5. Tolcap Example - Analysing a Design with Tolcap. How, When and Why

Tolcap Example - Analysing a Design with Tolcap - How, When and Why

This presentation shows you how to analyse a design with Tolcap; how to do it, when and why, by which we mean what you get out of going through this process.

Tolcap Example Objectives

We will work through a simplified example which sets out to show: how to analyse the components of a design using Tolcap; and how to calculate a stack tolerance. But it also shows when to use Tolcap (which will turn out to be as early in the design stage as possible), and how Tolcap raises questions about the design - questions you need to address with your supplier to help ensure you have a sound design.

Tolcap Example - a ‘Camera’

Here's the example: It's a ‘camera’ of some sort and it has three components – a heat sink, a body and a lens. The table also shows the dimensions and first cut tolerances in the ‘stack’ that defines the image distance in the camera.

The heat sink dimension 3mm +/- 0.05 is the height of a raised platform for a photodiode array that is connected to a surrounding printed circuit board.

The body which is 12mm long +/- 0.01 just spaces the lens from the photodiode array. The lens is injection moulded PMMA. The front part of the lens is 7mm +/- 0.1 from the mounting shoulder of the body.

Camera Tolerance Stack

Accepting the assigned tolerances, the distance from the front of the lens to the ‘image’ on the heat sink is 22+/- 0.16mm – worst case. We may ‘root-sum-square’ the tolerances and say the stack dimension is 22 +/- 0.11225mm (call it - 22 +/- 0.11) - but is ‘root-sum-square’ valid and meaningful? Anyway, are the component tolerances achievable? Let's use Tolcap first to estimate Cpk for the assigned tolerances.

Camera Heat Sink Analysis (1)

• Open Tolcap and log in

Camera Heat Sink Analysis (2)

• In the process group select ‘Casting’, select ‘Precision Pressure Die Casting for Copper Alloys’
• Check we are on the ‘Tolerance’ tab
• Enter the ‘Dimension’ 3 and for ‘Tolerance’ +/- 0.05
• and click ‘CALCULATE CPK AND PPM’

There is no prompt to select the material because this map is specific to the material of the part.

There is no modification needed in either the ‘GEOMETRY’ or ‘VARIATION WIZARD’: but Cpk is unacceptably low at 0.69.

Camera Heat Sink Analysis (3)

So lets find the capable tolerance:

• Click on the ‘Cpk’ tab
• Select 1.33 as the target Cpk
• and click ‘CALCULATE TOLERANCE AND PPM’

- So to achieve Cpk = 1.33 we find we need the minimum tolerance estimated is +/- 0.08mm.

Camera Body Analysis (1)

Next analyse the Body.

• Select ‘Machining’ and ‘Turning and Boring’
• Make sure we are back on the ‘Tolerance’ tab
• Enter the dimension 12 and tolerance +/- 0.01
• Click ‘CALCULATE CPK AND PPM’

The predicted Cpk is 1.64, but we need to:

• Click the ‘Material’ dropdown, ...and select Copper Alloy / Non-Ferrous Copper Alloy

The predicted Cpk will fall to 1.4

Camera Body Analysis (2)

The Body starts as a casting to save material, so it has to be turned round at some point in the machining process. So:

• Specify ‘1 additional set-up operation’
• and click ‘APPLY’

Camera Body Analysis (3)

The predicted Cpk is 1.29 .. Which is close enough to 1.33 to be considered an acceptable design.

Camera Lens Analysis (1)

To analysis the Lens:

• Select ‘Plastic Injection Moulding’
• and ‘Injection / Compression PETP, PF, PMMA.....’, etc
• Click the ‘Tolerance’ tab
• Enter the dimension 7
• and tolerance 0.1
• Click ‘CALCULATE CPK AND PPM’

- Cpk is 2.34 so far ...

