275 lines
13 KiB
HTML
275 lines
13 KiB
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<h1 align="center">Bellows</h1>
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<p>The linear slides I got off ebay didn't come with covers to stop swarf getting to the rails and ballscrews. So I decided to cover them with a bellows.</p>
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<p><img border="0" src="bellows.jpg" width="476" height="377"></p>
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<p>I did a little bit of research. There are some stock sized
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commercial bellows available; these are usually rubberised fabric, welded at the seams.
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I couldn't find any that fit, nor could I afford them. So
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I googled for DIY bellows folding. Again, I drew a blank.
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Then I checked the USPTO, and I hit a gold.</p>
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<p>
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<a href="http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO1&Sect2=HITOFF&d=PALL&p=1&u=/netahtml/PTO/srchnum.htm&r=1&f=G&l=50&s1=6,054,194.PN.&OS=PN/6,054,194&RS=PN/6,054,194">Patent number 6,054,194</a>,
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invented by Nathan R. Kane, is not just a description of
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bellows, but it is a complete thesis on optimal fold patterns
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for bellows. The optimal patterns will maximise the
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bellows extension length, while minimising the side wall
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distortion. By using just the fold pattern, a bellows can
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be created that holds its shape without external support.</p>
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<p>Polypropylene sheet was selected to make the bellows.
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Polypropylene is a good "hinge" material, meaning it can be fold
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back and forward repeatedly with fatigue. I tried a couple
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different thicknesses, 1.2mm, 1mm, .7mm, and finally settled on
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0.39mm - not that this is the best size, its just the thinnest
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stuff I could find in large sheets. I think thinner sheet
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would be better, but I couldn't find any. I found it at my
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local craft store.</p>
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<table id=polypropyleneGrain margin=5 bgcolor="#FFAAAA" border="2" bordercolor="#FF0000" cellspacing="6" cellpadding="6" bordercolorlight="#FF0000" bordercolordark="#FF0000" style="border-collapse: collapse">
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<tr>
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<td>
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<p>Update!</p>
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<p>Just a note on polypropylene. Polypropylene has grain!
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I'm sure my terminology is incorrect, but polypropylene, like
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paper or timber, has a preferred strong direction and a weak
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direction. If you try to tear polypropylene sheet in its
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weak direction, it will continue to tear in the direction it is
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forced. In the other direction, 90° to the weak
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direction, the polypropylene will not tear, in fact it will turn
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90° to the weak direction. This is because of the
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polymer chains (so I've read). </p>
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<p>Why is this a problem? If you try to fold the
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polypropylene with the folds running parallel to the weak
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direction, the polypropylene will crack. It is important
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to get the direction correct so the folds are perpendicular to
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the weak direction.</p>
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<p>So how do you find the weak direction? On the corner of
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a sheet of polypropylene, make a small cut, 5mm long and about
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5mm from the edge. Grab this tab and pull it. The
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pictures below show the results.</p>
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<p>
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<table>
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<tr>
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<td><img border="0" src="TearAgainst.jpg" width="250" height="356"></td>
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<td><img border="0" src="TearWith.jpg" width="250" height="356"></td>
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</tr>
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<tr>
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<td align=center>Against the grain</td>
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<td align=center>With the grain</td>
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</tr>
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</table>
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<p>
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The photo on the left shows trying to tear against the grain -
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you can't do it. The photo on the right shows tearing with
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the grain. The polypropylene tears easily. It is
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imporant that the bellows fold lines are placed perpendicular to
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this weak direction.</p>
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</td></tr></table>
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<p> </p>
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<h2>Building the Bellows</h2>
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<h3>1) Print the fold pattern</h3>
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<p>The first step is to produce a fold pattern for the bellows.
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For my first bellows, I used MS-Visio to lay out the lines.
