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Marcos Scriven

Sporadic poster of diversions

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Why the Mendel90?

I can’t recall when I first read about 3D printers, but it was a long while since then that I actually got to see them, and their output, for myself at the London 3D Print Show back in October. The biggest exhibitor by far was Makerbot, but there were lots of others showing what to me looked like DIY contraptions (looking back it’s clear they were all RepRap variations). I also saw the Formlabs Form1 in action, literally flashing within its curvy orange cube, which was absolutely fascinating.

androids.jpg Some of the examples at the 3D print show

Seeing some of the things possible, I was sold on getting a 3D printer, but deciding on which one was difficult. The Makerbot Replicator 2 seemed the best bet for someone like me - a complete out-of-the box product with a good support community. However, the cost in the UK, after being carted across the Atlantic, and slapped with a nice 20% so-called ‘Value Added Tax’ courtesy of Her Majesty’s Revenue & Customs, was just shy of £2000 (about $3200). For something that may well prove to be no more than a handful of weekends’ entertainment to a dilettante such as myself, it really was a little too much to justify.

Having narrowly avoided dropping a couple of grand on a shiny Makerbot, and reading up on the many experiences from people in the RepRap Forums, I decided to go for one of the RepRap family of printers. I settled on one variation called the Mendel90, a design that uses panels instead of rods to build its frame, and ordered this kit from +nop head (the guy who invented it), for a much more friendly price of £499.

The Kit

I was lucky enough to order the kit just as Nop Head had cleared his backlog of orders, and it arrived just a few days later. Unfortunately I had it delivered to my current client’s offices, and it ended up going to one of their other buildings (despite being correctly and clearly addressed), and opened up and unpacked by someone there before they realised it wasn’t for them. I thus received my kit with pieces all jumbled back in the box, which was really somewhat worrying.

androids.jpg The unpackaged kit and enclosed Bill of Materials

I immediately contacted Chris (and Mary, his ever efficient partner), and had my fears put at rest. They told me the most important parts (in terms of cost) were the hot end, Melzi control board, and stepper motors. To my relief, these all seemed fine.

androids.jpg Hot end, Melzi board, and stepper motors

It was only at this point I really looked at the instruction manual, and what seemed to be a million different screws, bolts, rods, and more soldering than I’d ever done in the 15 years since university. Despite having the right sort of knowledge in theory, it was still quite daunting to actually put it into practise.

Putting the Frame Together

As it turned out, the manual is actually very approachable, detailed, and organised into clearly defined sub-assemblies. Not only do these sub-assemblies make the whole thing less daunting, it also means it’s easy to spread the load if you’re building with other people (and a nice little real-world analogy for concurrency vs parallelisation) .

The first of these sections is putting the frame itself together, which though being the simplest is oddly the most satisfying, as it feels like one immediately has something to show for their efforts.

androids.jpg Don’t forget to degrease the bolts - I used acetone (nail varnish remover)

I say ‘simplest’, but I still managed to mess it up. I was attaching the very last fixing block (before putting the panels together), and got a little too heavy with the alan key. I emailed, tail between legs, to beg for a replacement, and Mary kindly sent off a couple straight away.

androids.jpg Don’t over tighten the fixing blocks

I moved on to other parts, but in the interests of keeping this log sequential, here’s the finished frame.

androids.jpg The completed bare frame

X Idler

While my frame lay frustratingly unfinished, I moved on to the X Idler Assembly, which is the part that moves along the Z axis, whilst supporting the X axis belt on its bearing. Being new to this I first had to identify which were the ball bearings and which were the linear bearings, but from the instructions it was clear the long ones (which move linearly - duh) along the Z axis were the linear bearings, and the ball bearings were the flat ones upon which the X axis pulley idles. Putting together was straight forward - my only worry was snapping in the linear bearings, having already broken one of the fixing blocks earlier.

androids.jpg Completed X idler

X Motor

As this was the first time selecting which ribbon clamp to use (you do use them on the frame, but in my case I had to abort that while awaiting replacement fixing blocks), it took me a while to work out which was which. Turns out the ‘N_33’ ones are 43mm, and the ‘N_14’ ones are 36mm. It also took me a while to find the prefitted ‘grub’ screw in the pulley, as it really is exceptionally small - basically a little black dot on the side. It required a 1.5mm alan key to tighten, the very smallest you can get, and not included in your average set. Soldering the motor and limit switch wires was easy enough - just remember to put the heat shrink on before soldering.

