Python Solution to Dimension Tracking

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This is about half written, if interested email User:Russ_hensel And since written it has been substantially expanded and revised. I will return to this documentation soon, I hope.


Contents

The Problem

To make FreeCad work you often need to enter values of various dimensions. There does not seem to be a method internal to FreeCad ( and probably there should not be ). These dimensions come from various sources:

  • Spec. sheets.
  • Measurements
  • Calculations
  • Design decisions.

You more or less need some notes to keep track of this stuff. Some of this may be paper sketches, paper notes, and digital documents ( many may choose spreadsheets ). I find spreadsheets to be very error prone so I looked for a python solution. The very simple version of this is shown in: FreeCad Gear Box This has now evolved, and this document will describe the solution. For the actual code contact: User:Russ_hensel

A Python Solution

My Python solution consists of a set of interacting classes:

PartDimension

This class holds a single dimension. Of course you can just go: a_dimention = 22. What is wrong with that? This dimension has a name and a value. What it does not have is units or a type that may help distinguish between pure numbers ( teethe on a gear ), angles, and linear dimensions. Also I have upgraded the class so that it does easy unit conversions and is easy to print.

Here is what you might get when you print a PartDimension:


print a_dimension

output:

          bolt_sep            ( mm ) = 25.0

And here is what you get with a fairly fancy getter method:


In:
    a_dimention   = PartDimension( "part name two", dim_type = "linear", radial = False, value = 27.3, units = "in")
    print( a_dimention )

    print( a_dimention.get_value( units = "ft" ) )

Out:
          part name two       ( mm ) = 693.42
2.275

A couple of implementation details.

  • Dimensions are always stored in some base units, linear in mm, angles in radians, the getter methods convert to the desired units. There are a couple of helper classes for the conversion:
    • LinearConverter
    • AngleConverter

PartInfo

Of course a part does not have a dimension it has many dimensions. So PartInfo is a collection of Part Dimensions all of which can be manipulated as a unit. There are methods to:

  • Support naming of PartInfos ( say "My Gear # 3 " ).
  • Add a dimension ( or a set of dimensions ).
  • Access dimensions or there values one by one or as a unit.
  • Print out all the part dimensions in any particular set of units.

PartStack

This is a collection of PartInfo items, the idea is that this represents several parts that are stacked up to be 3D printed as a unit. Perhaps more detail later.


Use

I am still working this out but here is an approach:

  • In an empty file import the code above ( now cad.py )
  • Write a function to generate a PartInfo object. I call my first one something like my_standards_factory:
# ---------------------------------------------
def standards_42_factory():
    """
    this holds standards
    """
    a_part                  = cad.PartInfo( "Cad Standards #42 "  )
    a_part.freecad_name     = "standards_42"


    a_part.create_part_dimension(  "washer_thick",       dim_type = "linear", radial = False, value = .1   )
    a_part.create_part_dimension(  "gear_clear",         dim_type = "linear", radial = False, value = .2    )
    a_part.create_part_dimension(  "gear_thick",         dim_type = "linear", radial = True , value = 4.5      )
    a_part.create_part_dimension(  "plate_thick",        dim_type = "linear", radial = False, value = 2.0      )
    ....... more
  • and use it:
    my_standards    = standards_42_factory()
    my_standards.print_part()

The print is just to verify the factory.

Then ....... more coming .....

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