Bottom Feature

Can someone please explain how the Bottom feature in SF 040 works? There are two parameters in there: Additional height over layer thickness ratio and Altitude. If I enable this feature and set both parameters to 0, then SF puts the first layer at half the layer thickness. If I turn off this feature, then SF starts the first layer at the layer thickness. I was expecting that having both parameters at zero would give the same as having the feature off since I thought that Altitude specifies the location of the bed.

When SF generates a height for the first layer as half the layer thickness, does it automatically cut the flow in half?

I looked in the wiki and this section is blank.

The documentation included with skeinforge is always up to date.

Quote

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 have a z of the altitude plus half the layer thickness.

(emphasis mine)

It does not cut the flow in half. At least for me, full flow is needed to that the bottom layer is squished down and sticks to the bed better.

I think if you turn off Raft you will get the expected start of z = 0, though Raft controls things like temperature setting, etc.

Edited 1 time(s). Last edit at 03/22/2011 07:12PM by NewPerfection.

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Thanks! Somehow I never looked in the SF folder for documentation. I was mislead by the help buttons in the dialog that doesn't work, at least on the Mac. I thought it was supposed to open a website on the browser but nothing happens when I click it. So, I assumed the the documentation is on the web.

I also figured it out by trial and error. With the default settings, additional height at 0.5 and altitude at 0.0, the z position of the first layer comes out the same as when Bottom is turned off. I now understand how to use this feature, but the explanation doesn't make sense since we don't extrude with the tip positioned in the middle of the slice, but rather on top of the slice. With both parameters set to 0, the first layer should be extruded with the z set to the layer thickness. Oh well, I'll just move on and learn another SF feature.

brnrd Wrote:
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> I now understand how to use this feature, but
> the explanation doesn't make sense since we don't
> extrude with the tip positioned in the middle of
> the slice, but rather on top of the slice. With
> both parameters set to 0, the first layer should
> be extruded with the z set to the layer thickness.
> Oh well, I'll just move on and learn another SF
> feature.

The problem is that if your first layer is extruded at your layer height, it won't stick to the bed (at least it doesn't for me). The first layer prints at z = half layer height so that it squishes the filament into the bed to make it stick. You're right though, the explanation for the feature is not very good.

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Squishing into the bed only works for me with PLA on blue tape or PCL on balsa because the plastic can soak into the fibers.

When the bed is solid like glass, Kapton or PET tape then there is nowhere for the excess to go, so it just makes a mess that takes several layers to recover unless I extrude the first layer at the correct layer height. Also the object ends up with a ridge around the bottom if the first layer is too low.

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What is this blue tape?
I have some problems of PLA sticking too deep and being difficult to remove, maybe it's a solution?
Thanks

3M blue masking tape is what I used before I went to using a heated bed with Kapton, PET or glass.

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nophead Wrote:
-------------------------------------------------------
> Squishing into the bed only works for me with PLA
> on blue tape or PCL on balsa because the plastic
> can soak into the fibers.
>
> When the bed is solid like glass, Kapton or PET
> tape then there is nowhere for the excess to go,
> so it just makes a mess that takes several layers
> to recover unless I extrude the first layer at the
> correct layer height. Also the object ends up with
> a ridge around the bottom if the first layer is
> too low.

This has also been my experience extruding on heated glass and kapton tape. It's also a pain to have to clean up the extra plastic.

PLA on glass works well as long as the temperature of the tip is higher than the rest (230 first layer and 210 for the rest with the glass at around 55-65C). But I've been having problems with ABS (250 first layer, 250 first layer and 240 for the rest with kapton on glass around 90-100C) coming off the bed and warping on large parts that print fine with PLA. I guess I'll have to get an aluminum plate and run the bed higher.