Saturday, July 31, 2010
Towards reprappable electronics
The steppers are being micro-stepped (1/16). This makes the whole thing virtually silent, and very smooth in its movement. The firmware supports this new configuration.
We're Going to the Faire!
Dear readers, the developers of the Michigan RepRap User Group cordially invite one and all to the Detroit Maker Faire!
(Have fun guys!)
Thursday, July 29, 2010
Continuous belt production
I think that (as with so many things) the original idea for this came from Ed Sells. And lots of people have subsequently had ideas of reprapping on a continuous conveyor belt running over a flat surface. See here and here and here.
The big advantage would be that you could print for as long as the plastic filament lasted, continually throwing reprapped parts off the end into a bucket. They would be split from the belt as it ran over the winding roller.
There are two problems that have to be overcome:
- Keeping the belt flat against the tendency of parts to curl away from it, and
- Driving it accurately without slip.
But the second problem is quite easy to solve (usual apologies if someone has though of this before) - you do continuous production without a continuous belt:
By winding and unwinding between two rollers you can cover the entire length of the bed, and pull the part off by running it over the end roller. You can then rewind to print the next part. A simple flag on the belt passing through an opto-switch would allow the system to be zeroed.
Driving the idler roller might require some thought. As the belt winds and unwinds, the roller diameters change, and so does the required velocity ratio between them. The stepper can compensate for this effect by changing the number of steps-per-millimeter it uses depending where it is on the belt to get the build right. But the idler has to do the inverse.
The first thing to say is that - with a thin Kapton film belt - this probably wouldn't matter and you could just run the two rollers with a timing belt between them relying on the elasticity of the system to take up the slack.
But if it did become a problem there are at least three solutions:
- Drive both rollers with steppers. I don't like this as they're bound to end up fighting each other.
- Put a wind-up spring on the idler roller to take up the slack. I'm not too fond of this either. It would probably load the stepper too much, and the forces required from the spring to achieve sudden movements would change with the mass of the object being reprapped.
- Keep a tension on the film by driving the idler clockwise (in the diagram) with a DC motor on a controlled constant current source. Constant current should give constant torque. By changing the current you could compensate for increased mass needing to be moved.
Labels: conveyor belt
Wednesday, July 28, 2010
Some thoughts and observations about having a Reprap machine in the design cycle
In which your narrator reflects on the rather radical difference between what he perceived the design process would be pre and post the advent of practical Reprap printers.
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Monday, July 26, 2010
Memories of plastic model airplanes
In which your narrator harks back to his youth and fingerprints ruined by trimming plastic flash off of model airplane parts with double edged razor blades.
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Friday, July 23, 2010
The phosphor pigment (no actual phosphorous used) needs to be mixed into the plastic at about 10% loading, which is an order of magnitude more than is normally used for colouring. Getting powder to evenly coat the granules at that density is hard, but Alan Booth at Imagin knows a trick or two and it came out beautifully even. The only problem is the cost, which by the time I've got 100m of 3mm filament in a bag is about NZ$60. That's using the cheaper phosphor too - not because I'm cheapskate but because the really good stuff isn't available in quantities of less than a tonne!
Saturday, July 17, 2010
Mendel & multiple materials
It has always been the intention to fit a head changer to Mendel, I've had a go myself on several occasions but always ran into the same problem i.e. I haven't been able to do so without substantial modifications to the core frame of the machine unless there is a substantial loss of build volume (...I once had a design that ended up having a maximum build height of 2cm.very practical:D). Moreover, it’s more difficult to have a head changer for extruders whose designs are still in the early stages (such as the paste extruder). The main problem I suffered with is that regular designs of extruders are just too bulky without increasing the overall size of the machine, which incidentally I think is the perfect size as it is. Any bigger, and I would find it too big to find houseroom for!
