FreeCad Gear Box

= What = This is a little record of my work to make and print a gearbox using FreeCad. Not everything here may be right, I am a beginner, but in some ways that is a plus, as I approach this with the ignorance of a beginner, and will answer some beginner questions that others may consider obvious. FreeCad Issues/Solutions

This is an article started by Russ Hensel, see "http://www.opencircuits.com/index.php?title=Russ_hensel#About My Articles" About My Articles for a bit of info.

= Issues =

FreeCad has a bunch of workbenches and types of objects that it can create. But in all cases you seem to know a bunch of values which are not directly generated by the FreeCad itself. This gives two poblems:


 * What are the values you need?
 * What workbench should you use.

Additionally:


 * You need to make printable parts, large over hangs are not printable. Objects like pyramids are easy, inverted pyramids are hard.  So make sure stacked up parts are broken up in the right way to be printable.  Some may be printed upside down, then turned over in the real world.

= A Python Helper Program =

In my approach to the problem each part needs to be positioned ( so far in the z axis ) so that all the parts come together correctly. I started to do this by making a sketch of a side view of the gearbox ( will add here later ). Then I began a spreadsheet of the z positions. I do not really like spreadsheets because I find them very error prone and compared to programming a bit tedious. So I wrote a Python program. Initially I just set up a large number of variables and did the math. This was using Python as pretty much a general purpose calculator. Then I began to see a pattern in the calculations.

A Pattern
The pattern goes like this:


 * Each part has a bunch of values that effect z positions
 * Bottom of the part.
 * Thickness of the part.
 * Midpoint of the part.
 * Top of the part.
 * Clearly there are some simple relations between these values.
 * Parts are often stacked up. In the gear box:
 * Bottom plate
 * Washer
 * Spacer
 * Lower Gear
 * Upper Gear
 * Spacer
 * Washer
 * Upper Plate
 * Some calculations:
 * A part's bottom z position + part's thichness = part's top z position
 * One part's top z position becomes the next part's bottom z position.

Pattern into Python
Two classes have been found useful so far.


 * Part3D This is a set of z values, top, bottom...., a name for the part, some simple calculations, and a print method to show the values.  Easy way to set values and see them.
 * PartStack This is an list of parts ( Part3D ) to be stacked up in the z direction. It has a name, an order of parts, simple relations between the parts, and a nice way of printing the values.
 * Nothing very complicated, but have proved to be handy and not error prone.
 * Email me for the code, or given the idea it is pretty easy to write yourself, perhaps better. Later I will find a good upload location.

Input/Code
import gear

print"""Set some standards """

washer_thick  = .1 gear_clear    = .5 gear_thick    = 5. plate_thick   = 1.

a_stack   =  gear.PartStack( "two gears" )

l_plate = gear.Part3D( "l_plate", plate_thick ) a_stack.add_up( l_plate ) a_stack.comp_z_top_mid

l_washer = gear.Part3D( "l_washer", washer_thick ) a_stack.add_up( l_washer ) a_stack.comp_z_top_mid

l_spacer = gear.Part3D( "l_spacer", 5.0 ) a_stack.add_up( l_spacer ) a_stack.comp_z_top_mid

l_gear = gear.Part3D( "l_gear", gear_thick ) a_stack.add_up( l_gear ) l_gear.comp_z_top_mid

u_gear = gear.Part3D( "u_gear", gear_thick + gear_clear ) a_stack.add_up( u_gear ) u_gear.comp_z_top_mid

u_spacer = gear.Part3D( "u_spacer", 5. ) a_stack.add_up( u_spacer ) u_spacer.comp_z_top_mid

u_washer = gear.Part3D( "l_washer", washer_thick ) a_stack.add_up( u_washer ) a_stack.comp_z_top_mid

u_plate = gear.Part3D( "u_plate", 5. ) a_stack.add_up( u_plate ) u_plate.comp_z_top_mid

a_stack.print_stack

Output
Set some standards

Stack: two gears l_plate bot = 0.0 thick= 1.0 mid = 0.5 top = 1.0 l_washer bot = 1.0 thick= 0.1 mid = 1.05 top = 1.1 l_spacer bot = 1.1 thick= 5.0 mid = 3.6 top = 6.1 l_gear bot = 6.1 thick= 5.0 mid = 8.6 top = 11.1 u_gear bot = 11.1 thick= 5.5 mid = 13.85 top = 16.6 u_spacer bot = 16.6 thick= 5.0 mid = 19.1 top = 21.6 l_washer bot = 21.6 thick= 0.1 mid = 21.65 top = 21.7 u_plate bot = 21.7 thick= 5.0 mid = 24.2 top = 26.7

WorkBenches

 * Most part basic seem to be easily made in the Part workbench.
 * I installed the gear workbench to make the gears.
 * I am planning to do parts step by step each in its own file and then combine into more and more complete parts.

Generic Parts
I first I made what I called generic parts. The plan is to make a part, copy it later to another document and customize it. Then the custom parts are put together to make a printable part. My generic parts are:
 * Generic Shaft: just a cylinder, I am not planning on printing my shafts, but rather using brass shafts, so this part will be used as a "drill" through other parts.
 * Generic Washer: two cylinders, one used as a drill through the other. Dimensioned to look like a washer.
 * Generic Spacer: two cylinders, one used as a drill through the other. Dimensioned to look like a washer.
 * Generic Gear: just a gear from the Gear workbench.

Individual Parts
Next I made individual parts, one document ( file ) for each part, these are copied out of the generic parts then tweaked:


 * Lower plate
 * Lower washer ( will use metal, will not print )
 * Lower spacer
 * Lower gear
 * Upper gear
 * Upper spacer
 * Upper plate
 * Gear shaft

Printable Parts
Some all of the parts above are printable but it makes sense to combine a few:


 * Gear plus: Lower gear, Upper gear, Upper spacer, shaft as hole through all.

For Cura
Downloaded and installed Cura. Now it looks like I need to export a file from my freecad format file, which to pick?

Look at *Manual:Preparing models for 3D printing - FreeCAD Documentation