The OpenBeam Kossel Famly - Part 1 - Architectural Decisions

Here's a blog post that's a long time in the making:  The introduction of the OpenBeam Kossel Family.  Due to the amount of information being presented, we've decided to break this into 4 segments, covering the architectural decisions, the Kossel Pro, the Kossel Reprap, and finally part availability and how to go about building one.

Terence's personal OpenBeam Kossel Reprap and Kossel Pro, built from T0 plastics.  Photo courtesy Chris Gilroy, Solarbotics Ltd.

The OpenBeam Kossel family is a branch of Johann Rocholl's excellent Kossel 3D Printer.  Since the beginning, we worked very closely with Johann and kept him well stocked with extrusions, linear ball bearings and other miscellaneous goodies.  The result was the Mini Kossel that replaced the legacy Kossel, which made its debut at the Seattle Mini Maker Faire a year ago.  Then, as one day, as I was driving, I came to the realization that the new vertexes Johann had designed for the Mini Kossel were basically a custom extrusion.  With that, I decided to redesign the Kossel Pro to merge the design tree with Johann's Mini Kossel, and here we are today.

The OpenBeam Kossel family consists of two forks, a Pro branch that features high grade mass produced components, as well as a reprap branch that features printed parts.  Great care went into designing both to ensure subassembly level part interchangeability.  Part commonality is an important design concept when designing these printers; because of the part interchangeability between the Pro and Reprap line, the same core vitamins kit will build both machines.  This allows Kossels to print their own repair parts in the field and offer Reprap machine builders an upgrade path.

Design Table showing the different Kossel family configurations

Mechanical Design Decisions:

In a nut shell, the mechanical design decisions came down to a simple premise: No compromises.

We wanted the Kossel family to be mechanically reliable printers, first and foremost.  After looking at various "Maker" style linear motion systems, we settled on the use of Chinese made copies of recirculating linear ball rails, coupled with proper cleaning and lubrication techniques.  It took a little bit of trial and error to get the lubrication correctly; we ended up designing custom fixtures that allows the balls to rotate through their motion range while applying the grease into the ball track.  This allows us to perform the grease application in a cost effective manner and offer the linear rails with grease preapplied.  

The Kossel Pro's linear rail, mounted to the vertical OpenBeam extrusion.  Note the self-terminating belt loop feature; this is a design that is ported into the new Kossel Reprap branch as well.  Photo courtesy Chris Gilroy, Solarbotics Ltd.

The rails add a lot of rigidity to the frame of the printer, and ensures that the printer's extrusions are true.  While norminal flatness tolerance for a hollow extrusion is about 1.5mm per meter, the rails are much stiffer and flatter, as a result of the grinding process used.  It also eliminates a huge variable in building the printers, as builders are no longer required to adjust the preload on the linear carriage.  Given the light load on the linear rails, the nature of induction hardened tool steel and ball bearings (the rails are so hard, they have to be cut by wire EDM when we order them), we expect these rails to last the lifetime of the printer.   

Kossel Reprap's full ball bearing pulley.  Photo courtesy Chris Gilroy, Solarbotics Ltd.

For the idler, we chose to use an actual timing belt pulley for the idler.  This results in much less wear and tear on the belts and eliminates the possibility of the belt skipping off flanged bearings.  It costs a little bit more to do it this way, but we believe the trade off in reliability is worth it.  For the Kossel Pro, we elected to design a ball bearing mount block and tension the block through a screw and nut mechanism.  This allows the top of the printer to stay flat relative to the base of the printer.  Future printer designs may use this space for a spool holder.

Finally we elected to use a nice, thick borosilicate glass plate for the build surface. Borosilicate glass, unlike laser cut acrylic, is temperature stable.  Reprap builders have found out long ago that repeatedly thermal cycling one surface of a sheet of acrylic (as heated plastic is melted onto it) is a good way to cause said plastic to warp.  By picking thick borosilicate glass as a build surface, we can be sure that the build surface is always flat.

Electrical Design Decisions:

Mike Ziomkowski, our electrical design engineer, will be writing a blog post in detail on the Brainwave Pro.  

Deltabots are unique machines:  with less mass on the end effector, they are fast and nimble, and execute GCode faster than a traditional 3D Printer control board can be fed through the FTDI interface.  Deltabots also have a problem with traditional board placement:  there just isn't that much usable space inside a triangular shaped frame to fit a rectangular board.  On top of all this, we really wanted a board that would handle competently 24V drive, and 1/32 microstepping for smoothness.  

Why 24V?  Motors are happier at 24Vs; accelerations are snappier, lower current will yield the same amount of holding torque.  Heaters are also happier at 24V; less current is needed to bring the same amount of wattage into the heating element at higher voltages. 

Early on, we toyed with the idea of creating a hexagonal control board, but we quickly abandoned that idea when we realized that the board had to sit between the power supply unit and the heated bed - in otherwords, between a toaster and an oven.  

