PDF's L-cheapo ignition

[doc1955]

Oh yes, for sure. However there is ONE modern convention I truly disagree with, and that is because I am both a draftsman AND a machinist and that is the convention of not dimming the last dim. What this does is forces me as a machinist to get out my calculator and waste time figuring it out. Had it been in already, I could just proceed in machining.

I started out a machinist then to the tooling shop then to n/c programmer and ending my job as a tool design engineer. I agree when you look at a drawing you should not have to pick up a calculator or pencil to figure out any dim's they should be on the drawing in an order that you can create part. But sometimes I know as I have many time been guilty of it as to forget a dim or dim it wrong.

 

[Richard Hed]

I started out a machinist then to the tooling shop then to n/c programmer and ending my job as a tool design engineer. I agree when you look at a drawing you should not have to pick up a calculator or pencil to figure out any dim's they should be on the drawing in an order that you can create part. But sometimes I know as I have many time been guilty of it as to forget a dim or dim it wrong.

Yup, but all that can be fixt. It's the "convention" that the last dim is not dimmed. It's generally built into the software.

 

[awake]

I'm not understanding what that means - the last dim is not dimmed, built into the software. Probably just reflects my ignorance, or maybe the software I'm using. but I've never noticed the software leaving out dimensions ... or putting them in, for that matter. Can you give an example?

 

[Richard Hed]

I'm not understanding what that means - the last dim is not dimmed, built into the software. Probably just reflects my ignorance, or maybe the software I'm using. but I've never noticed the software leaving out dimensions ... or putting them in, for that matter. Can you give an example?

I use AutoCAD 2004 Architectural which is clumsy compared to other 3D CADs, and with that, there is no problem in Dimming but I also use a very old version of Inventor which has the biggest problem of not running on all my computers properly. Usually what happens now, is that I am able to make a single part and it crashes, however, Inventor is MUCH easier to use than AutoCAD 2004. Unfortunately, Inventor does not allow that last dim. This is how it works: you can do an overall dim, say you 5 sub-dims, you can dim 4 of those sub-dims but the way modern thimking goes, the last dim is the overall minus the four. Which is correct, of course, but that still means the machinist or whomever, has to get out his/her calculator. If you've been there, making that part, you understand what I mean. I don't know how other 3D CADs handle this (Solidworks, Pro-E, MicroStation, etc) , nor do I know how later versions of Inventor handle it, but I do remember in drafting class the instructor told us that you leave out the last dim.

Well when you are in class, you do what the instructor says even when it contradicts what you know elsewhere or what another instructor might say. As for programs that automatically dim your project, I never heard of one, I do it all manually just like any program. I've never used Pro-E, would like to know how it is in comparison to Inventor and Solidworks which I HAVE used and like very much. What program do you use?

 

[DanLins]

Richard Hed said:"Oh yes, for sure. However there is ONE modern convention I truly disagree with, and that is because I am both a draftsman AND a machinist and that is the convention of not dimming the last dim. What this does is forces me as a machinist to get out my calculator and waste time figuring it out. Had it been in already, I could just proceed in machining. "

I certainly agree with you on this. Many times I work out all my dimensions in CAD, and get a nasty surprise when I get the work mounted and ready to begin machining. Out comes the calculator, last minute calcs for amendment dimensions.

Dan

 

[Richard Hed]

Richard Hed said:"Oh yes, for sure. However there is ONE modern convention I truly disagree with, and that is because I am both a draftsman AND a machinist and that is the convention of not dimming the last dim. What this does is forces me as a machinist to get out my calculator and waste time figuring it out. Had it been in already, I could just proceed in machining. "

I certainly agree with you on this. Many times I work out all my dimensions in CAD, and get a nasty surprise when I get the work mounted and ready to begin machining. Out comes the calculator, last minute calcs for amendment dimensions.

Dan

It's nice when a machine (computer) stops you from dimming something twice, but I really can't stand it when a machine tells me what I can or cannot do. Some programs allow you to put the last dim in like this: "(6.25)", that is, with parentheses. It's laughable, stupid and whoever creates these conventions must never have machined a part in their lives. I wish there were a way to turn that particular feature off in AutoCAD, Inventor, etc.

 

[doc1955]

It's nice when a machine (computer) stops you from dimming something twice, but I really can't stand it when a machine tells me what I can or cannot do. Some programs allow you to put the last dim in like this: "(6.25)", that is, with parentheses. It's laughable, stupid and whoever creates these conventions must never have machined a part in their lives. I wish there were a way to turn that particular feature off in AutoCAD, Inventor, etc.

Dimensions with parentheses indicate a reference dimension and not a solid dimension.

 

[Richard Hed]

Dimensions with parentheses indicate a reference dimension and not a solid dimension.

