MegaMendel

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MegaMendel

Release status: Experimental

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Description
Giant version of Mendel
License
GPL
Author
Contributors
Based-on
Categories
CAD Models
External Link

The idea for the MegaMendel came in the early 2010. The project was spurred by an idea of printing parts for a small wind turbine.

It is a mighty machine with a build area of 766mm x 453mm x 497mm. The X and Y dimensions are the freedom on the respective axes. The 497mm on the Z-axis is actually the height of the tallest object printed on it so far, please refer to the image below (the ruler is 600mm).

Record.JPG

It is possible to tweak it to print at least 55mm in height, but that will have to wait as I don't see the point in doing that right now :-)

The machine's dimensions are 1160mm x 880mm x 830mm and my guess is that it has a weight of some 30 to 35 kg's.

The whole project made me rethink parts of the construction. I had two reasons for that:

1 - I made the "plastic" parts in MDF - So there was a need for a design that would compensate for the weaknesses in MDF.

2 - I don't agree with some of the solutions in the standard Mendel, so I changed lots of details to what I consider simpler and better solutions.

So the MegaMendel is more of a RepStrap.

This page is meant to document the build process, and most important design decisions for others to adopt or comment.

What I really like about the Mendel concept is the frame and the idea of extruding material instead of removing material.

The design of the MegaMendel has been with emphasis on keeping things simple as well as ensure easy access to vital parts for repair or maintenance.

The frame

The first practical decision was the size and dimensions of the steel rods. My first thought was to simply double the size of the frame, but the I learned that threaded rod come in 1 meter pieces. That decided the length of the X-axis. The triangles are sized so the whole thing fits on a 80cm wide table. The rods themselves are 12mm instead of 8mm, that seems to be a good decision as the machine is fairly stable without to much weight.

The frame vertices were a bit of a challenge as I was working with a 9mm MDF sheet - I made a lot of small pieces and glued together to a larger piece that cut into shape so I got the thing in the image below.

Frame start.jpg

After some more cutting and drilling I had a frame that's just a big copy of the standard Mendel frame as seen in the image below.

Megaframe.jpg

Please note the beer can and the 60cm ruler.

The Z-axis

After the frame the turn came to the by far most complicated part - the Z-axis.

I made a few changes to the design here: The 60 degree angle between invites to tension, thus causing more friction than necessary, so I changed the angle to 90 degrees.

I also decided to mount the angled bearings on the Z-carriage itself, this is due to the fact that the angled bearings are the most important constraint, so I don't like having them on a part that has been weakened for adjustment purposes (I applied a similar philosophy to the other axes - more about that later).

It took two iterations before I was happy with the Z bearing arrangement:

Z 180.jpg

The one on the left was my first attempt, then I decided to go for a lower profile, giving about 10mm of extra length on the X-axis (I don't know what I was thinking, but I went for those stupid 10mm). This design will be changed a bit as I think a single retainer bearing is more than enough - That change will be when the machine is taken down for repair/maintenance anyway :-)

I decided to keep the lead screw in the 8mm dimension, but I added bearings to the top.

The belts, I got, are 10mm wide so I had trouble finding space for them at the bottom of the machine. The solution was to put the whole Z-drive on top of the machine, so the lead screws are actually hanging in the top bearing rather than "standing" on the bottom bearings.

This proved to be a good solution as I get a lot smoother run this way, and it runs very nice at 100mm/min.

At this time the MegaMendel found it's way into my study, where my cat took it into possession:

Catinframe.jpg

Please don't mind the date in the image - My camera was a bit of that day :-)

The main reason for the move was that more delicate parts were about to be added, so the dusty environment in my workshop was not the best place any more.

The Y-axis

The big challenge with the Y-axis was to prevent sagging, so I started out with a design applying six!! bars to the Y-axis. Then I came to my senses and designed it with just four bars. With three support points on each bar I would have just about the same distance between supports than the standard Mendel. The image below has not all the bearings installed yet, but it pretty much illustrates the concept:

Y chassis.jpg

Apart from the four bars there are two major differences from the standard Mendel here: The Y-chassis is running on top of the bars - This brings the chassis close to the print bed, allowing the support screws to be mounted in holes in the chassis rather than spending plastic on printing holes.

The next difference is the second bar from the left - That's where the angled bearings are located. This mean that uncle gravity helps us out here. In this has worked quite nice so far with no retaining from beneath the bars! (with the middle support bearings installed, that is).

Again the angled bearings are in 90 degrees as opposed to the 60 degrees - I even printed the height record with this set up!

The X-axis

The image above also have the X-axis installed - Again the angled bearings runs on top of the bar. Please note this design has got only seven (7) bearings on the X-carriage.

