So we have a guinea pig good friend testing out the idea of selling our smaller mill design as a kit. Below you can see all 77 parts all laid out prior to packaging. Since this was being shipped to Australia we really wanted to make sure we did not miss anything!
Or as I prefer to see it, the kit version of the mill all packaged up and ready for shipping!
Cassius V1.0 in Kit form, ready for shipping
As far as we could tell it looks like someone put together an initial minimum design for using the Allegro 3967, and all subsequent versions have been small revisions of that design. The problem is that looking at the data sheet for the A3967 and the schematic for the EasyDriver there were a lot of places I could not figure out what the original engineer was thinking. We looked at making our own board around the A3967 – but instead chose to directly drive the stepper motors with darlington transistors and control the phase timing from a microcontroller.
Cassius V1.0 PCB
We spent about a month having problems with Sparkfun’s version of the Easy Driver boards before deciding to make our own driver board. When they worked – which was most of the time – they were awesome. We started off concerned, becuase disconnecting a motor while the board is powered can fry the chip responsible for microstepping the attached motor, the Allegro A3967. Now this is a problem because it means if a motor fails it also is likely to take the electronics with it. We still decided to try the Easy Driver bord out because we had heard good things about it.
Once we started hammering on them we found it was also possible to fry boards so they would only drive the motors in one direction. The plans are open so we could look over the datasheets and review the design. That made us go from concerned about using the Easy Driver to flat out running in the opposite direction.
[Update: So after talking to Brian Schmalz, the developer of the EasyDrivers he was unable to reproduce our error after hammering on a number of boards and motors. It looks like we were either having a problem with our stepper motors or - what I think to be the more likely case at this point - I just made a mistake in how I wired up the motors. My best guess is the trick I used to wire up a unipolar stepper motor so it could be driven by a bipolar stepper motor driver was at fault. Only problem is I cant figure out why it worked at all if that was the problem. The important update though is that you should be fine using the EasyDrivers with bipolar stepper motors. ]
Stalk the wild streets of Seattle around the university district – and if you are lucky you may just run across one of Purple Crayon’s prototypes in the wild. Our intrepid photographer had to wait for hours to sneak up on the mill to get this rare picture. The pictured mill is one of the early software development prototypes being used in someone’s home to cut graham crackers as a test of using the mill to make parts for crazy custom gingerbread houses.
Cassius V1.0 software development station
This is probably how a large number of our units will get used by customers. The mill was designed to be in self contained boxes (the cover is off the mill in this picture) specifically so they could be used by customers who dont have access to a work space anywhere but in the home. In this case cracker crumps are prevented from flying everywhere and caught in the box for later cleaning – allowing the unit to be used next to someones main workstation in side the home.
The early prototypes are color coded, red, blue etc. There are no planes to offer one in fire engine red like the one pictured.
As part of prototyping production methods for our mills we tried playing with castible molds and plastic to provide a solution for the rapid production of multiple copies of a part. The results were fairly promising and the company may offer cast kits at some point.
Smoothon.com offers multiple lines of castable plastic resins with lines of compatible mold materials, and release agents. Smoothon.com also supplies tutorials for all their products – but the casting process is fairly simple and we will show what we tried here.
Start by gluing an original part to be copied down to a water proof sheet of flat material, and build a mold box around the parts. When casting multiple parts add small pieces of material connecting the parts. This will leave a channel in the cast mold through which the liquid plastic can flow, allowing the entire mold to be poured from a small number of points. Next add one or more pieces of material that will stick up out of the mold to provide pour points. Also add other pieces of material should also stick up out of the mold to allow places for the air to escape. The air vents can be very small – tooth picks work well.
Pieces mounted in mold box
Once the parts have been glued into the mold box hot glue the mold box to the underlying water proof sheet. This prevents the liquid mold material from leaking out of the box. If using a wooden mold box it is also a good idea to hot glue the interior corners of the mold box to make it liquid tight and able to hold the molding material. Pictured above is a mold ready for casting, and the same mold once cast is pictured below. You can see small pieces of wood used as risers for air and pouring points. Once the mold has hardened, you break down the mold, remove the original pieces and pull out the risers that were added for pour points and air venting.
Mold box filled and drying
To cast parts you just re-assemble the mold treating the inside with a release agent. You want to glue the box down to the underlying sheet again, this time to prevent any liquid plastic from escaping. When casting large pieces you should also put something heavy on top of the mold to prevent the mold material from floating up when you pour in the liquid casting material. At this point you just mix and pour your liquid plastic and fill the mold.
Make sure to have some place to pour off any excess material in case you mix too much plastic. Be sure to carefully read the directions on how the material used should be mixed and handled to avoid air bubbles in the finished casting. Stirring either to vigorously or not enough can both cause defects to result in most cast materials.
Once the plastic hardens break down the mold box and remove the pieces. A sample mold with cast pieces is pictured below.
Pieces in mold
Then just peal the parts out of the mold and cut off the flashing.
Removing pieces from the mold to clean up the flashing
Below are pictured some white sample cast parts next to their black ABS plastic pattern originals. The “wavy-ness” you see on the cast surfaces is because I used aluminum foil as a casting surface. They foil was not perfectly flat and this ended up being reflected in the cast surface.
Cast pieces next to the originals
So in summary casting parts turned out to be very easy and fast once you had an original. We anticipate that our customers who want a large number of a particular part will probably end up making molds or casting off parts made by one of our products.