Torque wrench for tightening flexvat bolts

[HTL]

If your vat has adjustable tension using a torque wrench when tightening the tension bolts allow you to keep your vats FEP film at an even and consistent tension for printing and between FEP replacements. If you use a gasket on your vat using a torque wrench when tightening your vats clamp bolts ensures an even clamp pressure on the gasket so that the FEP film will not slip under tension and that there is no possibility of resin leaking through a part of the gasket that is over or under tightened".

The torques needed for vat gaskets and tensioning can be surprisingly small, the clamp bolt torque for the bolts on the vat shown here is only 16 ounce inches on each bolt, the tensioning bolts are also torqued to 16 ounce inches. On a larger vat (since upgraded to a foam gasket) the bolt torques are 40 ounce inches for the clamp bolts and 130 ounce inches for the tensioning bolts.

Tools for setting torques this low are both expensive and hard to find (I know, I bought both Sturtevant PM5 and Sturtevant PM15 torque screwdrivers for my vat). This torque wrench design which is specifically for flexvats is a reasonable cost alternative to the other available options.

Uses standard 1/4" bits. The "torsion bars" are carbon fiber strips.

[hegykc]

Nice work. Yes, the low torque wrenches/screwdrivers are both hard to find and expensive. But, being an engineer I always see people going into DIY over-engineering stuff.I see here flex vat designs out of monstrous blocks of aluminum, cnc parts, custom made gaskets, adjustable torque wrenches and whatnot...

I just cut myself a flex vat out of 5mm acrylic, with NO gasket between the clamping pieces, and standoffs/distancers so the bolts are tensioned not to the same force, but the same distance, without any special tools.
There is no leaks even though the clamp has no gasket, because the fep film itself serves as a gasket. Again, no leaks even when I put water in it for a weak.
Instead of measuring the tension on each bolt, I just put a piece of 5mm plexi when tensioning the a bolt and once it was snug, pull it out, and on to the next bolt.

It is maybe 25% as complex as the stuff I see here, and it works flawlessly, and looks beautiful, and the only tool used was a hex key for the bolts.

So my question is, has anyone had problems with leakage when not using a gasket? Anyone even tried just clamping two pieces with the fep in between?

Or unevenly tensioned bolts?? Although I really don't know what kind of problems this would even bring you. I mean you tension until it's tight as a drum, so no way can a few mm of resin in that small vat sag the floor. Only thing that can happen is uneven release, which is in itself a passive tilt and therefore a BIG pro, not a con!

EDIT: here it is. No gasket, no tension measuring, no metal and no tools. 30$ laser cut at the most expensive local shop. And the build platform is also 5mm acrylic. Guess what? Ater the print, I can pick the model off with two fingers, or just flex the platform a bit and it falls off by itself. I will make it 3mm next time for an even easier removal. Next thing I'm getting rid off is that stupid platform leveling mechanism. All that needs leveling is the vat floor itself where the building actually takes place. Put the model on supports and it makes absolutely no difference if the build platform is 1-2 mm off balance, the flexible floor will take the hit and not brake and the support structure will take the fault and you throw that away anyway. I've tried this.

[dangre]

My vat does not use a gasket (acrylic/FEP/UHMW) but the resin does 'wick' in between layers by capillary action. No 'leaking'. I will add a gasket to my next design. I think the resistance of my tight UHMW tapped holes would make torque measurements inaccurate in relation to FEP tension. But nothing wrong with trying to add a little extra numerical measurable device to your product.

[HTL]

For me a gasket allows me to make a vat with a reasonably large usable area using a small number of clamp bolts. The gasket actually decreases the complexity of the vat, for example, the vat I use on my printer has a usable build area of 125mm X 175mm (excluding the curved edges) and I only need 8 clamp bolts and 4 tensioning bolts, without a gasket you would need many more. The ability to torque the clamp bolts down on a gasket means no resin leaks or slippage of the FEP film under any circumstances and torquing the tension bolts means that you can maintain a FEP film tension that is both uniform and known even if you need to change the FEP or it stretches. I think that there is a lot of benefit in a $2.00 gasket and a $10.00 torque wrench.

PS
I use rivet nuts in my designs, no tapping just make a hole the right size and press them in.

They are used flange end down, bolts screwed into them torque smoothly.

[android78]

@hegykc - that vat looks really nice. I've been trying to find out what materials are safe for use with the resins and wondering how much use you've had with the acrylic vat and if you notice any adverse reaction to the resin?

[hegykc]

@HTL

Do you have a "build thread" of your vat? Sure a 2$ gasket and a 10$ wrench would be awesome. I'd be really interested to look at what gasket and wrench you are using. My vat is 118 x 208.

