motor side. That pulley sees only a 1/4 revolution belt engagement. Could you then make the arm that holds the idler be extendable into the printer body? The idler then serves to tighten the tension on the belt, increases the pulley contact to something over 1/4 rev and uses a shorter belt. Belt track goes from a square to a tringle looking shape.

Yep I wasn't sure how long the belt was going to be, but it turns out the standard mendel z belt is a bit too long, requiring the square path, so i just threw a bearing and some fender washers on a screw and put it wherever; one less part to print. I like tensioning the belt by sliding the z motor bracket, since i can do that without having the put the printer on it's side. the weight and the two screws also help keep it in place while i tighten it. Even with only 1/4 contact, it runs soo much smoother than my old Sells, mainly because it follows a much simpler path with only the single idler (which could be even simpler, like you suggest, with a shorter belt)

the z axis top gets positioned correctly in the middle of the y axis just because of it's shape, but you are right that it can move a bit allong the x axis direction. The hard limits, however, are basically the m4 screws in the 6mm wide t-slots. So if the top bracket screws are hard against one of the sides of the t-slot channel, the bottom bracket screws should be hard against the same side, and you'll have pretty good allignment.

The same goes for the y axis z allignment. for the y axis, i'd say x allignment is harder than z allignment, though a jig, just like with the mendel, would help greatly with this.

If it is an issue with builders, as it might be for reprapers trying to acchieve extreme quality, it should be pretty easy to design a bracket with 3 m3 screws 120 degrees opposed, so that by tightening and loosening the screws you can adjust the position of the smooth rod ends to really dial in the perfect allignment.

The extrusion only cost ~$11 from Misumi. The t-nuts, on the other hand, cost ~$19! I just bit the bullet because i wanted to start building, but if anyone knows where to get cheap M4 tnuts, please add a link. Or you could always print them out yourself.

The maximum z travel has actually gone up for me. in the standard mendel frame, the x carriage hits the frame triangles befroe the top of the extruder hits, so you don't really benifit from the truncated design. moving the frame triangles outward gives me about 20-30 mm more z height. The Y axis is also much lower, which gives you another 20-30 mm increase. My max z build is right around 200mm now, though your milage may vary because it's highly dependent on your x axis and extruder and y axis. I think the lowest the y axis can go is ~20 mm above the frame/bottom of the z rod. this is because of the y motor, which juts up about 20mm.

Sounds good. You put a lot of thought into this.

I'll take your word on holding and alighing the upper Z axis guide rods. I'm not getting whats going on by the photos alone.. but no need to at this time.

On the less technical side....
I believe that in order for any of these designs to become popular they need to target one of the following user types:
1) I need a printer fast, don't what to build it, just by something that works (like going into a retail store) - high potential but doesn't really exist
2) I'm willing to buy it on line - Highly Popular but a minefield of poor suppliers that hurt everyone.
3) I'm willing to build it from a kit - Somewhat Popular and exists all over the net
4) I'm willing to build it based on standard parts I buy from many sources - also somewhat popular
5) I already have a printer and looking to build another one with improved quality - your design and many others in Reprap.org but I'm not too sure how popular.
6) I'm willing to build it from nothing more then plans off the internet (no printed parts) - You see less and less of this because printed components are so available, so not popular at all.

What makes this home FDM topic so interesting is that there is a huge market for this.
Is the next "Dell" out there and what will their winning stratigy be?

hmm i'll try with ascii until i can take pictures:

   /\
  /  \
 /    \
/  /[/b]  \
--------

The top "V" shape is the frame, and the bottom "V" in bold is the piece. the piece can only touch both sides of the frame at the very top, which is alligned right in the center of the bottom frame element. I think you understood that part, but i just wanted to clarify.

Cross section of Extrusion, with a screw in the channel:

=====    ||=====  Screw on right side of channel

=====||    =====  Screw on left side of channel

=====  ||  =====  Screw can only move 1mm to either side of center (6mm channel, 4mm screw)

So, this goes for any part screwed into the extrusion. There's some play, which allows for some missallignment/adjustment (depending on which side of the coin you look at ) Going to 5mm or 6mm screws/nuts might make sense, but i chose 4mm for the first build because you can potentially print 4mm tnuts. I don't know if there'd be enough plastic to hold a 5-6 mm screw/nut securely.


