Surface Callout for air-tight mating

[Freesum]

New to this site. Hoping I can get a question answered as I don't really have experience with surfacing.

I have two surfaces that need to create a gas tight seal but cannot be mechanically affixed to each other due to warping that will occur from thermal expansion in use. My solution was to have the two mating surfaces ground very flat so to butt up without any air-gap. See attached (red arrows mark the surfaces to mate).

I have a few questions...

1st) What is an appropriate callout to achieve this? I don't want to go overboard (over-budget) spec'ing out unnecessarily tight roughness/waviness requirements.

2nd) What's the thinnest material I can get away with that can be ground to that value?

3rd) What's the most economic process to achieve these results? (does that go on the callout as well?)

Thanks You!!

 

[drawoh]

Freesum,

You need to specify flatness and surface finish. This is easy, since all you are doing is applying numbers to your drawing. Unless someone has done something like this, you are going to have to experiment to find out the required values. Your big problem will getting your fabricator to make a reasonable quote on your drawings.

--
JHG

 

[Nescius]

Gasket? Flexible Adhesive? If the parts warp so severely in use that any mechanical bond is out of the question, no surface flatness/finish callout is going to matter.

How air tight is "air tight"? What pressure, if any?

 

[Freesum]

The top plate only bends when affixed to the frame. If it's unconstrained, it has room to expand and contract appropriately. From my understanding, a single sheet of metal heated evenly shouldn't warp from heat alone.

I'm considering a flexible sealant that will allow for thermal expansion. That may end up being the way to go.

The pressure is negligible. There are plenty of seep holes for the gas to flow out to the rest of the assembly. This is just a pooling chamber. Weird, I know...

@drawoh
Well that's the problem isn't it... I don't know what a mild flatness/surface finish is to begin with. So I don't know if I'm specifying a tight or loose condition.

 

[braddles90]

To give you some numbers, we manufacture parts that are typically 10-20mm flange face width (from pressure side to atmospheric side), surface finish 1.6, face flatness to 0.2mm and they will leak no matter what.

To be honest I'm not sure how far you could go to make a gas-tight seal - as mentioned it definitely depends on internal pressures, temperatures, etc.

How much thermal expansion is expected? A couple of hundred degrees typically doesn't lead to much deformation, there are many gaskets these days that can take big thermal gradients and deformations (think engine exhaust manifolds - dT around 300 degrees C, still have metallic or laminated style gaskets).

Give a call to Lamons or Flexitallic or another gasket manufacturer, they'll give you plenty of information when trying to keep something sealed.

 

[SwinnyGG]

You will never achieve a gas tight seal without contact pressure between these two parts.

The diaphragm you are subjecting to gas pressure is very large- even small fluctuations in pressure will cause large changes in contact pressure between the parts, and the interface will leak.