Proto B Brainwave Board, mounted to the inside of the Mini Kossel Reprap's lower triangular frame.  Note the vertical SD card reader slot.  Photo courtesy Chris Gilroy, Solarbotics Ltd.

So we designed a board from the ground up to mount vertically:  We  used a vertical full size USB-B connector for connectivity to the outside world (MicroUSBs are just a bit too fragile, and min-USBs are being phased out).  We used the AT90USB microprocessor, bypassing the FTDI USB chip, for much faster USB communcations.  We then mounted all the motor and end stop connectors on the other side of the board.  Finally, we designed the board to fit within the 75mm height vertex of the OpenBeam Kossel family.  75mm edge-to-edge, 60mm on center hole spacing is the same dimension as the OpenBeam NEMA17 motor bracket mount.  This allows us to mount the extruder motor in the base of the unit for a very compact and clean installation and use the aluminum structs as a heat sink for the motor.

Top view of the mini kossel.  This is a fully functional and wired printer with very clean lines and wiring.  Photo courtesy Chris Gilroy, Solarbotics Ltd.

There were lots of head bashing and threats of strangulation to get everything fitted into such a small package.  In the IATA kossel, the screws used to hold the corner vertex to the extrusion does double duty to hold the standoffs for the electronics. 

In keeping with the philosophy of designing for a "family" of printers, the Brainwave Pro's horizontal hole spacing matches the IATA /. Air Kossel's vertex hole spacing.  This allows us to build the smallest of the Kossel family - the IATA kossel that fits inside regulation hand carry luggage, without issues.

Other Improvements:

As we set out to design the Kossel, we found places where improvements can be made. One example is the J Head hot end.  Considered to be the reference benchmark for PLA printing, we sourced all our prototype hot ends early on in the program from the original designer.  However, there were a few things we weren't happy with on the J-Head design:

1)  The fit of a heater cartridge into the J-Head metal hole is a very loose slip fit, with no mechanical way of securing the heater cartridge.  We weren't happy about buying a precision machined part, only to have to resort to rolling the heater cartridge in aluminum foil to get it to the correct diameter to press the heater cartridge in.

2)  There is no mechanical means for securing the thermistor.  Pulling a thermistor off a J-Head in operation results in a catastrophic failure mode; without the feedback thermistor reading the temperature correctly, a heater cartridge on full throttle can become red hot and easily melt and destroy the J-Head housing.

3)  The default way of securing the PTFE liner with a hollow PEEK set screw is problematic; it creates sharp corners that make blind feeding of filament into the J-Head during filament change next to impossible.  Most users get around this by disconnecting / unscrewing the bowden feed clamp and manually fishing the filament through into the J-Head during filament change.

To remedy these issues, this is what we did:

A)  We added a set screw for retaining the heater cartridge.  

Undercut pocket and set screw hole for thermistor and heater cartridge retention.

B)  We added an undercut pocket for securing the themistor.  The thermistor is installed in protective PTFE tubing to prevent short circuiting, crimped with ring terminals (or Molex SL connector, depending on SKU) with a certified crimp tool, and then potted into the hot end heater block with high temperature non corrosive silicone.  It costs more in terms of labor cost to provide this as a pre-assembled component, but from a user's standpoint it is far more cost effective for us to spend the money on the tooling and potting compound and amortize the cost across a production run of hot ends and ensure that every hot end is built properly than to source their own tools.

4 hot end tips, set up ready for potting. The ring terminal variant is used for the Kossel Pro's end effector.

Completed hot end tip with potted thermistor.  We sourced a non corrosive silicone - a lot of Reprap builders uses "red engine silicone" which degases acetic acid and will corrode electronics over time.  We use Loctite 598.

Assembled Kossel Pro end effector.  Photo courtesy Chris Gilroy, Solarbotics Ltd.

C)  We changed the design of the PTFE retention liner.  Instead of a hollow PEEK setscrew, we custom designed our own retainer piece that features a large chamfered feed lip to feed the filament into the J-Head.  (This piece can also be swapped with another piece that features an M5 threaded hole to accept a push fit connector for direct bowden drive).  We had to design a special tool to secure this piece into our J-Head, so we designed the tool to be 3D Printable.  

Chamfered feed lip makes for much smoother feeding.  We designed a custom wrench that is 3D Printable to secure the PTFE retainer.

Additionally, these hotend parts are made by a local startup aerospace shop on their Haas VF3 machining center.  The threads are thread milled and holds a much tighter tolerance than traditional hand tapping with a regular tap.  As a result, we've logged over 200+ hours of leak free PLA printing without having to use any PTFE tape or thread sealant to assemble these prototype hot ends.

Points A and B are relatively small evolutionary changes, however, we were pleasantly surprised at how big an improvement C really is.  By having the big chamfered feed hole, filament can be fed blind into the hot end subassembly.  Nobody likes to have to disassemble half their end effector to change filament, so that is a very welcomed improvement.  

We hope you've enjoyed this update; we'll be back soon with another update on the improvements that went into the Kossel Pro.

-=- Terence & Mike, and the Kossel test team.