How to use Tolcap

This presentation introduces the Capability Curve and discusses target Cpk and shows how to use Tolcap to find the tolerance required to achieve a target Cpk.

Using Tolcap - A Gentler Introduction

You watched the earlier presentation about getting started but you're not sure you understand it all?
While our introduction describes the recommended way to use Tolcap, some users find it daunting.

- OK, you know the dimension and tolerance, you can choose a process and material, but the geometry and additional variation questions are too hard! - These slides go through a simplified approach: it doesn't have the precision of the full analysis, but it puts some reasonable brackets around the process capability estimate.

Using Tolcap - A Gentler Introduction

Let's go back to the example in our introduction.

We are machining – Turning or Boring a dimension of 100 +/- 0.035mm in a part made of mild steel.
As last time, you:

• Selected the ‘Turning and boring’ map in the ‘Machining’ group
• and entered the ‘Dimension’ and ‘Tolerance’ of interest, 100mm +/-0.035mm
• and clicked ‘Calculate’.
• Then you clicked the ‘Mp Wizard’ button and selected ‘Mild’ steel to get a predicted Cpk of 1.95.

Now instead of opening the ‘Gp Wizard’:

• click ‘show curve’ in the ‘Using Tolcap’ panel.

Using Tolcap - A Gentler Introduction

Look at the yellow dotted vertical line rising from 0.035mm on the Tolerance axis.
It meets the blue ‘best case’ line at Cpk = 1.95, but in the ‘worst case’, geometry and additional variation might result in the result on the red line – little better than Cpk = 0.5.
The script below the curve suggests that to achieve Cpk = 1.33 would not be possible were the tolerance reduced to 0.022mm, because that is where the blue ‘best case’ curve crosses Cpk = 1.33, and might need to be increased to +/- 0.063mm - because that is where the red ‘worst case’ curve crosses Cpk = 1.33.

In Comparison

In fact in our example, Gp and Vp analysis reduced the Cpk estimate to 0.97, and we can show that a tolerance of +/- 0.045mm would suffice to achieve Cpk = 1.33.
It is best to do the analysis, but the Capability Curve will give you safe though conservative limits.

Why Choose Cpk = 1.33?
You may well have noticed that Tolcap defaults to a target Cpk of 1.33.
Why?
Well, the Tolcap database is well established and it predates the ‘Design for Six Sigma’ bandwagon. The requirements of good practice at the time were similar: essentially it was still necessary to “design for six sigma”, because the first off-tool parts had to demonstrate Cpk = 1.67 (which is five sigma), and a margin for error is needed.
Series production parts had to maintain Cpk = 1.33 (which is four sigma) all the time, and the Tolcap database was tuned to estimate ongoing Cpk with that target in mind.
While Cpk = 1.33 allows 33 parts per million out of tolerance rather than 3.4 ppm if Cpk = 1.5, experience has shown it works.
But then again I suppose ‘Design for 5.5 Sigma’ wouldn't sound as good!

Design for the Cpk You Choose

That said it's your choice:
If you want to design to Cpk = 1.5 or any other value, change the Cpk threshold on the Capability Curve.

Rather than finding what Cpk is likely to result if you set a certain tolerance, you can use Tolcap to find the tolerance required to achieve a target Cpk:

• Click the Cpk tab
• and then enter the required target Cpk.

- Watch the tolerance change as you select the map and run the wizards.

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 organization significant costs and saves you a lot of 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 will explore the problems of tolerance stacks and how Tolcap provides solutions essential for statistical tolerancing.