A Second Mendel90

So, after months and months and the beginning of a whole new year, I have finally finished assembling my second Mendel90, a true RepRap, from parts which were made on my first RepRap, a RepStrap version of the Mendel90. The RepStrap was scrapped for parts which were used in this printer; almost everything - the electronics, extruder, motors, belts, bearings, bed and bars were all saved, the only bit that wasn't was the old frame.

So why destroy the old printer to make this one? Well first of all, I didn't really expect all that hot glue in critical places to hold up for too long, and secondly, I had a mistrust of the old printer's accuracy. The old printer also had a smaller movement area, about 150 x 170mm and 170mm on the Z axis. This new one can achieve 200 x 200 x 160mm, a little less on the Z axis, but much better in the X and Y which is where I wanted the build area. I do however limit this movement area to avoid hitting the bulldog clips which hold down the glass plate to the build platform. Lastly, the Mendel90 provides a very neat wiring solution using ribbon cables, printed cable clips and  mounting holes and so on. All this is accounted for in the drilling templates. If you've ever made a RepRap with wires all over the place and a power supply hanging off to the side on your desk somewhere, you'll understand how nice this is.

The wiring at the back, I know it doesn't look neat, but really it is when you're working with it, and  none of it can move around so no worries with wires getting tangled, accidentally pulled out of places, etc. 

The build went as smoothly as I could have hoped for. Nophead's amazing python script was spot on down to the last washer. However, in the process of building this machine I did modify a few things, and there was a bit of bodging - mainly to do with printed nut traps, but more griping about that later.

First and foremost, I had to modify the OpenSCAD file for the X carriage to fit a Budaschnozzle - the nozzle I already had from the old printer. After exploring the file for an hour or so, you soon figure out the gist of what's going on and can modify it and pray that it's all working out right. This file I managed to keep nice and parametric, though you have to open its SCAD file yourself and manually generate the stl file for it - it's not part of Nophead's python script, though it does have dependencies on many original Mendel90 SCAD files.

... and for more tweaking of OpenSCAD files. I realised that one of my smooth rods on the Y axis was 16mm too short, and instead of ordering a new one, I decided to change the distance between the brackets, of course this means that you'll also have to move the linear bearings inwards a bit on the Y stage. This involved putting "-16" on the end of some parametric value which sounded like the right one when I was looking through the files like so: "Y_bracket_spacing_or_something_similar = something - something + something/2 -16." Assuming Y_bracket_spacing_or_something_similar is actually what you guessed it to be and isn't some value that only gets outputted into the BOM rather than actually changing the model and thus the drill templates, it should work as you intended. However there is one more pitfall - you must hope that Y_bracket_spacing_or_something_similar is a sort of "higher level" variable from which ALL the other Y axis-related stuff is calculated, i.e. bar length, distance between the two Y bearings, etc. is calculated. If not, stuff gets harder and you have to repeat all this guessing and searching for yet another value. Though it should be noted that this approach of "just insert a number here" isn't very parametric - very much a quick fix, in this case what will happen is ANY machine you generate will have 16mm less than what the original OpenSCAD script thinks it has. There must have been an easier way to do this that I missed...

Also, nut traps in the printed parts came out too small, especially the smaller sized ones, however I noticed that this only happened with the ones where the hexagonal perimeter was extruded on the X and Y axes. For M3 sized nut traps, some turned out only just over half the size of the nut!
There are a few possibilities as to why:
  1. My X and Y axes weren't calibrated well enough, however they can't have been too inaccurate given how everything still seemed to be printed in about the right place, give or take 0.1 mm or so.
  2. Corner cutting, probably one of the biggest ones since the corners on the hexagons seemed a bit rounded.
  3. Too much plastic - this is probably also one of the main reasons. For one thing, the top layers of some objects came out a little "over stuffed" with infill.
  4. See also arc compensation, this also applies to corners, where too much filament ends up on the inside.

Thanks to the shrinkage, I spent a lot of time scraping out nut traps with a small screw driver, or sometimes I even just drilled out the nut trap and instead held the nut with small pliers or a spanner while tightening. I don't know how I'll ever replace the Y belt, or the Y belt anchors now...

For the build platform, I couldn't find any hex pillars, so I came up with another solution. Note the star washers used along with the plain washers, these are very important to stop things coming loose with all the vibration.

The Z endstop set up. Also, on the Z leadscrew you can see a
Z screw isolator which was added much later to try and reduce
Z wobble. (Note that this is a very recent photo, it has been
many prints since the machine was first built)

Another place where I didn't follow the design exactly was the Z endstop. Though I printed out the part for the lower endstop I never used it. Instead I used the same method as on the old RepStrap. In what is one of the very few uses of hot glue in this machine, I glued the endstop directly to the gantry, in about the right position, give or take a few mm. To get the adjustability, I glued a couple M3 nuts to the side of the X axis and put a 20mm M3 bolt through them. This solution is more than good enough to allow for leveling, a build surface change, etc. if there's ever a big change I'll just have to pull the endstop off and move and re-glue it.

However, other than a few things, the build went smoothly, whenever I had time that is.

Here is a video of the first print:

The results of the very first print:

Not a great set of parts, barely even usable in fact, but it didn't stop me using them.
Why were these parts unusable? Pretty much all due to the extruder missing steps
because I set the retractions to be too fast.

This was the third print I did - the second print was the big gear for this extruder:

By now I had all the settings well figured out and prints were looking great
except for some Z wobble, which was at least still not as bad as on the old printer.

The first few prints were parts for the Wade's extruder which the Mendel90 uses, I hadn't bothered printing these on the old machine due to time. The machine was at first using Greg's hinged extruder and the ribbon cable was kept in place with a bodge which can be briefly seen in the video of the first print.

All in all, I am very happy with the printer and I can now print reliably and the bed seems to stay level much better so I don't have to worry about re-leveling every time I want to print and it's great not having to worry about hot glue melting.