Cutting a hole for the stack?

[lhedrick]

I have a propane tank build.  The ends of the tank are spherical in shape.  I will have a 4 inch round stack and will need to cut a hole in the end of the tank for this vertical stack.  I can weld well enough but I'm not a metal worker and have question.  Is there a simple way of determining the shape of the hole I will need to cut as it won't be a circle.  I don't have a torch or plasma cutter.   Can I cut a round/oval hole with a 4 inch cut off wheel in the thin stock used to make these tanks, then grind the rough spots?.  Is it possible to find good blades for a jig saw or reciprocating saw/sawzall which will cut this steel?

 

[crankybuzzard]

Thin stock?  Most of your larger home propane tanks have a wall thickness of 3/16" - 5/16".

What size is your tank in diameter and length? 

You can cut a starter hole with a wheel, but it's going to be a PITA to round out.  Sawzall blades can cut this material, but it will be slow going, and still not give you a well rounded hole at 4" diameter.  However, since you have a grinder, you can cut the hole as best as possible, and then get a grinding rock for the grinder and slowly get it where you want it.

Depending on where you're located, a lot of welders will cut a hole like you need for a 12-pack if you talk to them right.

To get the pattern you need, find a piece of 4" OD cardboard tube, or make one, and cut/trim it until it will fit onto the outside of the tank the way you want it, then trace around it and begin cutting.

 

[lhedrick]

By thin I was thinking on the lines of the number you mentioned.   I was thinking thin as compared to 1/4 or more.

I had you cardboard idea in mind so I can give that a try.  

I have another option.  I have some 4 inch square stock and can easly get some 6 inch almost free.   Is a square stack is okay, that would be easy to do with a cut off wheel.

Does it matter if the stack is not round.

Thanks.

 

[crankybuzzard]

Square will work just fine.

 

[novasbc]

I asked at one point whether square would be fine on this forum, and it was indicated that it had more surface area, and throws of the equations (Here is the post: http://www.smokingmeatforums.com/t/216120/exhaust-stack-heights#post_1412432.).

I did use a torch myself, and drew around it first with a soapstone.  I managed to get it just right to where I could get the stack in, but it had enough friction to stay in place while I welded on it.

I cut the doors on my tank with a sawzall, it is slower, but more accurate (at least in straight lines).

I would just get it somewhat close, and put tack welds to hold it in place, and fill in where you need.  I had one spot that was particularly off, and I used some metal I had cut off other places to help fill it in.  You can use a larger rod if you are using a stick welder, that is better for filling gaps.