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Dimensioning Philosophy

B

BigJoe

New member
I'm curious to know about your detailing philosophy concerning the use
of dimensions created in the drawing or use of dimensions from the
model. Personally, I push the use of model dimensions (the
"capture design intent" philosophy). I have an acquaintance with
WAY more Pro/E experience than I have telling me to disregard the model
dimensions and just create what you need in the drawing. He does
a lot of contract work for a lot of different companies and I know he
rarely has anything good to say about the existing Pro/E models he
works on. Sounds like he spends a good portion of his time
cleaning up models. Maybe that has something to do
with it. I've been trying to set up lunch with him for a couple
weeks now to get more detail on what he's thinking, but I'd like to
pose the question to this Pro/E community in the meantime.



So, do you show/erase or do you insert->dimension->new references?
 
That's a good question. Our engineering manager is big on using the driving dimensions in the drawing. However, I've found (being relatively new to Pro-E) that the dimensions that make sense in the design process are rarely of any use to machine shop, thus I use what I can and add what I need without guilt...
 
Always, always use model dimension whenever possible!!!


If you want to dimension differently on the drawing, go back to model and redefine the feature.
 
Yeah, scottm. I hear that same argument from some of my
engineers, too; design dimensions aren't useful to the guy making the
part. I designed a pretty intricate part (about 70 features,
mostly cuts) that I had machined. The drawing was 8 pages of
detailed views and sections with NO created dimensions. Maybe
there just wasn't an opportunity on that part for a tricky feature
where the driving dimensions were different than what the machinist
needed to build the part.



When I have this conversation with other engineers we usually end up
changing dimensioning references and/or adding relations to capture
design intent AND give the part manufacturer the information he needs
to build the part.



If anyone has any examples of a situation where using model dimensions
just doesn't work or isn't practical, I'd like to see them.



Also, has anyone else run into problems with created dimensions when making changes to a part and/or drawing?
 
I agree 100%, use driving dimensions whenever possible. It is rare that I use draft dimensions.

The biggest exception I have had recently was an insert molded part that we wanted to change to 2 separate snap together parts. I started with the existing insert molded part, spilt it apart and then added the snap features. Many of the driving dimensions could not be used in either snap together drawing (because one leader was not connected to anything) so I had to create dimensions in that case. Starting both parts from scratch would have taken too long and might have introduced errors.

The other exception is CNC drawings where they want ordinate dimensions for programing. I use all created dimensions for that but we DO NOT use those drawings for inspection.

Created dimensions frequently fail when you redefine the part because they loose their references. Fortunately, it is now possible to edit their attachment 90% of the time.
 
Dimensions and tolerances appear on engineering drawings
in order to unambiguously define the FUNCTION of the part.


Function is king, period. One does not dimension and tolerance a
part so the model shop can make it. That is absurd.


That being said, features in models should be dimensioned
such that those same dimensions can be shown on the drawing
to convey the design intent and function of the part.


Yes, there is clean-up involved with showing dimensions on a drawing
but I still think it's the right thing to do and faster than creating them.


Mechanical engineers and designers state how a part needs to function
per the dimensioning and tolerancing schemes they apply to parts.


We shouldn't tell someone how to go about fabricating the part or even
how to go about inspecting it.
 
I agree completely Hacks.



When you said:

We shouldn't tell someone how to go about fabricating the part or even

how to go about inspecting it.
Click to expand...



I had to look at that twice. I think I understand though.
The drawing should be dimensioned and toleranced to sufficiently convey
part requirements (and design intent) but should NOT specify the
details of manufacturing or inspection. Did I interpret that
right?
 
I disagree with the statement regarding manufacturing or inspection, inferring a hard and fast rulein all cases. The purpose of designs and drawings is to communicate requirements. Some of those requirements may include a fabrication technique or an inspection technique in order for the component, assembly or system to meet the design intent.


I regularly indicate critical characteristics, inspection criteria, and occasionally manufacturing process requirements on drawings in order to assure that the design intent is carried through, in a clear and concise manner.


In some instances, I even require sampling plans and levels of statistical confidence requirements on the drawing. This is especially important when a component is outsourced.


Getting back to the model driven versus drawing created dimensions, where dimensions only are concerned, I prefer to use model driven data. This assures that the data is sound throughout the process.


Other information on the drawings, such as text notes, etc cannot be practically created in the modelling mode, and have to be created in the drawing mode.
 
Surfdancer,



I disagree with your response on manufacturing and inspection. We should
not care how a manufacturer makes a part as long as meets the print. The
manufacturer could file it by hand or machine it; if it meets the print then it
is a good part. Same with inspecting the part;the inspector will
verify the part to the print. You may have MIP's (mandatory inspection
points) on the print. They will inspect them using the tools that they
have available. All you should be concerned with is if it meets the
print.



Just my opinion,



Brian
 
I've designed inspection tools for some of our less-inspectable parts,
but these tools just make the parts easier to inspect. If someone
wanted to inspect w/ a CMM or laser mic, that would be fine with
me. I've never specified an inspection technique other than
referencing the tool designed to make the inspector's job easier.
 
