how do you cut a tube?
Printed From: the12volt.com
Forum Name: Car Audio
Forum Discription: Car Stereos, Amplifiers, Crossovers, Processors, Speakers, Subwoofers, etc.
URL: https://www.the12volt.com/installbay/forum_posts.asp?tid=108283
Printed Date: May 12, 2025 at 9:29 PM
Topic: how do you cut a tube?
Posted By: bigjohnny
Subject: how do you cut a tube?
Date Posted: October 20, 2008 at 8:45 PM
Long story short, I need to make a nice square cut on a 12.5" tube.
How do I make an accurate mark, and furthermore an accurate cut all the way around it?
the tube is a sonotube.... just hard cardboard.
Replies:
Posted By: i am an idiot
Date Posted: October 20, 2008 at 9:02 PM
Get 2 pieces of posterboard, usually 22 x 28 inches, Glue them together to make them 22 x 42 inches. Make sure they are square, the 42 inch piece should make a straight line. When the glue dries, wrap them around the tube matching the ends up, they will overlap by an inch or 2. With the edges lined up, using a marker of some sort, mark the tube. I have no help for you cutting it, the way I would do it is not safe and I do not reccomend it.
Posted By: 04nata
Date Posted: October 21, 2008 at 12:43 PM
Table saw and a miter gauge
Posted By: sneakycyber
Date Posted: October 21, 2008 at 2:40 PM
Ditto on the last post. That's how I do it. Although you usually have to remove the blade guard which is dangerous. I have a scar on my thumb that proves it.
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Posted By: i am an idiot
Date Posted: October 21, 2008 at 3:05 PM
sneakycyber wrote:
Ditto on the last post. That's how I do it. Although you usually have to remove the blade guard which is dangerous. I have a scar on my thumb that proves it.
I guess that is why I was not going to tell you how I would cut it.
Posted By: DYohn
Date Posted: October 21, 2008 at 3:27 PM
If you use a piece of paper to mark your line like was suggested in post #2, you can cut it using a fine-toothed blade in a jig saw or scroll saw if you and have an assistant to roll the tube while you guide the saw. I've seen them cut on table saws by clamping a piece of wood to the fence to hold the tube against at the proper length then roll the tube into the blade and turn it along the cut line. Again, two people make this job much safer. ------------- Support the12volt.com
Posted By: i am an idiot
Date Posted: October 21, 2008 at 5:20 PM
DYohn] wrote:
Again, two people make this job much safer.
Or twice as dangerous.
Posted By: KPierson
Date Posted: October 21, 2008 at 6:28 PM
Sonoco (makers of the sonotube) would cut it by spinning the tube and using a stationary round blade - I know because I've seen 'em do it! Not sure you would spin a 12" tube though, unless you have access to a decent size lathe. That would be a surefire way to make it perfect though. ------------- Kevin Pierson
Posted By: aznboi3644
Date Posted: October 21, 2008 at 7:15 PM
I use either my jig saw or my circular saw...it works.
Highly doubt my method of using a jigsaw is safe but oh well...I use what I have.
Posted By: bigjohnny
Date Posted: October 21, 2008 at 9:00 PM
Safety guards?? the only saw i have access to was built in like 1920 or something.... safety isnt part of its design lol.
My cut came out funny, as it was traced and cut AROUND the tube, instead of being able to make a cut straight across the tube, at an angle.
I might still be able to work with it, im not sure.... but I otherwise cant figure out a way to;
cut the tube without using this ancient tablesaw with no guides of any kind
and/or
mark the tube in such a way that the angle is marked across the tube instead of around, or the tube curve is compensated for or something.
I still have to figure out if i can use this piece i cut... i might be able to...
the next hardest part is marking the oblong hole onto the angled baffle board.
I have to stop taking on odd/angled projects heh.
Posted By: DYohn
Date Posted: October 21, 2008 at 9:17 PM
Why cut the sonotube at an angle? If I remember your project correctly you are putting a straight piece of sonotube into a wedge enclosure, right? Just make the wedge, cut the opening the correct diameter and insert the tube. Something like this: 
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Posted By: stevdart
Date Posted: October 21, 2008 at 9:54 PM
Next question: how would you find the correct dimension of hole to cut on that angled face? Is that trigonometry or something? ------------- Build the box so that it performs well in the worst case scenario and, in return, it will reward you at all times.
Posted By: aznboi3644
Date Posted: October 22, 2008 at 2:51 AM
its calculus...basically you have two intersecting planes...i think lol.
Posted By: bigjohnny
Date Posted: October 22, 2008 at 6:52 AM
DYohn, that would work, and I would love to do it that way, but as mentioned above, I dont know how i would go about finding the correct dimension for the hole.
Im pretty bad when it comes to math.... this is why i cut the tube first so i would have it to measure the hole....... im not sure it worked out that way so well.
I dont know the math to figure out how to measure/cut the hole.
