New user analyzing forum provided sketch (Solved)

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ajoeiam
Posts: 120
Joined: Sun Mar 03, 2019 9:36 pm

New user analyzing forum provided sketch (Solved)

Post by ajoeiam » Sun Mar 29, 2020 1:56 pm

Just updated last evening to:

OS: Debian GNU/Linux bullseye/sid (LXQt/lightdm-xsession)
Word size of OS: 64-bit
Word size of FreeCAD: 64-bit
Version: 0.19.20209 (Git) AppImage
Build type: Release
Branch: master
Hash: 5fc4a26a00390e91cbf91848b766b68ad87e5917
Python version: 3.8.2
Qt version: 5.12.5
Coin version: 4.0.0
OCC version: 7.4.0

For context see: "Wanting to 'pad' a part" earlier in this forum and the following quote from private correspondence.
Bance wrote:
Sat Mar 28, 2020 8:42 am
OK, I think that you should copy your PM to me and post it on the forum, Give it a title such as "new user analyzing forum provided sketch." I'll happily explain your queries and you 'll get the benefit of additional advice from members that are far more experienced than I.

Bance

I'm a little scared to because I had previously asked about what I see as inconsistencies between the different work benches. Basically got told that I needed to learn the 'freecad' way and/or ignored. I think that the developer crew might have a large tendency to see my questions as nuisance noise and would rather just code some more - - - - even though there are just very very few tutorials to help on figure out what to do. Its like I'm being told - - - - I had to figure it out - - - - you can too (that's where your assistance was so very useful!).
I am just finding that trying to model what I'm working on is really getting in the road of getting the job done, I have enough experience with paper and pencil so I can just quick and dirty get something down (trained where all too often its was a welding rod and dirt, great was a cigarette carton and a pen).
I Am really trying to understand the hows and whys in the methodology and am hoping that going through your example (thank you very much for this by the way!!) will help me to at least understand what in the bleeping blazes is going on.

Had already altered your model to what I need for my work so grabbed another copy and stored that as an independent file.
Started by opening every aspect (in the tree view) of the doc.
List is as follows:
1. File
2. Sketch
3. Body
4. Origin
5. list of 3 axis and 3 planes
6. Pad
7. Sketch001
8. Pad001
9. Sketch002

Initial observation(s)
1. illogical numbering scheme - - - there are really 2 Sketch #1s
2. question as to why more than 1 pad needed
(I now understand that Pad can only be used on a max of 2 geometries at once but its really not clear why that is necessary.)
Next I will look at each of the sketches.
Sketch
1. sketch is of 5 lines depicting 2 diameters, 1 radius and 2 thicknesses
5 degrees of freedom listed as an under-constrained sketch
Sketch001
1. sketch is of 2 coincident circles, 1 depicted by diameter and 1 by radius
sketch is fully constrained
Sketchoo2
1. sketch is of coincident circles, both depicted by diameter
sketch is fully constrained

(I hope the questions are not going to totally PO you but I'm trying to understand the work flow.)
1. both sketch001 and 002 are fully constrained yet I can't find a way to constrain sketch or any of its values - - why?
2. on the coincident circles was there a need to depict one sketch where a radius and a diameter are used and the
other uses 2 diameters - - - - is that necessary or ??????
3. it is possible to edit the values in sketch but not in either sketch001 or sketch002 - - - why?
4. how are the values in sketch linked to those in sketch001 and sketch002?
5. why is sketch done as 'lines'?

(Modeling the blade(s) for my project is only a very small part of the overall project. I have spent quite some time over a number of days trying to find a way to restrain (maybe better 'hold') a very long quite slender shaft containing a large number of these blades. As the shaft may be turning at greater than 10k rpm things are rather not straightforward so I've had to paper sketch and spec a seal/bearing/holding apparatus that is proving to be quite interesting in its engineering. This is a much more complex modeling exercise than that of a blade.)

What is really confusing me is that the process of using a 'Pad' seems illogical.

To me 'Pad' is adding the third dimension into a 2 dimensional figure.
That means that where there is thickness I 'Pad' the feature - - - where there is a hole or just nothing I could define the space as a hole (intrinsic to the part) or as nothing (extrinsic to the part).

Rather than being able to just attach values to the areas in the 2 dimensional structure it is necessary to break up the structure into pieces that only contain 2 geometries. As my next 'part' has at least 17 dimensions that means that I need to have at least 9 sketches - - - - the likelihood of error increases rapidly due to the very large increase in necessary connections.
I understand that you are (at least it would seem that you're not) not the originator of the 'logic' or the work flow - - - - I'm just boggled at how many extra steps and how many opportunities for error are created by using such an awkward shift from the 2 dimensional to the 3 dimensional model. )

TIA
Attachments
saw_blade_blank.FCStd
(21.27 KiB) Downloaded 5 times
Last edited by ajoeiam on Mon Mar 30, 2020 11:33 pm, edited 1 time in total.
drmacro
Posts: 740
Joined: Sun Mar 02, 2014 4:35 pm

Re: New user analyzing forum provided sketch

Post by drmacro » Sun Mar 29, 2020 3:26 pm

A decade ago I was working with a genius mechanical engineer. He could draw, by hand, the most amazing assemblies in isometric views. It was ture artistry. He would then draw third angle projection, dimensioned, versions of each part of the assembly.

