Rack Mount 1RU - Sheet metal assembly Stack Up Analysis

[y2k_o__o]

My current company doesn't perform any stack up analysis before they release product, and I am very new to this topic.

Here's the loop diagram for the shelf that consist of top and bottom chassis where screws are fastened through the side into.

1 - bottom of chassis to PEM centerline
2 - assembly shift from top chassis clearance hole
3 - PEM centerline to top of chassis
4 - Gap identified as a point of interest in tolerance stack-up analysis
5 - 1RU envelop




However, I was reading some online article that sheet metal flatness (0.2mm or +/-0.1mm) from top and bottom surface of the shelf also contribute to the stack-up. Can anyone educate me here when is flatness applicable ?

I have the impression that flatness is already accounted in the #1 and #3 dimension tolerance.

 

[drawoh]

y2k_o__o,

I don't know about flatness of sheet metal. Since it is not very rigid, flatness does not really matter.

Sheet metal shops bend at around ±.015", or ±0.4mm. If your hole position is controlled by a bend, your positional tolerance is ⌀.030", or ⌀0.8mm.

I would not think in terms of stack-ups. Assume your top cover is at the correct position. Work out the screw clearance holes needed to allow this. A stack-up is meaningless if you cannot insert the screws.

Let's assume you are using an 8-32UNC screw, with a diameter of .164". Your screw occupies space consisting of its major diameter plus the positional tolerance, .164"+.030"=.194". Your hole must clear this under all circumstances.

Your clearance hole is located to ⌀.030". The minimum acceptable diameter is .194"+.030"=.224". Your maximum diameter is whatever fits under your screw head.

 

[hendersdc]

Performing a tolerance analysis to determine how much the height of your unit can vary is a great idea but, also remember that the 1.75" for 1U is a nominal height. We worked off a rule of thumb that ~1.72" was the max height to make sure it fits between two already installed items above and below the open space in the rack.