androids.jpg Completed X motor assembly - four wires for motor, two for the limit switch

My biggest worry was wiring up the IDC connector - but as it turned out it was in fact very easy. The IDC connectors themselves automatically line up with the strands of the ribbon, so all you really have to do is clamp them together.

androids.jpg Behold the solderless power of IDC connectors (with incorrect strip)

In the picture above you can see the green plastic strip is actually wider than the ribbon cable - I discovered later that was a mistake on my part - the wider strip is needed along the Z axis, as it goes up and down the gantry.

Z Motors

Putting these together was very easy, my only worry was putting pressure on the motor shaft to get the PTFE tubing on. Incidentally, the manual introduced me to a cool way to twist pairs of wires - stick one end in a drill chuck, hold the other end, and give it a whirl!

androids.jpg Before the wires have had a cut and perm

Y Idler

Another easy piece - the only tricky bit is getting that last washer in.

androids.jpg Small but perfectly formed

Heat Shield

This is listed as optional in the manual, but it’s in my nature to dislike not having all the options! Finding aluminium tape in central London was the hardest part, but eventually located some in a local Travis Perkins. Felt more like an arts and crafts exercise than hardware building, but it gave a satisfying space age effect.

androids.jpg Looks like something for a satellite

Heat Bed

This was quite a fiddly part to make, down to the fact you have to combine the ends of multiple strands of the ribbon cable. It’s also a little confusing here, because it’s the first time I realised the ‘Hot! Warning’ text printed on the board is upside down as you look at it from the front of the machine.

androids.jpg My messy but working soldering job

Having wired up the heat bed itself, you then need to solder up the other ends to regular wires.

androids.jpg Other end of the heat bed cable

Y Carriage

The only tricky thing about this part is making sure you get the bearing mount with the added tab (bottom right in the picture) in the right place, which is there to hit the Y axis limit switch. I believe Nop Head has changed the kit for them all to have it.

androids.jpg Make sure you get the bottom mount with the tab in the right place

Z Axis

This was one of the most rewarding points in the build, where you actually start to put the sub-assemblies on the frame. It took me a while to find the place for the M3 nut in the bar clamps, as they’re actually inside it. I also wasn’t sure how long to make the blue wires for the Z limit switch (turns out they just need to be a little longer than the gantry, to reach the top of the Melzi board on the right hand side).

androids.jpg Finally putting the pieces on the frame

Y Axis

Having put the Z axis in place, this was the second part where you put together pieces you’ve made earlier. The only bit that had me scratching my head was making sure I rotated the pulley belt the right way.

androids.jpg Attaching the heat bed to the Y assembly

The instructions warn that the wires shouldn’t be touching the heat bed itself, which they were in my case. It was pretty easy to just bend them a little further though.

androids.jpg Make sure the wires don’t touch the head bed - I needed to bend them away

X Carriage

Like the Y axis, the only confusing thing was working out which way to twist the pulley cable. It turned out later I’d done it wrong, but it’s a very easy thing to fix in place anyway, so don’t worry about it too much.

androids.jpg Getting the right twist in the pulley was confusing


The instructions say to check the gears don’t ‘bind’, which I surmised (correctly as it turned out) that they don’t touch in the centre when you turn them. Nop Head provided further clarification by email later, saying that as both the gears have a prime number of teeth, you actually need to turn them many times to make sure this doesn’t happen.

Other than that, it was very easy to put together. Nop Head made it a solderless exercise, as he provides a little header board to wire in with screws.

androids.jpg No soldering required with a pre-made connector board

Cooling Fan

I didn’t realise it at the time, but this is an extremely useful feature of the kit - it allows you to cool the print while you’re printing, which in turn allows you to print small layers, or larger overhangs, without the previous layer sagging. Although the design is very clever, putting together was as trivial as it looks in the photo.

androids.jpg A really useful feature of the kit

Spool Holder

The kit comes with 20 meters of Village Green 3mm filament from Faberdashery (though I imagine the colour may change between kits). As such it doesn’t actually have a spool, so this holder is really only needed for future use. I did stress for a while about not having a spool, but with such small lengths it was fine just leaving it on the floor next to the printer.