If it's still not clear, bear with me for a couple more days till i get some more pictures up on the wiki.

Edit:
Oh and I don't intend to make any money from this design. I'm not selling parts or kits. I just wanted to share my design and give back something. I don't know the limits of the design yet, but i assume that i can print much faster because of the increased rigidity. The larger build area is also nice, but will require a wider bed/y carriage to take advantage of it, so that's a custom job until there's a standard. Wire management is also a pleasure, since they can be routed inside the t-slots and hidden. I've also included a printable clip to hold the wires in the channel.

The basics of the frame, the 90 and 60 deg pieces, are flat, and could be cut from steel, aluminum, or mdf, if you don't have access to printed parts. The other parts would be a little harder, but could probably easily be made with just a saw and a drill press, and some thought.

So, frankly, i don't think there's any need for threaded rod anymore (except for the lead screws). I would advise anybody who wants to build a Mendel to either go with the MendelMax or the Prism, as long as you can source the extruded aluminum. maybe it's presumpsuous, but the threaded rod mendels should be retired like the darwin has been (well, maybe after a lot of work on the build instructions for the new machines, and a lot of testing).

Edited 4 time(s). Last edit at 12/25/2011 01:36PM by Buback.

I'll keep an eye out for the photos. The design looks clean.

I have a threaded rod mendel and find no issues with rigidity. I have seen plenty of posts where folks complained about lack of rigidity so I know it exists. I believe that the weakness with the threaded rod mendels isn't the rod but the printed corner members, mostly based on a guess because I'm using corners that were built from 3/4" plywood, that can be bolted very tight.

Keep in mind that extruded aluminum has been used for a long time to build light machine frames so there is no argument about it capabilities. However you can't pick it up in a local home center. I tend to gravitate to things I can build from very common sources.

These days I'm mostly working with the combination of welding light gauge steel and printing. I find that welded parts, like printed parts have few limitations. For example I'm using a welded extruder design that is simple and robust compared to the popular printed version.

So, what are your plans to carry your prism design concept into the extruder?

Instead of T-slot nuts, wouldn't regular square nuts work?

Edited 1 time(s). Last edit at 12/25/2011 09:27PM by peqo.

I'm using the same nuts recommended by the mendelmax guys, which are rectangular pieces of steel with an m4 threaded hole (though i think they're using m5). But i just chose them for simplicity and compatibility. You could even use a plain m4 nut and washer, if not for the fact that the nut will spin endlessly. I don't know the dimensions of an m4 square nut, but if it is similar to an m4 hex nut it would probably work great.

The original Sells vertex parts were thicker and more rigid. I also remember that you could increase rigidity by canting the frame triangles in slightly at the top. So you could make a mendel more rigid, to a degree. I have had problems with rigidity when trying to increase print speed. Lots of fast, short moves caused a lot of back and forth movement along the x axis. I also want to try milling with a dremel toolhead, and i know that this is one of the key areas where the lack of frame rigidity was causing issues.

I think the weakest point on the Prism, just as with a Mendel, is the printed parts. Most of the parts are only 6mm thick, which is very thin. I chose 6mm so that the m4x12 screws wouldn't bottom out. You can easily adjust the parts to use m4x20 screws if we find that the 6mm parts don't cut it. Another option would be add more parts to reinforce the frame vertexes, since there are still unused slots on the extrusions in most of the vertexes.

I'm not discounting the mendel or prusa. I think the biological language adopted by the project is very apt, in that there are now diverging classes of machine designs for different situations and use cases. Maybe the Reprap community wasn't big enough during the Darwin-Mendel change in order to support multiple machine designs, but the community has grown much larger and less centralized, so I shouldn't say the old designs should be retired. I guess it's better just to let the evolutionary process play out on it's own.

Ok, I've got some preliminary visual instructions up on the wiki, and I've put all the files up on Github here.

I'm still putting together a BOM. Is there anything else I'm missing?

When I open your sketchup models all the holes are a series of flats. the larger the hole the more flats. Is this part of the design or sketchup? I got frustrated by sketchup some time ago and went to another free modeler called autodesk 123D.

The few parts I looked at look pretty good. They are all printable.