The X-carriage is not yet finished in the image above as I was expecting deliveries from MakerBot Industries. One guy there has my special gratitude - Isaac Dietz has been helpful beyond any reasonable expectation - Thanks a lot for that!!!

Deliveries

Having a bachelor's degree in mechatronics I could easily have built the electronic parts myself. But I saw lots of challenges elsewhere, so I went ahead and ordered the electronics and the steppers from MakerBot Industries in Brooklyn.

The electronics should be known by anyone in the community, MakerBot were the only ones to have them in stock at the time. Furthermore they were able to supply the mighty NEMA 23's for the big machine.

My deliveries was somewhat delayed by a volcano that decided to burp just as I was creating big things here - That's most annoying, they should put a cork in that thing!

The drives

The drives - Big machines need lots of torque. Or maybe not - I've simplified the drive mechanisms of two reasons: To get simpler mechanics and also to get less friction.

Common to all drives is that I bought the pulleys in standard sizes, and I had to drill them myself - One came out catastrophic, so I had to get new one. The others came out lousy, not too good and perfect. I decided to put the lousy one on the Z-axis as it has got the the longest belt and a bit of wobbling is not really that bad due to the high gearing ratio. The X-axis carries the least weight, so it got the not too good. The perfect one ended up on the Y-axis as this is the one that potentially carries the most weight.

All the pulleys got glued to the stepper axles with some two-component epoxy stuff - And they have stayed there for quite a few hours of building time. I did scratch the axles as well as the holes in the pulleys before gluing them - That might be why they just work flawless, or maybe they would've done just fine without the scratches :-)

The choice of the NEMA 23's may have been relevant for the Y-axis only - With a huge build volume that is. Furthermore the stepper drivers go into thermal shutdown when running the NEMA 23's at the full current. That could lead to losing steps during a build. I remedied this by turning down the current limiters a bit.

Z

The Z-drive was the first to be created, and it is kept quite simple:

Z drive.jpg

Please note that the are two 624 bearings in the guide tensioner parts, this due to the 10mm belt. The guide bearing could be omitted with a little redesign. And yes, it takes a bit of the build envelope, but this is the far end of the X-axis and my guess is that I will never need that. It would be fairly easy to redesign this so the stepper goes on the side of the frame so the build envelope would be complete.

I've heard arguments that this makes the construction more top-heavy, but that stepper has a weight of only a fraction of all the steel up there already.

In fact I'm really happy with this design as it also keeps vital parts in full visibility and it is easy to access for repair or maintenance - I may add that this design has worked without a single glitch for many hours of building time already.

X and Y

The X and Y drives do not need much explanation:

Xydrive.jpg

I don't think this can be done simpler than that.

The X-axis - This is where the NEMA 23 stepper appears somewhat ridiculous as my X-carriage has a weight just about the same as the standard Mendel's X-carriage - Then again, my Z-axis has a NEMA 23 to handle it :-)

Then there is something I just don't get about the standard Mendel, it has six bearings in the X-drive - MegaMendel does that quite a bit simpler. The Y-drive is also somewhat simpler.

X and Y tensioners

You may not believe it:

Xytension.jpg

Please note this after you're done laughing: They work like a charm - Never had a problem with them!!

A word on bearings

The MegaMendel will have a total of 49 bearings when the Y-retainers are in place - The same ideas applied to the standard Mendel would make it possible to build a standard Mendel with a total of only 30 bearings. I know some people don't as they get the bearing very cheap. I see reasons to reduce the number of bearings:

- Less friction - I have seen standard Mendels with even the Y-axis running a lot tighter than the MegaMendel's Y-axis!

- Simpler/easier assembly

- Possibly less plastics is needed as well

More about my design changes later.

The USB2TTL cable

I bought the FTDI converter cable together with the other stuff from MakerBot. That one turned out to selfdestruct before it even worked. I know the MakerBot guys have seen this kind of problem before - They might be interested to know if anyone have identified a pattern here.

The Plastruder

I was aware that the extruder is by far one of the biggest sources of problems, so I thought I played it by ordering MakerBot's Plastruder. I got a bit wiser. The incident occurred after some ten minutes of test extruding - The idler wheel broke. later on the thermal barrier deformed and leaked plastic (I think that was my own fault as I tried running at 240 degrees to avoid filament jams). That may in turn have lead to the heater retainer breaking.

I made a few hacks to the Plastruder, and had it running without any problems ever since.

The Plastruder hacks


Idler.jpg


Build surface

The first prints

Printing new parts for itself

The warping issue

Lessons learned

The good ideas

The bad ideas


Ideas for the future