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Chris3
Chris321-Topaz I
21-Topaz I
October 19, 2022
Solved

Absolute Accuracy Value for Large Assemblies

For those of you who have large car type assemblies, what absolute accuracy value are you using in your start part?

Chris3
Chris3Answer
21-Topaz I
October 27, 2022

Some other data points I have for others in the future:

 

PTC's absolute in start part uses: 0.00039 (converted from .01 mm)

In this thread, FPD Company says theirs is .0001 (smaller than car sized assemblies)

In this thread, @kdirth  and @tbraxton say they are suing .001 mm (no idea the size of their products)

I spoke with a large equipment MFG at the last PTC user meeting and they said they were using .0004 (in)

 

Our largest products are car to truck sized and I am leaving towards .0004 (in)

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14 replies

tbraxton
tbraxton22-Sapphire II
22-Sapphire II
October 19, 2022

I would not use a single value for absolute accuracy. I would suggest abs accuracy values based on the manufacturing process/tolerances of the parts. You will also need to manage the values in the context of data sharing features as the values should match between dependent models.

 

If you use a single value, it will be tighter than needed for many of the parts. The downside to this is a potential performance hit during regeneration. If you are not worried about the HW loading, then using a single value will work. I have no idea how much of a burden it is when tightening the abs accuracy by say 2x.

 

If you use a single value, you will need to use the tightest value of all of the components. An injection molded lens for the lights will need a much tighter value than the stamped body panels as an example.

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rreifsnyder15-Moonstone
15-Moonstone
October 19, 2022

The model accuracy does not have anything to do with manufacturing the part, It has to do with the number of decimal places Creo uses in computing the geometry. of the part. You're correct that setting everything to the tightest setting would increase the regeneration times for the models but with today's computers I don't think it would be a major issue. Also, in the past at least, things like interference checks could be unreliable if the accuracies of the parts were different. 

In your example you compare a molded lens to a body panel implying that since the body panel is stamped that it could use a looser accuracy value but in fact it could be the opposite. The complex geometry of a body panel often means that concave surfaces need to flow into convex surfaces with bends between some times. Basically the math to make sure the surface edges come together correctly means that it likely needs tighter accuracy.

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    tbraxton
    tbraxton22-Sapphire II
    22-Sapphire II
    October 19, 2022

    I am offering observations of how absolute accuracy affects the design and manufacture of components. I am offering this info for consideration for how one may choose to implement absolute accuracy. IME the same model accuracy is not used across all part designs in a car, airplane, etc.

     

    I work across many industries including transportation and aerospace in mixed CAD design environments. The model accuracy value is absolutely without question directly related to successful manufacturing of parts when the CAD model is used to program tool paths etc. I have had to fix accuracy problems that are preventing the creation of parts.

     

    I also have experience with the design of Class A surfacing of exterior panels used on ground vehicles and aircraft using Creo and ICEM surf.

     

    The assignment of the value used for absolute accuracy should in fact be related to the manufacturing process and the permissible tolerances on features. As an extreme example I have start parts that are used in sensor design for MEMs systems and the model units are angstroms. These sensors are integrated into much larger products. The MEMS start parts do not use the same abs accuracy as the start part for a sand cast engine block.

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    StephenW
    StephenW23-Emerald III
    23-Emerald III
    October 24, 2022

    Our assembly start part absolute accuracy is 0.008 (inch units). It's our basic assembly start part, no difference for large/small.  I don't know why .008 was chosen unfortunately.

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    Chris3
    Chris321-Topaz IAuthor
    21-Topaz I
    October 27, 2022

    PTC is again pushing me to accept a solution so I will mark @StephenW response as the solution because he provided one data point although I was hoping to get others.

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    StephenW
    StephenW23-Emerald III
    23-Emerald III
    October 27, 2022

    By no means do you have accept any answers as a solution. If you keep the post active, you will likely get a few more answers. To be honest, your question doesn't have a solution. You are just gathering data. 

     

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