The only concern I would have is strength of some features in these printed parts. I'm looking at the rod holder Z and Y bottom and seeing a 2.5 mm wall behind the nut. In PLA I wouldn't be able to trust the strength of the wall unless you plan to add a hard coating to the part (like CA). Could you go with a non standard component? for example I've taken 3mm thick sheet steel, tapped it and cut it out as a rectangle. Like a square nut but longer in one dimension. You don't need the strength of thread engagement (like the way a nut is normally designed) but do need to spread the load out because you are working in plastic. For all the effort maybe a square nut would be strong enouth.

I usually use polygons for holes instead of circles, because some firmwares have problems with processing all the gcode moves required for a circle fast enough. the code will just be dropped, and you'll end up with misshaped holes that are too small. it doesn't matter anyway, since they are hidden when it's all assembled.

The little m3 screw doesn't do much except keep the rods from sliding back and forth, so it doesn't have to be torqued down much. you could do without them all together, maybe use zip ties some way. Since there are only a couple, I'd rather get rid of them if possible, since it's almost too few to have to order, if you are starting from scratch.

Yeah i take care to make sure all my parts are manifold before i post them, but thingiverse is full of bad sketchup files.

Interesting comment about dropping code. I had that problem when using Gen3 electronics. It was just not keeping up. Switched over to ramps and have no issues keeping up, but I'm not trying to drive the printer at high speeds.

All modeling software outputs STL in a series of triangles that look like flats on any curved surfaces. One issue I had with Sketchup is that the flats are more pronounced and the STL converter is a free download, unless you pay for the pro version. Autodesk flats are far less pronounced and the stl converter is part of the package, free beta. No doubt that my files are larger, but thats not a concern.

I understand that some of these holes are clearnace but I'm sure they all are not. For example I think I saw a 22mm bearing pocket in your design. Attached is an image of the back of a timing belt pulley I printed from an autodesk model. It has a 1/4" hole held by a 4mm cap screw. You can see how the flats can hardly be seen on ID and OD. There is a custom built nut in the slot, similar to what I described above.

I also design 22mm bearing pockets at 22mm and print them as round as I can. The bearing won't fit in a cold PLA part. So I drop it in a cup of water at about 100C for about 10 seconds. Then push the bearing in and let it cool. The result is a snug fit and I can take the bearing out and in with no issues.

I uploaded the pulley to the Autodesk 123D gallery

Timing belt pulley

Thanks for the input Dean. You are printing at much better quality that I am. I don't really like working with arcs and rounded parts, but that's just a personal preference for angled shapes.
Anyway, all the parts work fine, since I've printed them myself, but I'm very willing to incorporate parts that work better into the design. And anybody that wants to fork the design is, of course, welcome to.

I can't wait to hear some feedback from anyone that decides to build the prism, so if you do, please give me a quick review.

I think I am going to choose that design for my next reprap.
I suppose that would be compatible with those printed T-Slots ?

yes, but either the parts or the tslot profile will have to be modified a bit.

Some of the parts have a 5mmx2mm tongue that goes into the slot. This keeps them centered and properly aligned. The tslot I used has a 6mm wide channel which gives about 3 degrees of misalignment between two screws 20mm apart. the tongue printed closer to 5.5mm for me when i printed the parts, so the 6mm channel is necessary. So you would have to adjust the tslot to have 6mm channels and 2mm thick walls for it to work. I don't know if it will print as nicely with these adjustments, but i'll ask them.

The other option would be to take off the tongue from the parts. if you made the channel 4mm wide, you wouldn't need the channel to maintain alignment. you can use their m5 profile as-is if the parts didn't have the tongues, too. maybe i'll do that and make a 'fully printable' branch?

On another note, I tried printing at 150mm/s last night and the results were much better than expected. I printed a perfectly usable y idler in 10 mins, which is at least 2/3 faster than my regular settings! I still have a lot of tuning to do, but it's only extruder issues. So the prism can certainly handle high speeds. I'd like to upgrade to a more robust x axis before i try printing faster, though.

Edited 1 time(s). Last edit at 01/05/2012 10:56AM by Buback.

I gave up with printing the profiles for now, will try that when I have an aluminium version working.


Follow-up on thread [forums.reprap.org] that I want to stop polluting.