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This was a bit tedious, so I wrote a simple program.</p>
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<p>
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<img border="0" src="Application.png" width="669" height="559"></p>
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<p>The program will only create a fold pattern for a simple bellows with either
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regular or alternating folds. The "generate g-code" option
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is not functional. I want to be able to generate g-code to
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guide a knife to score the bellows material, however all the
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bellows I need to create are larger than my mill, so this
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feature was postponed.</p>
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<p>The parameters are...</p>
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<table id="table1">
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<tr>
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<td width=150px>Config</td>
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<td>Different configurations can be selected.
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Configurations are automatically saved when the program
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exits, or a new configuration is selected.</td>
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</tr>
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<tr>
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<td>New</td>
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<td>Creates a new configuration. </td>
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</tr>
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<tr>
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<td>Shape</td>
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<td>Only a "Half Cover" is supported (2 sides and a top)</td>
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</tr>
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<tr>
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<td>Inversion</td>
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<td>Only 2 inversion are supported</td>
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</tr>
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<tr>
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<td>Mounting Folds</td>
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<td>The total number of extra folds to add for mounting
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the ends of the bellows</td>
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</tr>
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<tr>
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<td>Alternate Folds</td>
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<td>If true, adjacent folds will alternate up and down,
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rather than being in the same direction. This
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makes the compressed bellows smaller, but the top of the
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bellows has folds that may collect swarf. </td>
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</tr>
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<tr>
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<td>Inside Width</td>
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<td>The inside width that the bellows is going to cover.</td>
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</tr>
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<tr>
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<td>Inside Heigth</td>
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<td>The inside height that the bellows is going to
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cover.</td>
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</tr>
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<tr>
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<td>Protected Length</td>
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<td>The length of the area the bellows is going to
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cover.</td>
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</tr>
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<tr>
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<td>Fold Width</td>
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<td>The width of one fold.</td>
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</tr>
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</table>
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<p>The program will print out the fold pattern. It will
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tile the output over multiple pages. The diagonal line is
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used for lining up the pages when they are stuck together.</p>
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<p>The image below shows a snippet of the fold pattern.
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Note the solid and dotted lines. A solid line is a peak
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fold, and a dotted line is a valley fold (or vice-versa, it
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doesn't matter).</p>
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<p><img border="0" src="Pattern.png" width="451" height="447"></p>
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<p>The picture shows the fold pattern with a sheet of
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clear/translucent polypropylene on top.</p>
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<p><img border="0" src="Layout.jpg" width="600" height="435"></p>
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<p>The program and source can be downloaded here
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<a href="Bellows.zip">
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<img style="background-color=white" border="0" src="../../gplv3-88x31.png" width="88" height="31"></a>.
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The code isn't terribly stable. It was only used to
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generate a couple of patterns. The displayed pattern will
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only be updated when you tab between fields. There is no
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zoom - make the window bigger to see more of the pattern.</p>
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<h3>2) Trim</h3>
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<p>The polypropylene sheet is cut to size. It is then
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stuck to the fold pattern using tape.</p>
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<h3>3) Score the lines</h3>
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<p>The next step is to encourage the polypropylene sheet to fold
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at the correct place. If these were being produced
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commercially, they could be vacuum formed, or pressed.
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Since this is overkill for a couple of one-offs, I tried a
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couple of manual techniques.</p>
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<p>First I tried scoring with a hobby knife. This was
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fine, but in a few spots the cuts went a bit deep and made the
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fold weak. Next, I tried using my soldering iron to melt a
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grove. This wasn't terribly successful as shown by the
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video below. I did learn though that the scoring and
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folding needs to be very accurate, or the bellows will not fold
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properly. Finally, I settled on a scriber - a pen with a
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sharp carbide tip used for scratching metal, to scratch the
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polypropylene.</p>
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<p>Use a metal ruler and carefully trace over the peak folds.