Thankfully, eD had the idea of bowden extruders some time ago, which quite a few people seem have gotten work. Thus I think way forward for the time being is to implement a carriage for multiple bowden style extruders. Although this is almost certainly going to reduce build quality slightly. Some have already designed multiple bowden extruder carriages; here is my take on it:
The main differences are:
1. Support for three extruders, either bowden or paste. (limited syringe size of 10cc).
2. All the multiple head machines I've seen rely on the extruders being exactly the same height in order to get the bed clearance just right to produce good builds. I've always found getting adjusting bed clearance is tricky enough just for one extruder, and I think it's likely that there will always be a build up of tolerances that extruder heights will always change by significant magnitudes. Even still, with warping and so on in the carriage, it may still be difficult even if the extruders were identical. Thus, the carriage treats each extruder clearance like pretty much everything else on Mendel, by allowing for adjustment. All extruders are attached to a mounting plate, of which on the underside are of series three captive nuts for each extruder arranged in an equilateral triangle. Three compression springs are then placed between the plate and the extruders, and cap screws are used to adjust the compression of each spring independently. (In the picture above I’ve only used the springs on the left hand extruder, it makes sense to fix the main extruder and adjust the other two to match it)
3. In the same style as the mini carriage, the bearings are all located within the carriage itself. Having the bearings located on the outside surfaces of the carriage( like the standard Mendel design) ensures that the walls of the carriage must lie between the bearings and any extruders, eating into the working volume. However this does result in quite a wide carriage.
4. I needed to make some compromises, neither of which I'm particularly happy with, so that I don’t have to redesign the whole x-axis. Firstly the belt travels through the carriage itself on the 360 side, and secondly the belt is clamped from the inside out.
5. Finally its NOT makerbotable (its about 115 x 95mm), but it should be a fairly easy redesign to allow for only two extruders. . Even still, if you have a RepRap I'd recommend printing in PLA or using a heated bed or you'll probably run into trouble
Anyway, I’ve heavily modified the Paste Extruder, and used it in parallel with a bowden extruder I designed that based on Erik’s design but using a worm gear to do the driving (Designs to be on the SVN/thingiverse shortly). Gerrit Wyen (an intern at the RepRap Lab @ Bath Uni) created a simple script to take account of the offsets between the extruders and modify the GCode accordingly. Also Adrian has done so much I’ve forgotten exactly what he’s done, but its a lot:D. Here is a video one of the first prints; it’s a cube using PLA for the outline and top and bottom layers, and silicone sealant for the interior infill:
Whilst I did reverse the extruder to prevent ooze, and this is sufficient during a "normal build", its sat there for such a long time doing nothing whilst the paste extruder is working it still oozes horribly. For this reason I built little barrier to collect the ooze from hitting the part, but I think we may need to lower the extruder temperature a little when not in use. Anyway, here is the final part, which came out surprisingly well:
Friday, July 16, 2010
RepRap in the Year 3000
Here is the homework of Alpha Perry, aged nine. She was asked to imagine herself as a news reporter in the Year 3000, and to write a piece on the history of a technology.
I thought that this was so good (despite its containing bits from me) that I asked her and her parents for permission to add it to the RepRap site...
Labels: school essay
Thursday, July 08, 2010
Overhang support and PCBs
With a lot of work going on on putting pen-plotters in RepRaps for PCBs (and other things), it seemed to me that we might re-visit an old idea.
That idea was using oil to assist separation of support material. Now that the Java host software does full overhang support calculations, it seemed that we might combine the two: a pen plotter with an oil-filled felt pen would run over the top layer of the support after it had been laid down to make it easier to break away when the part was finished.
Above is a brief experiment I did by hand. The L shape was built 'the wrong way up' forcing it to need support. Both the part and it's support were PLA. I paused the build at the last layer of the support deposition, and wiped corn oil over the support with a Q-tip. Then I resumed the build. The support material did indeed separate much more easily than it does without the oil.
If a support layer was simply plotted twice when it was the last support layer under the part, once with the polymer, and then once with an oil-filled felt tip, this would give the same ease of separation. The same pen plotter could be used with an etch-resist pen to make PCBs, such as these designed to make reprappable electronics simple. Two birds; one stone.
Wednesday, July 07, 2010
Vectorization of pixel defined print roads actually working properly
In which a very kind Adrian Bowyer takes pity on your narrator.
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