noob question

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buick455
Posts: 14
Joined: Sun Mar 10, 2013 8:55 pm

noob question

Postby buick455 » Sun Mar 10, 2013 9:14 pm

OS: Windows 7
Platform: 32-bit
Version: 0.13.1828
Python version: 2.6.2
Qt version: 4.5.2
Coin version: 3.1.0
SoQt version: 1.4.1
OCC version: 6.3.0

I am attempting to learn how to use FreeCad so please go easy on me :lol:
I will be explaining what Im trying to do to the best of my ability but I think my lack of terminology is going to bite me in the butt..

( MAIN BLOCK ) I have made a cube h-47, l-57, w-45
( RECESS ) Then I made a recess in the top h-5, l-43, w-43 & placement x-5, y-7 , z-42

I am trying to make this model parametric (printing with a reprap and the 43x43 might need to be bigger or smaller after a test print) so I can change the size of RECESS later if need be. I cant seem to find a way to lock the RECESS part to a center orientation

When I go into the data tab for the cut out (RECESS) and change the H, L & W it stays locked in one corner instead of adding to all sides..

I just re-read this and i hope someone understands they above, lol...
jmaustpc
Posts: 10382
Joined: Tue Jul 26, 2011 6:28 am
Location: Australia

Re: noob question

Postby jmaustpc » Sun Mar 10, 2013 10:47 pm

Hi welcome :)
It sounds like you are using either the Part work bench or the Complete workbench.

For a beginner it is best to say do not use the Complete workbench....it has too many features missing. It was originally called "complete" meaning its contents were completed and of appropriately high quality, bug free....it does not mean "complete" as in it contains everything.

From the sound of it you could start in Part and insert a Box, however you could just stay in PartDesign.
Create a sketch...
create and constrain a rectangle,
Pad that rectangle to desired height,
High light just the top of the Pad, select new sketch which will automatically "map" to the selected surface, Note that you can also9 create a sketch and use the "map to face" tool to map the sketch to any flat face (e.g. a cylinder end is OK but a cylinder wall is not).

In the second sketch, create another rectangle.
Use the "link to external geometry" tool to insert a construction line (hidden line when sketch is not in edit mode) linked to external geometry on two sides of the Pad,
Then use lines from those externally linked construction lines to constrain the position of the rectangle, use the "equal" constraint to constrain the rectangle to the centre of the pad. Then set those lines to construction lines using the "construction line" tool.

Then use the Pocket tool on the second sketch.

Jim
jmaustpc
Posts: 10382
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Location: Australia

Re: noob question

Postby jmaustpc » Sun Mar 10, 2013 11:07 pm

Here is an example to your dimensions
padandpocketcentred.FCStd
(13.28 KiB) Downloaded 59 times
Jim
buick455
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Re: noob question

Postby buick455 » Mon Mar 11, 2013 12:26 am

Ok, I must not be doing this correctly at all... :oops:
This file below was all done on the Part Workbench.. I was kinda working from what I knew worked (might not have been right but worked) in Openscad. I think I'm in over my head. I thought I could just place and size different shapes and use shapes as subtractions like in Openscad but without all the typing & and with a mouse to move stuff around...
Attachments
FreeCad_Testrun_001.FCStd
(8.74 KiB) Downloaded 49 times
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NormandC
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Re: noob question

Postby NormandC » Mon Mar 11, 2013 3:16 am

Hello,

This looks OK to me. What you used is what we call constructive solid geometry (CSG) which is what OpenSCAD does, basically applying boolean operations on primitive shapes.

It's the most basic method of solid modeling. It's easy, but it has disadvantages. In FreeCAD, a solid box's placement is located on one of its corners, as you've already found out. Also, in this mode you cannot apply relationships between shapes and say put one shape always centered to another on two axes. You have to do it by hand, by editing each shape's coordinates. And at the moment it is not possible to move objects with precision with the mouse.

Which is why feature-based parametric modeling done in the Part Design workbench is a more powerful method of modeling. But it does have a steeper learning curve. You almost always start with a sketch to generate what is called a "feature". The way features are ordered in the tree is different from solids created in Part and combined with booleans.

If you're willing to learn this method, you can look at the wiki to get the basic principles. Start with those two pages:
https://sourceforge.net/apps/mediawiki/ ... _Workbench
https://sourceforge.net/apps/mediawiki/ ... _Workbench

There are good tutorials on YouTube, for example John Durston's. Because of FreeCAD's rapid development, they may be a little dated - but the basics stay the same. If you prefer written tutorials there's forum member kwahooo's excellent FreeCAD Tutorial blog.