I was chartered, by others in the company, with teaching him AutoCAD. The intent being for him to transition from paper to CAD. You remind me a lot of him.

In many cases the answer to your question is simply: "because that is the way it works". And, new tools typically need new ways at looking at the how to accomplish the task. If you've only ever used an open end wrench and you get a ratchet wrench, do you set it on lock and use it like an open end wrench because you don't understand the pawl mechanism?

There is a lot about your post here that I'd love to be sitting next to you to explain because typing words to explain these things takes a lot of time and thought.

One notion you seem to be stuck on is two pieces of geometry in a sketch. Other than placement of the sketch plane in space, there is no 3D information stored in the sketch. The sketch plane is a 2D drawing surface in space. It can contain many circles, just not many concentric circles, if fact two is the number. You can, have one large square containing 10 smaller squares, but only two can be concentric. There is an exception. If you ave a large square that encloses a smaller square...AND the smaller square also contains any number of squares, when you attempt to pad this sketch, the modellers behavior is to assume shapes within shapes designate a void in the resulting model, it doesn't know how to handle a void, inside a void.

As for the names, it is a convention, that the first Sketch, Body, Pad, etc. are named with no number. Later ones are named by appending a number starting with 001. In fact you are free to name them anything you want after they've been created. And you are encouraged to do so.

I this case you will note Sketch is outside the body and is being used as a master sketch to provide parametric dimensions to the subsequent sketches.
drmacro
Posts: 740
Joined: Sun Mar 02, 2014 4:35 pm

Re: New user analyzing forum provided sketch

Post by drmacro » Sun Mar 29, 2020 3:40 pm

(Modeling the blade(s) for my project is only a very small part of the overall project. I have spent quite some time over a number of days trying to find a way to restrain (maybe better 'hold') a very long quite slender shaft containing a large number of these blades. As the shaft may be turning at greater than 10k rpm things are rather not straightforward so I've had to paper sketch and spec a seal/bearing/holding apparatus that is proving to be quite interesting in its engineering. This is a much more complex modeling exercise than that of a blade.)
What you describe here is an assembly, not a part. The blade is one part of that assembly. The shaft is another, bearings another, etc. Each piece of the assembly can, and would, be modeled as separate parts.

And the parts would then be assembled in 3D space using, for example the blades, with a concentric constraint to the shaft.
openBrain
Posts: 3898
Joined: Fri Nov 09, 2018 5:38 pm

Re: New user analyzing forum provided sketch

Post by openBrain » Sun Mar 29, 2020 3:42 pm

Also be aware that the file you posted (I admit I only quickly read the text) isn't obvious to advise on because this isn't very well modeled on itself.
Actually the solid can be modeled in only one sketch and a revolution, without the whole shebang around. ;)
I guess this is part of a more global project, but hard to say how it will work in this context.
It looks like your knowledge of 3D modeling is rather low (this is absolutely not insulting) and I think you could improve faster if you target first a reasonably complex object to model, and do baby steps.
Browsing tutorials, from basics to more advanced, is also useful. But it should be done to understand concepts, not just to "click where you're told to". ;)
Finally polishing how you handle your forum topics may help to. I'd say that a maximum of 3 questions (clearly stated) at a time is good. If you have more, start with the ones that seem the most basic. When answers are clear, go forward for the next 3 ones. ;)
ajoeiam
Posts: 120
Joined: Sun Mar 03, 2019 9:36 pm

Re: New user analyzing forum provided sketch

Post by ajoeiam » Sun Mar 29, 2020 6:12 pm

drmacro wrote:
Sun Mar 29, 2020 3:26 pm
A decade ago I was working with a genius mechanical engineer. He could draw, by hand, the most amazing assemblies in isometric views. It was ture artistry. He would then draw third angle projection, dimensioned, versions of each part of the assembly.

I was chartered, by others in the company, with teaching him AutoCAD. The intent being for him to transition from paper to CAD. You remind me a lot of him.

In many cases the answer to your question is simply: "because that is the way it works". And, new tools typically need new ways at looking at the how to accomplish the task. If you've only ever used an open end wrench and you get a ratchet wrench, do you set it on lock and use it like an open end wrench because you don't understand the pawl mechanism?

There is a lot about your post here that I'd love to be sitting next to you to explain because typing words to explain these things takes a lot of time and thought.

