<p>You can go about this by doing the arithmetic for volume of the extruded lines that go in the first layer, and figuring how their width will be affected by their thickness differing from the planned thickness.</p>
<p>For isolated lines or lines that are just part of a double wall, any excess will be &quot;elephant foot&quot;. For solid fill, the error will accumulate until there's nowhere for the material to go, and it will either pile up above the nozzle height at the sides of the nozzle or cause the extruder to skip or slip after building up pressure that leaves the nozzle over-primed. It's also possible that the bed springs or compressibility of the build surface (buildtak, etc.) take up some of the error past this point.</p>
<p>For cases where the distance between the nozzle and bed is too great, on the other hand, it's possible that the extrusion will only slightly smash against the bed, or not at all. In that case you have to work out what dimensions the non-squished, likely-circular cross sections will have.</p>

CC BY-SA 4.0
2022-10-02T02:59:58.490Z
I feel like the case of too-great distance is particularly pernicious, as the result isn't just lack of squish, but warping and unmoored prints leading to failure. Do you have any guidance for evaluating the risks and results?
Kenn Sebesta
CC BY-SA 4.0
2022-10-02T13:56:00.897Z
You asked about "a priori" ways to determine, so rather than trying to give anecdotal or empirical findings I focused on how you'd go about predicting the behavior. I think it shoul be possible to get a rough idea at what point lack of adhesion will occur this way - you'd look for the distance where the extrusion becomes mostly round and has very little surface area left to squish against the buildplate.
R.. GitHub STOP HELPING ICE
CC BY-SA 4.0
2022-10-02T17:35:45.270Z
I see what you're getting at, although I'm not sure what I can do with "very little" in regards to surface area. I think it would be helpful to have a rule of thumb, if such a beast were to exist. Is "very little" a percentage of surface area? Volume? Or is it more likely to be a fixed width no matter the layer height and width.
Kenn Sebesta
CC BY-SA 4.0
2022-10-02T18:30:58.640Z
If you imagine the cross section as a bubble expanding from the nozzle orifice, I think you'll find there's a fairly rapid transition from making no contact at all with the bed (fully round bottom) to covering a large portion of the intended line width. You'd have to work out the math to be sure, but this is my intuition at least.
R.. GitHub STOP HELPING ICE
CC BY-SA 4.0
2022-10-02T02:31:46.837Z
2022-10-02T02:31:46.837Z
R.. GitHub STOP HELPING ICE
<p>What are ways to a priori determine how bed flatness affects prints, as a function of the initial layer height? It's pretty obvious that a 50 µm difference from one edge of the print to the other is not a big deal when layer heights are 500 µm, but it's huge when the initial layer height is 100 µm.</p>
<p>So where's the elbow between <strong>good</strong>, <strong>okay</strong>, and <strong>bad</strong>?</p>
<p>Motivation: I'm working on making a flat bed and want to know when to stop.</p>

<bed>
How does relative bed flatness affect print quality?
Kenn Sebesta
# How does relative bed flatness affect print quality?

(Asked by: username_0 on 2022-10-02)

What are ways to a priori determine how bed flatness affects prints, as a function of the initial layer height? It's pretty obvious that a 50 µm difference from one edge of the print to the other is not a big deal when layer heights are 500 µm, but it's huge when the initial layer height is 100 µm.

So where's the elbow between **good**, **okay**, and **bad**?

Motivation: I'm working on making a flat bed and want to know when to stop.

**Tags:** bed

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## 1 answers

### Answer by username_1 on 2022-10-02. Score: 1

You can go about this by doing the arithmetic for volume of the extruded lines that go in the first layer, and figuring how their width will be affected by their thickness differing from the planned thickness.

For isolated lines or lines that are just part of a double wall, any excess will be "elephant foot". For solid fill, the error will accumulate until there's nowhere for the material to go, and it will either pile up above the nozzle height at the sides of the nozzle or cause the extruder to skip or slip after building up pressure that leaves the nozzle over-primed. It's also possible that the bed springs or compressibility of the build surface (buildtak, etc.) take up some of the error past this point.

For cases where the distance between the nozzle and bed is too great, on the other hand, it's possible that the extrusion will only slightly smash against the bed, or not at all. In that case you have to work out what dimensions the non-squished, likely-circular cross sections will have.

> **Comments:**
> **username_0**: I feel like the case of too-great distance is particularly pernicious, as the result isn't just lack of squish, but warping and unmoored prints leading to failure. Do you have any guidance for evaluating the risks and results?  
> **username_1**: You asked about "a priori" ways to determine, so rather than trying to give anecdotal or empirical findings I focused on how you'd go about predicting the behavior. I think it shoul be possible to get a rough idea at what point lack of adhesion will occur this way - you'd look for the distance where the extrusion becomes mostly round and has very little surface area left to squish against the buildplate.  
> **username_0**: I see what you're getting at, although I'm not sure what I can do with "very little" in regards to surface area. I think it would be helpful to have a rule of thumb, if such a beast were to exist. Is "very little" a percentage of surface area? Volume? Or is it more likely to be a fixed width no matter the layer height and width.  
> **username_1**: If you imagine the cross section as a bubble expanding from the nozzle orifice, I think you'll find there's a fairly rapid transition from making no contact at all with the bed (fully round bottom) to covering a large portion of the intended line width. You'd have to work out the math to be sure, but this is my intuition at least.  

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