Although Tolcap is fairly intuitive to use this video offers a little help to get you started. It
guides you through the steps of a tolerance analysis and suggests the sort of questions you need to address to get an
accurate process capability prediction.
1. How to Use Tolcap
Getting started ... This presentation is to help get you started
It guides you through the steps of an analysis and tries to give you
the sense of the questions you need to address to get a process capability
2. When to Use Tolcap
An introductory presentation addressed WHY you would use
Tolcap, this one explains HOW.
- But first, WHEN do you use Tolcap?
If you're a
designer, use Tolcap from the very earliest stages of the Concept Design. You are
probably sketching out some fundamentally important features, and these
elements will doubtless give rise to certain key dimensions where the required
tolerance is not negotiable. You don't need a formal engineering drawing to know
this. So at this point and thereafter whenever you are about to specify a tolerance
for a vital dimension on a part, use Tolcap and check for process capability before
you get stuck with a design with problems.
If you are maybe a Quality Engineer
and you want to check why a problem feature is proving difficult to make – does the
problem go back to the design? Use Tolcap to see if there is likely to be a process
3. How does Tolcap work?
Process capability of a toleranced dimension depends on
the manufacturing process to be used, the material and geometry of the part – and
the state of the tools and machinery and the capability and maybe the past
experience of the supplier. It sounds a complex interaction – and it is! But Tolcap
leads you through the main factors to a good working estimate of process
4. Process Maps
The prime factor determining process capability is the
manufacturing process. Parts are formed by such process families as:
- Forging - is hitting parts with a hammer to deform them plastically into shape
- Turning - is cutting pieces of swarf away from a rotating bar
- Casting or Moulding - involves melting the material, pouring it into a mould and letting it refreeze
The point is that these processes essentially come down to physics, and
the process capability that is ultimately achievable depends on how well an
experienced manufacturer can control all the variables to get the best out of the
basic physics of the process.
5. Process Maps
Tolcap incorporates capability maps for a variety of manufacturing
The maps plot dimension against specified tolerance for each process.
The diagonal lines on the maps are lines, or sometimes contours, of constant process
capability. The higher up the map, the higher the process capability.
6. Process Maps
To use Tolcap:
- Log in
- ... and select the map for the manufacturing process for which you are designing
... let's suppose this is ‘Turning and Boring’ and it's in the ‘Machining’ group
- Enter the dimension and tolerance of interest – for example 100mm +/- 0.035mm
... do that even if the drawing shows 100mm -0 +0.07mm, because Tolcap does expect a balanced +/- tolerance
- Next click ‘calculate’
... You will see numerically and graphically the predicted process capability (Cpk = 2.35) - and where you are on the map.
7. Material Effect
So far the process capability estimate is for the best a competent
supplier should achieve in the ideal case.
Here we see that is Cpk = 2.35.
Just thinking again about the physics of the process – the properties of the material
being processed will have an effect: hardness, malleability, thermal expansion
coefficient, and so forth.
Where there is a strong interaction between the process
and the part material, Tolcap provides alternative process maps for groups of common materials.
For other processes, the prediction from the standard map is modified by a ‘material to process factor’ Mp.
This is probably a good time to
- Click the ‘view all processes’ link
... to see which processes are for specific materials and
which are not. There are several groups for injection or compression moulding for
various plastics for example.
8. Material Effect
For ‘Turning & Boring’ there is an ‘Mp Wizard’ button.
- Click this and the drop down arrow and select a material
... In this case mild steel.
Note you don't need the precise specification of the material – and should your material not be
shown, you can select one with similar relevant properties.
9. Material Effect.
... and notice the decrease in predicted cpk to 1.95 – which is still
Next we consider factors loosely related to the geometry of the part:
10. Geometry Effect.
... and we are presented with several questions – these may vary
with the process we have selected.
- First we are asked how many planes we are machining at one time
– are we perhaps facing while turning?
- Next, is the part long & slender?
– it is more difficult to hold tolerances than for a squat sturdy part.
Let us suppose our part is long and slender and so we select answer ‘B’.
- Does the part have repetitive features?
– a source of more variation?
This question is clearly applicable to such processes as gear hobbing, and here it could be relevant if we
were turning a series of grooves which we wanted to be identical.
- Are there extra set up operations?
Suppose we have to turn our part around to bore this feature.
So we select ‘1’ extra process
- Then click ‘Update Chart’
11. Geometry Effect.
...and we see the effect of the slender part and turning it
round is to reduce the Cpk prediction to 1.5.
And we still have to consider some additional factors.
12. Additional Variation
...and we are presented with some more questions:
- We would hope the manufacturing machinery is up to date and well maintained.
- The feature geometry may be at the limits of feasibility,
- or the part may be at the limits of size or weight – too big or too small.
Let's suppose our part is not only slender it is long, so it is large – near the capacity limit of the lathe
- and we
answer ‘Yes’ to the component size question
13. Additional Variation.
... and notice that Cpk has fallen to an unacceptable 0.97.
Well that's a tear up then?
Maybe, but not yet. OK we are predicting a low Cpk,
but we know the factors that caused that low value: the part is big and it's long
Let's take the result to the supplier and discuss the problem first:
the part is near machine capacity – is that a real problem?
Does it matter?
a similar sized part in production?
The part is long – but can it be supported in the
middle, or maybe held in some sort of collet, so that the part is supported and the
Positive answers here – backed up with data from the process
on similar parts will allow us to go back to the analysis and get a better prediction.
14. Summary & Next Step.
This presentation hopefully leaves you ready and confident
you can start using Tolcap.
If you're still puzzled, try using Tolcap anyway – it may
make sense - or go through this material again, or see the
next presentation that
gives some more explanations and an alternative simpler approach to process