Best Practices with SpreadSheets

= Introduction = This is a draft of what I imperfectly understand as best practices ( and tips, and gotchas ) for parametric modeling in FreeCad using SpreadSheets.

Why
For me the point of a parametric model is that measurements ( and sometimes other features ) can be changed by changing the value of an input variable or parameter. The parameters may be stored in many different locations in your model but I find that a spreadsheet is the most useful. You can just look at a table of values, their names, and perhaps some notes about them. The spreadsheet can also do some calculations like changing radii to diameters and vise versa.

How
Use the spreadsheet workbench to add a spreadsheet. Then put you important dimensions in a cell. I find it most useful to give each cell a name or alias. If you do not do this the cells have non-informative names like B5. I normally also type the alias to the left of the column with the data, so when I look at the spreadsheet I can see both the name and the value. It is often useful to tile your workspace so you can see the spreadsheet and the model as you enter formula.

= Names in the SpreadSheet = Like programming good names can really help you keep track of what you are doing. So be systematic in making up your names. Some bad names:


 * part_size   ( a radius, diameter....? )
 * distance    ( what direction, from what? )

Better names:
 * bolt_diameter
 * bolt_length
 * shaft_dia

But note that you should try to be consistent: try to use dia or diameter but not both.

Finally all names in a spreadsheet are referenced outside of it by using the spreadsheets name, so I usually rename it from the default "Spreadsheet" to the easier to type "ss".

= SpreadSheet values in the Model = Most values/measurements in the model that can be entered as numbers can also be entered as formula, and the formula can use the names from the spreadsheet as values. Suppose for example your model has several shafts in different sketches which should all have the same radius, and also that you have holes in your model for the shafts to fit into. How do you change the radius of all the shafts and holes?

Here is an approach:


 * In the spreadsheet have a cell shaft_radius, and another shaft_clearance, each with a numerical value. The clearance is the amount ( in radius ) that the hole needs to be larger than the shaft for the parts to fit.

ss.shaft_radius. Note that as you type FreeCad will try to help, if you type ss.s then FreeCad will suggest shaft_radius, and you can just click on it.
 * In a sketch where you have a circle for the shaft apply the radius: when the value box come up press the formula icon at its righthand side, or hit the = sign. Now you can enter the formula:


 * For a shaft hole try the radius ss.shaft_radius + ss.shaft_clearance

= Master Sketches =

Rather than use a spreadsheet I have seen some people use a "master sketch" The idea here is that all the important dimensions are drawn out in a sketch. This sketch is never padded, pocketed or drafted, but other sketches ( usually parallel to it ) use the connect to external geometry feature to synchronize the parameters. I have tried this and like the way it is more visual than a spreadsheet, but it also has limitations: it is harder to do math in it, it is unclear what to do if you have three dimensions to parameterize, it is harder to annotate. Despite the limitations you may find it useful. When I use it now, and I still do, I link its parameters back to the spreadsheet and more or less get the best of both worlds.

= The Topological Naming Problem ( TNP ) =

This is a problem which causes a model to break .... you do not get what you expect, and it usually includes elements in your model that report errors so they will not recompute. The problem seems to be particularly severe in highly parametric models which are fine until you change one of the parameters. I do not want to go too deeply into the problem here ( partly because I am not an expert ) but I think many of the best practices are developed for highly parametric models so they will not fail. I think the crux is that a model, like computer code, is held together by names and numbers. When you recompute an object, say even a cube, you get a new object and the names of its faces, edges.... may change or be in different places. Other objects are attached to the names and now may not be where you want them or even in a place that makes sense. This can easily come up when you attach a sketch to the face of an object. So we will recommend practices that avoid this sort of attachment.

= Placement of Sketches =

You should place sketches on objects that will not move, or at least will not move in a way that changes it name. Here are some ways.

Planes Through Origin
A sketch attached to any of the 3 planes passing through the origin is safe from the TNP as these are named invariant "things" If you can use these planes, if you are making holds all the way through an object you can start from the bottom as well as from the top.

Move the Sketch Placement
When the drawing of the sketch is "closed" there are still a lot of changes that can be made Data tab of the sketch in particular see:

Base -> Placement -> Position ( x, y, z )

The directions x, y, and z are not the x, y, and z of the overall model but of the plane of the sketch. z is always out of plane and x and y in the plane so the whole thing is a right handed coordinate system. Move z to get the plane to move perpendicular to z. This amount can ( and should ) be entered from the spreadsheet. Here is an example of the idea.

Suppose you have a cube constructed from a sketch sitting on the xy plane and extending up in the z direction ( lets this be cube_z_size in the spreadsheet ). You want to "drill" some cylindrical holds on the top face half way down through the cube. The simple thing to do is attach a second sketch sitting on the top face. Don't. It is very liable to break on resizing. Instead place you sketch on the xy plane then set the sketches z position to cube_z_size. It now moves with the cube as if it were attached but is not subject to TNP. You set the depth of the holes ( in the pocket ) to cube_z_size/2 and now one parameter in the spread sheet changes everything in the model that needs changing.

Datums
A datum is another way of moving a sketch placement. A datum ( for planes ) is simply a plane that you can position anywhere in a model body at any angle ( all probably controlled by a spreadsheet ) and then you can attach sketches to the datums. I am not sure of what the trade offs are between "placed" sketches and datums so can not tell you when to use one or the other. It seems a bit more work to construct a datum, but then it is a bit more obvious how the sketch is being placed.

= Name the parts of the Model =

After I have constructed a model I am often a bit confused as to what is what in the model and how the pieces relate to each other. Because of this I often name parts of the model and think that this is a best practice for some parts of some models. Say you have bolt holes based on a sketch. You could name the sketch bolt_holes, then pocket them and name this bolt_holes.... well not quite two things with the same name is one route to insanity. So I prefix them po_ for pocket (pd- for pad ), s_ for sketch. The names are then s_bolt_holes and po_bolt_holes. As usual with names try to pick a scheme and stick to it.

= In the Sketch =

Center around the Origin of the Sketch, if it is in the wrong place use the sketch

= Order of Operations = I have seen some stuff recommending you model in a certain order of operations. I do not understand this well enough to make recommendations.

= What Ends the Parametric Model Chain = When you add some features to a parametric part of a model, you may get a result that is not longer parametric: Say feature A is the basis for feature B. If changes in feature are not reflected in a change to feature B then the parametric nature of the model is broken at the transition A -> B breaks the model chain. In most cases operations do not break the chain, the following table documents some modeling operations and effects:

= Models the Illustrate these Practices =

All these models may not be perfect, but attempt to show best pratices. The links may be direct offsite of this wiki or to an intermediate page in the wiki.

Links to Related Material

 * Spreadsheet Workbench - FreeCAD Documentation


 * Forum Search and Specific Pages: best pratices site:forum.freecadweb.org at DuckDuckGo []

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