cutting 5" aluminum pipe [mast]

jimer · Sat Jan 28, 2017 3:55 am

I'm trying to match up two 5" aluminum pipes and I must cut the base plate off the original in order to extend the piece another 7 feet. I'll put a insert pipe in then weld to finish it for the extension.
Right now I am simply thinking of wrapping a piece of straight cardboard around the whole thing and drawing a line with this as a guide. Then I'd take a cutoff wheel and cut the line around the whole thing.
Anyway, am I on track or how does one deal with aluminum pipe when cutting it and keeping some semblance of being square. I hate to find out I did it the hard way or something - story of my life. lol
Anyway thanks.

Jim

Rupes · Sat Jan 28, 2017 4:35 am

Thats all I'd do, use the cardboard. Maybe string if it was bigger. Main thing will be keeping the to pieces straight wild welding. Unless that insert pipe is a tight fit. If it is it doesn't matter if its not perfectly straight as you can fill the gap. I'd probably put a mild bevel on the two pieces and leave a gap so I could get in there and weld it to the insert pipe for both pieces then just fill it up.

A3 sheet of paper will make it around and as long as the ends are parallel you can tape it on and trace a line. Might need some second opinions on welding it though, filler, technique etc. Gonna be a fair bit of load on it.

Poland308 · Sat Jan 28, 2017 9:22 am

You need a wraparound. They sell them at the LWS. It's the standard way to mark out pipe for a straight cut. It comes with instructions. As to cutting I'd use a portaban.

mpete53 · Sat Jan 28, 2017 11:01 am

I have used a piece of paper several times wit good results. Marking the cut is only half the problem, making the cut is the other. I know hand tools are not the fastest, but, accuracy not speed is the main goal here so I would use a hand hacksaw. Block it up on several saw horses so it doesn't roll, start cutting at the top just leaving the line. once you have cut through onto the open pipe start working your way cutting only one edge and rotating the pipe to keep the cut in a good working position.
While I have not done anything this large, I have done 1.5 stainless tube this way and had no visible gaps when mated back to the machined end it mated to. A tight insert that came with the radar mast kept them aligned.

Mark

Poland308 · Sat Jan 28, 2017 2:40 pm

That's the method used with the portaban on large pipe. You just work your way around following the edge of the line. I've used it on pipe over 12 inch diamiter. Depending on wall thicknesses.

jimer · Sat Jan 28, 2017 3:18 pm

Thanks guys.
Portaban is one tool I don't have. I'll have to google some of the other things said here.
I do have a Miller plasma cutter, sawsall, 4.5" cutoff tool, a larger grinder, jigsaw. I also have a 12" chop saw. I could never get the plasma cutter to do a clean cut on aluminum, not sure why.

I appreciate the answers.

Jim

noddybrian · Sun Jan 29, 2017 8:44 am

Cutting aluminum with plasma oxides the edges badly as it reacts with the oxygen in the air - it improves a lot if you use a bottle of nitrogen in place of air - as to whether the job justifies the cost is debatable - the fit up is not that critical if you can use a back up insert ring - my worry with a mast is that it has high cyclic loads imposed likely to cause cracking and is usually a 1 piece extrusion of heat treated and anodized material not regular pipe so welding it will be challenging and will likely weaken it significantly.

Poland308 · Sun Jan 29, 2017 10:04 am

I missed the part where he said mast. After rereading I agree

noddybrian wrote:review

there is that possibility. And so I'll ask more questions. Is there a lot of weight mounted to the top of this mast? Can you keep the joint towards the top of the new assembly? I.e. Leave most of the original bottom.

noddybrian · Mon Jan 30, 2017 4:23 am

I'm going nut's - I was sure OP said " mast " but between my vision seeing double & blurred of an evening combined with reading another similar post elswhere I now don't see it ! maybe OP can clarify purpose of said tubes & we'll start over ! Sorry guys.

Rupes · Mon Jan 30, 2017 5:34 am

Yeah I saw it then couldn't find it when I reread. Its in the thread title

Ï thought the pipe insert would take strain off the join especially if long enough but I guess you'd still have tension and compression constantly fighting either side of the weld every time wind blows. And its probably designed to flex in an even manner top to bottom.

It must get done though. Probably one of those thing that has have an engineer look over.