Newbie issues

[xuraax]

Hi,
I am starting to learn FreeCAD, having played with CAD extensively on AutoCAD, Rhino3D and on a parametric CAD program called Multisurf. I have 2 issues for which I need some help:

1. Is there a function which does the inverse of what the "create a pocket" or "create a hole" does in part design? basically I have a 3D object and I want to shave off from the object anything which lies OUTSIDE a cylinder place concentrically on it?

2. This is more baffling to me. I have created 2 concentric circles placed on one of the faces of a 3D object. The circles have their centres locked on the origin (or so I think). When editing the sketch itself the circles are shown correctly centred but as soon as I close and go back to part design the circles are correctly placed on the right surface but their centres are shifted to one side. A couple of pictures and the program itself will explain all this better but since this is my first post I understand I am not allowed to attach anything. I an using FreeCAD 0.19.

Regards

Joe

[chrisb]

Hi and welcome to the forum!

1) Draw a big rectangle around the Shape you want to cut off and make a pocket. Study SketchCutGrip and Pocket002 in the attached file.

2) Upload your file.

[xuraax]

@chrisb,

Thank you for your speedy response. I am currently studying the file your have provided. In the meantime here are 2 screen shots and the program drawing I am working on.

The 2 concentric circles in green are correctly placed which I will eventually need to extrude.

image2.png (25.36 KiB) Viewed 1192 times

The white circles is what appear after I exit sketcher.

Image1.png (10.24 KiB) Viewed 1192 times

gooseneck.FCStd

(47.54 KiB) Downloaded 21 times

Incidentally, re my issue no.1, the 4 small right angles triangles in grey are the parts I want to shave off. Imagine if you want to machine this part out of stock 60mm diameter aluminium which is the diameter of the biggest circle in the drawing.

Regards

[chrisb]

You probably want to use Sketcher External.

[Shalmeneser]

I have created 2 concentric circles placed on one of the faces of a 3D object.

If you modify your piece, face names will change and you will meet Topological_naming_problem :
Your sketch doesn't find Face23 no more.

[xuraax]

If you modify your piece, face names will change and you will meet Topological_naming_problem :
Your sketch doesn't find Face23 no more.

Thank you. Your solution certainly works.

I am currently reading up to get to grips with this Topological Issue. According to my understanding so far, it seems that renaming will occur if the intermediate block is changed to extend outside the limits of the first block which creates a new face and perhaps force renaming.

In my case what i had changed was to reduce the outside diameter from 64mm to 60mm so no new face should have been added. Also the 2 concentric circles remained correctly locked to the top surface. It seems to me that the constraint of the alignment of the centres that got lost.

Most important for me is to learn how you went about diagnosing the issue and what did you do to get round it.

Regards

[Shalmeneser]

https://wiki.freecadweb.org/Tree_view/en#Overlay_icons

[drmacro]

...

I am currently reading up to get to grips with this Topological Issue. According to my understanding so far, it seems that renaming will occur if the intermediate block is changed to extend outside the limits of the first block which creates a new face and perhaps force renaming.

Hmm...not quite, see below.

In my case what i had changed was to reduce the outside diameter from 64mm to 60mm so no new face should have been added. Also the 2 concentric circles remained correctly locked to the top surface. It seems to me that the constraint of the alignment of the centres that got lost.

Most important for me is to learn how you went about diagnosing the issue and what did you do to get round it.
...

Any time a recompute happens, changes can occur edges TNP can occur.

Your change may have simply changed the order and not the number of edges. When something is referencing a generated edge and the number now refers to some other edge TNP has occurred.

To fix it you typically start look for the at the highest place in the feature tree where things are good. Most of the time you just need to reselect the edge that visually represents what was selected previously. Sometimes, if a sketch was referencing the generated geometry, the attachment of the sketch may change and you need to figure out how to reattach. This is why it is best to attach sketches to base planes...they don't change.