Nozzle wiper/purge bucket/bed clamp upgrade.

Readers will know that I have a strip of silicone rubber running across the back of my printer which is used to wipe the nozzle. Beyond this silicone strip is a “bucket”, which isn’t really a bucket because it has no bottom.

Here is a picture of how it was.

Old purge “bucket”

The silicone strip is the white part. This has to be in a fixed position on the printer frame because the nozzle is fixed in the Z direction. But with the bed moving in Z, and having linear guides at the sides, there isn’t much room to fit anything, hence the plastic brackets which fix to the back of the frame.

The maximum travel in Y before the carriages hit the frame is 364 mm. When the nozzle is at the edge of the build plate, it is at Y=350mm. So I only have 16mm to play with. There is an aluminium trim at the back of the bed which is 3mm thick and which functions as a back stop to prevent the glass from being pushed too far back. The part that forms the groove needs to have a wall in front of the silicone strip, and 2mm is about the minimum width that this can be. The silicone strip itself is 3mm thick. This takes another 8mm out of my 16mm leaving only 8mm where the nozzle can be behind the silicone rubber. But, when the nozzle brushes over rubber the strip, it deflects it by about 3mm before the rubber springs back. So that leaves 5mm. So the clearance between the part holding the rubber strip and the (moving) bed has to be less than 5mm, so I aim for 1mm.

I have tried just having the silicone strip stretched across the frame but it’s impossible to get the right amount of tension because the silicone rubber is somewhat elastic and stretches. So I’ve found that it works best if it’s held in a groove along it’s length.

The other semi-critical dimension is the height of the strip in relation to the nozzle. If it’s too high, it doesn’t deflect properly, or it deflects but doesn’t spring back to the vertical as the nozzle passes over, but instead it remains bent over. I’ve found that having the top of the strip about 1 to 2mm higher than the nozzle tip is about the optimum.

Which leads to a problem with the plastic mounts because the nozzle tip of my 5 colour hot end sits lower than the 3 colour version. So when I swap between them, I have to adjust the height of the silicone strip so that it is about 1 to 2mm above the nozzle tip across the entire width of the bed. That isn’t easy to do because those mounting brackets are simply bolted to “tee” nuts which slide in the extrusion. Some fine adjustment was required.

So I milled a 3mm slot, 3mm deep into a length of 8mm x 8mm aluminium, 2mm from the edge. A new strip of silicone was glued into this slot. Two further slots were milled in each end to fit around an M4 bolt with a washer either side, which sits in these brackets (one at each end).

So when fitted it looks like this from the top:

The bolt has a lock nut, so to adjust the height, I simply undo the lock nut and turn the screw. From top to bottom, the picture shows the edge of the (glass) bed, then the 3mm polished aluminium trim, then a 1mm gap, then the aluminium silicone strip holder which has a 3mm groove, 2mm from the front face. Here it is from another angle but with the bed lowered.

Here is a close up of the nozzle tip in relation to the strip:

All that remained was to make a “bucket” to catch the bits:

Actually it’s a tray not a bucket and only has a lip on one long side (to make it easier to clear the bits away). This is fixed to the bed frame and has a hole for the rear lead screw to pass through. Here is another view looking down from the top.

From the top of the picture down, that’s the glass build plate, the 3mm thick aluminium trim, the 1mm gap, then the silicone rubber strip sitting in the grooved aluminium, then the tray to catch the bits with the hole for the lead screw, then the tray lip.

Positioning is quite important because if the strip holder is a few mm further back the nozzle won’t get wiped and if it’s 1mm or more further forward, the bed will collide with it. The aluminium trim on the back of the bed sticks up 1mm above to form a lip. This is to prevent me accidentally pushing the glass too far back when I slide it in.

That’s the wiping/purging part take care of. The next issue was the clamping mechanism for the glass. This is it looking at the side of the bed.

It hasn’t changed much. Basically it’s just aluminium angle with a small lip at the top which clamps the glass in place. The wing bolts screw through slots in the plate and into “Tee” nuts in the bed frame extrusion. So to remove the glass, I just slacken those bolts, raise the aluminium slightly and slide the glass out from the front. To fit the glass, I just slide it in until it touches the trim at the back of the bed, press down on the aluminium sides and tighten the wing bolts. It grips really well and I’ve never had a part shift, despite the glass being a couple of mm smaller than the aluminium plate (it wasn’t supposed to be but the 400mm x 400mm aluminium plate I ordered turned out to be 402mm x 402mm).

The side plates have to be cut out to to clear the bed frame at the front, and there has to be slots for the wing bolts. I had to do this with a hacksaw and file before, but now that I have a milling machine I have been able to make a neater job. Also I hand painted the old parts to match the frame which was all black. They looked…..well like they had been hand painted. Now that I have many more aluminium parts, I decided not to paint them again.

The trim at the back of the bed is also 3mm wider at each side as well as having a lip to prevent the glass from being pushed too far back. This extra width is to prevent me from accidentally pushing those side plates too far back, which can also foul on the silicone rubber strip holder.

Finally, I replaced the trim at the front with a new piece which is polished. That’s purely for aesthetic reasons. The top of this trim sits just below the top of the aluminium plate to allow the glass to slide off easily. Here is a picture of the corner detail.

When I home Z, I heat the nozzle to around 140 degrees C because the nozzle itself is the probe and I want to soften any plastic which might have oozed out. This can sometimes leave a small dab of filament on the bed. So I’ve modified my homing macro such that the nozzle goes to the extreme rear of the bed while it is being heated. Then when it moves to the probe point, which is the centre of the bed, any plastic at the nozzle tip gets wiped away as it passes over the silicone strip. My “pre-print” macros that I use at the start of all my gcode files do the same thing.

As ever, I hope there might be something in some of the above which someone somewhere might find useful.


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