Camera Lens Analysis (2)

Next click the ‘GEOMETRY WIZARD’

• ...the characteristic is across the parting line, (which it wasn't on the heatsink) so select Yes
• And click ‘APPLY’

The predicted Cpk falls to 1.22

Camera Lens Analysis (3)

Next click the ‘VARIATION WIZARD’

• Does the need for optical properties impose any limitations on material flow etc? Well I don't know? Never mind then let's select ‘Yes’ in the last question, just to be on the safe side
• And click ‘APPLY’

... and the predicted Cpk falls to 0.66.

Camera Lens Analysis (4)

We better find a tolerance that gives an acceptable Cpk.

• Select the ‘Cpk’ tab and set target Cpk to 1.33
• Click ‘CALCULATE TOLERANCE AND PPM’

... to find the tolerance should be at least +/- 0.16mm.

Camera Design - Capable Tolerances

So now we have analysed each component using Tolcap. We can update our assembly drawing and our table with tolerances that will be capable to Cpk equals 1.33.

Camera Capable Tolerance Stack

Now with these capable tolerances, the distance from the front of the lens to the ‘image’ on the heat sink is 22 +/- 0.25mm worst case. 0.25 being the sum of the tolerances. The root-sum-square of the tolerances is 0.18.

Using the formula derived for Cpk = 1.33 tolerances, tstack is 3/11 of the sum of the tolerances plus 8/11 of the ‘root-sum-square’ of the tolerances, we get t_stack is 0.19848, ok 0.2 plus or minus.

Camera Example: Discussion

This design example 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 deliberately as a diagram: just to show that Tolcap is best used early in the design process, it’s hard to overstress this, once you’ve got to a drawing your committed to some concept and it very difficult to back track if you find you’ve got some problems.

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 ... because this can steer your design and your project as we shall see.

Lens Discussion (1)

Let’s consider the lens first as it required the widest tolerance.

Does the requirement for optical properties really affect process capability? An obvious question to ask the supplier, who may have evidence that it doesn’t.

Lens Discussion (2)

Then again is dimensioning to the front of the lens avoidable – or even appropriate? If it is, consider mounting the lens somehow inside the body onto the front face, so that the dimension of the thickness of the lens is on one side of the die.

But look at the optical equations: of course the thickness of the lens will have an effect on its focal length, but maybe the shape should be adjusted and the optical centre brought in line with the mounting feature?

Review optics and their sensitivity to dimensional errors.

Heat Sink Discussion

The design assumes a raw cast heat sink will be satisfactory. It may be necessary to machine the cast anyway to ‘clean up’ the faces to mount the sensor, but machining is also a way to get better tolerances if it is necessary to improve the stack tolerance.

Camera Body Discussion

The Camera Body tolerance is an order of magnitude smaller than for the other components in the stack. So this may be an opportunity for cost reduction. Do we really need to machine the casting? Is there an alternative process to produce the part at lower cost?

Tolcap Example Objectives

The objective of this example has been to show you:
- How to analyse a design using Tolcap
- and how to calculate a stack tolerance.
You should do this analysis as early as possible, right back in the concept stage of the design.

Use these results in discussions with your supplier to make sure you’re going with the processes you selected and to get the best out of your design.

Before we close I’d just like to tell you the story of this example. Of course it’s set up for the objectives I’ve described. Why else would we have a copper alloy camera, other than to show you that for some processes we have different maps for different materials, and for others we have one map and a material correction factor. It was based on a real case. The organisation concerned were just rolling out Tolcap and I was invited to a meeting were the design team were presenting the concept of such a camera to the production people. Well, they were very concerned about the distance between the lens and the sensor, and that the image should be in focus. And they were describing some design where by there would be an assembly fixture, and all the parts would be held in place, and a light shone through onto the sensor and some measurement taken to show if was in focus, and shims would be fitted, and glue, and as you can imagine the production people were getting quite agitated about this. I asked to take the design away and I applied Tolcap in just the way we’ve done it, and to honestly my surprise when I put in Cpk equals 1.33 tolerances for each component the stack came out entirely within the requirements the design team wanted. Leading to a very simple assembly. And I tell you this for two reasons: again to underline that you should do this design analysis very early in the concept stage; and two just to say that just occasionally Tolcap gives you the solution rather than a list of problems with capability issues.

Thank you for your attention.