Exactly--that is what it is called, however, in MY (not humble at all) opinion, for that last dim, it is assenine. It is assinine (hmm spell checker must be wrong) to have to work that last dim when you are turning a part.

 

[Peter Twissell]

The reason for avoiding that last dimension is that it over-constrains the part.
If the part is dimensioned correctly, you should never have to work out a last dimension. The critical dimensions (e.g. journal lengths) should be toleranced and the non-critical dimension (e.g. a flange thickness) will be what it is when all the critical dimensions are achieved.
If you have to work out the last dimension, either the part has not been correctly drafted or you've made it in the wrong order.

 

[Richard Hed]

The reason for avoiding that last dimension is that it over-constrains the part.
If the part is dimensioned correctly, you should never have to work out a last dimension. The critical dimensions (e.g. journal lengths) should be toleranced and the non-critical dimension (e.g. a flange thickness) will be what it is when all the critical dimensions are achieved.
If you have to work out the last dimension, either the part has not been correctly drafted or you've made it in the wrong order.

Yes, that is true, but as a machinist I want that last dim. I don't mind if it is put in as a "reference" dim, I just don't want to get out the calculator.

 

[Peter Twissell]

A reference dimension is what it should be - often the overall length of the part.

 

[Richard Hed]

A reference dimension is what it should be - often the overall length of the part.

Ah, I was thimking that too, however, the overall length is often the first dim needed (not alwahys), as when you needs to cut a 5.5" piece of round stock.

 

[awake]

It's nice when a machine (computer) stops you from dimming something twice, but I really can't stand it when a machine tells me what I can or cannot do ...

The reason for avoiding that last dimension is that it over-constrains the part.

Hmm, light may slowly be dawning for me - I confess that I have been having a hard time figuring out the problem under discussion. No doubt a large part of my confusion comes from the fact that my experience with CAD programs is limited to LibreCAD (2d) and FreeCAD (3d) - I've never used AutoCAD, or Inventor, or so on.

In LibreCAD (2d), I can produce lines and arcs and such to a particular "dimension" as I build them, or I can use snapping and trimming and so on to get what is needed. But these are not what LibreCAD calls dimensions; dimensions are the markings that are added to the drawing, and you can add dimensions all day long, anywhere you want - nothing prevents adding competing dimensions or "the last dimension" or so on.

When it comes to 3d CAD, when creating a 3d model, FreeCAD does object if a part is over-constrained in the sketch mode. This makes sense, because unlike LibreCAD, which just puts lines or arcs of a given size wherever you say, without worrying about whether or not that makes sense, FreeCAD is building an object that actually fits together. If it allowed over-constraints, the computer could get into endless loops trying to reconcile tiny fractional errors imposed by the limits of binary representation on two competing dimensions. Here I am using "dimension" to refer to the constraints, not to something added on to the existing part to explain it.

But here's the thing in FreeCAD: there is a big difference between creating the 3d model, and producing plans from it. I can make a set of plans of a part (using the TechDraw workbench), and on the plans I can add all the dimensions I want - including the overall dimension or last dimension or 15 competing dimensions - all work just fine. Now we're talking, not about dimensions that constrain the part, but rather dimensions that are taken from the part and displayed. Ordinarily, the constraints used to build the part are never displayed except when building the part - or at least, I haven't found / used that functionality. (Likely it is in there somewhere, since there are a number of "workbenches" in FreeCAD that I have barely touched.)

So here's what I'm wondering - maybe it would be helpful to distinguish between build dimensions and display dimensions - or some terminology like that. And now I'm wondering, in CAD programs such as Autodesk, can you only display the "build" dimensions - which can't be over-constrained without causing problems?

I hope the rambling above makes some sort of sense. As I said, I have been confused by trying to understand the problem that folks are experiencing. Have I gotten any closer, or just hopelessly confused things further??

 

[tornitore45]

We have hijacked the original thread but this is so interesting.
Just to add a twist to the topic...
For the stubbornness of not learning yet a new complex software application like CAD system I do all my drawings in... drum roll please EXCEL

Before you poop on it, I am very good at EXCEL, the extra effort to make a drawing look right and accurate is highly rewarded by the freedom in many other respects.

The beauty is that I can force dimension a feature based on what I want and calculate a dimension based on all others.
Some dimensions are number and are Bold because I can change it as I please
Other dimension are formulas using all other dimension.
I can use trig functions, or Average to place a feature between to locations, often I place a label "Wall" and calculate the metal thickness to make sure I do not end up with a fragile part.
I can place all dimensions I want
A change in one of the original forced dimensions ripple through all the formulas and readjust all other dimensions.
I can work a mixed metric/imperial by entering mm and displaying inches via a formula
I can add/subtract a few thousands for clearance or to make an odd size come out a standard fractional stock.

