Ok so how flat is flat enough.

Ok so how flat is flat enough.

Postby KD6HQ » 2016-Sep-Mon-21-Sep

I just received my latest piece of aluminum and it is supposed to be "flat".
Well it's not of course, but I don't think it's bad. From what I can tell it's about 0.302 MM out.
That just a tad more than a standard (if there is such a thing) piece of printer paper.
I will say it is better than others I've gotten. Not as good as another one that I have.

The other one that I have, has a flatness call out, this one did not. Using a straight edge
on the one with a flatness call out you can see just a hint of light when holding a flashlight
up against a straight edge but can not get a piece (same piece) of paper under the straight edge.

So what do you think, is 0.302 MM good enough or should I try and "fix it"?
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Ok so how flat is flat enough.

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Re: Ok so how flat is flat enough.

Postby Mooselake » 2016-Sep-Mon-21-Sep

I'd put a piece of glass on it. That's more than a layer thickness.

With RJ's dial indicator my double strength glass is flat within a few hundredths of a mm

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Re: Ok so how flat is flat enough.

Postby plexus » 2016-Sep-Mon-23-Sep

I have some experience with this. I tried to make my own aluminum bed by buying custom cut stock from an online metal place. long story short, wrong move: I don't know much about metal stock but got a lesson. stock cut like that will have warps. its not guaranteed to be flat to any great accuracy. the only way to make it flat is to machine it properly. in the end I was lucky to get a refund for the metal. I ordered some really hard rigid (expensive) stainless. and it was more economical to buy 6 pre-cut pieces 300mm x 300mm (I think it was 4mm thick). they let me keep the pieces. however they are of little use to me.

I ended up getting someone on here to machine me a flat bed. it has to be quite flat. I would say you'd need the bed to be +/- whatever your min layer height is you want to be able to work with. in other words, pretty flat.
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Re: Ok so how flat is flat enough.

Postby frankv » 2016-Sep-Mon-23-Sep

I'd say 0.3 is possibly good enough... that's about the thickness of the first layer. So (assuming your bed is leveled properly to the XY axes), you could adjust the height of the first layer to maybe 0.1mm above the high point on the bed, which probably won't jam the hot-end. At the low point it will be 0.4mm above, which will just stick. If there's large areas of high or low, that's probably worse than a wavy surface where high and low points are close together.

You could try adding a layer or two of tape on top of the bed at the low points to flatten the surface a bit more... I flattened my (cheap Delta) bed with dozens of layers of tape, with strategic bits cut out here and there. The bottom surface of prints is a bit unsightly, but other than that it does print just fine. OTOH, I've now got glass to put on it.
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Re: Ok so how flat is flat enough.

Postby KD6HQ » 2016-Sep-Tue-01-Sep

Not having a way to attach my dial indicator to the printer I decided to place the new bed on top of my existing bed and use the probe to measure the hills and valleys.
I started off by getting the 4 corners as close as reasonable time would allow.
The readings were:
0.24 0.28
0.28 0.29
not bad, then I took 9 point readings.
0.25 0.30 0.26
0.43 0.53 0.50
0.26 0.29 0.24

0.53 - 0.25 gives 0.28 change from low to high. With my feeler gauge and straight edge I got .302 a difference of 0.022 - not bad for a feeler gauge.

My normal 1st layer is 0.3 mm, but I have went down to 0.1 mm on occasion.

I think I will continue to head in the direction of the glass top. It may be more time consuming to initially setup but I think I may have a little more confidence in it.

Is there a specific reason to use double strength glass - other that it won't break as easily?

Next I have some questions about the heated bed makeup but I think that should be in another thread.
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Re: Ok so how flat is flat enough.

Postby Mooselake » 2016-Sep-Tue-06-Sep

DS is stiffer, so it flexes less with the binder clips. I could measure a bend with the thinner single strength. I think it cost me ten bucks for four pieces with the corners clipped for the screws. Next time I'd do that with a diamond bit in the Dremel and have the edges rounded instead

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