This matter isnt really this cut and dry, is it.......there is no doubt the drawing (and tolerances) have to convery the deisgn intent of the part, otherwise the part will not serve its intened purpose; however to suggest that a mechanical engineer should not consider the manufactuing aspects is insane. A good ME needs to be very aware of manufactuing process when designing parts, otherwise you have a part with the right design intent, but isnt manufacturable, or more commonly is way more difficult to manufacture than it needed to be.


Manufacturing concearns and part design are inter-retlated, and a good solid model will have critical dimensions defined in the model, but some dimensions will probably have to be added in the drawing since the processes used to develope the model are not shop processes.....you can very easily create a model that is very hard to manufacture.
 
I agree with SKREM. It may have to do with the type of company you work at. I work at a medium to small sized manufacturer - there is no one between me and the shop, i.e. no process engineers or manufacturing engineers.


The idea that the engineer doesn't care how its made is very old school. You can't reduce costs and optimize a design if you don't design for manufacturability.


I suspect as always that the answer depends on the situation and lies in the middle somewhere.Good discussion, I'll have to think about it while I'm defining my references on my next design...





Scott
 
First of all I would would agree with SKREM & scottm. The Engineering designing the part should be connected with manufacturing. As Engineers we should care how it is made. We should understand the manufacturing process. This is how we can keep costs down in manufacturing from the beginning and not wait until the manufacturing begins and they come back with ideas to save money. That just creates more work for us and everyone else.


An Engineer designs the part for functionality. A good Engineer designs the part for function and knows the manufacturing process. A better Engineer designs the part for function and has experience inmanufacturing.The experience is just more continuing education that will make him a better Engineer.


You must care how the part is made. Lets say you design a part from sheet metal. On the print do you just say connect these three point somehow? Sure you may have modeled it as a stamping, but what would stop manufacturing from making this from a solid? or from welding two or more pieces together? Is that what you want? Did you do the FEA on all the possibilities? The point that I want to make here is that manufacturing should be involved early in the part design process. Either by the designer and/or with the actual Manufacturing Engineer. It will make you both look good and as a bonus your product will be ahead of the game. Oh. and as far as how it is inspected...that is part of the Manufacturing experience. It would be great if the model had the in process datums and tolerances as well.


Drawing dimensions should be DRIVEN! Model it right. If the dimensions are not right on the drawing, go back and redefine the model. Add reference dimensions in the sketcher when needed. I believe that while creatingevery feature in a model the look of the drawing is kept in one's mind.Last month or so, someone here wrote10 or so steps to be a goodDesign Engineer. It started with learning and understand drawings & detailing. It ended up with learning mCAD. BRAVO! For the most part mCAD isa tool for Engineers to convey their ideas. Now back to the driven dimensions, sometimes it is easier to go back andbuild the model from scratch. You'll usually find that you do a better job the second, third, & forth time.


That is how I view things here. I understand that everyone has a different situation.
 
The beauty of showing dimensions whenever possible is in the ec change processes between models and drawing. Increasingly the models are being sent to diff mtg's and no design intent is transfered if all or most dims are created. The models can be redefined to show the dim exactly the way they are intended. Nuff said
 
A friend of mine works at a big telecom co. and everyone he's talked to
there is telling him NOT to show driving dimensions in drawings, but to
create them. If you happen to be in favor of creating dimensions
please give us your perspective.
 
In my opinion there is a right way and another way. the right way just makes life easier.I've been doing this for over 10 years and seen it done all different ways and it always comes back to showing dimensions. the easy way out is to create them on the fly and get through and look good. but not always correct.


g
 
Just forget about Shown dims on complex parts and sheet metal parts. It's not going to happen or parts that or made in the Asm.
Edited by: pmack009
 
This discussion has come up in past companies I have worked for. I to have been working with Cad as a tool for 12 years. It comes down to using the tool given in the most efficient way. If you create dims and use GTOL. you are not using the systems checks and ballances helping you to insure that you have done things correctly. If you show Dims all intent is captured, GTOl's go on correctly and modification to the model later does not cause the drawing to blow up. In this you have used the system more efficiently. Also kschauer is correct.


If you have ever place datum's in a model. (A,B,C,) You are telling the inspector how to check the part.


Thanks all. Good discussion.....
 
As I noted earlier in the discussion, you are telling the
inspector what to reference on the part (surface, axis, etc...). You however
are not dictating the measuring tool they need to use to verify the part.
You do not care as long as the part falls within the given tolerances and they
are taken from the noted references. You
are also not going to tell the machinist that they have to use a certain tool
to do their job. You do not want them to
go out and buy a tool when one that the have in house will do the trick. You are opening yourself up for additional
tooling cost and or schedule delays if the tool is not available. The
point of the drawing is to convey to the machinist,
assembler, and inspector what the final product is supposed to
be. Let them decide what tools that they want to use to ensure
that the parts meet specification.



Brian
 
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