Why a TOP PLATE? - BainbridgeArtisanResourceNetwork/Mark4_printer GitHub Wiki

The Mark4 printer is designed with a thick plate at the top of the frame. This "TOP PLATE" holds the the guide rails for the bridge mechanism, which in turn holds the moving extruder. This page gives some description of why we chose a TOP PLATE for our design.

Let's start with a couple of critical functions that the are performed by the TOP PLATE (or something else in some other printer design). At the heart of our printer is a nozzle that dispenses molten plastic. We need to control where this molten plastic goes if we want to make something. In most (maybe all) core XY designs, the nozzle is moved around a horizontal plane to create a single layer of our part. We define an "X" and "Y" direction on this plane. We then use a complex arrangement of two belts and two stepper motors to position the nozzle where we want it to be. The belts and the pulleys constrain the the X and Y position of the nozzle - almost. Due to the fact that the belts can stretch and flex, the nozzle can move a little from its intended position. We need to counteract the unintended movement that belt stretch allows. We do this with three linear guides.

A linear guide is a clever arrangement of a stiff rail and a block that moves on the rail. Once the rail is fastened to something, the block can move in only one direction - hence the name linear guide. If two linear guide rails are set perpendicular to each other, the location of the intersection of their moving blocks is very precise. The picture below shows takes the top plate out of the picture at the top of this page and shows only the rails, the extruder (where the nozzle lives), and the build surface where we want to deposit our molten plastic.

We've labeled build surface a "surface". We really want this "surface" to be a plane - perfectly flat. We'll call it a build plane from now on. The reason is - the software that slices our parts into layers for printing assumes the printer will print flat layers. If we print layers that aren't flat, we won't get the parts we want. So, what do we need to do to ensure the layers are flat? Several things:

1. The 2 Y GUIDE RAILS in the picture need to be mounted so that both ends (and the whole middle) are all the same height above the build plane.
2. The X GUIDE RAIL in the picture also needs to be mounted so that both ends (and the whole middle) are all the same height above the build plane.
3. This positioning needs to not change while the printer is in use, or gets transported, or leaned on by someone watching their part get printed. The structure that makes 1 and 2 happen needs to be robust.

There are as many ways to achieve these three requirements as there are printer designers. More, if you allow for designs that need to be adjusted when they get out of whack. The decision of which way is used on a particular design can depend on a lot of things that are not "what is the very best way to achieve all three requirements, no matter what it costs". Many commercial designs opt for a four sided aluminum frame to hold the Y GUIDE RAILS, and hope to achieve flatness through careful assembly (even though it can be very hard to do). But an aluminum frame is light and can be shipped in pieces, making for a lower-cost printer.

We have opted to use a thick, very flat cast (not extruded) aluminum plate to support our Y GUIDE RAILS. This lets us start with a single surface to meet condition #1 above. We aren't worried about fitting the parts in a small box, and we have access to the BARN machine shop where we can cut out the middle of our plate very easily without messing up the flatness. And we are using a thick plate (3/8") so that it will be less likely to flex during its lifetime. As we proceed with the design details, we may choose to add some stiffening to the bottom of the plate...