The final chassis assembly brings together the four elements already constructed and adds the final two stepper motors.

I ran out of another bolt at this point - this time the 6-32 ¾inch UNC socket cap bolt. Fortunately, not only did I have some my extra ½inch bolts left over, these were also sufficiently long to do the job of the ¾inch screws - so I didn’t need any more extras. Clearly this slight mismatch between the Bill of Materials and the assembly instructions needs to be made right.

At  this point in the assembly I also had to make up the cables for the limit switches; this proved to be the trickiest assembly routine so far. As with the cables for the motors, each switch has a different length cable and these need labelling carefuly. The tricky bit is that each switch has a snap-in connector and the wires attach to these connectors by crimp-pins - and you have to attach the crimp-pins to the ends of the wires. There are full illustrated instructions for this on the Fab@Home site. This is not a desperately difficult process, but does require attention to detail if the pins are to slide nicely into the connector and snap securely into place. It is the first part of the whole assembly that I think might present some difficulties to at least some pupils - and the first one where I messed up… I ruined one of the crimp-pins and, of course, there are no spares. This time Maplin came to my rescue. Schools embarking on this process with pupils will either need to give them some initial practice, buy a proper crimping tool (probably the best bet) or, perhaps, have this one part of the assembly done by the teacher or technician.

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This part includes another stepper motor. A bit of care is needed when dealing with these motors because they are not all the same and the shafts they use differ as well. In particular the shafts are of different lengths with different thread pitches (the z-direction shafts for the platform and the syringe have much finer threads). The syringe motor is different from the others in that the shaft moves up and down through the centre of the motor as it turns to allow it to press and release the syringe plunger. The other motors simply hold and turn the shaft so that the parts attached to the shaft move in the x-, y-, and z-directions.

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The z-carriage includes the platform that objects will printed onto. From a learning point of view, this is a good example of how to construct a light yet rigid structure.

During construction of this part I ran out of one of the many kinds of bolt employed in the construction of this fabber; the 6-32 ½inch UNC socket cap bolt in fact. I have to admit that I, like the average European reader I imagine, was initially nonplussed by this - being used to metric nuts and bolts I had no idea what either 6-32 or UNC meant. A short time on the web left me knowing more than any man should about imperial screw measurement systems and a bit longer found a very few UK suppliers who could supply imperial bolts. ModelFixings appeared to be the best of these, having a wide range of parts and the ability to supply in realtively small amounts (I bought 25 for £3.75, inc. shipping, which I thought wasn’t bad for a rare (on this side of the Atlantic, at least) part - though expensive per bolt). The bolts arrived a couple of days after ordering.

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Here is a view of the assembled x-y carriage. This assembly provided no (new) particular difficulties but did introduce a new aspect of assembly; building the wiring.

The principle of Fab@Home is build it yourself and the wiring is no exception. The x-y carriage includes two more limit switches (in the central structure that moves along the main thread), but I have left the wiring of these until later. It also includes the stepper motor that will move the platform in the x-direction.

There are four stepper motors altogether in the final build (x, y, z, and the syringe driver). Each comes from the manufacturer pre-wired, but these need extending - by different lengths. Clear labelling of the extension wires is important, both to ensure that each motor gets the correct extension and also so that each motor’s wires are easily identifiable when wiring to the control board since at this stage the wiring will be threaded carefully through the structure and encased in braiding to keep it safe and tidy - so tracking back along the wires will be tricky.

The connections to the extension cables are sheathed in heat-shrink tube (see image).

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Here is the fully assembled base; at first galance a little like a storage box with some bits cut out of the sides but closer insection shows some mechanical and electrical components already in place:

• In the upper picture the small black squares on the left hand side are limit switches for the x-y- and z-carriages.
• The lower picture shows, on the reverse of the box, the timing belt pulley that will ensure even motion along both of the y-shafts.

As the previous post indicates, I found some ambiguities in the instructions that I would like to have ironed out before setting secondary school pupils lose to follow these instructions. A key part of this ambiguity for me is that the assembly instuctions use one ‘part number’ system to refer to the various parts but the parts themselves (supplied in a myriad of small plastic bags) are labelled with different ‘part numbers’ from the manufacturer; the bill of materials spreadsheet provides the vital link between these two numbering systems. The result of this is that I spent (wasted) a great deal of time cross-referencing between these; each threaded insert, nut, bolt etc. etc. needed to be carefuly checked, not least because each of these comes in a number of different sizes. It will be, however, a simple matter to edit the assembly instructions to include the manufacturers part numbers and I would strongly recommend this before pupils start work.

These points apart, there is nothing here that should prove difficult for secondary pupils and there is quite a lot of incidental learning to be gained about how to create a well-engineered structure based on thin sheet material. In particular the use of a soldering iron to melt the threaded inserts into the acrylic was very straightforward and effective and the use of inserts for nuts and bolts to secure the structure (bottom image) is exemplary.

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It turns out the parts I thought were missing aren’t…. In the case of one part I simply can’t count .

The case of the other part is worth logging. Line 23 of the Bill of Materials shows a part “Linear ball bearings for X; Y axes”, item number 60595K73, that I thought should match Base assembly step 2 , item No 3 (see diagram). In fact I was looking at the wrong kind of bearing and then confused by finding that what you get in the kit is 4 white + green boxes manufactured by INA, item number 60595K13, whose contents look nothing like item 3 in the image.

In the end all the right bearings were present and the only error was in the single digit difference between the part numbers. I will press on and see what else emerges.

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I’ve suggested to the Fab@Home folks that there would be merit in placing the Model 1 CAD assemblies onto Ponoko as a free download; most people would still probably go to the Fab@Home site for them, but the advantage of Ponoko for some is its ability to link the downloader with a local manufacturer who could make the parts.

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So this is what a Fab@Home looks like when it arrives; both smaller and lighter than I expected (to give a sense of scale, I’ve included a 150mm measurer in the images).

Unpacking reveals three smaller packages: a set of laser cut acrylic parts, very carefully protected in layers of thick corrugated card (lower right of second image); a box full of a large number of mechanical, electronics and assembly parts (upper right of second image); a polystyrene box holding the stepper motors - pretty much the most expensive elements in the package (left of second image).

The acrylic parts are not only safely but very compactly packed; spread out across the floor it looks an impressive collection.

All the assembly instructions are on the Fab@Home website; I didn’t feel as though I could proceed working off the screen, so I’ve printed the full set - which you can see is substantial.

So I set to with enthusiasm, but came to a grinding halt very quickly when it seemed a part that is required quite early in the process was missing.

Moral: check the parts of the Bill of Materials before you start… Checking revealed that two critical parts (ball bearings for the Z, X and Y axes) were missing. So we are now chasing these up.

There is still a great deal can do to prepare all the parts while I wait, but I won’t be able to finish the assembly until the bearings turn up.

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