There’s a dust filter which clears any dust off the filament before it goes into the tubing, which was particularly handy if you’re in the same situation of leaving the filament on the floor.

androids.jpg Nice spool holder - but not needed for a small amount of filament

Power Supply

This part of the build had the most soldering which, although not particularly challenging, was a little fiddly. I would recommend not cutting the dummy load leads until the PSU is in place, as despite cutting to exactly the specified length, they weren’t quite long enough to run neatly along the top of the PSU itself. Combining all the 12V cables, with a suitably neat and small tip to fit into the Melzi board connector, was the bit that took the longest time.

androids.jpg The dummy load resistors - needed to ensure the PSU actually turns on

I did think it might be easier to solder up some female Molex plugs, so if the PSU ever failed, you could just plug in a new one, without the hassle of resoldering.

Wiring up the Melzi

The ultimate step in the physical build was very easy, with clear instructions in the manual, albeit one you obviously need to keep double checking. I did manage to make it look fairly messy though, so it might be worth taking some time adding some extra cable ties.

I subsequently added a hacked female USB port to the 5V dummy load (the gold one), which made a convenient (and high load) power supply for a Raspberry Pi.

androids.jpg Wired up - added a USB connector for Raspberry Pi power

The Finished Machine

About 24 hours in, I finally finished the machine. By this time it was just past midnight on a Sunday, so decided to not risk powering up. My partner took a picture the next day.

androids.jpg Oh my goodness it actually looks like it might work

First Print

This was the moment of truth - as someone that hadn’t built anything more complex than a piece of Ikea furniture in many years, I felt sure something would go catastrophically wrong. But yet again, the good instructions came to the rescue, with very clear test steps, which gave me confidence all was ok.

androids.jpg Connecting up, and going through the checks

This involved checking resistances/continuity of various components, and issuing M commands via the console section of Pronterface. Examples included typing M119 to check the status of the limit switches matched in the firmware, and M106 & M107 to turn the cooling fan motor on and off. It was truly satisfying when it did!

androids.jpg My first dealings with Pronterface

Once all the checks passed ok (even now to my great surprise), I kicked off a print from the included Android GCode.

About a minute later (while the bed and hot end heated up), the printer whizzed and whirred into life. I was astounded when it traced the clear outline of the Android figure.

androids.jpg First print from the provided Android GCode file

Although I was rather chuffed that it had worked at all, I was obviously concerned that there was some kind of issue in the X axis (I.e. left to right). After checking the X axis motor, it was clear the grub screw was loose, allowing it to wobble slightly on the flat part of the motor shaft.

androids.jpg Wobbly on the X axis

After tightening with a 1.5mm alan key, I printed the same file and, to my relief, the issue was fixed.

androids.jpg Before and after tightening the X axis pulley grub screw


As mentioned earlier, this took me 24 hours (in one evening and two weekend sessions) to put together. I don’t have any experience of other kits, or even other printers, but I can definitely say that this kit, and this printer exceeded all my expectations. I would imagine if doing this a second time, or simply being more confident about doing this sort of thing, you could literally halve that. Further, with a helping hand you could easily get printing within a day.

For a quarter of the price of a Makerbot Replicator 2 I’ve had a thoroughly interesting and entertaining time building my own printer, and as such have a much more in depth feel for how it all works.

I can’t directly compare building a Mendel90 from this kit to sourcing all the components yourself, but from what I’ve read of other people’s experience it definitely saves an awful lot of time. A guy on the RepRep forums called Mike said the longest part of his own build was just getting the panels cut right.

To summarise, I can recommend this kit for the following reasons: Mendel90 itself is considered one of the most stable designs Comes directly from the designer himself The glossy black Dibond panels give it a very professional finish The team (Chris and Mary) are exceptionally helpful and responsive There’s a huge attention to detail A great price, which saves a huge amount of time compared to sourcing yourself Contains absolutely everything you need

androids.jpg Took 5 hours - but surprisingly good quality