15 May 2016

Seek and Geek 11: Ultimaker 3D Printer

The Ultimaker is a great 3D printer that is known in the desktop 3D printing community for printing quickly and reliably. It can use both ABS and PLA filament, and the original is made from a lasercut plywood construction and assembled using slotted "t-nuts" (The community calls these bolted joints t-nuts even though these aren't real T-Nuts, which you can use to clamp parts to a mill bed, among other things).

This sounds similar to my FASBot! Here I will look into the similarities and differences between the two.

To start, the material, while also wood, is plywood, as opposed to MDF. According to some online sources (http://www.diffen.com/difference/MDF_vs_Plywood), plywood is stiffer than MDF an is less sensitive to moisture. 

How much stiffer is plywood than MDF? According to Matweb, the Young's Modulus of plywood is 9.8 GPa. According to MakeItFrom, the Young's Modulus of MDF is only 4GPa. Plywood is over twice as stiff! 

This is probably bcause MDF is compressed bits of wood fibers while plywood has long grains. It's the difference between baking a cake of carbon fiber shavings and epoxy versus making a tube from longer strands of carbon fiber epoxied together. The latter will be stronger. 

The main structure forms a cube that is held together with slots and tabs for alignment, and "T-nuts" to keep everything together. 

The printer uses belt drive for the X-Y axes, and a hefty leadscrew for the Z axis. Here are the main guides that transmit force to the printer head. A single, long bushing is pressed through each, that looks about 3X as long as the diameter of the shaft. Saint Venant strikes again! The belts are tensioned using those little clamps sticking out the sides. 

Each axis has a pair of these guides, spaced far apart. Each slides along a shaft that also transmits motor torque. The axis shafts are held at each end with a radial bearing that are held in place with a pair of plates (shown above disassembled). 


Like my FASBot clamp axes, the Ultimaker transmits motor force to one direct leader slide and one follower slide through a belt transmission. Using this construction, force can be applied to both sides at once, preventing jamming. Each pair of slides pushes another shaft sideways, causing the printer head to move. The printer can still slide along this shaft, but is constrained by an orthogonal shaft coupled to another pair of slides. 

The build platform is the carriage of the Z axis. It has two beefy sides to allow it to be cantilevered without much deflection, and a pair linear bearings allow it to move only up and down.

The bearings are held in place with stacked parts. Note that the length of this bearing is about 3X the diameter of the guide shaft. Saint Venant! 

The leadscrew is constrained at the bottom using a rigid shaft coupling that attaches a stepper motor located underneath the printer bottom plate. A bearing supports the shaft coupling, and thus the leadscrew, axially. The leadsrew is not constrained at all from the top, which would allow it to. This is acceptable, because the printer process is only sensitive while the bed is moving downward, and this occurs in tiny steps of 0.001" (~ 20 microns) at a time. The only forces the leadscrew will see are downward gravitational forces. 

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