3. How to use Tolcap - Achieving Capability

How to Use Tolcap - Achieving Capability

We hope that the previous presentation got you started with Tolcap and showed how to predict the process capability of a toleranced dimension from process maps, and consideration of material and geometry of the part, and some sources of additional variation.

This presentation will show you some useful different ways to apply Tolcap by turning that question round.

Using Tolcap - From Capability to Tolerance (1)

As I said the previous presentation showed how Tolcap provides you with a prediction of process capability for the dimension and tolerance you specified.

Now if the Tolcap algorithm can get from dimension and tolerance to process capability, then it should be – and it is – possible to rearrange the algorithm to calculate the tolerance required on a dimension to achieve a specified capability.

Let’s establish why you might want to do that.

Using Tolcap - From Capability to Tolerance (2)

Working from required capability to tolerance has three useful applications:

Firstly is the final process - and by the way we always look at the final process that determines the dimension of interest – is the final process good enough to achieve the required tolerance capably? Do you need to ream those holes or will simply drilling be perfectly adequate?

Our next video addresses tolerance stacks or stack-ups (if you will) – and it turns out that a good strategy to maximize capability of the stack, or to minimize stack tolerance capably, is to specify the tolerance for the stack elements for equal process capability, and Tolcap shows you the tolerances to achieve that.

Thirdly, when working in this mode, Tolcap will show an optimistic and a pessimistic tolerance to achieve the require process capability. And this is particularly helpful if you are one of our users who is not at all confident in choosing geometry or process factors.

Using Tolcap - From Capability to Tolerance (3)

To show how to use Tolcap in this mode, we will work through the same example as in the previous presentation – a dimension of 100mm on a part that is ‘Turned and Bored’.
Then we specified a tolerance of plus or minus 0.035mm, but this time we will specify a process capability, Cpk.

Using Tolcap - From Capability to Tolerance (4)

So as before, log in and select the process category for the manufacturing process that defines the dimension of interest in your design.

Here ‘Machining’ and ‘Turning and Boring’ …

Using Tolcap - From Capability to Tolerance (5)

This time

• First click the ‘Cpk’ Tab
• Then enter 100 as the ‘Dimension’
• And click on the ‘Cpk’ box – to see a dropdown choice for target Cpk

Design for the Cpk You Choose (1)

You may enter any value between 0.05 and 4 as target Cpk. The dropdowns provide prompts for popular values.

You may well have noticed that Tolcap defaults to a target Cpk of 1.33. Why? The Tolcap approach reflects good practice at the time the algorithm was constructed – as represented the Ford motor companies ‘Production Part Approval Process’ or PPAP - which required the supplier to demonstrate Cpk of 1.67 of the first off-tool run of the new part, and to ensure Cpk be greater than 1.33 throughout production.

The Tolcap prediction addresses that last clause, so our recommended default Cpk target is 1.33.

Just to link this to ‘Design for Six Sigma’, if you can design to get a theoretical Cpk equals 2, i.e. a band of plus or minus six sigma around the target dimension, then you have a margin of plus or minus sigma for that first production run.

So click 1.33 then ‘CALCULATE TOLERANCE AND PPM’ …

Design for the Cpk You Choose (2)

… and you will see the required tolerance to achieve Cpk equals 1.33, or the equivalent 33 ppm reject rate, shows plus or minus 0.017mm.

But that of course is for the ideal case …

Design for the Cpk You Choose (3)

…and as before we need to:

• Select the appropriate material – Mild Steel
• Apply the ‘GEOMETRY WIZARD’
• And apply the ‘VARIATION WIZARD’
• And recalculate

And we will see to achieve a Cpk of 1.33 the required tolerance is plus or minus 0.046mm.

Design for the Cpk You Choose (4)

…you can also see the result on a Map.

And let’s recap the results we got from this example …

Design for the Cpk You Choose (5)

And you have the figures that when we selected a required tolerance of plus or minus 0.035, Tolcap predicted we could expect a Cpk of 0.97.

But if we ask Tolcap what tolerance we need to achieve Cpk equal 1.33 it predicts that a minimum tolerance of plus or minus 0.046mm should be specified.

A Tolerance Range? (1)

While our introduction describes the recommended way to use Tolcap, some users find it quite daunting.

Ok, you know the dimension and tolerance, you can choose a process and material, but the geometry and additional variation questions don’t necessarily have right or wrong answers – you have to apply some thought and judgement.

Let’s work the example once more, this time just supplying the process, dimension, target Cpk and the material…

A Tolerance Range? (2)

So select

• Machining – Turning and boring
• The Cpk Tab
• Enter the dimension of 100mm
• Cpk 1.33
• Select the material as Mild Steel
• Click CALCULATE

and Tolcap suggests that under favourable conditions, minimal geometry effects and no additional variation, plus or minus 0.022mm might be achievable.

A Tolerance Range? (3)

Well we are well aware there might be part geometry effects or additional variation problems, but we don’t know quite what those issues are at this point.

But Tolcap can show us a range tolerances

• From that required for the most favourable conditions – that we just obtained
• To, well – not the worst case, but the least favourable that we can imagine in practice.

So if you click on the Range Tab …

A Tolerance Range? (4)

… Tolcap shows that while plus or minus 0.022mm tolerance may be process capable under the best conditions, at the other end of the scale the tolerance may need to be plus or minus 0.063mm.

That puts our original starting tolerance of plus or minus 0.035 into some sort of perspective and context.

A Tolerance Range? (5)

Of course, the more you put in, the more you get out of Tolcap.

If we actually apply the ‘Geometry’ and ‘Variation Wizards’ as before, the ‘Range’ chart will show the minimum tolerance predicted to achieve process capability

• For the entered conditions now
• For the most favourable conditions
• And for the least favourable conditions

Using Tolcap

We hope these introductory presentations have shown you why Tolcap is a unique and vital tool for designers.

Setting achievable tolerances saves your organisation significant costs, and saves you a lot of that stress and distraction sorting out designs that could otherwise turn out not capable.

Tolcap helps you check the capability of a tolerance, or generate a tolerance with a target capability.

Our next presentation explores the problems of tolerance stacks, or stack ups, and how Tolcap provides solutions.

Thanks for Listening

Our next presentation will explore the problems of tolerance stacks and how Tolcap provides solutions essential for statistical tolerancing.