Friday, 7 December 2012

Mendel90 updates

It has been a long road but I am now in a position to sell complete kits for Mendel90. My original plan was to use laser cut acrylic but the companies I got quotes from could not guarantee the holes sizes would be accurate enough to be tapped. I also found out that 3mm Dibond is stiffer than 6mm acrylic as well as being a lot lighter and cheaper. The downside is that the polyethylene core is too soft and the aluminium wall too thin to tap a thread into reliably. That meant replacing all the screws with nuts and bolts, which made the machine fiddly to put together. To fix that I reversed all the bolts and put nut traps in the plastic parts. The design still supports the MDF and acrylic variants with screws, but I only sell the plastic parts for these.

Picture courtesy of  Alzibiff, a Mendel90 owner who is also a photographer: 
Because the underside of the base now has screw heads protruding, the machine has to have some form of feet to raise it. I added two aluminium square tubes, which also provide stiffness as they are directly under the stays. An added  bonus is that the wires for the Y axis can now run underneath the base to improve the appearance. Another advantage is that the printed cable clips can now be replaced by zip ties that go through holes in the panels.

I changed the default electronics from Sanguinololu to Melzi because it includes the SD card interface and fan drive on board and uses screw connectors rather than friction fit. These are the only connectors I have found to be reliable in the long term and it will allow me to offer a no solder version of the kit. After a couple of false starts I now have a reliable source of these and they are 100% functionally tested using the same motors as I provide in the kit.

I also changed the PSU to an ATX500 because I can buy them in the UK with CE approval and they come with integral mains inlet and switch, obviating the need for the kit builder to do their own mains wiring.  It is also shorter, allowing room for the Melzi (which is very long and thin). The downside is it needs a pair of load resistors on the 5V and 3.3V to get the 12V rail to be close to 12V under load. These are provided in the kit.

To stop the deeper power supply sticking out beyond the base I moved the right hand stay inwards. It is now directly behind the  Z bar that it braces, which is better from a structural point of view and gives maximum space for the electronics bay. A couple of plastic brackets hold the ATX PSU in place.

The build height is increased to 200mm as there is very little downside to doing so and it makes room for the Melzi electronics.

I added a spool holder which suspends the spool between the stays on 608 bearings using a couple of triangular brackets.

In order to turn the filament through 90 degrees I use a PTFE Bowden tube. That also has the benefit of removing any drag on the head when the extruder pulls filament from the reel. The only force on the head is that required to bend the loop of filament and the tube, which has only a thin wall.

The end of the tube is terminated by a printed connector with a flange that just sits on top of the extruder. That allows the extruder to reverse without having to push the filament back up the tube.

There is dust wiper on entry to the tube which consists of a block of foam squeezed into a smaller box with the filament running through a slot in the middle. It prevents dust that settles on the spool being dragged into the extruder.

I made a few tweaks to the extruder. I now use J-Head nozzles so I integrated a groove mount into the Wade's block. The nozzle is a tight fit and I press it in with printed jig and a vice with rubber jaws.

After the first five kits I found I could no longer get the J-Head MK4B and had to switch to the MK5B. That isn't long enough for a machine like Mendel90 because the bottom of the carriage ends up too close to the bed. I had to extend the bottom of the Wades' block so that it now protrudes below the carriage. In order to do that I had to make it a little slimmer to fit through the hole and that had the side effect of making the bearing housing symmetrical about the filament path. I think the reason it wasn't before is because there isn't room for the bolt heads on the motor plate side. I solved that by turning the bolts around and using captive nuts in the bearing block. That also makes the springs much easier to fit as the nut engages before the spring has to be compressed. The hob position is now further up the bolt at 25mm meaning more of the shoulder enters the bearing.

Other minor changes were that I lowered the motor a few mm so that it traps the head of the mounting screw  making it easier to fit to the carriage. I also replaced the spring that I used to retain the nut on the hobbed bolt with a lock nut and a star washer. They only need to be finger tight.

In order to be able to offer a solder-less version of the kit I made a tiny break out PCB for the extruder motor and heater connections.

I also made the socket on the end of the X cable an IDC version. That meant the pin to wire connections had to be 1:1, so I had to increase the number of pins from 9 to 15. The heater wires were previously doubled up to handle 2 Amp heaters but now I have enough pins for three wires giving about 4 Amps. There are also wires for a fan and a Z probe.

I simplified the X ribbon cable layout. Previously I had a pair of grounded wires acting as guard between the X limit switch and the noisy motor wires. Instead I simply moved the limit switch wires to the other side of the cable, where the quiet signals are. It actually makes the X end wiring neater and gives me the extra two wires for the extruder heater so that the cable remains 20 way, which is a standard size.

