Question about vacuum chuck forces:

Originally posted by twhitehead
Except for the fact that the 'orifice' consists of multiple slot whose exact arrangement we don't know.
But otherwise I generally agree with your analysis.
What I don't quite understand is where sonhouse plans to put a seal and how this will change the situation. If I am correct and he places a ring around the edge of the flat objects so that it now has ...[text shortened]... r seal, then he is mistaken in thinking it will have no effect on the difficulty in removing it.

I never said the force to remove would be the same. The force to remove would be dozens of times higher because it is the area connected to the vacuum pump that gives the downward clamping pressure, and of course it can only be as strong maximum as the air pressure above and total vacuum underneath, like 1 E-6 torr or some such, the absolute force would not be much different if the vacuum level was 1 E -3 torr or 1 E- 9 torr, for all intents and purposes, the force required to pull the pieces apart would be just about the same, maybe a gram or two more with more perfect vacuum.

The way I see it, the two surfaces when I grind the chuck metal with a find grit diamond I will be smoothing out scratches and such and I see the results in the fact that less photo reisist gets into the vacuum piping. When the chuck starts getting scratches, more photo resist gets into the guts of the vacuum system.

That is why I want to use an O ring seal. Don't know why they didn't design one years ago, the way it works now is awful. Keep having to clean out the vacuum lines, filters, control solenoids and such. An O ring seal avoids all that.

And the bonus is the area under the O ring is now MUCH larger than the area of a few circles of channels 1 mm wide, a circle about 60 mm diameter and some straight lines maybe another 60 mm total which would be about 100 to 120 square mm, not a whole lot of potential for real holding power.

In our system, the chuck spins at around 3,000 RPM and fairly vibration free so the chuck pressure doesn't have to be 100 kg, just a kg or 2 works fine.

The real problem is the contamination of the guts with photo resist.

I worry the force will go up literally to 100 Kg and if the O ring sticks up too far maybe breaking the 1 mm thick alumina substrate which is very brittle, doesn't bend much at all.

The chuck is aluminum but about 10 mm thick so it is a lot stronger than the alumina.

My O ring seal will avoid all the problems we encounter, low holding force when there are too many scratches and build up of solid material on the chuck due to poor cleaning and getting photo resist into the guts of the vacuum system. If you imagine a circular O ring say 4 inches diameter, 100 mm, close enough🙂 and 12 square inches area or about 8000 mm squared and half vacuum, say 7 PSI the clamping force would be about 80 pounds or around 35 Kg. Just rough numbers for illustration. That would be about 25 times the force we now experience which will be great for keeping the substrate from moving around under the spin cycle of 3,000 RPM.

I just want to make sure the height of the O ring when compressed won't break the alumina substrate.

Originally posted by twhitehead
Except for the fact that the 'orifice' consists of multiple slot whose exact arrangement we don't know.
But otherwise I generally agree with your analysis.
What I don't quite understand is where sonhouse plans to put a seal and how this will change the situation. If I am correct and he places a ring around the edge of the flat objects so that it now has ...[text shortened]... r seal, then he is mistaken in thinking it will have no effect on the difficulty in removing it.

"Except for the fact that the 'orifice' consists of multiple slot whose exact arrangement we don't know."

Whether it is multiple slots or not, deflection will occur around each slot, and superposition still holds. So the arrangement isn't important in regards to the initial set up described.

"But otherwise I generally agree with your analysis."
I'm glad that we are on the same page.

"What I don't quite understand is where sonhouse plans to put a seal and how this will change the situation. If I am correct and he places a ring around the edge of the flat objects so that it now has a much better seal, then he is mistaken in thinking it will have no effect on the difficulty in removing it"

You are correct in asserting this. If he places a ring around the perimeter of the plate enclosing all the slots, the area of the slots will be meaningless. What is truly important is the enclosed perfectly sealed area (which will substantially grow the holding force and undoubtedly have negative consequences on the substrates deflection and internal stress)...please read this Sonhouse It just so happened that these two areas (the areas of the slots and the areas of the seal ) in the initial set up coincided,... but it is certainly not necessarily the case when you start adding seals to the mix.

Originally posted by joe shmo
"Except for the fact that the 'orifice' consists of multiple slot whose exact arrangement we don't know."

Whether it is multiple slots or not, deflection will occur around each slot, and superposition still holds. So the arrangement isn't important in regards to the initial set up described.

"But otherwise I generally agree with your analysis."
I'm g ...[text shortened]... incided,... but it is certainly not necessarily the case when you start adding seals to the mix.

I already have that problem in mind, the bending of the substrate and I have plan B ready in case I break one, using only junkers for testing. I am making a rectangular shaped O ring slot with curved corners because there is a minimum turn radius to keep the O ring happy.

So it is going to be about 5 mm in on all sides so close to the edges.

I KNOW the clamping force will go way up over what we have now.

But I have a plan in place if it in fact breaks the alumina substrates.

Up to the point the alumina breaks, more clamping force is great.

But the main thing I want is to keep that dam photo resist out of the guts of the vacuum system and the O ring will do that just fine.

Also, (according to the model) it is much better to have a perfectly sealed contact surface, than it is to have a vacuum pump at all ( that is if you have the former a pump is rendered useless). Its doubtfull that you could make this happen with the material you are using, but in theory if the mating faces seal out atmopheric pressure entirely the force will be maximized for any given area.

I remember a Physics demo where our professor lifted his steel desk with nothing more than flat circular sheets of plastic with some handles attached to them, and another demonstration was breaking a wooden ruler using a sheet of newspaper (and atmosheric pressure). I guess I never really gave it that much thought until twhitehead pointed it all this out.

Originally posted by joe shmo
Also, (according to the model) it is much better to have a perfectly sealed contact surface, than it is to have a vacuum pump at all ( that is if you have the former a pump is rendered useless). Its doubtfull that you could make this happen with the material you are using, but in theory if the mating faces seal out atmopheric pressure entirely the force wi ...[text shortened]... re). I guess I never really gave it that much thought until twhitehead pointed it all this out.

That method would be great if we could pull it off but the surfaces have to match on a microscopic scale I think anyway.

Our chuck surfaces get roughed up to regularly to be able to use that method. We are stuck with just vacuum. But with my O ring the vacuum pump could be mostly gone and we would still get enough holding pressure to be able to spin at 3000 RPM safely.