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Back to Tutorials |
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Designing a casting. Part 1 Positioning the components |
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By
now you should be fairly familiar with Blenders modelling capabilities.
The 608 Bearing and V-roller tutorials have covered most of the
commonly used tools and techniques, so this won’t be so much
of a
tutorial as a discussion on the steps needed to use Blender as a tool
to help you design a casting. As
the next stage of my CNC router upgrade is to redesign the X-axis
slides I will use this as the motive for designing the casting. We will again reuse the
bearing and
V-roller as the building blocks for the casting layout.
There are many casting techniques that could be used for this component
such as sand casting, die-casting and centrifugal casting, but because
of my limited workshop facilities and the availability of a pottery
kiln, I have chosen to use the investment casting technique. This is
where a wax model of the part is fabricated then dipped into a
refractory slurry which is dried, the wax is melted out leaving the
refractory casing which when fired becomes the mould for the molten
aluminium. The mould is then broken away from the aluminium when it has
cooled.
The benefit of this method is you can produce an accurate wax model
without worrying too much about withdrawal tapers or undercuts, If you
can make the component in wax you should be able to produce an
investment mould from it. I will go into a little more detail about the
mould making and casting process in the foundry section. Here I will
concentrate on using Blenders modelling capabilities to design the
casting. |
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If you haven’t already got blender running this is the time
to start it up? Delete the default cube X and then append
into the scene Shift F1
the V-Roller, bearings, shaft, nut and washer assembly created in the
V-Roller tutorial. They will be placed into the scene rotated so the
roller will run along the X-axis. Go into side view NumPad 3
as we need to add a shaft for the V-Roller to run along. To make sure
the shaft is centred to the V-Roller, select the V-roller and in the
Edit Buttons click “Centre New” this will ensure
the object
centre is on the centre of the V-Roller. We can now snap the cursor to
the V-Rollers object centre Shift-S
Cursor>Selection. |
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Add a cylinder to represent the shaft SpaceBar
Add>Mesh>cylinder and in the popup menu set Vertices: 32,
Radius:
9, Depth: 100. In its current position the shaft penetrates through the
V-Roller so we need to move it down so that the shaft sits below the
‘V’. |
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Tab into Object Mode then Grab the shaft and
move it on the Z-axis
until it sits just below the V-Roller. Zoom in fully and repeat this if
necessary to get the closest possible alignment.
You
can also zoom into a selection box by pressing Shift-B and
dragging a selection box over the zoom area you require.
In front view NumPad 1,
Tab
into Edit mode and Box
select the right-hand vertices. Move these vertices G along the X-axis 300mm. This will
make the shaft 400mm long. Tab
back into Object Mode. |
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| It's possible to have the
V-Roller rigidly aligned to the X-axis using
just 3 rollers, as I am going to reuse this set-up on other areas of
the machine I am going to use 4 rollers. In front view NumPad 1, Box
select the V-Roller assembly and copy it Shift D. Move it
along the X-axis
150mm.
Now Box
select both V-Roller assemblies and copy them Shift-D, moving the
copies on the Z-axis
until the ‘V’ is below the
shaft. In side view NumPad
3 Zoom in close to position them as
accurately as possible |
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| Its
not feasible in a home workshop to make components to micron accuracy
so some form of adjustment will be necessary to position the V-Rollers
tight to the shaft. I am going to fix the top two rollers into the main
casting, but allow the bottom rollers to be adjusted on the Z-axis. For
this I will use a simple rebated block with elongated bolt holes. This
can simply be located within a rebate in the main casting. |
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| The
Adjuster Block | |
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| To make the adjustable
block, RMB select the bottom right hand V-Roller axle and go into Local
View NumPad
/.
Select Shift-Alt-RMB
the loop of vertices at the back of the fillet and snap the cursor to
the centre of them, Shift-S
Cursor>Selection. This gives us the location point for the
adjuster.