On the subject of ribbon cables: I increased the one for the bed from 24 way to 26 way making that a standard size also and two extra wires for the bed heater is not a bad thing.

In order to be able to print small items and items with steep overhangs in PLA I took a leaf out of Richard Gain's book and added a ducted fan to the carriage. I couldn't use his design directly because he has a longer nozzle mounted in a different orientation. This is my compressed version.

 The exit of the duct produces a ring of air directed inwards. The idea is to direct the air onto the part close to where the new plastic is being laid down without cooling the nozzle.

Because the Melzi only has a single fan output I removed the bed cooling fan but left the hole and fan guard for it. It is something I use to speed up production by cooling the bed rapidly at the end of a build, but the 80mm high airflow fans are expensive. They used to be cheap when they were used in PCs but they have all switched to quiet ones nowadays with less flow. If you want to add one you can hack a MOSFET onto the expansion port of the Melzi and control it with M42 in Marlin.

The OpenScad model now includes everything in the kit, which is everything needed to build the machine apart from some sticky tape used to secure the PTFE tubing. Most things are visible in the rendering but a few generate BOM entries only. These are things like wires and ribbon cables, which are hard to draw. Resistors, thermistors, sleeving and heat shrink are all drawn in places close to where they are used.

Having everything in the model is the only way to keep the BOM 100% accurate. I also re-structured the sub assemblies so that they reflect the order things are assembled rather than their placement in the machine. For example the Z lead nuts were in the X end assemblies because that is where they end up, but when assembling they are fitted to the leadscrews and then inserted into the X ends, so they need to be part of the Z axis assembly.

I created a detailed build manual for the kit in OpenOffice format. It links to a lot of pictures generated by the model so that they automatically update. The manual is also checked into GitHub, so there is a version matching each revision of the machine. I had started with instructions in the Reprap wiki but that soon become impractical as it can only represent a single version and all the images need to be manually updated. It can't link to images on GitHub for instance. Although the manual is for the kit version of the machine, it will also be useful to people making the other versions. The main differences are that different fasteners are used. The PDF version is here.

The first five kits were commissioned for a build a weekend hosted by the GIST lab in Sheffield during the university's Festival of the Mind. Due to various supply problems I only just managed to get all the parts together in time and then only with a lot of help from a couple of friends. I ran out of time completely to do the instructions so I stayed up all night and hacked some together. Despite that the build weekend went well.

Two of the teams completed their machines on day one and were extruding plastic. By the end of day two all the machines were completed, two teams were printing items downloaded from Thingiverse, another was extruding. Unfortunately two were held up by faulty Melzis. I hadn't tested them beforehand because I had five spares, but it turned out one of the originals had a small fault and none of the spares worked at all. Needless to say I won't be using those suppliers again!

The Derby Makers team won the "Golden Spanner Award" awarded for "the best demonstration of the Craft and Magic of Technology". Here they are printing.

Picture courtesy of Derby Maker Glyn Smith.
More pictures of the weekend here.

The build weekend was very helpful for ironing out snags. This lead to tweaks in the design and the instructions being greatly expanded.

Kit details:


Build volume 200mm x 200mm x 200mm.
Filament 3mm.
Nozzle size 0.4mm.
Footprint 465mm x 419mm.
Height 400mm, with spool 609mm.


The kit contains everything needed to get printing including ~ 50m of Faberdashery PLA. You will need a PC and some tools to put it together and run it, details at the start of the manual. Some things included in the kit are: -

  • Melzi electronics with Atmega1284P, ROSC shorted for correct microstepping, tested and programmed with Marlin firmware configured for Mendel90
  • Micro SD card for pause free printing, comes pre-loaded with all the software needed to print on Windows (Linux equivalents can be downloaded) and a USB to SD adaptor for direct connection to a PC.
  • 0.4mm J-Head MK5B hot end for 3mm filament, pre-assembled with thermistor, resistor, wires and sleeving.
  • ATX500 PSU with quiet fan and dummy load resistors.
  • USB cable.
  • Prusa MK2 heated bed with 2mm glass sheet.
  • 3mm aluminium composite panels CNC cut and drilled.
  • 5 NEMA17 43Ncm stepper motors.
  • 8mm hardened high carbon rods with h6 tolerance as required by the linear bearings.
  • T2.5 metal pulleys and polyurethane belts with steel reinforcement.
  • CNC hobbed bolt.
  • Extruder break out PCB.
  • 1% thermistors to avoid the need for temperature calibration.
The complete BOM is here.

The big advantage of the design is that the axes need no alignment to ensure they are orthogonal. The only calibration required is bed levelling, Z height and extruder flow rate. Details in the manual.

The price of the kit is £499, plus VAT in the EU, plus shipping. For availability check the forum post here.