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Part 3 Detailed Thread Profile
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Before
we
can model the thread we need to understand the thread form.
ISO
Metric threads come with a Standard profile and as the V-Roller axle
will use an M6 thread, I will describe the set-up of this size.
Layout
of a Metric Thread
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| Key: |
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| D |
Major Diameter |
| P |
Pitch |
| H |
Thread Height |
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The
ISO Metric thread profile is a fairly simple shape to construct in
Blender and with the screw tool only one instance of the thread profile
is required. As a model engineer I am not going to discuss thread
tolerances or fits in any detail. You should however be aware that the
finished male thread profile (bolt) should be kept below the bold blue
line of the diagram above and the finished female thread profile should
be kept above the bold blue line, otherwise the two won't fit together.
If you need them tolerance classes are defined in ISO 965-1.
For
this
exercise we will make the thread to the nominal profile above. To
construct the thread only two pieces of information are required the
Major Diameter and the Pitch. A few of the more common sizes are
detailed in the table below. |
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| Major Diameter |
3 |
4 |
5 |
6 |
8 |
10 |
12 |
16 |
| Pitch |
0.5 |
0.7 |
0.8 |
1 |
1.25 |
1.5 |
1.75 |
2 |
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Tab
into Object Mode
and select the profile. Tab
back into Edit Mode.
We now need to work out the Thread height. It could be done with
trigonometry given that the pitch is 1mm and the thread angle is 60
degrees. But I find it easier to do in Blender.
De-select
All
vertices then to
the left of the axle Ctrl-LMB
click to
add a new vertex.
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Extrude
this vertex on the X-axis
1mm.
The thread form is an equilateral triangle so the sides of the thread
triangle will be the same length as the pitch. Link
select both vertices of the edge and copy them Shift-D on the Z-axis 1mm (the thread
pitch).
Select
the top left
vertex of the two edges and snap the cursor to it Shift-s
Cursor>selection. |
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Then select the vertex on the other end of the
edge and with the Pivot set to 3D Cursor Rotate it 30 degrees.
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Snap the cursor to the
bottom left vertex and Rotate this -30 degrees.
Select All
and remove doubles W.
Select the centre vertex and Extrude
it on the X-axis
then scale it to the cursor Scale
X
0. |
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the Mesh Tools 1 panel press Edge Length and select the centre edge.
The Edge Length shown is the Thread Height. |
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Delete X the vertex on the
left-hand side of the central edge. Select the three vertices of the
thread profile and move G
them towards the undercut at the bottom of the thread.
If you haven't already got
one
open split the 3D view and open the Outliner.
Click the eye icon for
the axle to remove it from view.
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| Select the vertex on the
major diameter of the thread and Extrude
it on the X-axis
-0.108mm
(1/8 H) |
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We
can now use the snap tool to position the thread V profile.
Click
on the magnet Icon on the view header. Select the 3 vertices of the V
and move them G,
whilst moving hold down Ctrl
and move the cursor towards the end vertex. The V will snap into
position. |
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We now need to add
reference geometry to allow us to cut new vertices on the major and
minor diameters.
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Extrude
vertex A on the X-axis
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Extrude
vertex B on the Z-axis
as detailed, then Extrude
this X -0.216mm
(1/4 H)
then Extrude
again on the -Z-axis
ensuring the edge crosses the V.
To finish the reference edges, extrude a vertex from the major diameter
across the v. |
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Select the three vertices
of the V and cut Knife
(Exact), snapping the cut Ctrl-LMB
to the vertices of the major diameter.
Repeat the cut snapping the knife to the vertices of the minor diameter.
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vertices, but ensure the top chamfered edge of the
thread is not deleted, as this will be used to locate the cursor when
we
spin the thread. |
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Add Edges F to the vertices of
the major and minor diameter. Threads usually have a radius at the root
of the thread so we will add a fillet.
Select the vertex at the bottom of the
minor diameter and snap the cursor to it Shift-S
Cursor>Selection.
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Select both vertices of the minor diameter and copy them Shift-D, then Esc to leave them in
place. With the Pivot in 3D Cursor Rotate
the edge -30
degrees. |
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Select the vertex at the
top of the
minor diameter and snap the cursor to it Shift-S
Cursor>Selection.
Again Shift-RMB
Select both vertices of the minor diameter and copy them Shift-D, then Esc to leave them in
place. This time Rotate
the edge 30
degrees.
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is the centre point for the bottom fillet.
Cut a vertex on the intersect Knife
(Exact) and using Ctrl-LMB
to snap the cut to the rotated
edge. |
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Snap the cursor to the
intersect and Delete the reference vertices.
Select the vertex at the top of the major diameter and in the Mesh
Tools panel set Degr:120 and Steps:4 Press Spin and the
profile
for one pitch of the thread will be created.
Select All
and Remove Doubles W. |
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Select the vertex on the
thread centreline (right hand vertex on the top chamfer profile) and
snap the cursor to it.
De-select All
vertices, then Link
select the thread profile.
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We
need the thread to continue past the chamfer so we can later combine
the chamfer detail to the thread. Counting the grid squares we will
need 10
turns.
