Poly Vats- Ears not flat? Causes Rocking

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NCSU_Mike
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Poly Vats- Ears not flat? Causes Rocking

Post#1 » Mon Mar 13, 2017 10:41 pm

Hi All,

I'm getting to the end of a full teardown and rebuild of a 2-year old v1.2 B9, hitting several maintenance items and alignment checks along the way.

One of the big things I'm trying to correct in this rebuild is the machine's geometry. I haven't studied the flatness of each panel before, and was surprised to find out just how bowed each panel and major component of the machine was! In fact, the only things that weren't significantly out of flatness, were the linear motion guide rail (the back plate most DEFINITELY was out... I had to shim by over 0.018"... the THK specs call for a surface with less than 50um deviation, lol... soft aluminum or not, it was pulling the rail out of shape). Most components were checked with a known good granite surface plate, and several ground machinist's straight edges and squares. I'm amazed how well the printer was doing previously, even with these loose tolerances. Solid AK-like design philosophy! Put the precision where it counts.

Long story short, I've found that the polymer vats themselves are still rocking from front to back (a rotation about the x-axis) on the red x-slide plate. This happens regardless of thumbscrew tension, or with the thumbscrews removed or installed. The plate itself is very flat, and isn't rocking in the rails (rails have been sufficiently, but not perfectly shimmed). Also, it's pretty apparent that everything but the thumbscrew ears are raised up by a few thou' above the level of the plate. So, the vat is effectively floating, only held down by the two ears.

I did a quick search and couldn't really find much info relating back to the poly vats, and how they should be behaving.

So, I checked various vats for flatness. The bottom surfaces were indeed extremely flat... no noticeable rocking on the granite surface plate. Also, I was measuring from both the frame's ridges and the raised locating features on the corners of the vats WITHOUT the ears on the plate. Both were very flat! However, as soon as I found a way to measure the ears and the rest of the plate (without the raised locating features being measured), boom, rocking, just as when mounted on the red sliding plate.

Measuring across two vats (I have more that I can check as well), I found that one was raised a lot higher than the other, but both had the same rocking behavior. So, I'm assuming it's a design feature.

Is this a normal thing? I could see the extra clearance and flex acting to protect both the vat and x-axis components from damage due to compression during curing and movement.

Does sanding the vat ears flat to the level of the rest of the vat (or shimming it so it doesn't rock) improve printing performance at all?

I've read through a few older posts on vat position repeatability, and it sounds like there's a definite improvement in print quality if the pdms comes back to approximately the same place for each layer. Is this still the consensus?

Of course, it's not as big of a factor as projector to z-axis alignment changes during printing. But, I'd like to eek out every ounce of performance from our printer if possible.


Thanks in advance,

Mike

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Metalcaster
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Re: Poly Vats- Ears not flat? Causes Rocking

Post#2 » Tue Mar 14, 2017 8:26 am

Both, my 1.1 and my 1.2 machines have numerous areas that I had to square up so that I could get the quality that I wanted. If you look at the front of the machine, the Flat shelf that everything rides on is slightly bowed. This is caused by the method used to bend the Aluminum. Squaring the shelf with the back required me to put adjustable rods on the front down to the feet...so I could adjust the shelf front up or down on each side. I also squared the feet of the machine and reinforced them so the machine is now truly movable without bending. However, all of that, did not fix the Bow in the shelf that is visible when you look directly at the machine. The interior of the shelf is now flat with the interior back plate.
If you look on the bottom of the orange vats, you will notice tiny little bumps near the
screw holes ... on each side of the hole. this is to put the tray at a very slight angle... perhaps to help with release. The problem it can create is that when you tighten the screws holding the Vat, it can / will bend the vat in the center... with an acrylic bottom, that can get bent too! I replaced all my acrylic windows with Glass so that part stays flat... I removed those bumps on the bottom of the trays, sanded down all contact points until everything was square on the tray.

That is overkill for PDMS... PDMS does a great job of making the tray surface flat if you make the tray filling and drying areas perfectly flat.

NCSU_Mike
B9 Maker
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Posts: 34

Re: Poly Vats- Ears not flat? Causes Rocking

Post#3 » Sat Mar 18, 2017 1:24 pm

Hey Metalcaster,

Great answer, thanks! I did indeed notice the nubs... they weren't present on my older vats. On the vats that did have them, they made contact before the rest of the vat ribs, and kept the windowed side from rocking as much. Indeed, I could see them causing a few hundred microns of distortion when adding-in the screws.

Did you notice a big difference with all of those mods to your poly vats?

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Metalcaster
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Re: Poly Vats- Ears not flat? Causes Rocking

Post#4 » Sat Mar 18, 2017 6:12 pm

The difference in the modified polyvats is that I have squared the vats and put Glass in place of the acrylic.
Makes them easier to maintain and flat to the rest of the machine.
Other then speed, I would put the prints from my 1.2's against any existing printer.
I am pretty sure some prints I did are in the 15 to 20 mn. Squaring up the machine has increased the accuracy of the prints by quite a bit.
One machine set up for large bracelets.... The other for everything else :-)

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