Botton Plug-in - Use and Settings......

Hi,

Can anyone explain the Skeinforge Bottom plug-in operates in simple terms to me?

I've read the manual (http://fabmetheus.crsndoo.com/wiki/index.php/Skeinforge_Bottom) but don't understand how it translates into practical advice about printer calibration. (Wiki manual wording at bottom of post)

I have drawn some (quite probably wrong) conclusions based on what I experience with it. I'd be interested in people's observations.

I decided (for no good reason at all and wrongly) that I could just turn Bottom off and it wouldn't affect my prints in an important way. However, I then noted that all my objects were approx 0.5mm short (no matter how tall – always approx 0.5mm never, say, 5% shorter). So, I turned it back on and the height was correct (more or less – obviously other Skeinforge settings are in play here too).

My Bottom settings are:
- Additional Height over Layer Thickness – 0.7 (note that my layer thickness in Carve plug-in is 0.3, in case that's important)
- Altitude - 0

I'm not using a raft.

This would mean the Wiki manual descriptive text translates (for me) to:

- No 'Additional' height – so ignore that.
- Take an 'additional height' of 0.7 (since I set that)
- Add the layer thickness (0.3 in this case, as provide by Carve/layer Height).Total = 1.0mm
<*** Or am I interpreting the word "over" here incorrectly – is it a division not an addition? ***>
- Multiply by the layer thickness (0.3 in this case, as provide by Carve/layer Height).
- Resulting in a final number of = 0.3mm

So, I guessing that the Bottom plug-in somehow (as I would want it to) raised my object height by 0.3mm ('approx 0.5mm') – i.e. it compensated for the fact that the height of my extruder nozzle when I begin a print is approx 0.3mm (according to feeler gauges & 'the thickness of 1 (or maybe 2) piece(s) of paper'?)

Does that sound right to anyone???

Cheers

Pete

p.s. Very hard to avoid bottom jokes in this post 

Wiki Wording:
"Additional Height over Layer Thickness
Default is half.
The layers will start at the altitude plus the 'Additional Height over Layer Thickness' times the layer thickness. The default value of half means that the bottom layer is at the height of the bottom slice, because each slice is made through the middle of each layer. Raft expects the layers to start at an additional half layer thickness. You should only change 'Additional Height over Layer Thickness' if you are manipulating the skeinforge output with your own program which does not use the raft tool.
Altitude
Default is zero.
Defines the altitude of the bottom of the model. The bottom slice has a z of the altitude plus the 'Additional Height over Layer Thickness' times the layer thickness."

The way I understand the documentation, in your case Bottom would make the print start at 0.3 (your layer height) times 0.7 (Additional Height over Layer Thickness) = 0.21 mm above the print surface.

If you disable it, I believe your first layer will hardly be printed because the extrusion hole will still be stuck right onto the bed.

The Additional Height over Layer Thickness parameter is a fraction of your layer height, so the correct calculation of the nozzle height in your case would be:

0.7*0.3mm + 0mm = 0.21 mm

The other setting that affects the nozzle height is the Operating Nozzle Lift over Layer Thickness in the Rafts plug-in. This is also expressed as a ratio over the layer thickness and is defaulted to 0.5 unless you changed it. With a layer thickness of 0.3 mm this calculates to an addition lift of 0.15 mm. So the nozzle position on the first layer should be:

0.21mm + 0.15mm = 0.36mm

If you adjusted your z endstop to exactly touch the bed, then this would only be 0.06 mm higher than the ideal. If you had 1-2 pieces of paper clearance, then that's 0.1-0.2mm. Added to 0.36mm, would mean that your nozzle first layer was at 0.56 mm which is about 0.26 mm larger. In my experience, this much would be enough for the plastic not to stick to the bed if the flow rate is set properly. Since you were able to make it stick, you might also have the flow rate set too high (extruder calibration).

Note that as the manual states, you should keep the Additional Height .. parameter at the default of 0.5 and then adjust the altitude to take account of your bed position when your z is at the endstop. In my case, I always use a piece of paper (about 0.1 mm) to set the bed clearance. So, I set my altitude to -0.1. This way, I don't have to worry about crashing my nozzle into the bed.

Edited 3 time(s). Last edit at 10/26/2011 07:56AM by brnrd.

DeuxVis Wrote:
-------------------------------------------------------
>
> If you disable it, I believe your first layer will
> hardly be printed because the extrusion hole will
> still be stuck right onto the bed.

It should be printed at half the layer thickness or 0.15 mm in his case if he didn't change the default setting of 0.5 for the Operating Nozzle Lift over Layer Thickness in the Raft plugin.