|Simple Heated Print bed Levelling for RepRap Printers|
Since building my first RepRap Prusa V1 printer over a year ago, I have been gradually updating the machine, improving it’s print quality as I go on. One of the areas that I have managed to improve upon quite a bit is the y Axis of the machine. The original Prusa had a fairly complex sprung print bed which was less than ideal. in order to improve upon it I both used ideas from the V2 Prusa and developed my own very simple adjusting mechanism to all but eliminate backlash in the Y direction. Here is how.
The original Prusa arrangement (Which I used originally) you can see in the picture to the left. There were two sheets of MDF, the lower which attached to the smooth rods (via bearings) and the drive belt formed a Y-carriage. The upper which formed the build platform of the printer attached to the lower MDF sheet via 3M bolts and was held apart by springs.
This arrangement allowed for the print bed to be levelled by adjusting the four screws that attached it to the lower Y carriage to the upper build platform. Adjustment was a frustrating business to say the least, but this was not the worst problem caused by this arrangement. The springs and bolts were designed I think to protect the hot end (the print head) and the build platform from, damage should they collide for any reason. The problem with this was that it adds a lot of movement to the print bed losing the printer some precision.
The first thing that I changed was not my idea. Shortly after completing my V1 RepRap Prusa, Josef Prusa, the original designer of this RepRap version updated his design to the V2. Amongst many alterations that Josef made was to simplify the Y-axis. The simplification was on the face of it an obvious one. Instead of attaching the build platform to the Y-axis smooth rod bearings via a Y-carriage, Josef’s new design simply attached the bearings directly to the build platform. This removed entirely the troublesome springs and removed the ability to adjust the levelling on the print bed, except by adjusting the corner bolts that attached the heated print bed to the wooden build platform.
This was fine, but in practice I found that it introduced two more problems in the Y-Axis. The first came down to the ease of adjustment. The level of adjustment required here is very sleight, moving the corner of the heated print bed up by a fraction of a mm at a time for instance. To do so using nuts and bolts meant holding the nuts in place with a spanner or pliers whilst using a screwdriver to turn the bolts. It was far from ideal. The second problem was that drilling holes in the soft MDF platform was difficult to be precise and inevitably the holes turned out slightly wide than the M3 bolts they contained. There was no doubt a lot less movement in the Y-axis compared to the V1 design, but still a tiny bit left.
So I was looking to improve upon what was there and my solution had to fulfil three criteria:
The idea that I eventually decided upon was pretty simple. I would use a nut trap that glues to each corner of the build platform. The nut trap would hold a M3 locking nut which would be held still. I designed this in a design package called OpenScad using a belt clamp from Josef Prusa’s files as basis for the design.
To fit the part I first fed four M3 washers into four M3 x 20 bolts
I then inserted each of the bolts through the corner mounting holes of the heated print bed
I then fed a M3 washer onto each of the four protruding M3 Bolts
I then screwed a M3 locking nut onto each of the four protruding bolt. This needs to be tightened just enough that the bolt can still turn in the mounting hole
Next I inserted four M3 Locking nuts into the plastic nut traps. The traps should be printed so that just a small amount of force is required to insert the nuts.
Now I twist the Y-Axis corner nut traps so that they screw almost fully onto the M3 x 20 bolts.
It may be necessary to widen the holes on your build platform to M4 size before performing the next step.
Now apply the glue of your choice (I used 2 part epoxy) to the Y-Axis corner nut traps and place the heated print bed onto the wooden build platform. Clamp or apply some weight at each corner to ensure that they bond well to your build platform. Once the glue has set, the build platform is ready for use.
The files required to create your own build platform leveller can be found here... http://www.thingiverse.com/thing:21503