Rolling a pipe smaller for firebox

[DoTheEyeThing]

I have some 20" OD x 22" Long x 1/4" that I need to roll down to 18" OD for the inner part of my firebox. I think I could cut out a strip, pull the tube tighter with ratchet straps and weld it, but I'm wondering if the residual stress could cause some warping of it and what it's connected to from all of the hot/cold cycles in it. Would it be better to have it rolled to 18" at a machine shop? More worried about doing it right than cost.

 

[thirdeye]

Is this going to be a sleeve for your firebox (creating an air gap for insulation)? Does it have to be 18" OD, or can it be 19" OD? The less you remove the easier it will be to re-weld and keep round.

Typically welding shops will weld tabs on each side of a longitudinal cut and bring them together with bolts, then tack the long seam and work the piece back into round. It might be cheaper to buy a piece of 18" pipe.

 

[DoTheEyeThing]

Yes the 18" OD is the inner sleeve to sit inside the 20" OD pipe for an air gap as semi-insulation (won't have anything in the gap). I'm trying to make an accurate copy of Aaron Franklin's backyard smoker, and his is 18" inner, so I'm ideally aiming for that.. bringing the sides together with bolts sounds like a good way to do it. Do you think the worry of warping from the fire is a valid concern? Maybe I could thoroughly heat it with a weed burner to try to relieve the stress before welding it together?

Maybe I could find an 18" pipe but I suspect it'll be tricky to find anything close to the short "22" long piece I need. I got pretty lucky finding these old 20".

 

[thirdeye]

Those are good questions, and I can't answer from experience, but I will toss some ideas out on the table for conversation purposes:

• The tabs I've seen were made from angle iron with one elongated hole.
• When you bring the seam together I don't think it will be a perfect cylinder, but would heating the pipe as you are tightening the bolts help keep it round?
• If you get seam on the 18" pulled together and the can is somewhat round, you could cross brace the ends to hold it round when welding the seam.
• You don't want to fully weld the seam from one end to the other, rather make some tacks, then let them cool. Make tacks between those and let them cool. Repeat until the seam is fully welded.
• I don't know about warpage during cooking. Are there any supports welded to the 18" pipe?

 

[DoTheEyeThing]

Great ideas. Thank you for the info!

I'm not sure if there are supports. I suppose a few small ones can't hurt. I believe it's welded to the same round plate on the ends that the outer pipe is welded to. I think I'll need to weld the outer pipe to one round plate, weld the inner to the other round plate, slide the inner one inside the outer and weld the other inner pipe edge through the door cutout in the round plate. Attached a pic of a model to better demonstrate.

 

[SmokinEdge]

Do you have a steel supply yard near you? I live in a small town 60 miles from the nearest medium-ish town, and we have a steel yard with a fab shop that will gladly sear, roll, precut whatever you want. They even have a water jet that will cut 3” plate all CNC.
You might look around for a yard that will make that 18” roll then all you have to do is weld it up.
You can try to build it, it can be done, but you will struggle And probably ruin a couple before you get one close to round.

 

[bill1]

If you want it to be reasonably round when you're done, rolling it is your best approach. But how close is a shop with a 3-wheel roller that can handle 22" wide widths? At .25" thick? Even if you find one, you may find it cheaper just to buy 18" stock.

If you do cut and roll or just cut & clamp, remember you're dealing with circle circumferences so you're cutting out 22*pi - 18*pi or 4*pi = 12.6" wide section. When you think of it that way, it's maybe easier to visualize why keeping it round using clamps, straps, or bolts could be difficult.

 

[thirdeye]

Great ideas. Thank you for the info!

I'm not sure if there are supports. I suppose a few small ones can't hurt. I believe it's welded to the same round plate on the ends that the outer pipe is welded to. I think I'll need to weld the outer pipe to one round plate, weld the inner to the other round plate, slide the inner one inside the outer and weld the other inner pipe edge through the door cutout in the round plate. Attached a pic of a model to better demonstrate.

I was thinking about spacers in the form of flat bar stitch welded lengthwise on the OD of the 18" pipe. Maybe one at 0°, 90°, 180° and 270°. But this might be overkill.

bill1 had a good point about the change in arc length on the two sizes as 1° on the circumference of 20" is different than 1° on 18". And if you can find a shop like SmokinEdge mentioned, consider rolling a 19".

 

[DoTheEyeThing]

I was thinking about spacers in the form of flat bar stitch welded lengthwise on the OD of the 18" pipe. Maybe one at 0°, 90°, 180° and 270°. But this might be overkill.

Yeah I think that sounds worth doing. Could make it easier to get everything lined up too.

I did some more checking and I believe the inner diameter of the inner sleeve is 18", so at 1/4" thick it's 18.5" OD.