Go into Object Mode and in front view Insert a mesh circle, SpaceBar
Add>Mesh>Circle, in the popup menu set Vertices:
32. and Radius: 3. |
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| Whilst still in Edit Mode
add a Plane and Scale
it by a factor of 22
to give a 44mm square. Delete the face from the plane X “Only
Faces” then subdivide the edges 7 times. W
“Subdivide Multi” and set 7
in the popup menu. This will give our outer square the same number of
vertices as the inner circle. Add faces between the inner circle and
outer square by pressing F
and selecting "Skin Faces/Edge Loops". Select all and Extrude "Region"
along the Y-axis
10mm,
to add thickness to the component. |
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In
order for the adjuster block to be able to be positioned in
the casting it needs some side flanges that will also incorporate the
elongated bolt holes. In side view NumPad
3 cut a loop of vertices K "Midpoint". Grab the new loop of
vertices and move them on the Y-axis
-2mm,
this will give us a 3mm face to form the flange from.
Select the flange face's, one side at a time and extrude them 10mm.
The top and bottom flange will shield the components of the slide from
dust produced by the router. |
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Delete the two corner
faces indicated, X
"Faces" on each of the corners. Make sure the edge between the
two faces is also deleted.
Select the vertex indicated and snap the cursor to it. Shift-S
Cursor>Selection. Then select the four vertices from one of the
deleted faces.
In front view spin a corner radius with five vertices. |
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When all the four corners
have been spun Select All and remove doubles W. |
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We
now need the centre line of the flange to position the elongated bolt
holes. It can't be loop cut because the corner faces are triangles.
Use the Knife tool (Midpoints) and cut the loop indicated. |
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To
add a chamfer to the edge of the adjuster loop cut Ctrl-R the outer
loop of faces. With the Edge Length button activated, slide these to
0.5mm from the edge. Do this on the front and back of the flange.
Now select the front outer edge loop and move it 0.5mm on the Y-axis.
Do the same on the back outer edge but move this -0.5mm. |
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Snap the cursor to the
middle of the two vertices on the left-hand picture.
In front view add a circle, Vertices: 16, Radius: 2.2 (clearance for a
4mm bolt). |
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Copy
the vertices from the circles centreline and above and move the copy
4mm on the Z-axis. Then copy the vertices of the circle from the
centreline and below moving them -4mm on the Z-axis.
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Delete the original circle and the flange vertices closest to it.
This leaves you with the outline of the slot positioned centrally on
the flange. |
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Alt-RMB select the
vertices of the elongated hole, then Shift-Alt-RMB select
the vertices of the flange that surround it. Shift-F to fill the
area with faces. Alt-F
to beautify the fill and Alt-J
to convert to quads.
Copy the new faces and move them 3mm on the Y-axis.
Shift-Alt-RMB
select the two elongated slots and loft faces between them F "Skin Face/Edge
Loops".
Select all and remove doubles. |
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Select the vertices central to where the other three slots are required and delete them. Copy the slotfaces and surrounding
faces. and use Blenders snap tools to snap the copy to the other locations. (Snap described here) |
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First Rotating the two left-hand copies 180 degrees.
Finally select all and remove doubles to complete the adjuster. |
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Once the flange is complete go back into Global view NumPad / and copy
the adjuster to the other V-Roller
Shift-D
X -150 |
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| The Lead Screw & Roller-nut | |
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| WE
have now got the locations for the fixed rollers and the adjuster
blocks. A few other components are needed on the casting because it
will also hold the lead screw roller-nut. | |
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I am using an M16
threaded rod as the lead screw and have designed a
simple roller-nut based on the V-Roller and axle. It’s simply
a
series of slots at 2mm pitch to marry in to the M16 thread.
To stop the
lead screw from flexing away from the nut a couple of bearings will be
located above the lead screw and offset slightly to either side of
the
centreline. These bearings will use a shortened version of the V-Roller
axle. |
| The roller-nut and
bearings will also need to be adjustable, so the
brackets that hold them back to the main casting will also need to
allow some movement in the Z-axis. |
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My CNC machine is
configured with two X-axis slides, one either side of the table. The
Y-axis runs between them forming a bridge type layout. The X-axis
therefore also has to locate the mounting points for the Y-axis. If you
have a look at the photos of my old machine on the CNC page this will make more sense.
At
this stage of the design, things are very flexible, none of the
component positions are fixed and it's easy to move things around to
optimise their location.
We
have now accounted for all the parts of the X-axis, so in part-2, I will start constructing the main casting around them. |
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