Part and PartDesign

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Overview

There has been much discussion over the years about the differences and ramifications of using the Workbench Part.svg Part and the Workbench PartDesign.svg PartDesign workbenches.

It is a good idea to use one or the other until the user is comfortable with one, then learn the other. It is also typically recommended that new users not mix them until the ramifications of doing so are understood.

Let's talk about those ramifications.

Part Workbench Concepts

Part Workbench is essentially CSG style modeling. The operator combines various primitives to end up with a representation of the desired shape. (In fact, Part Workbench goes one step further than just primitives and allows the operator to use a sketch+extrude operation (or sketch+revolve, loft, sweep ...) to create random shapes as well.) When each primitive or shape is created, it has no relationship to other objects created (except sketches and their attachments), it is a single solitary solid.

Solitary solids


This condition remains so, until, the operator uses some operation to combine them (typically a Boolean that adds or subtracts them). Each starting solid remains accessible separately and the operation creates a new object.

The take away is the single solitary solid bit and the combining them bit.

PartDesign Workbench Concepts

In the PartDesign Workbench the Body object is constructed directly as a single solitary cumulative solid.

The 1st step in a body must be a block of material, either from an additive primitive or an extrusion from a sketch, or an imported shape (then called Base Feature).

This initial block of material will be changed sequentially until the desired final shape (solid) is obtained.

It is cumulative in the sense that each operation adds or removes material.


By default, the "tip" of the body - unless there is a voluntary change in the visualization of a particular feature - is the last operation performed on the body. This is the current and visible state of the body, ready to be changed again by new feature.

Any function under the body represents the cumulative shape of the solid from the 1st feature to the feature considered.

So to have the complete solid, on the one hand the Tip feature must be the last stage of the construction of this solid, and on the other hand it is the body which must be selected and not a stage of its construction.

This will make it possible, in the event of a modification, to always have the last version of the solid represented.


Note and additions : At each time of the construction, the last function used is the "Tip", which can be defined too as "active stage in the construction of the object" or "stage preceding the next action in the construction of the object". When the object's drawing is complete, Tip is naturally the last stage or feature of the construction. But if desired, in case of forgetting, any feature of the construction can be provisionally declared as Tip: it then becomes the step preceding the next action in the construction of the object, which means that new feature(s) can be inserted anywhere in the construction, on condition not to create any incompatible with the suite.

When everything is finished, you have to redeclare the last feature as Tip, which corresponds to the finished object.

Cumulative Body Solid

This image shows a Body. It is a cumulative solid that consists of a padded sketch and a cone primitive. This is a single solid.

If Tip on Pad, the pad can exist separately, but if Tip on Cone, the cone cannot exist separately (Tip on cone = pad + cone).

(Another thing mentioned often is a Body MUST be a single contiguous solid. This means all geometry created by a feature in the Body must touch it's predecessor.)

The Ramifications

Although not recommended for newcomers, it is possible to combine tools from Part WB and PartDesign WB, provided you know what you are doing. For example :

People get caught when they attempt to use some feature under the Body (rather than the Body itself) as one selection of a Part Workbench Boolean operation. This is a problem, because the selected feature does not represent THE complete solid.

In a sense, from a Part Workbench standpoint, the Body represents another primitive. So, using a Body (remember it is a proxy for the tip) and a Part Workbench object to do a Boolean is valid. But the resulting object is a Part WWorkbench object. And, thus PartDesign Workbench tools can't be used on it any longer.

And, it can get even more complicated. If you create a new Body and drag the result from the previous paragraph into it, a BaseObject is created. And you can go off an use the PartDesign Workbench tools on it.

The Caveats

There is a caveat with the Tip and it's representation of the single solid in the Body. If the tip is a subtractive feature and is used in a dress up operation, for instance a Mirror, the Mirror is operating on the underlying feature (a pocket for example). Thus the cumulative solid is not mirrored, but the subtractive feature is. The result of this must create a single solid.

