This brings to mind the detail that I deliberated when I was setting the "zero" that may have some bearing on this. After reading various remarks from, I think Mark W, Chris, and maybe others, about various levels of friction on the paper when setting the zero, including adding a bump or two to the point of clamping the paper, I thought it through and decided to try it one bump "loose" on the paper. I came to this after realizing that various factors contribute to the whole point of getting a setting and considering the mechanical factors in play as well as the curing method with additional base curing time built in to the program.
The first thing I realized as I came to understand the functions is that in normal operation there is no actual mechanical/ physical pressure applied to the part, nor should there be. Not from the start to the end of the buildup. And that's a large part of what affords the accuracy of the parts that these produce. There is a gap (as set in the program) that is filled with resin and that resin is in turn cured by the light emitted from the screen, gap by gap, layer by layer.
With that in mind and understanding the functions, there would be absolutely no benefit to physically clamping the bottom of the vat between the build plate and the screen. Not only would there be no benefit but it would be detrimental to that function not to mention the unnecessary physical strain put on all parts as it would just displace any resin to nothing (and be hard on the film) that was intended to be part of the base. So that means there will be no base started until the machine has finally progressed upward until there is a gap for the intended process to begin. It leaves the base thinner than intended but also puts undue strain on mechanical parts and especially undue forces on that, apparently somewhat delicate, film.
So with that intended result in mind it's apparent and logical that that's the whole point of getting the platform level to the screen/ film and the precise starting point set. I say precise but that's qualified and accomodated in the design, using a feeler gauge that's specified as "a sheet of paper" but without a specified supporting dimension. That's OK and I give them credit for keeping it simple as standard paper really is pretty reliably about .003" but I also suggest it's accepting a less than perfect dimension as perfectly okay.
Then there's the base "cooking" time, eight layers suggested, that's measurably longer than the rest of the cycle. Suggested times that are four and more times the build baking times. I suspect there's a two-fold reason for that. One is to more completely cure/ harden the base for a more assured solid attachment to the build plate but also, with the potential for various opinions of setting zeros as well as the potential for minor errors in levelling and all of the other physical tolerances, whatever the source, to still allow the base to properly cure. It is the correction for all of the variables affording an assured, very accurate, starting point for the actual part. Within reason, of course.
So with that in mind, how does that help us understand the only thing we need to do, set the "zero". And how much force or lack thereof, do we apply to the paper. Well, right out of the chute, the film measures .005" but we're using .003" as a feeler gauge.
Hmmm. And the increment that we must advance by, built in to the machine, is near a half thousandth. And the build platform, at least on mine, has a very slight center crown to it. Very slight but it's there. So the process apparently has that error and others like it covered. In fact, it makes more apparent how these things can produce parts with such accuracy. They are relying heavily on the base layer finding its own self corrected starting point, serving a purpose beyond just sticking the part.
Okay. With all of that in mind, what's actually happening? The film that's going in is .005". The paper we will use as a gauge is .003". That's already .002" tight in a perfect world. The travel of the base is part of the chosen settings that can be as little as .01mm (a half thousandth) so if we aren't setting the the base number higher than that we get basically nothing there. I have used .05mm layers which is basically that last .002". Bottom line, if everything was set ideal the very first layer can be as little as .003", not allowing for tolerances in the machine parts, levelling, accuracy of the zero setting, etc. Now, why the extra curing time? It will assure a pretty good cure even with a pretty broad range of first layer thicknesses whatever the source of those variations. Thank goodness for tolerance.
Now, with all of that in mind, how does that translate to actions when I'm standing there with paper in hand and an itchy zero button finger?
I lower the plate in the defined .01mm (half thousandths) increments until the paper has a slight drag. One more tap and it gets tight. I can still force the paper in tension but by now I'm most likely taking out screw lash and maybe even springing the structure slightly. For our purposes that's too tight already. So I back it up one, and then again, two taps. I call that zero and am within .001" even if I missed the call on the drag. The extra cure time specified in the base setting will cover me and I will not EVER be pinching the expensive film nor overloading the machine mechanisms and all to ZERO benefit.
It'll even save pinching a small bit of debris that's easy to miss yet can do damage if it's solidly forced against the delicate sheet. As far as I can tell, there's just NO reason to set the zero tighter than specified and in fact, allow all error to the loose side rather than tight. The process will handle it, no hammers required.
If in doubt, up the bottom layer cure time but don't pinch the sheet. That's not where to save time when it's only for a few layers and the potential problem it creates is hours later finding a screwed up print or worse, damage.
I'm sharing what I've surmised after hearing some of the start up horror stories about failed efforts, a good dose of overthinking, and my extensive multi run experience.
Hope it helps someone. After all of this effort I'm running with it even if it's total hogwash.
