The Body starts as a casting
to save material, so it has to be turned round at some point in the machining process. So:
The predicted Cpk is 1.29 .. Which is close enough to 1.33 to be considered an acceptable
design.
- Cpk is 2.34 so far ... next:
Click the ‘Gp Wizard’
...the characteristic is across the parting line, (which it
wasn't on the heatsink) and
The predicted Cpk falls to 1.22
... next
Does the need for optical properties impose any limitations
on material flow etc? I don't know? Let's admit ignorance here and:
- Select ‘Yes’ in
answer to the last question and
- Click ‘Update Chart’
...
The predicted Cpk is 0.66.
To find a tolerance that gives an acceptable Cpk select the Cpk tab and set target Cpk to
1.33:
... and find the tolerance should be +/- 0.160mm.
Camera Design - Capable Tolerances
So now we have Cpk = 1.33 capable tolerances:
The Heatsink 0.082, the Body +/- 0.01 and the Lens 0.160.
Now with capable tolerances, the distance from the front of the lens to the ‘image’ on
the heatsink is 22 +/- 0.25mm (0.252 exactly).
The ‘root-sum-square’ of the tolerances is 0.180.
Using the formula derived for Cpk = 1.33 tolerances, which is
tstack is 3/11 of the sum of the tolerances plus 8/11 of the ‘root-sum-square’ of the
tolerances, we get tstack coming to 0.2.
Camera Example: Discussion
This design is presented to illustrate how to use Tolcap and
how to calculate a stack tolerance. That said it also serves to highlight when to use
Tolcap and how it can improve the design process.
The example is presented as a diagram: just to show that Tolcap is best used early in the
design process, before a drawing commits you to some concept.
Don't just look at the Cpk values or tolerances you get from Tolcap: Look at the processes
you selected and the answers you gave in the wizards ... these can steer your design and
your project.
Lens Discussion 1
Take the Lens for example: We wondered whether the requirement for
optical properties affects process capability. We didn't know so assumed it did. We need
to find the real answer to the question, so we should ask the supplier. Don't ask whether
they think it does or doesn't – ask for an explanation – and vitally for evidence that it
does not. Have they made similar lenses before? Don't they have process capability data
on them we can use for comparison? If the optical requirement doesn't in fact affect
process capability, we can go back to the analysis and reduce the tolerance.
Lens Discussion 2
Is dimensioning to the front of the lens avoidable - is it even
appropriate? If it is necessary, consider mounting the lens inside the body onto the front
face, so that the dimension is on one side of the die.
But look at the optical equations: the thickness of the lens will have an effect on its focal
length, but maybe the shape should be adjusted so the lens is convex on both faces.
Maybe the optical centre could be brought in line with the mounting feature? Maybe
rather than include the lens in the stack it is better to review optics and their sensitivity
to dimensional variation.
Heat Sink Discussion
On the Heat Sink the design assumes a raw casting will be
satisfactory.
It may be necessary to machine the casting to ‘clean up’ the faces or to achieve tighter
tolerances if it is necessary to improve the stack tolerance.
Body Discussion
On the Body, the capable tolerance is an order of magnitude smaller
than for the other components in the stack. This may be an opportunity for cost
reduction. Do we need to machine the casting? Is there an alternative process to produce
the part?
Tolcap Example Objectives
We hope this example has demonstrated:
- How to analyse a design using Tolcap
- and how to calculate a stack tolerance.
- Also when to use Tolcap - as early in the design stage as you possibly can.
- How Tolcap raises questions about the design - questions you need to address with your
supplier to ensure you have a sound design, and questions to pose about your design
- and tasks to incorporate into the design project.