How to change an unknown diameter of a hole to a specific size?

Question

I have a cube with several holes in it. Now I need to change the size of one hole to 6.34 mm diameter. What is the best way to do this?

Answer 1

The LoopTools add-on proposed by TLousky, is obviously massively convenient and exactly what you want.

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Given the above statement, the rest of this answer might be slightly academic, but can still be useful if you are interested in how something like this is scripted. My solution works on the assumption that:

• the hole is a circle, and its vertices are the only things selected.
• the object has all transforms applied
  • find the average coordinate (center of circle)
  • let it find the diameter by checking the distance between the average coordinate and any of the vertices.
  • use the average vector as a pivot point to scale away from.

code:

# This example assumes we have a mesh object in edit-mode

import bpy
import bmesh
from mathutils import Vector

def get_average_vector_of_selection(bm):
    avg_vector = Vector()
    num_vecs = 0
    for v in bm.verts:
        if v.select:
            num_vecs += 1
            avg_vector += v.co
    return (avg_vector / num_vecs)

def get_radius(bm, average):
    for v in bm.verts:
        if v.select:
            return (average-v.co).length

def scale_circle_verts(bm, new_radius):
    average = get_average_vector_of_selection(bm)
    # bpy.context.scene.cursor_location = average
    current_radius = get_radius(bm, average)
    print(current_radius)

    if new_radius == current_radius:
        return

    amount_to_rescale = current_radius / new_radius
    for v in bm.verts:
        if v.select:
            new_vec = (average).lerp(v.co, 1/amount_to_rescale)
            v.co = new_vec


obj = bpy.context.edit_object
me = obj.data
bm = bmesh.from_edit_mesh(me)

scale_circle_verts(bm, new_radius=0.2)

bmesh.update_edit_mesh(me, True)

Answer 2

Another way to achieve this is using the LoopTools addon's 'Circle tool'.

If the addon is not active on your system, go to the User Settings (CtrlAltU) > Addons > Mesh > LoopTools and activate it using the small checkbox on the right:

In Edit Mode select the loop or loops you want to resize, then either from the Tool Shelf > LoopTools or the Specials Menu (W > LoopTools) choose the 'Circle tool':

Toggle the "Radius" option on, then type in the radius you want (diameter / 2) for the hole:

Answer 3

1. Make a cylinder at the right place and size.

2. Create a vertex group and add the vertices around the hole to it.

3. Add a shrinkwrap modifier (to the cube), select the cylinder as the object and limit it to the vertex group.

You may have to move the vertices from the cube into roughly the right position. As you see, they follow the cylinder but as they are not evenly spaced, the resulting hole is not very regular.