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What's the best way to cut a tank?

post #1 of 25
Thread Starter 

Hi there,


I have a new tank that needs to have the end cut off for the FB end.


What's the best way to cut it straight? I have a plasma, but I'm having a hard time measuring/marking a straight line on the tank. I don't think the original end cap weld is even remotely close to straight.


I've heard it can be done with a large paper, wrapping the paper around the tank so that the ends line up perfectly. I've tried that method with some other circular cuts and haven't really had good results...


I might be overstressing the importance of good cut, I guess I just want to have decent fit for the new end plate being a novice welder :biggrin:

post #2 of 25

Hello.  Datum Lines for your build:


Hello.  I KNEW this question was coming!!  I have been meaning to post this for a VERY long time but I wanted to have pictures to go along.  I just haven't got around to it so I'll do my best to explain.  IF you have questions please feel free to PM me or ask on this thread.  This is gonna be a long read but must be so that I can explain all.


The long axis seam SHOULD be straight but let’s not assume that.  Besides you don't want a seam top, bottom or 90 degrees.


There is a welder’s tool that will make this simple but as it is expensive to buy, we'll do this the hard way.  You will need a builders square, 2' level, torpedo level, flat blade 25’, 50’, or 100’ tape measure and more than 2 hands.


Your tank MUST be level along the long axis and chocked so that it can not roll.  I would put the welded seam at 135, 225 or some obscure angle around there.


With you standing facing either end of your tank:  place the builders square near the end of the tank or just on the tank side of the cap weld with the long side of the square running down the side of the tank to your left or right and the short side running horizontal to the top of the tank.  Now place the torpedo level on top of the short side of the builders square and move the square around the tank to get the short side level.  KEEP BOTH SIDES OF THE BUILDERS SQUARE TOUCHING THE TANK!  In other words both sides of the “L” shaped builders square are touching the tanks at all times.


NOW.  Put the 2’ level on the long side of the builders square.  Now roll the square around the tank so that both levels are as perfectly level as possible.  Take note IF either bubble in just slightly off to one side and remember which side.  IF it is a larger thinner tank with no end caps it MAY be egg shaped.


The square MUST be touching both the top and side of the tank on the inside of the “L”.  The long side of the square MUST be hanging straight down ( think plumb bob ).


NOW that you have all things in place and as close to level both ways as possible, place a mark on the top of the tank at 1” on the short side of the level ( can be 2”, 4”, whatever is closest ( close counts here ) to top dead center.

Now move to facing the other end.  Follow the same steps.  REMEMBER if one level was bit off and remember off which way.  Set the square so that it is exactly the same as the original end ( off to the left on the first end, off to the right the other ).


Place your mark at exactly 1”, 2”,4”, whatever you did on the first end.


NOW you should have a straight level line to start with.  Snap a line between the marks with a chalk line.


Using your flat tape measure, measure the circumference of the tank.  You MUST make sure the tape is as straight as possible.  Holding both ends of tape measure work it back and forth so that the tape is as straight up and down as possible to achieve a true measurement.  Once finding the circumference, divide by 4 and the mark the quarters.


I would use the first line you establish using the levels as 0 degrees.


If  you need to transfer your quarter lines to an end cap a broken bandsaw blade or something similar works well.  Hold the blade on the quarter line straight for about 6”.  Do the same 180 degrees from your top quarter line. Now slide the blade along those quarter lines letting it find it’s own natural path until it comes in contact with the weld cap.  So long as you hold the blade on the quarter lines and don’t force it in a certain direction it should lie flat across the weld cap and allow you to then draw a straight line over the weld cap from one quarter line to the other.

To cut an end off straight where both end caps are there; start on the 0 degree line.  Measure from center of the weld to the center of the weld.  Now mark the center point.  You will need this for the door.  We used to use adding machine tape or a wide flexi band saw blade or wide banding material to draw the line around the tank at the center line and then mark center on each quarter line.  Without access to any of this it is really just a leap of faith that the ends were straight when caps were welded on.  Use the measurement from 0 degrees and mark each center from the same end.  Now using that same measurement mark from center line to the end to be cut on each quarter line.  Draw between the marks on each quarter line and that hopefully is close enough.