The reason why I question "extra even" tension is that FEP is a highly flexible, rubbery like material. And even if you had 4 bolts tensioned at 100%, 80%, 60% and 40% respectively, due to the high material flexibility the tension will uniform itself across the whole fep area, so in the case above you would get a 70% average tension across the whole vat. Even though at the corners you would have unevenly tensioned bolts, the fep itself would even out these tensions because of its high flexibility.
Just imagine two people pulling ot two sides of a big fep sheet, one guy pulling with X force, the other guy pulling with Y force. Does that mean that somehow there will be two sides of the sheet with different tensions? No, the tension will average out to X+Y/2. The same as it wil on a 4 bolt tensioning system: bolt 1+2+3+4 / 4 = uniform tension across the whole fep surface.
That would be my theory at least, I'd love someone to prove me wrong.

That's why I ask has anyone had problems due to uneven bolt tension? Any failed or deformed prints??

@android78
Cast acrylic and FTD resin, about two weeks, one print of 20 hours every other day, I've NEVER taken the resin out or cleaned the vat in that time. Still good as new. Prior to putting resin in, I poured water inside and left it for a week, no leakage at all.

[HTL]

The FEP (.005" Dupont Type A) I use is not in any way rubbery (see the photos on this thread: ). I torque my tension bolts for the same reason a drummer in a band will torque the bolts on his drums, consistency. A consistent and repeatable tension across the drumhead makes for a consistent and repeatable end product. I'm a believer in process control, the more consistency the better. As for torque wrenches I have a Sturtevant PM5 a PM15, the wrench shown above which incidentally covers the torque range of both the Sturtevant's plus the big ones that I use to work on the engine of my 89 silverado (5.7l v8, short cab, short bed with cap, step side, roll pan, side bar running boards, two tone maroon and silver) but I don't use those on the vat.

My gaskets are made from common 3mm craft foam (EVA) that I cut on my laser cutter, takes about 30 seconds to cut and in my opinion well worth the time. I'm sure that there are plenty of people doing just fine without a gasket or the use of a torque wrench to tighten the bolts on their vats but if you are building a flexvat from scratch and want it to work right out the box my suggestion would be that you use a gasket. A laser cutter is not needed you can cut a gasket out of craft foam with an exacto knife in a few minutes.

[dangre]

hegykc,
I have had failure due to improperly tensioned FEP. I was pulling my hair out when I could not get adhesion to my build plate after a FEP change. After draining my vat thinking it was the resin, I noticed the FEP did not sound the same as normal. When I 'thumped' the FEP it had a higher pitch on one side than the other. I added more tension to that side to make it sound even across the FEP and added a bit more on all my tension bolts. The next print worked flawless like it did before I changed the FEP. If I had smooth threads and a torque wrench I would have noticed this error on my part. I don't have smooth threads which would mess up any bolt torque measurements so I just you my ear to get even tension. Maybe it doesn't matter much on small vats but it does on larger vats. With your two guy analogy, the FEP would be taught between the two guys but very loose in the perpendicular direction. I suppose if you only had two tension bolts your analogy works but if you have multiple bolts around your vat then each has to be the same otherwise you get uneven tension pulling in different directions. My reasoning for why the part did not stick to the build plate was because during the peel move the FEP just moved up with the part so never released on the 'loose' side so it eventually choose to stick to the FEP instead of the build plate after successive exposures (just my theory).
P.S. Funny, I just replaced my vat a couple minutes ago and after my first tension I could 'thump' the FEP and it sounded very different in each corner. A little more tension in the lower pitched corners made for a consistent 'thump' and an evenly tensioned vat.

[hegykc]

@HTL

Oh you're the guy behind the blog, major respect! I got my fep from you. I looked up the torque screwdriver, isn't it 105 $ ?
Anyway I am ALL FOR precise measurement and knowing the real reasons behind things, so I will test this properly before theorizing further. I'm pretty sure what saves me is the 24 bolts and that's what evens out the tension. And I guess what I was saying is anyone just getting started with sla is better to go with many bolts that cost a few cents each, than a 105$ tool. I myself will get one for experimenting.

@dangre

Thanks for sharing the experience. Most likely your problem was not enough tension overall. I think once you get beyond a certain point (and that point is fep being firm enough for pealing), what difference does it make? So lets say your fep has to be tensioned to 60% to be firm enough to release the layers when printing. Then, what does it matter if some of your bolts are tensioned at 65%, some at 85% or some at 90%, or eve some lower than 60% (provided that you have more than just 4 of them). As long as your overall tension is greater than the pealing limit, you're fine. What's more, uneven tension will force the pealing effect to happen on one point, instead of evenly across the whole section, thereby greatly reducing pealing forces and serving as a kind of a passive tilt effect. That's why I'm concerned that an even tension might actually be a problem, not an advantage. I will test this out.

[HTL]

Note that the reason I designed this wrench is so people who wanted to torque the bolts on their vat would not need to spend a lot of money on a torque wrench or screw driver.

Torque wrench final design. The lever clamps down on the torsion bar and locks the bit into place.

Torque range for "thin" torsion bar: 8 - 40 ounce inches.
Torque range for "thick" torsion bar: 16 - 250 ounce inches.