One notion you seem to be stuck on is two pieces of geometry in a sketch. Other than placement of the sketch plane in space, there is no 3D information stored in the sketch. The sketch plane is a 2D drawing surface in space. It can contain many circles, just not many concentric circles, if fact two is the number. You can, have one large square containing 10 smaller squares, but only two can be concentric. There is an exception. If you ave a large square that encloses a smaller square...AND the smaller square also contains any number of squares, when you attempt to pad this sketch, the modellers behavior is to assume shapes within shapes designate a void in the resulting model, it doesn't know how to handle a void, inside a void.

As for the names, it is a convention, that the first Sketch, Body, Pad, etc. are named with no number. Later ones are named by appending a number starting with 001. In fact you are free to name them anything you want after they've been created. And you are encouraged to do so.

I this case you will note Sketch is outside the body and is being used as a master sketch to provide parametric dimensions to the subsequent sketches.
I wish there were a way of interleaving my responses - - - - - would make far more sense that way.

If you want a new tool comparison - - - - its a little bit like using CADD software feels like I've gone back to a hammer and chisel and I would rather be working on that VMC with a twin axis table on it that I have been using and I really want to have a full 5-axis mill.

As I have spent a VERY large amount of time trying to understand the work logic in FreeCad and still failing to understand I can understand your hesitation to use 'enough' words. I am quite comfortable with words (somewhat unusual in trades guy circles) so I am asking questions because I am NOT getting the information I'm looking for from anything I can find.

As a logic exercise take a sketch, #1 is in the X:Y plane, make another, #2 in the X:Z or Y:Z (your choice) - - - - merge them - - - - bingo - - - - your model is complete. What I don't know - - - - is how to program that intersection.

Re: names - - - - OK I will use a different convention in my own drawings. Just seems awkward to have 2 #1s - - - - for any reason.

Thank you for your comment on 'Sketch' - - - - how are the values listed here connected to those in 'sketch001 and sketch002'?
(That would be quite useful to know!!!

Thanks for the ideas!
ajoeiam
Posts: 120
Joined: Sun Mar 03, 2019 9:36 pm

Re: New user analyzing forum provided sketch

Post by ajoeiam » Sun Mar 29, 2020 6:15 pm

drmacro wrote:
Sun Mar 29, 2020 3:40 pm
(Modeling the blade(s) for my project is only a very small part of the overall project. I have spent quite some time over a number of days trying to find a way to restrain (maybe better 'hold') a very long quite slender shaft containing a large number of these blades. As the shaft may be turning at greater than 10k rpm things are rather not straightforward so I've had to paper sketch and spec a seal/bearing/holding apparatus that is proving to be quite interesting in its engineering. This is a much more complex modeling exercise than that of a blade.)
What you describe here is an assembly, not a part. The blade is one part of that assembly. The shaft is another, bearings another, etc. Each piece of the assembly can, and would, be modeled as separate parts.

And the parts would then be assembled in 3D space using, for example the blades, with a concentric constraint to the shaft.
Exactly how I was envisioning it - - - - except - - - - it is necessary to model the blade as an assembly.
As well - - - - even after the blade system (the assembly) is fully modeled - - - - well then there is the rest of the machine.
drmacro
Posts: 740
Joined: Sun Mar 02, 2014 4:35 pm

Re: New user analyzing forum provided sketch

Post by drmacro » Sun Mar 29, 2020 6:23 pm

ajoeiam wrote:
Sun Mar 29, 2020 6:15 pm


it is necessary to model the blade as an assembly.
Are there multiple blades? why is the blade an assembly?
As well - - - - even after the blade system (the assembly) is fully modeled - - - - well then there is the rest of the machine.
Indeed, machines of many parts have many models, that are assembled just like it would be in real life. ;)
Bance
Posts: 380
Joined: Wed Feb 11, 2015 3:00 pm
Location: London

Re: New user analyzing forum provided sketch

Post by Bance » Sun Mar 29, 2020 6:24 pm

Well done for posting this I'm sure it will benefit many others as well.
I'll try to address your points one by one.
ajoeiam wrote:
Sun Mar 29, 2020 1:56 pm
1. illogical numbering scheme - - - there are really 2 Sketch #1s
Sketch and in fact all Freecad objects as far as i know use programmers numbering which traditionally starts from zero.
ajoeiam wrote:
Sun Mar 29, 2020 1:56 pm
2. question as to why more than 1 pad needed
This is a plan of three bodies made from sketches containing circles that are concentric.
ThreeConcentricTop.jpeg
ThreeConcentricTop.jpeg (52.81 KiB) Viewed 100 times
Then the profile.
ThreeConcentricSide.jpeg
ThreeConcentricSide.jpeg (27.81 KiB) Viewed 100 times
Then an angled view.
ThreeConcentricTilt.jpeg
ThreeConcentricTilt.jpeg (44.82 KiB) Viewed 100 times
These are just a few of the possible outcomes of a pad operation, but how is the software supposed know which is the desired outcome?