I asked Misumi for a list of distributors for individuals, and they sent me a quotation as an answer. So I confirmed command and the profiles should be there by next week.

I have to hurry getting the rest of the BOM now - and need to repair my Huxley so I can print the repraped parts. So many things to do and so few time

Received the profiles and nuts from Misumi europe, despite of what the saleswoman who called me told - she said no order without a VAT number would pass through. Go figure.

No I really need to get my Huxley printing all those RP parts.

Great. I can't wait for someone else to make one and share their thoughts.

I'm watching this topic so keep posting as you progress. I'm sure it'll help others who decide to take the plunge.

Progress report :

1) Printed most of the parts - the Huxley finally is starting to get reliable.




2) Have gathered/ordered most of the vitamins now. Still need smooth rods and a few nuts and bolts.


I have a couple of questions :

About mounting the 2 large pulleys on the Z axis threaded rods : I suppose it has to be done à la Mendel, but I fail to see how the pulleys will not tend to rotate loose on the lead screw. Tightening the nuts which hold it in place is enough to avoid rotation ?

Also I believe the rod holders parts are supposed to fit a M3 nut, but for me it is rotating "nearly" freely in its slot, I will maybe switch to M4 there. Is that my printer or did your parts have that too ?



Edited 1 time(s). Last edit at 11/22/2013 08:13AM by DeuxVis.

the parts look great!

The mendel instructions really suggest lock nuts or thread lock for all m8 nuts. I've been using lock washers and it seems to work fine.

My old Mendel was printing thick lines when i originally printed the parts, so i might have scaled those channels up a bit (can't remember). Frankly I'm not sure the smooth rods even need to be locked in place. With my pieces, the nuts also spin unless i hold them in place with a screw driver. I didn't want to make them a complete trapped nut because it is a pain to put them in place and they often strip.
Please also tell me whether the smooth rod fits. the pieces needed some reaming when i printed them. This also made the m3 screws a bit unnecessary because the rods are so tight they don't move much. I'd like to eventually take them off the BOM completely just to have one less part to order.

Ah yup, lock washer of course. Thanks for the tip.

I only had to ream the smooth rod holes a little bit. It actually might have been possible to force fit the rods without that, but I don't have them yet to test. I use the handle of my cutter to test, it happens to be exactly 8mm diameter - but it's difficult to force a lot on that while grabbing the cutting end

:-) been there myself. The right tool for the job is the one within arms reach :-D

As long as it isn't loose, it's probably unnecessary to secure the rod on both ends.

@DeuxVis: Please also give some comments on if the build instructions work out for you {Or where they fail you} when you eventually get building.

thanks!

Sure, not there yet but I will.


Edit : now that I think about it, one information missing (did some guess work instead) is the length of the closed belt (Z axis). I took one that is sized for regular mendel (1 m), as I figure it has to travel around 80 cm plus some width.

As for the open belts, well that's less important as you can get a long one and cut it.

Mmm now that I reread the wiki, that should have hinted me :

If you currently have a Mendel (particularly a Sells Mendel because of the belts and Z gears) this frame is quick, relatively cheap upgrade

Edited 2 time(s). Last edit at 02/04/2012 02:56AM by DeuxVis.

yeah I used the standard size belts, just because that's what i had on my sells. The Z belt could be shorter, but i don't know what size to recomend so i won't. I should have noted that i just screwed a bearing, with some fender washers, right onto one side of the profile to act as an idler. This makes the belt follow a mostly rectangular path (see the picture at the top of the wiki page showing the bottom of the prism)

I moved the idler farther away from the z pulley after the picture so that it would have a bit more contact with the pulley, but you really have a whole lot of freedom on where to put that idler.

Little update on progress.
Made a heated bed for the Huxley so I can print the X carriage in ABS, it seems to work fine : [www.thingiverse.com]
Now I need to decide on the carriage I'll print.

Apart from that I just need the smooth rod, extruder hobbed belt and a few bearings to arrive and I'll be ready to start the build.

Started to ream all the parts.

---

Most of my technical comments should be correct, but is THIS one ?
Anyway, as a rule of thumb, always double check what people write.

Neat design. To address the problem of fitting the Z rods, how about mounting them outside the frame, opposite the threaded rods rather than within the frame. Then the Z rods could be simply dropped into place when the frame build was finished.

T Case