Help setting up a tolerance stack up analysis for a door latch assembly

[Luca Rutigliano]

Hello everyone, I have designed the following latch in CATIA but I've encountered some issues trying to set up the tolerance stack up analysis of a back door latch assembly (shown in the pictures). I graduated 2 months ago and I've never done a tolerance stack up analysis, from what I've read i should start from defining the KC of the assembly. Now, the basic feature of the latch is to guarantee imo the necessary gap and alignment during engagement between the striker and the ratchet. The requirements specified by the OEM for this latch are the following ones:

- The Striker could be allowed misalignment with Latch Y ±2mm maxi, Z ±3mm max relative to vehicle axis system
- Overtravel: 2.5mm (to be confirmed) (distance beyond closing point in which the striker could travel before stopping)
- Travel to open the latch 7.4 ±1.6mm, Full travel 12 ±1.6mm

My company uses Cetol 6 sigma for tolerance stack up analysis (using both RSS and Worst case method). Which is the most optimal/critical KC to take into account in your opinion? I was thinking about the 1st one to monitor, but again I am struggling to understand which components of the assembly to include in the stack up calculation.
Then again, another major doubt: this analysis would take into account the operation of the latch, but the striker and latch are actually integrated in 2 separate assemblies which are assembled separately and enter in contact with each other only during functioning. Is this a step too far ahead?
Thanks again for your support.

 

[3DDave]

You probably need the model of the car body and of the lid and the hinge between them. There is a reason for the slots on both parts - to allow alignment at the assembly line. Someone should build a par of fixtures - one to align the catch with the lid/hinge and the other to align the catch with the body/hinge so that no adjustment is required by the final assemblers. The slots should have been sized to account for previously gathered statistical data/measurements made on the assembly line to ensure there is enough travel. More than that, part of the gap is the rubber seal and that will change characteristics with temperature.

In addition, the modeled variation should be extracted from actual delivered part measurements, not just the engineering values, if one is expecting to predict the rate of rejections. There are many times where the distribution of variation is skewed and neither RSS nor worst case is going to give a useful answer.

Someone should have already done this tolerance analysis before any drawings are released. I don't know why a critical design function would be dumped on a new grad with little experience, but maybe it is some sort of test to see who you go to in your company for support.