HTH Steve
ajoeiam wrote:
Sun Mar 29, 2020 1:56 pm
1. both sketch001 and 002 are fully constrained yet I can't find a way to constrain sketch or any of its values - - why?
How were you trying to constrain Sketch? The lines were constrained to point on axis and vertical length, the only other way they could be constrained is horizontally.
ajoeiam wrote:
Sun Mar 29, 2020 1:56 pm
2. on the coincident circles was there a need to depict one sketch where a radius and a diameter are used and the
other uses 2 diameters - - - - is that necessary or ??????
I constrained them the way you specified in an earlier post, vis :- 1. Arbour (not specified, so out of habit used radius.) 2. ID of the carbide teeth. 3. OD of the blade.
ajoeiam wrote:
Sun Mar 29, 2020 1:56 pm
3. it is possible to edit the values in sketch but not in either sketch001 or sketch002 - - - why?
Sketch is as drmacro noted, a master sketch, it's purpose is to centralise parameters, other sketches are linked to master sketch so changing a parameter there is propagated. It is because these parameters are linked via expressions that they cannot be edited in situ.
ajoeiam wrote:
Sun Mar 29, 2020 1:56 pm
4. how are the values in sketch linked to those in sketch001 and sketch002?
They are linked using the expression engine. It is a very powerful mechanism that can utilise formulae and logic to manipulate values.
ajoeiam wrote:
Sun Mar 29, 2020 1:56 pm
5. why is sketch done as 'lines'?
I deliberately left them as lines to show that the sketch was abstract and not for using as a sketch in the modeling sense, I could quite as easily have sketched a section of the blade, which as has been pointed out could have been revolved to produce the model. I didn't feel that using a revolve would help in this case because you were focused on padding circles.

A good way of thinking about modeling in Part Design is, imagine you are using plastecine you can add to it or take from it but it is just one piece in your hand and for the most part you can only do ONE thing at time to it.
ajoeiam
Posts: 120
Joined: Sun Mar 03, 2019 9:36 pm

Re: New user analyzing forum provided sketch

Post by ajoeiam » Sun Mar 29, 2020 6:25 pm

openBrain wrote:
Sun Mar 29, 2020 3:42 pm
Also be aware that the file you posted (I admit I only quickly read the text) isn't obvious to advise on because this isn't very well modeled on itself.
Actually the solid can be modeled in only one sketch and a revolution, without the whole shebang around. ;)
I guess this is part of a more global project, but hard to say how it will work in this context.
It looks like your knowledge of 3D modeling is rather low (this is absolutely not insulting) and I think you could improve faster if you target first a reasonably complex object to model, and do baby steps.
Browsing tutorials, from basics to more advanced, is also useful. But it should be done to understand concepts, not just to "click where you're told to". ;)
Finally polishing how you handle your forum topics may help to. I'd say that a maximum of 3 questions (clearly stated) at a time is good. If you have more, start with the ones that seem the most basic. When answers are clear, go forward for the next 3 ones. ;)
Your response caused me to smile - - - - - very broadly.
I have been told for a lot of years to 'slow down' - - - - in fact - - - I worked hard to limit myself to 5 questions. Likely could have had a LOT more.
You are seeing this project as complex - - - not sure how to say this - - - - but I don't find this kind of a mechanism to be complex.
(If you're interested the whole 'contraption' is only one small part of a highly complex system where I am trying to figure out AND optimize a whole bunch of things. I've also started to look into Monte Carlo Analysis because I'm finding that what I'm working on - - - - well think of something with 12 to 15 dimensions and some of those dimensions themselves are multi-dimensional . . . .)

It would be wonderful to see a model like you describe in your first sentence. There is a video that purports describing exactly that. Impossible to see what is going on - - - - I've tried LOTS of times.

What I'm understanding very clearly is that I should not have asked this question at all.
Oh well - - - - lesson learnt!

Over and out
ajoeiam
Posts: 120
Joined: Sun Mar 03, 2019 9:36 pm

Re: New user analyzing forum provided sketch

Post by ajoeiam » Sun Mar 29, 2020 6:27 pm

drmacro wrote:
Sun Mar 29, 2020 6:23 pm
ajoeiam wrote:
Sun Mar 29, 2020 6:15 pm


it is necessary to model the blade as an assembly.
Are there multiple blades? why is the blade an assembly?
As well - - - - even after the blade system (the assembly) is fully modeled - - - - well then there is the rest of the machine.
Indeed, machines of many parts have many models, that are assembled just like it would be in real life. ;)
Well a blade is actually comprised of multiple parts.
Oh yes - - - - a LOT of blades.
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