20" OD rolled down to 18.5" OD is a 4.7" gap to close - sounds more doable than 12" :) As much fun as trying to close it myself might be, I got a recommendation for a local guy I'm going to take it to to see if they can roll the gap down.

If that doesn't work out, I'll explore the other suggestions.

 

[bill1]

Oh, and I apologize I confused your length and diameter. So the width you're cutting out is half what I calculated since it's 20pi minus 18pi equals 2pi or 6.3".

 

[DoTheEyeThing]

Quick update - I cut out a 4.7" slice and it sprung open quite a lot. Took it to a local shop and they rolled it down to 18" for $30 :) It looks like it got a little flattened at the joint so it's 17 3/4" there... about 18 1/8" at the widest part.. don't think it will matter really. Maybe I could put a jack in it to try to push it back out a little?

 

[thirdeye]

Well the $30 was certainly a good deal. I wonder why they didn't put it back into the roll and improve the roundness. Anyway you can do hot mechanical straightening with jacks, a beam or a piece of channel, but it's not a symmetrical process and easy to over-do it. The flat spot is warmed up and worked back into round, then the piece is allowed to cool, then braced before removing the jacks.

 

[SmokinEdge]

Looks close enough for me. I’d roll on ahead. That 30 bucks was cheap Money.

 

[bill1]

Looks close enough for me. I’d roll on ahead. That 30 bucks was cheap Money.

Agree 100%. The joint is a little flat because once a free end clears the 3rd of the 3 rollers, it is no longer being bent.
Even if you could have gotten an 18dia section for cheap, this way you supported (helped capitalize) a valued tool at a local brick&mortar shop. I'd call it win-win-win...win for you, win for the shop owner, and win for your local technological prowess.
Capabilities like this are being lost daily at both the local small shop and in large corporate settings. It's so easy for a manager to say, "we never use that old tool any more" and then out it goes.
Personally I think this country needs more steel-working tools and fewer MBA's.

 

[thirdeye]

Agree 100%. The joint is a little flat because once a free end clears the 3rd of the 3 rollers, it is no longer being bent.
Even if you could have gotten an 18dia section for cheap, this way you supported (helped capitalize) a valued tool at a local brick&mortar shop. I'd call it win-win-win...win for you, win for the shop owner, and win for your local technological prowess.
Capabilities like this are being lost daily at both the local small shop and in large corporate settings. It's so easy for a manager to say, "we never use that old tool any more" and then out it goes.
Personally I think this country needs more steel-working tools and fewer MBA's.

I agree with everything you said, and in fact built my small businesses around that same philosophy.... but the can was so short, I think they sunk it when welding the long seam and a pyramid plate roll could have worked it a little. It's no different than rolling a skirt for any number of vessels or small tanks. In fact, don't you think they welded it in the roll?

 

[DoTheEyeThing]

Yeah I'm really pleased with the outcome and thinking I will be going to them again when my cook door inevitably springs.. though I wonder if it would have the same flattening issue when it clears the third roller? Maybe the jack and chain method I see a lot of builders use would be better for that?

The guy warned me he rolled it tight and it could curl in on itself if the edges got out of line so I think you're correct regarding sinking it. They tacked it for now - more welding practice for me haha.

I believe it's time to start a build thread Still have more questions!

 

[thirdeye]

Yeah I'm really pleased with the outcome and thinking I will be going to them again when my cook door inevitably springs.. though I wonder if it would have the same flattening issue when it clears the third roller? Maybe the jack and chain method I see a lot of builders use would be better for that?

The guy warned me he rolled it tight and it could curl in on itself if the edges got out of line so I think you're correct regarding sinking it. They tacked it for now - more welding practice for me haha.

I believe it's time to start a build thread Still have more questions!

I mis-understood.... I was thinking the shop welded it fully, not just tacking it.

What you will be up against is distortion or warping due to the heat input when welding the long seam. I mentioned using short welds and moving around to balance the heat, and giving your short welds (and the pipe) time to cool and settle back down. You may find that moving from an OD weld to an ID weld can reverse distortion. And in some cases I've seen welders heat items at 180° to where they are welding so that the heat in the piece is balanced.

 

[bill1]

Like ThirdEye, I thought they welded the entire seam too. I'd have thought they'd roll it in many iterations, especially as they approached final diameter, and then when the gap was <1/4" they could have closed it and made the seam weld with just a couple pony clamps putting a hundred pounds or so of closing force on the ends.

I'm sure you'll do just fine welding the length. As you've noted, take several stabs at it. It may not be pretty, but it can be rotated to the bottom of your final build.

Thanks for the last pic. You did some nice cutting and they did some nice rolling. My complements to you both.