need a geometry lesson

I have a 10 foot length of 8 inch pipe that I want to use for a heavy duty gate post. I need to snap a line the length of the pipe to drill holes for my gate hinges. Question--- How do I make sure that the line I snap is truly running parallel with the pipe? I hope my question is clear. Thanks, Ellis
 
Put the pipe vertically on a flat surface and use a plum bob. Mark both ends then snap a line from those points.
 
Does the pipe have a seam? Look inside and check. Usually the seam is pretty dead on. The key is usually. I have used the seam to rip a length of pipe and it was very close.

OTJ
 
ellis,

Lay the pipe down, set a long angle iron on it, vee down. Mark your line. Should look like a circle with a dunce cap on it from ends view.

D,
 
I would use a 10' length of angle iron or c-channel and trace a line along one edge. If your 8" pipe is straight, it should give you a decent plumb line.
 
Lay the pipe on flat surface. Don't let it roll. Using a piece of 4" cardboard (or whatever") Stand the cardboard up on both ends of pipe, mark your snap line.
 
This would do it, but probably overkill! For what you want an angle iron is easiest.
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Lay the pipe on a flat surface, secure it so it can?t move, place a framing square on the flat surface next to the pipe, measure 4" to the center for the first hole. Repeat for the second hole.
 
Just lay a piece of angle iron or say 3"channel iron on your pipe lengthwise, and draw a line along one side of it. It will center itself and give you a straight centered line to mark your places you want to drill holes from top to bottom. No fancy calculations need to be made.
I just used that little hack yesterday to make drawer pulls from inch wooden dowels. You just need to think bigger than I was.---------------------Loren

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Here's a low-tech, low-cost, and easy method to make a straight line. First, ensure that the pipe is perfectly level from end to end. If the pipe is rusty on the inside, cut a piece of waxed paper to line the inside. Make sure the strip of waxed paper is fully expanded, lining the full inner circumference of the pipe.

On the waxed paper, use a single drop of soapy water. The water will flow to the lowest point. The dish soap is to cut any surface tension which ensures the water is perfect. It only takes a drop or two of soap per GALLON of water, so doesn't take much.

Now do the same for the other end. Those two low points are then marked into the edge of your pipe. And there you have it - a straight line using low-tech.
 

My boilermaker neighbour showed me the same trick , but in my circumstance I wanted to make sure two ends of pipe were perfectly square to each other before welding them together . It worked brilliantly , eight 12 foot long pipes in total made from three pieces each , those 8 inch steam pipes now hold up my barn .
 
The big problem is if you want holes on each side for a rod to go thru. I have not found a way to get holes centered on anything round to come out in same place on both sides. If anybody has a soloution for that let me know.
 
Easiest way to mark a straight line around a pipe is to get a length of paper long enough to go around. Then wrap the pipe with the paper tightly and make sure the top edges are lined up. You will have a straight line as long as the paper is straight.
 
Do you know a pipefitter? See if you can borrow a Flange Wizard mag centering head. They're pricey new but I got an excellent used one from eBay. It is dead-nuts accurate.
 
Here?s a link, didn?t post the first time.
Won?t let me post it so you?ll have to copy & paste.
Flangewizard dot com/magnetic-centering-heads/
 

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