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set Degr: 360 Steps:32 and Turns:10 Make Sure Clockwise is not selected
Press
Screw and the thread will be formed. |
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Go
into top view NumPad 7.
De-select All
then Link
select the centre and chamfer vertices. Delete these X.
Select All and Remove Doubles W. |
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Tab
into Object Mode
and in the outliner open the eye icon for the axle,
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bringing
it back
into view. If
necessary move the thread in the Z-axis to align with the axle. Close
the eye for the thread.
We now need to prepare
the axle to accept the thread. |
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back into front view and select the axle. Tab into Edit Mode
and select the top right vertex of the chamfer and copy it Shift-D moving it
to the right of the axle on the X-axis.
Extrude
this vertex on the X-axis
to the left of the axle. Repeat this on the bottom vertex loop of the
thread section. |
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Go
into edge select mode and paint a selection B B across the
straight section of the thread.
Delete
these X (edges).
Go back into vertex select mode. |
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Tab
into Object Mode
and in the Outliner bring the thread
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back
into view. RMB select the
thread then Shift-RMB
select the axle. Press Ctrl-J
to join them into one mesh.
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into Edit Mode
and De-select All
then Link
select the thread. With
the Knife tool, cut two rows of vertices across
the thread, Knife
(Exact) and snapped Ctrl-LMB
to the reference edges. |
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De-select All
then Box
select the vertices below the bottom reference edge. Zoom in MW to make
sure all the vertices below the edge are selected (not the vertices on the edge)
Delete these vertices X. |
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Box select the vertices above the top reference edge and
delete these. Select the two reference edges and delete them.
The thread is now the correct length for the axle
but there is still more to do. We must join the base of the
thread to the under-cut and the top of the thread to the chamfer.
If you zoom in on the base of the thread you will see the
flange that was originally on the axle sticking out past the
edge of the thread (highlighted in green below). |
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To correct this we need
to manually merge the vertices of the original axle flange onto the
corresponding vertex of the thread.
RMB Select a vertex on the flange then Shift-RMB select the
corresponding vertex on the thread. |
Press Alt-M to merge the
vertices and choose "At Last" from the popup menu.
Repeat this around the circumference of the axle. |
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The
intersect between the
top chamfer and the thread is a much more complicated detail. Because
the intersect doesn't occur on a set plane there are no vertices on the
line of intersection. Therefore we cannot simply merge vertices to
obtain the correct profile. The first part of combining the thread and
chamfer will be to create a line of vertices on the intersect between
the two sets of faces.
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Go into face select mode
and box select the top section of thread and chamfer. Press Shift-H to hide all
the other vertices. |
We
can now use another of the python scripts to cut the vertices on the
intersect of the faces. Split the 3D view and open a Script Window
(detailed description in 608-4). With all the faces
still selected, in the Script window click on
Scripts>Mesh>Geom Tool and from the popup menu click on
"intersect: face(s) (cut)". A new loop of vertices will be created on
the intersect between the faces. Go back into vertex select mode,
select All
and Remove Doubles W.
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With the new edge created
on the intersect its now simply a matter of deleting the vertices,
edges or faces that are not required. It sounds simple but this will
test your patience and call on many of the editing techniques you have
learned.
Only delete X a few vertices
at a time, if necessary Ctrl-Z
will undo the change. In places you may need to merge
vertices Alt-M and other
places you will
need to add new faces F. Its
also worth noting on one side of the axle
the thread intersects the chamfer in two places.
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amount of time, but as your
experience grows this will only take a few minutes and the end result
is well worth the effort. |
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Once complete press Alt-H
to bring back all the hidden vertices. Select All, Remove Doubles W and Set Smooth W. |
That
completes the top thread. Rather than repeating all that work on the
bottom thread we can simply copy the top one and reposition the copy to
complete the axle.
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In front view Box select
the thread vertices, then loop select Shift-Alt-
RMB the inner
loop of vertices on the
undercut next to the main body of the axle.
Copy these Shift-D and move them to the side.
In front view Rotate the
copied vertices 180 degrees.
Separate
the copied thread from the axle P.
Tab
into Object Mode, Select the copied thread and then Tab back into Edit Mode.
Select the top row of vertices and snap the cursor to them Shift-S Cursor>Selection. |
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Tab into
Object mode and in the Mesh panel click the Centre Cursor button. This
will move the object centre to the centre of the top row of vertices. |
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Select the axle and delete the
vertices of the bottom thread.
Alt-RMB
select the inner loop of vertices where the under cut was.
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Snap the cursor to the centre of
these vertices.
Tab
back into Object Mode and select the thread. |
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Snap the thread to the cursor Shift-S Selection>
Cursor.
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With the thread still selected Shift-RMB select the axle and joine
the two objects into one Ctrl-J.
Tab back into Edit Mode, select All
and Remove Doubles W. to
complete the axle.
This axle will have 7274 vertices, compared to 1418 vertices of
the axle without the detailed thread.
You may also want to set up a camera and lights and render an image of
the shaft as detailed in 608-9.
Tab into
Object Mode and save your work. |
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In the next
section we will do a dimensioned drawing of the V-Roller components.
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