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: thefullpicture.co.uk 

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:

Specification

Build volume 200mm x 200mm x 200mm.

Filament 3mm.

Nozzle size 0.4mm.
Footprint 465mm x 419mm.
Height 400mm, with spool 609mm.

Contents

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.

More accurate thermistor tables

A couple of weeks ago I wanted to create some thermistor tables for Marlin. At that time it had a copy of createTemperatureLookup.py, which I think was written by Zach Smith based on my article "Measuring temperature the easy Way". It uses the simple two constant thermistor equation based on the resistance at 25°C and beta.

The two constant formula was adequate for my own software because I have separate constants for each thermistor that I use and I calibrate them against a thermocouple at the working temperature and room temperature. Marlin however has a thermistor table for each type of thermistor, so if you use the same thermistor for the bed and the hot end they share a table. The problem with the simple equation is that beta is not very constant and depends on temperature. There are several values given on the datasheet for different temperature ranges, none of them very applicable to our application. If you have beta correct for the hot end at say 250°C it is about 7°C out at the bed temperature, say 130°C.

I decided to make a new script which uses the three constant Steinhart–Hart equation. The graph shows the difference between the two equations over a large temperature range: -

The two constant equation is only accurate around the two temperatures the constants are calculated at (in this case 25°C and 256°C). When these are at opposite ends of the thermistors range the error in the middle is quite large.

The script I made is MakeTempTable.py. Its parameters are resistances at three temperatures. The tables is makes look like this: -

    {     344,       300     }, // r=   101 adc=  21.47
    {     369,       295     }, // r=   108 adc=  23.08
    {     397,       290     }, // r=   117 adc=  24.83
    {     428,       285     }, // r=   126 adc=  26.75
    {     461,       280     }, // r=   136 adc=  28.84
    {     498,       275     }, // r=   147 adc=  31.12
    {     538,       270     }, // r=   160 adc=  33.63
    {     582,       265     }, // r=   173 adc=  36.37
    {     630,       260     }, // r=   188 adc=  39.38
    {     683,       255     }, // r=   205 adc=  42.69
    {     741,       250     }, // r=   223 adc=  46.32
    {     805,       245     }, // r=   243 adc=  50.31
    {     875,       240     }, // r=   266 adc=  54.71
    {     953,       235     }, // r=   290 adc=  59.55
    {    1038,       230     }, // r=   318 adc=  64.88
    {    1132,       225     }, // r=   349 adc=  70.77
    {    1236,       220     }, // r=   384 adc=  77.26
    {    1351,       215     }, // r=   423 adc=  84.42
    {    1477,       210     }, // r=   466 adc=  92.32
    {    1617,       205     }, // r=   515 adc= 101.05
    {    1771,       200     }, // r=   570 adc= 110.68
    {    1941,       195     }, // r=   632 adc= 121.30
    {    2128,       190     }, // r=   702 adc= 133.01
    {    2335,       185     }, // r=   782 adc= 145.91
    {    2562,       180     }, // r=   872 adc= 160.11
    {    2811,       175     }, // r=   975 adc= 175.70
    {    3085,       170     }, // r=  1092 adc= 192.81
    {    3384,       165     }, // r=  1225 adc= 211.53
    {    3711,       160     }, // r=  1378 adc= 231.95
    {    4066,       155     }, // r=  1554 adc= 254.15
    {    4451,       150     }, // r=  1756 adc= 278.21
    {    4866,       145     }, // r=  1989 adc= 304.15
    {    5312,       140     }, // r=  2258 adc= 331.99
    {    5787,       135     }, // r=  2570 adc= 361.68
    {    6290,       130     }, // r=  2934 adc= 393.15
    {    6820,       125     }, // r=  3357 adc= 426.25
    {    7373,       120     }, // r=  3852 adc= 460.80
    {    7945,       115     }, // r=  4433 adc= 496.54
    {    8531,       110     }, // r=  5116 adc= 533.16
    {    9125,       105     }, // r=  5921 adc= 570.31
    {    9722,       100     }, // r=  6875 adc= 607.60
    {   10314,        95     }, // r=  8007 adc= 644.61
    {   10895,        90     }, // r=  9356 adc= 680.92
    {   11458,        85     }, // r= 10968 adc= 716.13
    {   11998,        80     }, // r= 12903 adc= 749.86
    {   12509,        75     }, // r= 15234 adc= 781.80
    {   12987,        70     }, // r= 18051 adc= 811.66
    {   13428,        65     }, // r= 21469 adc= 839.27
    {   13832,        60     }, // r= 25635 adc= 864.50
    {   14197,        55     }, // r= 30732 adc= 887.30
    {   14523,        50     }, // r= 36995 adc= 907.68
    {   14811,        45     }, // r= 44725 adc= 925.72
    {   15064,        40     }, // r= 54309 adc= 941.52
    {   15284,        35     }, // r= 66249 adc= 955.23
    {   15472,        30     }, // r= 81195 adc= 967.02
    {   15633,        25     }, // r=100000 adc= 977.08
    {   15769,        20     }, // r=123783 adc= 985.58
    {   15883,        15     }, // r=154025 adc= 992.71
    {   15978,        10     }, // r=192694 adc= 998.64
    {   16057,         5     }, // r=242427 adc=1003.54
    {   16121,         0     }, // r=306773 adc=1007.56

The ADC values in the table are multiplied by 16 because Marlin uses oversampling to give four more bits of precision. The old tables just multiplied the integer ADC value by 16 but I multiply it before rounding it to an integer so the table has the same precision as the oversampled ADC reading.