Center punching each mark is a good idea for future reference.  Put 2-3 punch marks to identify 0 degrees.  Clear as mud!


If I left anything out let me know.


post #3 of 25
I wished you had some pictures. Good information though.
post #4 of 25

Good info.  Makes sense and will be useful when I cut mine.  (Waiting for warmth.) Thanks.


post #5 of 25

Go to the welding supply and buy a wrap-a-round. They are made for just that.

Happy smoken.


post #6 of 25

Sorry, am moving right now so can't even find the camera.  I will repost this with pictures as soon as.


Hello David.  Was a long time ago, can you still buy a wrap-a-round for under $20?  That would be best to transfer the lines from quarter to quarter or all way around the tank if not too large a tank.  As you know a center finder would be useful but those darned things were about $40 20 years ago for the small one.  Did some pipe fitting rework for refineries back when a young man.  Wrap-a-round, center finder and contour marker were MUST HAVE tools.  Weld that 6" 90 degree elbow at a 17 degree angle to the center line.  6" pipe and then a flange at a 7 degree down angle and a 10 degree angle to the right.  Darndest stuff I ever had to do.  I guess they just built the pipework in place and made it fit but to build replacements in the shop was a nightmare.  Reading those pipe isos you looked like a person talking sign language.  OK, this goes off that way and then turns this way....  :ROTF  Sorry to go a bit off topic.  If you can afford one, David is right on with the wrap-around!  Good luck.


post #7 of 25

Look for Ribwizzards thread on building a RF smoker.....

post #8 of 25

I would think fan-fold computer label/paper would give you a good transfer since it is relatively wide and if the edges align, you should be good.  At least that's the way I plan to transfer measurements on mine.  I have a roll of ups shipping labels and they will work just fine.

post #9 of 25
I use a wide roll of masking tape. Just mark reference in a few places and connect the dots. Either leave tape in place or scribe/ mark along tape edge and remove.

post #10 of 25
Use a piece of banding material as a wrap around.
post #11 of 25
Thread Starter 

That wrap-around looks to be just what I need. Have to get a bit creative with it, I have never seen such thing for sale anywhere in Finland.


I actually once thought about using a magnetic strip for marking, since it would also hold still easily. But usually they are are too short for this.

post #12 of 25

If your tank has no fittings, you could also turn a furniture dolly upside down and place the tank on the dolly wheels with the tank against a wall. Use a fixture to mount a Sharpie and rotate the tank on the dolly wheels, it will give you a reference with respect to one end that is against the wall and the lines will be parallel. It does not have to be perfect, just close enough to weld up the gaps. The arcs you will marking will be less than 180 degrees.

post #13 of 25

Be sure and post some pictures, they always help



post #14 of 25

I cut my door without incident.  Anyone got any tips on how to lay out the cut on the end of a domed tank?  Using a paper guide gives a smiley face with turned up corners.  The only things I can think of are


1. with tank level and at TDC, measure up from the concrete floor and mark the cut out in several places for the rectangular fire box and connect the dots  or


2. use a transit with tank at TDC and do the same.



Edited by storeman0913 - 2/7/15 at 3:21am
post #15 of 25
One of my welders used a Lenght of 11/2 wide sand paper, grit paper or what ever you call it, wrap around pipes or tanks then marked with pencil or engineers chalk.

Smokin Monkey 🇬🇧
post #16 of 25

A 2x6 cut to the proper height works well to mark the dome with the tank on a level surface 0 degrees up.

post #17 of 25
Mark centerline on top of tank. Measure down to one side. Use a string, chunk of cardboard to get your endpoints. Use level to mark near center of tank.....connect the dots.

I made a lot of cardboard templates.

post #18 of 25

It's not hard, lay it out, look at it check your measurements check them again and cut it



post #19 of 25
Originally Posted by LenDecaturAL View Post

A 2x6 cut to the proper height works well to mark the dome with the tank on a level surface 0 degrees up.

So you are saying the same thing I concluded.  Measure up from the ground (level surface and mark and connect.  I had reached the same conclusion.  I don't see how it can be done from the top surface of the tank.  Thanks.


post #20 of 25

Yes, wider board is more stable, 4 x 4 would work also, just make sure you have a good square cut.

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