Easily levelled Platform (With only two bolts). by Mecano

Last crawled date: 3 years ago

One of the most tedious jobs for RepRap owners is levelling the platform. When dealing with the platform, we talk about levelling the printer platform about X and Y axis so that we do not encounter problems in printing by keeping the print head at an even distance from the heated print bed surface thereby ensuring adhesion of the molten plastic. This is an onerous task which is generally not very well documented for RepRap users as the assembly is tricky to adjust and indeed the entire process is a black art with individuals generally developing their own variations.
The vast majority of RepRap printers use a platform bed heated by a PCB. The PCB is a square (214 mm) with holes drilled at the corners to secure it with screws (usually M3). This however does not possess the necessary rigidity and flatness which is why either a piece of glass or solid piece of aluminium is usually used to deposit the heated plastic. Because we do not print on the PCB directly, the approach is to use a piece of glass or other material that is smooth and rigid and flat. But the weight of this on our four M3 screws and springs places noticeable strain on the assembly with sometimes disappointing results.
The difficulty of accurately levelling the platform through the four M3 nuts can drive one to total madness. Clearly, one of the four nuts is redundant. That is, it is there really just to accommodate the other three. The solution then, is to place the PCB heated bed on a piece of Aluminium which is in turn supported at only three points. Three points is all that is necessary to define the plane of the platform. Now if one of these points is a ball pivot, we only have to vary the other two support points (two screws) to level our platform perfectly.
In my opinion the heated bed PCB is a component which solves in a simple, clean and efficient manner the problem of heating the printing surface. While this approach is not something that works brilliantly, the truth is that compared to the other options (wired resistors below the plate for example) I prefer the PCB heated bed.
Here are some pictures of how this could be achieved:
SEE FIRST PICTURE
In the illustration above, we have a layered view of first, green glass, then in red, the PCB heated bed, then in brown, the 5mm cork sheet and finally in gray, the piece of laser cut 3mm aluminum.
This entire “sandwich” would be supported by the three screws shown and fixed to the ABS parts drawn in dark red. Just to ensure that this “sandwich” stays solidly assembled the M3 screws are placed in the front part (not shown) to secure the aluminum plate and PCB Heated Bed. The glass platform will be fixed with spot clamps.
Finally the linear bearings are mounted on ABS supports and positioned with the aid of another aluminum plate which defines the frame and holds the belt.
Bottom view:
SEE SECOND PICTURE
The front screws, with which the build platform is levelled, are positioned as shown below:
SEE THIRD PICTURE
The M6 screw is fixed to the ABS platform support in a way that allows the supporting assembly to rotate without having to unscrew the nut. The Aluminum platform “sandwich” base, is threaded, which causes the platform to raise or lower when the screw head is turned. Also, these are Nylon screws which do not transmit heat to the ABS supports.
SEE FOURTH PICTURE
The rear screw will act as a ball (pivot) thanks to the elasticity of the two cork washers (5 mm thick) to allow slight pivoting rotation of the platform. Here, the Aluminum is not threaded so the height is always the same and is determined by the thickness of the cork washers. Naturally, more cork washers can be added to raise the platform and provide greater pivoting capacity.
SEE FIFTH PICTURE
Here is a dimensioned drawing of the assembly:
SEE SIXTH PICTURE
This design will suit the 148 mm standard RepRap but not the 258 mm width RepRap nor the AIR 2 design. This design is really intended for a future AIR3. Nonetheless the intention here is to present the design concept to the Open Hardware community so that we all have the opportunity to consider and modify the design for the benefit of all.
The platform plan view dimensions are shown below:
SEE SEVENTH PICTURE
As can be seen the platform is not exactly symmetrical and is centered at about 245 mm. To obtain a print area of 200 mm would require guides with a displacement length of about 445 mm and it should also should be noted that the hot-end performance must not be at the center of the 445 mm but offset at 26 mm.
We could round this off to a free travel length 450 mm with the hot-end at about 210 mm on one side and 240 on the other side.
Another important dimension to consider is the 45 mm deviation from the center of the guides to the printing surface. This distance depends largely on the thickness of the glass, aluminium and cork as well as the vertex of the upper support which has to sit above the belt. Perhaps the 45 mm clearance will be of better use to people who still use resistors to heat the bed. In any case, what is certain is that the shorter this distance is, the better as this will allow more travel of the platform in the Z-axis.
You can see the original of this article written in Spanish at:
http://reprappower.blogspot.com.es/
Or you can follow my work on twitter @RepRapPower
Solertron Thank you for helping with the translation.