Louis Rossmann: Autodesk turns lifetime licenses into subscriptions [video]

Each sketch can be placed independently anyway (offset from its attachment), and you can place them parametrically. So if you want the sketch on the top surface of another pad you can simply set the attachment position with e.g. >.Length referring to that object. Or by using a named reference from another sketch, or whatever.

Local co-ordinate systems can be useful, and sometimes I have added planes attached to those, because setting up attachment in truly arbitrary places is fiddly, but I almost never use datum planes.

TNP isn't usually triggered by resizing a sketch, unless the resizing causes some topology to change -- an edge being added in the edge list before a critical one is what will break attachment. For example if resizing one sketch in a body causes an earlier-numbered edge of a face you're attaching to, to disappear, that will do it, I think.

So generally TNP is something you don't always have to worry about, if you plan your design; sometimes you can attach to faces without concerns.

This isn't actually truly unique to FreeCAD. All CAD kernels have to solve this problem somehow; it's just that they are usually really narrow edge cases rather than big ugly obvious ones!

For me the bigger problem with FreeCAD (that won't go away quickly) is fillets and chamfers. OpenCascade can't allow a chamfer to consume other edges so you often run into difficulties where you have to tweak some measurement by a fraction of a millimetre so it doesn't.

I am getting around that by considering carefully which chamfers are merely presentational and which are effectively structural. The structural chamfers I may solve in some other way -- within the sketch, or with a subtractive operation, or whatever.

I agree with the recommendation for the Mango Jelly videos, and if you're on Facebook, the main FreeCAD group can be useful (Mr Mango Jelly hangs out there).

Datum planes are usually unnecessary; cluttering up a design with a plane per sketch is certainly the wrong way to go about it.

You can place each sketch with the same tools you place a datum plane -- by editing the attachment and attachment position. (You just can't do it at the point of creating the sketch in the same way as you can when creating a plane).

You can then use expressions in those positions, including e.g. the width/depth/length of other objects (using the length of another Pad would be one very common scenario). Or you can use values from a Spreadsheet.

And TNP won't always bite you anyway. It's possibly better to learn how to fix attachment issues than clutter up your design with datum planes.

Learning how to place sketches in arbitrary space is so core to FreeCAD that it's probably worth going through the pain on this.

Datum planes do have uses -- like when you want to attach large numbers of sketches to a single plane, or when you want to model some arbitrary plane without using physical geometry. They are also useful for e.g. cutting holes up to a face (because the plane can substitute for a face)

They are also helpful, I find, when I am using an LCS for some feature.

But quite often you see people trying to work around TNP with a datum plane, only to attach that plane to the same object that will have the TNP.

This tutorial on attachment may help you (it's a bit involved)

https://wiki.freecad.org/Basic_Attachment_Tutorial

But the various Mango Jelly videos about sketch attachments are well worth watching.

Indeed -- it's actually used in the FreeCAD Assembly 3 workbench as a solver (I think that supports switchable/pluggable alternatives now though)