And of course don't hesitate to ask questions, we're here to help. :)
jmaustpc
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Location: Australia

Re: noob question

Postby jmaustpc » Mon Mar 11, 2013 4:05 am

Edit:- Norm and I posted at the same time hence a lot of what I have said is a duplication. :)

The method you have used is CSG (constructive Solid Geometry)

See wiki pedia here
http://en.wikipedia.org/wiki/Constructi ... d_geometry

You can use this method if you want to but it has limitations. In FreeCAD there are usually several was to achieve something.

You said you wanted to make the position of the "cut out box" have a position constrained to the centre of the face of your First Box, hence I suggested you use the PartDesign workbench.

The assembly workbench will eventually be able to position items in 3d space with constraints relative to each other, but Assembly is still in early development and is probably going to be reasonably complicated.

In your very simple example the power of sketcher may seem less obvious, however imagine you cut out box were to be some odd shape rather than a regular shape. For example, sketcher could pocket using a star shaped cross section.

CSG in CAD can just mean using a CSG method with the GUI as in FreeCAD however it is also possible for it to be how the CAD software works behind the GUI. Some old fashioned CAD applications work that way, like BRLCAD. But it has limitations. FreeCAD is based on a more modern CAD kernel which uses BREP.

BREP see wiki pedia
http://en.wikipedia.org/wiki/BREP

The following quote is from Wiki pedia, in the page linked to above
Compared to the constructive solid geometry (CSG) representation, which uses only primitive objects and Boolean operations to combine them, boundary representation is more flexible and has a much richer operation set. This makes boundary representation a more appropriate choice for CAD systems.[citation needed] CSG was used initially by several commercial systems because it was easier to implement.

OpenSCAD according to its founder is only supposed to be a quick and simple utility, which presumably is why they based it around CSG. There is nothing wrong with that, it is great for what it was intended for.

OpenSCAD can be used within FreeCAD. There is file export/import which by its very nature will always have some limitations. Also you can actually run OpenSCAD from right inside FreeCAD. See the FreeCAD wiki on how to do that. Basically you have to install OpenSCAD and then tell FreeCAD where to find OpenSCAD on your computer and then go to the OpenSCAD workbench in FreeCAD.

Jim
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NormandC
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Re: noob question

Postby NormandC » Tue Mar 12, 2013 12:18 am

jmaustpc wrote:The assembly workbench will eventually be able to position items in 3d space with constraints relative to each other, but Assembly is still in early development and is probably going to be reasonably complicated.
The assembly workbench will deal with separate parts. I doubt boolean operations will be possible in the assembly wb. And if booleans are then applied in the Part wb, I wouldn't be surprised if the 3D constraints between the affected parts get nuked. Driving 3D relationships between solids which get fused afterwards does not make sense, at least in the parametric CAD packages I've used.
jmaustpc wrote:OpenSCAD can be used within FreeCAD.
With the caveat that it is still very experimental, and does not yet work reliably on most shapes you throw at it.

For example I exported our big block's FreeCAD_Testrun_001 file to .scad then opened it in OpenSCAD. The shape is wrong.

* That's what a Buick 455 is right? :P
keithsloan52
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Re: noob question

Postby keithsloan52 » Tue Mar 12, 2013 12:26 pm

Where do I find the big block's FreeCAD_Testrun_001?
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NormandC
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Re: noob question

Postby NormandC » Tue Mar 12, 2013 2:32 pm

Uh...

Big block = buick455 (the Buick 455 was a big american Buick V8 engine, hence "big block")

FreeCAD_Testrun_001.FCStd is the file buick455 posted above...
buick455
Posts: 14
Joined: Sun Mar 10, 2013 8:55 pm

Re: noob question

Postby buick455 » Tue Mar 12, 2013 3:20 pm

normandc wrote: * That's what a Buick 455 is right? :P
Yes, it is :) Way back when my wife was my GF and I had $$$ and time to do foolish things I shoe-horned the BB from my totaled 70 Riviera boattail into my 85 Regal just b/c I could and wanted the Riv to live on.. Still have the Regal to this day, just wish it aged as well as I have, lmao..
keithsloan52 wrote:Where do I find the big block's FreeCAD_Testrun_001?
hey Keith, the file is in 4th post. BTW I'm 6of1 over on the RR boards.


Ok, as far as my FreeCad adventure (might be an understatement) goes I have been using the Sketcher Workbench and I think I'm getting a grip of things. Getting things constrained is for the lack of better terms a pain in the butt (I'm probably doing it wrong, workflow wise, it just takes me forever). What I have been trying to do is re-create an object I created in Openscad (which also took forever b/c I was learning as I went). I think FreeCad will give me more flexibility (the ability to use the mouse and see whats being made before hitting F6 helps too). I am uploading a pic of my object from Openscad & my attempt in FreeCad so you can see my progress (or lack there of). Some things from the pic are missing from the FreeCad model (didn't want to bite off more then I could chew).

Image
Attachments
freecad day 002.FCStd
(26.91 KiB) Downloaded 52 times