What's a reasonable flatness tolerance for a 250 X 60 X 2 sheet metal panel

[Zibraz]

I'm having a minor debate with an engineer in work on flatness tolerances on sheet metal. We have been getting panels which aren't sufficiently flat (bowing in the centre by 1.5 mmm roughly).

I suggested putting a flatness tolerance of 0.5.

Is this flatness across a 250mm long 2 thick reasonable? I am able to find the tolerance of standard hot/cold rolled metal, but unable to find a reasonable tolerance guide for drawings.

 

[greenimi]

The theory says that the maximum flatness whatever is functional.
What means sufficiently flat for your application?

You can use flatness per unit basis meaning overall flatness (1.5mm) and 0.5 per each area of 20x20 (for example)

 

[3DDave]

The Aluminum Association has handbooks of typical aluminum processes. In this case I expect it to be H35.2

See

https://www.aluminum.org/ansi-h352

$150 for non-members.

 

[Belanger]

I second greenimi's suggestion of a "per-unit" flatness callout. It looks like a double-decker feature control frame, where the upper number controls the flatness over the entire part, and the lower number is a tighter tolerance for any localized area. This allows you to be somewhat generous over the entire panel while still preventing sudden bumps/spikes from developing.

 

[drawoh]

Zibraz,

Just about anytime I specify sheet metal parts and I use the main face as a datum feature, I specify that the datum applies when the face is clamped to a flat reference surface. A sheet metal panel will sag under its own weight, unless it has gussets bent in, or screwed, riveted or welded on.

What are you trying to do?

--
JHG

 

[CheckerHater]

Food for thought?

http://www.bstoolsthai.com/pdf/12).pdf

"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

 

[Zibraz]

After researching and looking around on the internet, it seems that flattening sheet metal isn't advised or easy. I think i'm going to have to go into work and look at these plates myself to determine an acceptable flatness tolerance. I was hoping there were some standards or common flatness applied to modified sheet metal parts.

The sheet metal part is fixed in four corners, but our production engineer has complained about the part bowing in the centre. There is a runner with an assembly fixed on top on the sheet metal part. The assembly fixes onto a fragile quartz glass tube. The production engineer is anxious on whether the bowing can cause difficulties with alignment.

Personally, I think an out of flat part is alright (Unless something stupid like 5mm difference over the length of the component). I was hoping of putting a reasonable tolerance on the part to make everyone happy, but it appears it's not that easy.

I found standard tolerances for non modified sheet metal parts, but there is nothing for modified sheet metal.

 

[3DDave]

In very thin but large pieces of material very small differences in strain that get captured in the sheet can cause significant bowing, which is why most such parts get at least one crease to increase the moment of inertia to something far larger than the plain sheet. Even heat applied to one side can cause enough thermal differential to make the part deflect in an annoying way if there is no other constraint.

 

[jassco]

Hi, Zibraz:

Did you ask for tolerance for material (sheet) or finished goods?

If you ask for sheet metals, you need to contact your vendor or manufacturers.

If you ask for finished goods, then it is determined by what you can live with and what you can get economically.

If I were you, I would measure them to get current capability of this flatness. And then you get back to your vendor(s) and ask what they can do to improve it.

Best regards,

Alex

 

[MintJulep]

They are hoping for "Standard for flatness tolerance of Zibraz's sheet metal part fixed to fragile quartz glass tubes".

Other than Zibraz, I am unsure what standard making organization would have reason to publish such a standard.

 

[Zibraz]

Jassco:

I'm going to go in to the factory and look at these panels myself. Use some feeler gauges and determine the actual bow in the plates. Personally, I think there isn't a need for a flatness on these plates, but the production engineer disagrees.

I will have a chat with our suppliers and see what they can reasonable produce economically & reliably. I was hoping there was some sort of common practice with this, but there doesn't appear to be any.

I wasn't necessarily hoping for a standard, but a common practice, or something reliably achievable with sheet metal. It seems much more complicated than what I originally anticipated.

Thanks for the help guys.

 

[MintJulep]

Perhaps the real problem is stiffness, not flatness.