tell me why this tolerance is wrong

Hi everyone

I have been having an animated discussion with the design engineers this morning as to why I think there drawings is wrong. The design engineer is sat there with the standard which is ISO 8888 and is adamant that he is correct and he can dimension the feature like this and use whatever tolerances he likes in whatever order.

That is the offending tolerance, A,B & C are the overall datums for the whole part and I have no problems with that segment. The lower segment however to me is wrong in so many ways, maybe I am wrong as I am someone who is actually trained in ASME and not ISO, maybe he is right and it can be done like this in ISO,

He claims the F in the circle means measure in free state however ive never seen it placed where it is now. I also believe from my ASME training that the bottom segment refines the top segment so cannot use different datums, and I think this is the same for ISO.

I would usually argue my case louder but this is the senior design engineer here and he is digging his heels in at the moment and not changing the drawing, plus I don't want to look like an idiot if he is right and can tolerance the feature like this.

Anyone have any thoughts?

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bigtallanddopey over 7 years ago

I would say he's right...
I found this picture from a gdt article, which seems to be case close to yours. The article has been written for a well known teacher of gdt. (from ISO10579)
{"data-align":"none","data-size":"medium","data-attachmentid":399558}

Maybe the inclusion of the F meaning free state basically changes it into two different tolerances depending on how it is measured. So far I haven't found any reference as to how to use the F.

What is strange on your dimension is that the tol is smaller in the free state than in the fixing state...

The free state measurements are to a more local datum system so in theory there should be less error, but I also questioned the design engineer on this

In ISO standards, the first principle is the independancy of dimensions, so you can write the symbol in the same box without linking both dimensions...

In your case, you should tell us where are Q and J from ABC.

Q is two points that control in the Y direction located at the top of the drawing I posted, J is three points locally around the hole controlling Z, No X control.

Another point : 8888 is a BS standard, not a ISO . Maybe NB can confirm it or not ?

you are right that it is a BS standard BS EN ISO 8888, but also still ISO I believe?

I cannot find an ISO definition, But ASME states that "unless otherwise stated all measurements are to be taken in a free state condition". Therefore if there is a note stating how to constrain the part but you want to measure a certain feature in free state you need to use the F modifier.

Again that was in ASME so may not apply, but in my case there are no notes on the drawing stating how to measure or constrain the part, just an F on some features meaning measure this in free state. I would have measured all the features in free state as that is the only way to measure the parts on the CMM and get to all the features that need measuring. For me the designer has tried to have less tolerance in the Y direction compared to the X direction and so has created a local datum system. I cannot understand his thinking in this instance why he feels the need to split it up and create the FCF as he has done.

for me I would have just written it like this, which maybe wrong but it gives him what he is asking for.

|0.5|J|Q|C|
|0.4|J|Q|
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bigtallanddopey over 7 years ago

I would say he's right...
I found this picture from a gdt article, which seems to be case close to yours. The article has been written for a well known teacher of gdt. (from ISO10579)
{"data-align":"none","data-size":"medium","data-attachmentid":399558}

Maybe the inclusion of the F meaning free state basically changes it into two different tolerances depending on how it is measured. So far I haven't found any reference as to how to use the F.

What is strange on your dimension is that the tol is smaller in the free state than in the fixing state...

The free state measurements are to a more local datum system so in theory there should be less error, but I also questioned the design engineer on this

In ISO standards, the first principle is the independancy of dimensions, so you can write the symbol in the same box without linking both dimensions...

In your case, you should tell us where are Q and J from ABC.

Q is two points that control in the Y direction located at the top of the drawing I posted, J is three points locally around the hole controlling Z, No X control.

Another point : 8888 is a BS standard, not a ISO . Maybe NB can confirm it or not ?

you are right that it is a BS standard BS EN ISO 8888, but also still ISO I believe?

I cannot find an ISO definition, But ASME states that "unless otherwise stated all measurements are to be taken in a free state condition". Therefore if there is a note stating how to constrain the part but you want to measure a certain feature in free state you need to use the F modifier.

Again that was in ASME so may not apply, but in my case there are no notes on the drawing stating how to measure or constrain the part, just an F on some features meaning measure this in free state. I would have measured all the features in free state as that is the only way to measure the parts on the CMM and get to all the features that need measuring. For me the designer has tried to have less tolerance in the Y direction compared to the X direction and so has created a local datum system. I cannot understand his thinking in this instance why he feels the need to split it up and create the FCF as he has done.

for me I would have just written it like this, which maybe wrong but it gives him what he is asking for.

|0.5|J|Q|C|
|0.4|J|Q|
Cancel
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