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Participant

Hi all,

After doing so many complex design and engineering calculations, i am stuck with this simple plus minus calculation.

I have to do this addition and subtraction of dimenssions with tolerances, as shown in the attached image.

As i want similar representation of Tolerances in mathcad sheets.

Thanks,

Shashank

1 ACCEPTED SOLUTION

Accepted Solutions

Here is a sheet with an added unary minus which is a bit more explicit-friendly.

The drawback is the way you have to write your subtraction (either A + - D, or A + (-D)).

17 REPLIES 17
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Hello Shashank,

hope this small routine will do. Add cells to the nested array Y as much as you need

Cheers

Raiko

P.S.

This is a MC 15 sheet

Regular Member
(in response to Raiko)

Another example to add along with Raiko's for your Mathcad toolbox. Also, a MC15 file but will convert to Prime.

I hope this helps.

Hi,

I tried various ideas for this approach.

Most of the applications, however, became too complex to make it useable for what I needed.

attached is a simple sheet with a few functions - hopefully they will give you some information about mathcads capabilities

If you make any useful modifications to these functions, I would be grateful if you could publish them here.

Regards

Andy

Nice idea. Do you know why your inverse +/+ does not work as modificator placed in the unit field at the end of an evaluation? It should in my opinion but it keeps moaning about the argument is not a vector?

Working with tolerances is easy if you stay with addition and subtraction only, but going further your approach may fail because you assume you get the minimum/maximum value of any calculation by applying the minimum/maximum value of every variable involved. Unfortunately thats not true as the following simple example shows.

Furthermore it is not for sure that every calculation will end up in a symmetrical tolerance interval.

Hmm, all these approaches only work OK if you just use addition and subtraction in your calculation. It may fail if using multiplication and/or divison or functions, absolute value, etc.

A more general (but also more cumbersome) appraoch can be found here: http://communities.ptc.com/message/199660#199660

Its in Prime2 format.

I looked at the example in your sheet and think its wrong, too.

You get

but the correct result should be

considering that

While in this example you can get the correct result by making the calculation for ever combination of the extreme values of the variables, this must not be the case for every formula. So I think the most general solution would require a brute force calculation of a whole range for every variable and searching the min/max results as i did here(Prime2): http://communities.ptc.com/message/199660#199660

Attached is the file with the calculation of your formula (Prime3)

Thank you all for your kind responses,

But what i need is actual tolerance representation and the logic behind upper and lower tolerance calculation while adding or subtracting parent dimenssion values.

Thanks

The best I could come up with:

But the actual result of A-D should be 0.8 +0.4 -0.1

(where +0.4 is the upper tol & -0.1 is the lower tol)

as when we subtract parent dimenssions, their tolerances gets added in cross way

for example, tolerances of A-D will be calculated by following logic:

0.2 - (-0.2) upper tol of A(0.2) - Lower tol of D (-0.2)

-0.1- (0) Lower tol of A (-0.1) - Upper tol of D (0)

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