Impressions of the Cupcake CNC, 26 April 2009

Greetings all,

I (unexpectedly) got to see a cupcake CNC fabber yesterday at the Microsoft Startup Labs near MIT.  This was part of  

http://www.themakerrevolution.com  

(I had prior commitments yesterday, but some luck with logistics, and that enabled me to catch 

the last 90 minutes of this event.) Following are my impressions of this device, 

please forgive the choppy grammar/stream of consciousness style.  

It's warm enough here that I'm functioning at only a low level, 

and thinking of crawling into my glass of cold soda to cool off....  

(I'm also apparently having a disagreement with this editor about where/if lines are wrapped.

It's sure not wrapping in any way I like/understand.... Grrrr)

They (Bre P. and Zach H.) were printing ABS, but they say it prints HDPE and PLA as well.

Small size: 

form factor roughly that of the original "tombstone" MacIntosh computers, 

for those old enough to remember those. Maybe a bit smaller.

The outer structure is built from thin (6 mm or 1/4" inch) plywood, cut via laser.  

Attachment used a combination of tabs/slots and threaded 

fasteners. The box structure, even with large cutouts in the faces, 

gives a nice sturdy structure, since the walls resist shear/racking.

Some of the smaller structural pieces were 1/8" plywood. (I think it cuts faster via laser, and it cheap.)

Some pieces (exp. the portion that moved up/down) are made of laser-cut, clear acrylic.  

This is nice, because one can see the inner working of the extruder 

(e.g. see how fast the filament is being consumed.)

The cupcake uses metric threaded fasteners, rods and bearings.  

(I was anticipating use of imperial sizes from designers in the US.)

The power supply is housed in the base of the unit (below the build bed), and the electronics are all mounted 

onto the device, mainly on the outer back wall. So, it's a single "brick" 

and was pretty easy for them to break down and pack it up. I presume that setup was similarly quick, 

but I didn't see that.

[IMHO, calling this CNC is a slight misnomer, because CNC usually implies conventional machining, 

at least to me. But I suppose they can't use FDM (R), due to the trademark on that term, 

from what I understand from past traffic on the reprap forums. This is a nit/pet peeve]

X and Y used belt drives. Z used a combination of belts driving threaded rods.  

The extruder used a gear motor for the pinch-wheel design.

Fast motions, ballpark of 40 mm/sec, maybe even faster. Similarly fast accelerations; 

it really whipped through printing the raft for the polyhedron.

Smooth motion, using dual rods (and some sort of plastic bushings) as slideways, 

along with some (obligatory) skate bearings. The size 17 steppers didn't seem to be taxed, 

and the gen-3 (surface-mount) stepper driver chips

were warm to the touch, but not hot.

Fine (0.5 mm or under?) extrusion -- made finer by moving (the nozzle) faster than the melt came out, 

stretching it and thus reducing the diameter of the melted plastic being extruded.

I watched it print a little polyhedron (with a waffle pattern internally to reduce plastic volume 

and speed the printing. The print quality was quite good (though I haven't seen many printed objects 

to compare with.)

The build volume is small (100 mm cube AFAIK);  IMHO, this might be better with one longer horizontal axis, as is usually done for machine tools such as mills or lathes.  Apparently the cupcake's build volume 

is constrained by the size pieces they can cut on the CNC laser cutter they have.

It has survived the "tender" attentions of the TSA, and they proudly showed me the TSA inspection certificate.  

[IMHO, this may have been an unwise tempting of fate, since they had a return trip ahead of them.  

Maybe they drove and the TSA document was from a prior trip.]

Walk-away operation. The machine worked well enough (and they were apparently confident enough) to walk away from it 

while it printed out the polyhedron. It printed the the part out with no user intervention, 

except setting the nozzle-bed height at the start of the print.  

The coil of ABS was lying on the coffee table beside it, slowly being drawn up and into the pinch-wheel extruder.

They used skeinforge to slice .stl representations of objects, and replicatorG to drive the machine. 

I don't know what version of the Gcode firmware they were using, but it seemed to behave well.

Printed parts: 

I saw gears, a pulley, an airfoil section, and a pair of tweezers, printed previously.  

Apparently, they had forgotten to bring tweezers (to remove the first blobby bit of extrudate) from the build bed, 

when the extruder was warming up, so they printed themselves a pair, thus avoiding further singed fingers 

-- quite handy. got to take the tweezers home; thanks guys!

(Why are they called a pair? They're actually less of a pair than a pair of scissors....)  

The gears meshed pretty well (not as good as hobbed metal gears, but not too bad, either.)

I don't know how well they hold up in use.

The view from the Microsoft Startups Lab, on the 11th floor of a building on the bank of the Charles river is 

*gorgeous* -- especially watching the sailboats and rowing sculls out on the river!  

Again thanks to the MS folks for making this venue available for the makerrevolution event!  

There were a number of other exhibits and activities, but I got their late, 

and focused my attention on the cupcake.  

My apologies to the people who brought lots of other cool projects.  

I wish I'd been able to get there earlier to check those out.

Zach, Bre or anybody else who was there, please free to comment/correct on what I've written.

Seeing this small, simple machine working, and working well has me re-thinking my repStrap game plan.  

There is something to be said for modest goals (printing small parts, without self-replication), 

as opposed to a more ambitious machine -- one that's not yet finished 

(translation: my much of sub-assemblies down in the basement, that can't yet print anything.)  

I'll have to sleep on this, and decide whether to change course on Cerberus, 

or perhaps divert and build a pre-cerberus machine as a first step.

-- Larry