Tolerances for 3D printing

[felipe-m]

Now tolerances for parts to be 3D printed are in kcomp.py and kparts.py
There are various tolerances, such as:

• TOL: the general tolerance
• STOL: a smaller tolerance
• TOL_BEARING_L: larger tolerance for linear bearings, since they don't always fit
• MTOL/MLTOL: which are also smaller tolerances, but MTOL is now the same as TOL

Tolerances depend on the 3D printer used, and also if it is about a lenght of a radius/diameter
Usually 0.4 mm is recommended, but it varies.

It would be great to have these tolerances more tunned, and also to have an option to increase or reduce them in case you are using a different 3D printer. And also to allow the user to set a specific value.

As an example, we have just printed the Linear Bearing Housing kcomp.SCE20UU_Pr30b, with this tolerance
MLTOL = kparts.MTOL/2. -0.1
Being MTOL=0.4, then
MLTOL = 0.1
Making the diameter to fit the bearing:
bearing_d_tol = bearing_d + 2.0 * self.MLTOL

So the diameter of the housing is 0.2 mm larger that the bearing diameter:

But with the other printer, it seems that it was ok with the original tolerance, which was:
MLTOL = kparts.MTOL
bearing_d_tol = bearing_d + 2.0 * self.MLTOL

Which would be 0.8mm larger
In previous designs, we have used 0.7 as tolerance for the diameter

Anyhow, I think that it was too much, and maybe the formula would be:
bearing_d_tol = bearing_d + self.MLTOL

And adjust from that