This page looks at one way of making a port tube to whatever diameter you need. Select some PVC pipe that is the next size up from what is required
If we cut a piece lengthwise from the tube, we can then squeeze whats left to form a smaller tube. All that remains is how to get it to stay that size. A second tube with slightly less taken out of it could be placed around our first tube and the whole arrangement glued with a suitable glue to give us a composite tube. It would look like this in cross-section....
Calculating how much to cut out
Let the following abbreviations apply
| id_start | Inside diameter of a new unmodified tube |
| id_required | Inside diameter of composite tube we end up with |
| wall_thick | Wall thickness of a single new unmodified tube |
| remove_inner | Width of section removed from inner tube |
| remove_outer | Width of section removed from outer tube |
Consider just the inner tube, and for the moment, ignore the wall thickness
This theoretical tube will have a diameter equal to the inside diameter of our starting tube
The amount to be removed is the starting circumference minus the circumference after cutting and joining
change in circumference = pi * id_start - pi * id_required
change in circumference = pi * (id_start - id_required )
Because we will be marking our cut on the outside of each tube, we will need to to know the width of the strip at that position. This means we now need to consider the effect of wall thickness. This drawing shows the wedge that is to be removed.
| r | radius ( which is circumference / 2 ) |
| t | wall thickness |
| a | reduction in inside circumference ( just calculated above ) |
| b | reduction in outside circumference ( what we need to know ) |
The theory of similar triangles tells us that ....
a / r = b / ( r + t )
re-arranging terms
b = a * ( r + t ) / r
substituting for known terms
remove_inner = pi * (id_start - id_required ) * ( ( id_start / 2 ) + wall_thick ) / ( id_start / 2 )
The second tube, which will become the outer layer, has the same id_start, but id_required will be larger by twice the wall thickness.
remove_outer = pi * (id_start - ( id_required + ( wall_thick * 2 ) ) ) * ( ( id_start / 2 ) + wall_thick ) / ( id_start / 2 )
Alternatively, you can make a pipe larger by inserting a strip. Use the same equations and you will get a negative result for the "remove" figure - this is the width of the strip to be inserted. In this situation you will lose the width of the saw cut from your initial tube. This needs to be added to the width of the strip to be inserted.
I've added an extra page to the downloadable calculator to include these calculations for you
Note that in practice, variations in the fit of your DIY tubes will mean that the width of a strip may vary slightly from these figures.
Some Quick Tips
-
When altering the size of pipe, use wooden disks cut to the required diameter. Some threaded rod will hold them in place and a simple stand will make the heating and gluing much easier
-
Wrap the pipe around the disks, using wire to hold it in place, and then work over the pipe with a hot air gun. The PVC will soften, relieving any stresses, giving a perfectly round cross section and a tube that is happy to stay at the new size.
Work outside and keep a bucket of water handy in case of fire. If one happens, DONT PANIC. Make sure you turn off the power to the heat gun before you throw water everywhere.
- When
gluing the layers, air will be excluded, so don't use a glue that relies
on evaporation to set. Use one that sets using chemical reaction such
as epoxy resin
- Have
plenty of those disposable latex gloves handy. Everytime you go to pick
something up, you will want to change the gloves.
- When
it comes to removing the finished tube from the jig, some holes drilled
in the disks before assembly will make the job much easier. A file with
a wooden handle can be used through the hole to tap on the next disk,
persuading it to move. If this doesn't work, it may be neccessary to
destroy the wooden disks. The holes make it easier to break it up.
- It is easier to make a pipe smaller than it is to make it larger
The next page has some example concentric vents sizes