And finally sum up the total to cut out my blank piece.

All the rules are broken but I am the one that machine the part and know which dimension determine the cutting and which dimension is just what is left over.

The core of the problem is that there is a fundamental difference between the information fully defining the "geometry" of a part and the information necessary to machine a working part with given tolerances.

The best cad would give full freedom to over constrain a part and give warnings and errors when things are wrong, such for example a negative thickness.

 

[Richard Hed]

Hmm, light may slowly be dawning for me - I confess that I have been having a hard time figuring out the problem under discussion. No doubt a large part of my confusion comes from the fact that my experience with CAD programs is limited to LibreCAD (2d) and FreeCAD (3d) - I've never used AutoCAD, or Inventor, or so on.

In LibreCAD (2d), I can produce lines and arcs and such to a particular "dimension" as I build them, or I can use snapping and trimming and so on to get what is needed. But these are not what LibreCAD calls dimensions; dimensions are the markings that are added to the drawing, and you can add dimensions all day long, anywhere you want - nothing prevents adding competing dimensions or "the last dimension" or so on.

When it comes to 3d CAD, when creating a 3d model, FreeCAD does object if a part is over-constrained in the sketch mode. This makes sense, because unlike LibreCAD, which just puts lines or arcs of a given size wherever you say, without worrying about whether or not that makes sense, FreeCAD is building an object that actually fits together. If it allowed over-constraints, the computer could get into endless loops trying to reconcile tiny fractional errors imposed by the limits of binary representation on two competing dimensions. Here I am using "dimension" to refer to the constraints, not to something added on to the existing part to explain it.

But here's the thing in FreeCAD: there is a big difference between creating the 3d model, and producing plans from it. I can make a set of plans of a part (using the TechDraw workbench), and on the plans I can add all the dimensions I want - including the overall dimension or last dimension or 15 competing dimensions - all work just fine. Now we're talking, not about dimensions that constrain the part, but rather dimensions that are taken from the part and displayed. Ordinarily, the constraints used to build the part are never displayed except when building the part - or at least, I haven't found / used that functionality. (Likely it is in there somewhere, since there are a number of "workbenches" in FreeCAD that I have barely touched.)

So here's what I'm wondering - maybe it would be helpful to distinguish between build dimensions and display dimensions - or some terminology like that. And now I'm wondering, in CAD programs such as Autodesk, can you only display the "build" dimensions - which can't be over-constrained without causing problems?

I hope the rambling above makes some sort of sense. As I said, I have been confused by trying to understand the problem that folks are experiencing. Have I gotten any closer, or just hopelessly confused things further??

What you are saying makes great sense to me. I never thot of it in those terms, but you are right. Yes, the part constraining should be different from the dimming part. I thimk that is exactly what the problem is, is that the programmers have made the dimming part into a constraining part when (In my not so humble opinion) it should not. The program should, however, attempt to put an orderly group of dims, and not more than one dim of each dimension.

 

[Richard Hed]

We have hijacked the original thread but this is so interesting.
Just to add a twist to the topic...
For the stubbornness of not learning yet a new complex software application like CAD system I do all my drawings in... drum roll please EXCEL

Before you poop on it, I am very good at EXCEL, the extra effort to make a drawing look right and accurate is highly rewarded by the freedom in many other respects.

The beauty is that I can force dimension a feature based on what I want and calculate a dimension based on all others.
Some dimensions are number and are Bold because I can change it as I please
Other dimension are formulas using all other dimension.
I can use trig functions, or Average to place a feature between to locations, often I place a label "Wall" and calculate the metal thickness to make sure I do not end up with a fragile part.
I can place all dimensions I want
A change in one of the original forced dimensions ripple through all the formulas and readjust all other dimensions.
I can work a mixed metric/imperial by entering mm and displaying inches via a formula
I can add/subtract a few thousands for clearance or to make an odd size come out a standard fractional stock.

And finally sum up the total to cut out my blank piece.

All the rules are broken but I am the one that machine the part and know which dimension determine the cutting and which dimension is just what is left over.

The core of the problem is that there is a fundamental difference between the information fully defining the "geometry" of a part and the information necessary to machine a working part with given tolerances.

The best cad would give full freedom to over constrain a part and give warnings and errors when things are wrong, such for example a negative thickness.

Yeah, well like you, I don't like machines telling me what to do (no less other people telling me what to do), however, I DO, indeed, appreciate advice, help, etc. and if a machine does as you suggest, that is, warn me, I'm fine with that--just no loud beeps or cattle prods, please.

BTW, that's why the thread gets hijacked, because it is so interesting. As for myself, I like to talk about anything that comes up, it always comes back to the probs we are having (CAD is part of engineering, afgter all). It's fine with me to talk about anything at all.