In this example, a mirror of the tip (which is the pocket of the slot) around any of the base planes, or even a face of the solid will not produce a mirrored solid of the entire model. (In fact, it will produce a Mirrored feature in the tree that is essentially empty.)

Solitary solids

In this example, a mirror of the tip (which is the pocket of the slot) is performed around the datum plane and produces a mirrored slot:

Solitary solids

See the PartDesign Mirrored.svg PartDesign Mirrored tool wiki page for more information.

Comparison

You can see below the same example built with each of the two workbenches. Of course, there are always several possible construction timelines with each workbench.

Compare constructions with Part WB and PartDesign WB
In Workbench PartDesign.svg PartDesign WB In Workbench Part.svg Part WB
01- PartDesign Body.svg New body > Sketcher NewSketch.svg New Sketch in XZ plane 01- Workbench Sketcher.svg Sketcher WB > Sketcher NewSketch.svg Sketch in XZ plane
01sketchXZ PartWBvsPartDesignWBn.jpg
01Psketch PartWBvsPartDesignWBn.jpg
02- PartDesign Revolution.svg Revolution / Z 02- Part Revolve.svg Revolve / Z
02revolutionZ PartWBvsPartDesignWBn.jpg
02revolveZ PartWBvsPartDesignWBn.jpg
03- Sketcher NewSketch.svg New Sketch in XY plane 03- Workbench Sketcher.svg Sketcher WB > Sketcher NewSketch.svg New Sketch in XY plane
03sketchXY PartWBvsPartDesignWBn.jpg
03sketchXY PartWBvsPartDesignWBn.jpg
04- PartDesign Pocket.svg Pocket 04a- Part Extrude.svg Extrude
04pocket PartWBvsPartDesignWBn.jpg
04aExtrude PartWBvsPartDesignWB.jpg
04b- Part Cut.svg Cut
00nothing PartWBvsPartDesignWB.jpg
04bCut PartWBvsPartDesignWB.jpg
05- Sketcher NewSketch.svg New Sketch in XZ plane 05- Workbench Sketcher.svg Sketcher WB > Sketcher NewSketch.svg New Sketch in XZ plane
05sketchXZ PartWBvsPartDesignWB.jpg
05PsketchXZ PartWBvsPartDesignWB.jpg
06- PartDesign Pad.svg Pad sym/XZ 06a- Part Extrude.svg Extrude sym/XZ
06padSymXZ PartWBvsPartDesignWB.jpg
06aExtrude PartWBvsPartDesignWB.jpg
06b- Workbench Draft.svg Draft Draft PolarArray.svg Polar Pattern
00nothing PartWBvsPartDesignWB.jpg
06bDraftPolarPattern PartWBvsPartDesignWB.jpg
06c- Part Fuse.svg Fusion
00nothing PartWBvsPartDesignWB.jpg
06cFusion PartWBvsPartDesignWB.jpg
07- Sketcher NewSketch.svg New Sketch on base planar face 07- Workbench Sketcher.svg Sketcher WB > Sketcher NewSketch.svg New Sketch in XZ plane
07sketchBaseSupFace PartWBvsPartDesignWB.jpg
07PsketchXZ PartWBvsPartDesignWB.jpg
08- PartDesign Hole.svg Hole - counterbore 08a- Part Revolve.svg Revolve
08hole-counterbore PartWBvsPartDesignWB.jpg
08aRevolve PartWBvsPartDesignWB.jpg
08b- Workbench Draft.svg Draft Draft PolarArray.svg Polar Pattern
00nothing PartWBvsPartDesignWB.jpg
08bDraftPolarPattern PartWBvsPartDesignWB.jpg
09- PartDesign PolarPattern.svg Polar Pattern of Hole and Pad 09- Part Cut.svg Cut
09polarPatternHoleAndPad PartWBvsPartDesignWB.jpg
09Cut PartWBvsPartDesignWB.jpg
10- Construction tree in PartDesign WB 10- Construction tree in Part WB
PartvsPartDesign1 arbre.jpg
PartvsPartDesign2 arbre.jpg

Conclusion

Part and PartDesign workbenches can be used together with some care, creating quite complex models.

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