A new guy confused about top down printing

[Malber]

Hi
I'm about to begin a project building a top down DLP SLA printer, but there is one thing that I have wondered and that I cannot seem to find an answer to anywhere, possibly because it's a stupid question. My question is, what do you do to set the platform at the right distance under the surface of the resin for the first print layer? Do you just eyeball it and sand off the excess after the build is done, or is it crucial that it has the exact right depth?
Also, another question regarding the z-axis for my printer, I am thinking of using a screw driven actuator made using V-slots. Is that an alright choice? Or is there a better/cheaper solution?

Thanks in advance

[PacManFan]

You should focus the projector directly onto the surface of the resin from what I understand. It is important that you maintain an even level of resin at the surface. With a top down, the build sequence typically dips the model in the resin and surfaces to either settle the resin, or use a wiper to maintain the slice thickness at 25,50,75,100 microns. For a top-down printer, you don't need as much torque as a bottom-up. The system you described should be fine as long as it can maintain the vertical and horizontal stability.
-Steve H.
PacManFan.

[Malber]

Thank you for your reply, that is very nice feedback. I think we might have misunderstood each other slightly though. What I meant was wether there is a way to make sure the very first slice is the right thickness, as I do not see how the printer would know the exact level that the resin was poored to. If I understood u correctly this could be done using a wiper, although if I go for a wiperles design will I just have to estimate the very first slice?

[Scatterplot]

AFAIK you do just need to ballpark the first slice. I know with bottom-up printers the first few layers get overexposed anyway (to make a solid foundation), so typically if you need precision on the base of the part just put the whole thing on supports, then snap it off when you're done. I've never used a top-down before myself, but I'd aim to get the thinnest possible layer for my first layer. You're only looking at 50-100 microns of error in that zone anyway, so it doesn't really matter too much (for most things).

[android78]

If you are coming from FDM 3d printers then some of your confusion may be because you are used to building directly on the build platform. I don't believe this is very common with SLA and you will usually have Support base attached to the build plate upon which you have the supports, then at the top of the supports, you have your model. If you have a few layers of the support base, then you can start with the build platform level with the surface as it will sort itself out after the first few layers.

[Malber]

Thank you guys, that is very nice information. I had not even thought about the fact that you build on supports, but now that you mention it, it makes good sense. My parts should have arrived today, so let's see how it goes and if I'll be able to make it print