Posted By: DYohn
Date Posted: October 22, 2008 at 9:49 AM
You will have to cut an ellipse into the angled side. Calculate the diameter of the ellipse by measuring the angle of the plane you are cutting into in relation to the desired angle of the cylinder (the Sonotube.) This becomes the hypotenuse of the resulting triangle created by the intersection of the two shapes. Then it is a simple geometry calculation to determine the length of the long side of the resulting right triangle (Z): the diameter of the sonotube is one side (X,) the depth of insertion is another (Y.) The formula which you may remember from 8th grade geometry is the Pythagorean Theorem, Zsquared = Xsquared + Ysquared. Here's a reference. Once you know the length of the long side, this becomes the long axis of the ellipse. The diameter of the Sonotube is the short axis of the ellipse. The formula for calculating the diameter of the ellipse is PI X Square-Root of 2 X ((1/2 long axis)squared + (1/2 short axis)squared). Here's a handy-dandy calculator for you. Me, I hate Sonotubes and wouldn't build the enclosure that way at all. There must be easier ways to skin that cat. ------------- Support the12volt.com
Posted By: stevdart
Date Posted: October 22, 2008 at 6:33 PM
If I were trying to do this, and being able (using DYohn's info) to figure out the short and long sides of the ellipse, I would still find it difficult to draw the correct arcs between each of the four points. Since we are talking about a wood enclosure, a miscut ruins the project. So what I would try is to construct a cardboard replica of the wedge box. Then, "walk" the tube into the angled side, snipping away the cardboard as you walk it in. The pressure of the tube will cause a slight dimple in the cardboard as you work it slowly in, and so it's a process of snugging the razor blade along the tube wall and cutting the cardboard along the pressure points. Eventually you will have it walked all the way through. When the tube is inserted where you want it, double check the angle of the tube in relation to the bottom of the box (right angle?). Use tape to smooth the cutout in the cardboard so that you can reinsert the tube and see no air gaps. The face of the cardboard replica box will serve as the template for your cutout. One thing of note: the thickness of the MDF will inhibit the tube from proper insertion, since the wood would be cut perpendicular to its surface (which is angled in relation to the tube). You will have to back cut the lower half of the ellipse, which can be a time-consuming endeavor. Sort of shaving the hole out from the backside but being careful not to cut all the way to the surface. A coping saw might be the tool to use for this. ------------- Build the box so that it performs well in the worst case scenario and, in return, it will reward you at all times.
Posted By: bigjohnny
Date Posted: October 22, 2008 at 8:54 PM
ok....wow.... that equation there lost me... i dont learn well from reading alone.
Anyway that aside, I'll look into further.... it sounds complicated and math was never my strong point.
also like stevdart said drawing the arcs may become difficult.
The tube solves a couple of problems, I'm aiming for a box as close to 1.75Ft3 as possible, but due to trunk restrictions I need to use an angled box, and this is as big as it can possibly be. Adding this tube should bring me pretty close to what I need, but will still allow me to remove my spare tire should I need to do so.
secondly, I didn't want to mount the subs on an angle. What I have going on in my mind should look pretty decent when its all finished and (hopefully) fiberglassed, but still achieves the proper sound.
the actual tube itself isnt very long, isnt a "full tube" and I should be able to reinforce well enough so that it doesn't alter the sound, thus keeping the sound quality intact...... at least in theory anyway, since the box is the proper size.
as for the thickness of the MDF causing a problem instering the tube properly..... not a problem, I have a spinsaw, which has this nice router attachment that will allow me to put it at an angle.
All I gotta do is set the angle, and cut away, as long as i cut it in the right direction the tube will slide in nice and straight.
I think that I might have to do is just sort of rough it out, seeing as how i lack alot of the professional grade tools to do it all precisely.
If i cut the hole a shade smaller i should be able to sand it to the proper size with a drum sander. Some screws and PL premium will ensure a damn snug fit.
Posted By: bigjohnny
Date Posted: October 22, 2008 at 8:57 PM
say, DYohn, since your so good with math, perhaps you know how to figure out the volume of that angled piece of tube??
figuring out the volume of a cylinder is easy, but how do you figure out the volume of a cylinder with an angled bottom like that?
Posted By: i am an idiot
Date Posted: October 22, 2008 at 9:13 PM
Length 1 + Length 2 = X X divided by 2 = the length in which you use as the length of the cylinder.
Posted By: bigjohnny
Date Posted: October 26, 2008 at 8:13 AM
ok i think i got the "template" setup.
ill try to take some pics a little later on, but what i did was put the angled tube on my table with the part that didnt sit flush on the table, hanging off the edge.
I used my laster level and shot a line across the tube, marked it and cut it.
this gave me a tube cut at an angle that sit flush on a flat curface.
using the piece i cut off, I stuck it to the other side of the tube to fill in the gap.
Now i can use this tube to draw out a template on some cardboard, and use this to make my ellipse on my baffle board.
I can set my rotary saw to cut it at an angle, then i will just be able to slide a straight piece of tube directly into the hole.
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