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Take care and do this accurately or the bellows will not fold
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properly. Don't score the diagonal guide line running the
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length of page.</p>
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<p>The picture below shows the scored peak folds. The
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lighting in the picture makes them appear black. </p>
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<p><img border="0" src="ScoredTop.jpg" width="500" height="362"></p>
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<p>When scoring fold corners (line intersections), make sure you
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overshoot the intersection by 2 or 3mm. This will help
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with the folding later.</p>
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<p>
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<img border="0" src="ScoredOver.jpg" width="682" height="511"></p>
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<p>When the top is finished, remove the polypropylene sheet,
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flip it over and stick it back down on the fold pattern.
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You can use the scored peak folds to line up with the printed
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peak lines as the bellows patterns are symetrical. Then
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score the valley fold lines.</p>
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<p>The photo below shows a scored polypropylene sheet.</p>
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<p><img border="0" src="Score.jpg" width="682" height="511"></p>
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<h3>4) Crease the Folds</h3>
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<p>Next, each of the scored lines needs to be creased.
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Here, we are not trying to fold the bellows, just get the folds
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going in the right direction. Just bend each fold and
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squeeze them with your fingers, like in the photo below.</p>
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<p><img border="0" src="Crease.jpg" width="500" height="303"></p>
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<p>It is important to get all the small angle folds.
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The folds must also be done in the right direction. If a
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scored line is on the top of the sheet, the two sides of the
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fold should fold down, like this...</p>
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<p>
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<img border="0" src="FoldDirection.png" width="497" height="312"></p>
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<p>The creased bellows will look like this...</p>
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<p><img border="0" src="Creased.jpg" width="568" height="407"></p>
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<h3>5) Folding</h3>
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<p>The final folding is the tricky bit. Although the
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initial creasing does help the bellows want to go in the right
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direction, it still takes a lot of finger muscle and patience to
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fold the bellows. It is important that the corners are
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sharp, otherwise they will be a source of ballooning.</p>
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<p>I found it easiest to fold the bellows if I worked on one
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side, folded 3 or 4 folds, clamp it, then do the other side.</p>
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<p>
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<img border="0" src="FingerFold.jpg" width="568" height="426"></p>
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<p>The quick release clamps worked well holding a few folds at a
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time.</p>
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<p><img border="0" src="AndClamp.jpg" width="568" height="358"></p>
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<p>Finally, all folded.</p>
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<p>
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<img border="0" src="DoubleClamp.jpg" width="594" height="248"></p>
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<p>The bellows is clamped between two chunks of timber to
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encourage it to stay in place.</p>
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<p><img border="0" src="BigClamp.jpg" width="409" height="408"></p>
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<p>The completed (blurry) bellows.</p>
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<p><img border="0" src="Folded.jpg" width="392" height="352"></p>
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<h3>6) Mounting</h3>
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<p>Obviously this will depend on what you plan to use the
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bellows for. I used some 3mm steel plate to mount on each
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end of the bellows. The bellows are held to the plates by
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M4 screws and small lengths of 3x10mm bar, tapped for the M4
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screws. These were then mounted to my X axis slide.</p>
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<p>
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<img border="0" src="MountingParts.jpg" width="417" height="362"><img border="0" src="Mounted.jpg" width="414" height="343"></p>
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<p>3mm steel plate probably wasn't the best choice for mounting
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the bellows, as I have already hit tall hold down bolts with it.
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In the future, I may tray thick plastic, and self adhesive
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velcro strips.</p>
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<h2>When good bellows go bad!</h2>
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<p>The video below shows what happens when a bellows isn't
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folded correctly. The bellows on the right was bubbling
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out when it was closing. This snagged when moving in the Y
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direction, leaving what you see now...</p>
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<object width="425" height="350"> <param name="movie" value="http://www.youtube.com/v/1O0ln1kLFCI"> </param> <embed src="http://www.youtube.com/v/1O0ln1kLFCI" type="application/x-shockwave-flash" width="425" height="350"> </embed> </object>
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