The script can also take ADC values as parameters instead of resistances. This allows you to calibrate a thermistor in situ. If you set the temperature to a value in an existing table and let it settle and then measure it with a thermocouple you know that the ADC value for the measured temperature is the value in the table for the set temperature. You can then produce a new more accurate table.

Two days after I put it on Github ErikZalm added a new script to the official version of Marlin to do exactly the same thing: createTemperatureLookupMarlin.py, amazing coincidence! It is different code but I think it uses exactly the same maths to find the three coefficients using simultaneous equations that I lifted from here. 

Peel-able support?

One of the few advantages commercial FFF machines have over Reprap at the moment is that breakaway support works much better. In particular the UP printer from PP3DP is reputed to have easily removable support using only the same material it builds with, i.e. ABS.

Today I was redesigning the Mendel90 ribbon clamps to have nut traps to make assembly easier and came up with this design: -

I thought I could print it using the bridging ability to span the slot in the base but it failed abysmally. I think it is because it is so close to the heated bed the bridge sags a lot more that it would normally do.

I tried the support option in Skeinforge but I have never got it to work well. It puts a sparse zigzag under the bridge and the flow rate can be reduced to make it weak. The problem is then that when it is removed the top layer of the support bonds more strongly to the part above than that it does to the support below, so it gets left behind. Worse still the bottom layer of the object is more strongly attached to the top of the support than it is to the layer above, so it is very hard to remove just the support.

I think the reason for this is that when the support is sparse the layer above drapes down in between the gaps. That reduces its contact to the layer above and increases its contact to the layer below. This sketch illustrates my theory: -

When I watch videos of the UP printer it looks like the top of the support is solid and flat. This reminds me of the way I used to do rafts. I made the top layer of the raft almost solid and raised the bottom layer of the object a little to make it peel-able. Indeed support is just the same as a raft, it is just that it is elevated.

To test the theory I hacked my host software to load a separate file for the support so that it could be sliced as a normal object and so have a solid top. It also has a solid base of course, which is another advantage over Skeinforge's sparse support as that can easily become detached from the bed.

When extruding I did the support for each layer before the object's layer and did it a bit lower. I also missed off the outline to give a gap of one filament width at the ends. I worked out the diameter that the object's infill would be if it was not squashed into an oval. I offset the support downwards by the difference between that diameter and the normal layer height. That means that when extruding the underside of the object that is being supported the filament is not being squashed, so has minimum contact with the support. It doesn't droop though and the next object layer is squashed against it making the bond above stronger.

The bottom layer of the support is thinner than the rest because of the downwards offset, so I had to reduce the flow rate accordingly. 

It wasn't peel-able by hand but I could separate it cleanly with a penknife, something I have not been able to do before.

The bottom layer of the bridge has round filaments that do not touch (as they are not as wide as they should be) but that is always the case with bridges. The difference is they do not droop and are well bonded to the layer above.

They are of course a little lower than they should be. A better scheme might be to have the support at normal height and raise the head as it passes over it. That would give even better bonding to the layer above which would tend to fill in the gaps. It would need rapid Z movements though.

I would be interested to see what the bottom of a supported surface of an object from an UP printer looks like.

To test the idea further I tried making a sphere. I made the support in OpenScad by subtracting it from a cylinder. To get some lateral clearance I did a Minkowski sum of the sphere with a thin disk.

$fa = 10;

R = 20;
clearance = 0.5;
h = R - R * cos(60);

module sp()
    translate([0, 0, R])
        sphere(R);
 
if(0)
    sp();
else
    difference() {
        translate([0, 0, h / 2])
            cylinder(r = (R * sin(60) + 2), h = h, center = true);
        minkowski() {
            sp();
           cylinder(r = clearance, h = 0.01, center = true);
        }
    }
 
color("red") sp(); 

The support was pretty difficult to remove because it ended up quite dense as Skeinforge makes solid layers when there are shallow sloping sides. Also the sparse infill ends join up to make a complete outline. It is more a proof of concept rather than a practical way to make support.

This is the underside of the sphere where it met the support. It looks quite good but above it there is some distortion to the spherical shape that I cannot explain.

So I think having a solid top surface on top of sparse support is the way to go and a dense bottom layer to anchor it to the bed. In between it can be very sparse but it would then need several solid layers to become flat.

It still takes some effort to remove, so I don't know if it is as good as the UP support yet. The difference may be the plastic.