pie cutting straight tube for bends?

[Tony Hanna]

I'm almost ashamed to admit I don't know how to do this, but in my defense, it was touched on only briefly in apprentice training years ago. Since 90% of the tube work we do deals with high pressure steam tubing, all the bends are preformed and it's usually just a matter of cutting to the proper length and welding it in.

What I need to do is make a short radius 90 out of 3" exhaust tube. Would anybody care to walk me through the procedure for laying out the cuts so that I end up with pieces shaped right for the bend I need? I've got a couple pieces cut now to make the 90* with 2 45* angles, but it's ugly and doesn't look like it will flow very well.

Before anybody jumps in and says "just buy a 3" manderal bend", I'm short on cash, but have a nice stick of 3" tubing and a full spool of wire for the welder.

Thanks in advance,
Tony

[looneytuner]

I did it. cut 4 22.5 angles. But let me tell you it ain't as purty no matter what you do.
My opinion after done is that a mandrel bend is like Mona Lisa compared to a welded bend.

Notice the "rolled" humps at the ends. Two shaped pieces of metal JB welded on vice grips and a lot a squeezing.

I my 3" exhaust, I did V cuts not all the way through to get more perfect bends. These were only cutting maybe a 1/4" max v in the pipe. I didn't cut completely through the pipe. These look a lot better.

[BadAssPerformance]

Make the 90° bends out of at least 3 sections shaped like this, but with the same angle on both sides:

_________
\.........../
.\......../
..\___/

the angles needed depends on how many sections you use to make the '90' out of...

[Dez]

Yep, it would of come out curvier in 3 or more pieces.

[phaxtris]

i belive i worked it out to 15 deg still giving you a nice looking bend (that is 15 deg per side)

sometimes you cant buy a tight enough radious without buying a big bulky cast fitting so pie cuts are the only way

if i could post attachments i would show you a picture of a still decent looking pie cut in 2.5"

[Tony Hanna]

So if I go with 6 15* cuts to make the 90*, how do I control the length of the radius? I'm looking at it like this: A 15* cut on the straight piece to start the bend, then 2 wedges cut 15* on each side, and a final 15* cut on the straight piece at the end of the bend. Would it just be a matter of varying the length of each wedge to change the length of the radius?
Thanks for the help everybody!

[phaxtris]

exactly, except how i have them is it is 1 striaght with a 15 deg cut, then two wedges, the last wedge piece gets you you to 90 ish, then attach your straight pipe

the extra 5" deg is pretty easy to make up in gaps between the wedges

like you said, radious is determined by the size of your wedges

Edit; oops! i do myne at about 17 - 18 deg, that way i can get away with only 2 wedges and 1 straight

[Tony Hanna]

Gotcha! Thanks man!

Now I just need to figure out a way to make nice repeatable cuts with what I have on hand. I'm thinking maybe a miter box made out of plate steel and a long blade for the sawzall...
I really wish I had a chop saw.

[shadow88]

Wow. You must have been surfing my brainwave. I was just looking at my ducting in the basement and noticed all the nice sectional bends. I think the angle to cut the tube on is a direct result of how many pieces you want to make up a bend. I took one of the bends out and straightened it out. I was just about to measure what angle the cuts are on and realised I don't have a compass You're making anexhaust. I was going to re-do the IC pipes in 2-1/2 rather than 2-1/4"

Good luck and a big +1 on the chop saw.

[Tony Hanna]

Thanks! Yeah, I'm trying to put something together for a 90* bend to connect the Holset to the flex coupler in the downpipe.
The more I think about it, the more I think I may just go ahead and use the 2 pieces I cut with 45's. I really need to get the car on the road quickly, and I can always rework it later when I have a little more time and money.

[BadAssPerformance]

I typically use the big band saw at my work... draw a dotted line all the way around the pipe at the angle I want then cut is with the band saw, 'driving' it around as needed to stay on the line. Also have used the compound mitre saw that I bought for doing molding in the house, just swapped to a fiber metal cutting blade

[Tony Hanna]

The miter saw would definately make things easier. I'll probably have to settle for making a miter box to use with my sawzall though. Aside from that it would be laying out the entire pattern and using a grinder/cutoff wheel. That worked ok on the 45*s, but I ended up with slightly imperfect cuts that need a bit of grinding for decent fitup.

[phaxtris]

no chop saw! thats essential!

[Tony Hanna]

Yeah, that's definately on my list of must have tools the next time I have some spare cash.