 

[awake]

We have hijacked the original thread but this is so interesting.
Just to add a twist to the topic...
For the stubbornness of not learning yet a new complex software application like CAD system I do all my drawings in... drum roll please EXCEL

Before you poop on it, I am very good at EXCEL, the extra effort to make a drawing look right and accurate is highly rewarded by the freedom in many other respects.

The beauty is that I can force dimension a feature based on what I want and calculate a dimension based on all others.
Some dimensions are number and are Bold because I can change it as I please
Other dimension are formulas using all other dimension.
I can use trig functions, or Average to place a feature between to locations, often I place a label "Wall" and calculate the metal thickness to make sure I do not end up with a fragile part.
I can place all dimensions I want
A change in one of the original forced dimensions ripple through all the formulas and readjust all other dimensions.
I can work a mixed metric/imperial by entering mm and displaying inches via a formula
I can add/subtract a few thousands for clearance or to make an odd size come out a standard fractional stock.

And finally sum up the total to cut out my blank piece.

All the rules are broken but I am the one that machine the part and know which dimension determine the cutting and which dimension is just what is left over.

The core of the problem is that there is a fundamental difference between the information fully defining the "geometry" of a part and the information necessary to machine a working part with given tolerances.

The best cad would give full freedom to over constrain a part and give warnings and errors when things are wrong, such for example a negative thickness.

Mauro, what an interesting approach - may be unique! I'd love to see an example of how you do this in Excel.

 

[tornitore45]

Mind, the XLS files attached are the converted from XLSX some functionality may have been lost.
The files were made for my use only so some of the mental gymnastic may not appear obvious to someone else, as a matter of fact sometime I wonder myself "what was I trying to accomplish here?"
Accuracy is pursued only to the end of getting the part done, details that are obvious to me like for example the drill for a threaded hole or its depth are not displayed.

 

[awake]

Very interesting - I didn't even know you *could* draw in Excel. And of course, you are able to store all sorts of detail in the spreadsheet cells, both text and numbers/formulae.

When you draw in Excel, can you then take dimensions directly from the drawing, or vice-versa, have a dimension (say in a cell) adjust the drawing automatically?

 

[johnmcc69]

In Pro-E, (like many 3D CAD programs) when creating the geometry, the system will "warn" you if you do over-dimension/over-constrain a feature but will ask you if you want to convert that dimension as a "reference" dimension & will display it. Note: this is in the " Sketcher" environment when creating the initial 3D geometry. When I am ready to create the detailed drawing, I create my new drawing, add my views, title blocks, ETC. I can then select a toolbar that will allow me to show those dimensions that were created initially in the 3D model & it will automatically create & show those dimensions on the part. I then can move those dimensions around as I please & from view to view, or views on other sheets of the same drawing if required. This also holds true for geometric tolerancing, notes, center lines, & other items. I can also set this up to "clean up" those dimensions & place them all nice & equally spaced. Keep in mind I can add additional dimensions to my hearts content & happily add as many unnecessary dimensions as I want (I don't do this, my experience knows better...). I know it used to be a constant argument with 3D CAD users of how to dimension drawings, one side says use only "Shown" dimensions (those created initially in the geometry & frown in creating additional dimensions. I believe that group are programmers & eat & sleep zeros & ones, or software salesman who want to make you think their software will "automatically" create working drawings.
The other side says use "created" dimensions (manually) creating the dimensions. (All dimensions, "shown" or "created" update parametrically as things change) I tend to use this method. As I started out as a machinist, I fully understand what dimensions are required to manufacture a part. Additionally, I can also pick & choose what "shown" dimensions/notes/ETC. I want, allowing me to use both methods. I think like a machinist while detailing drawings & walk the machining steps through my mind as I go. I have tried using the first method, but found it took longer to "clean up" the automatically shown dimensions & not productive to the way I work. This holds true if I am designing model engines or industrial tooling/assemblies/fixtures.

Formulas can also be used to drive geometry via "relations" & also has its place (personally, I avoid them because they can be hard to manage & create problems farther down the road). One "feature" I really got a "kick" out of was when I was working on a belt or track design & was able to turn my belt/track length into a fixed perimeter dimension (think belt length..) & let the computer "suggest" pulley dia. & center distance, from that I was able to further develop Pulley sizes & a workable center distances.

3D CAD is an amazing tool, granted they all have their problems (undocumented "features"?)

More than you wanted to know huh?

Don't let any of this deter you from learning it, what I mentioned are some more advanced techniques in 3D software, I spent many hours banging my head on the desk learning how to sketch a symmetrical square & "revolving" a sketch to make a simple "wheel".

We all start from the beginning, & whatever works for you, just keep using it.

John