Preparing copper tube for bending?

rons · Jun 13, 2019

Different copper tubes:
Type K - OD = 0.875 ID = 0.745
Type L - OD = 0.875 ID = 0.785
Type M - OD = 0.875 ID = 0.811

I have bent Type K annealed tube with a bending jig. The 7/8 tube was rolled for a 5" radius.
My question is how to change Type M drawn tempered tube to the annealed state. Object is to bend it.
The wall is 0.033 less in thickness compared to type K. It should take less force to bend the tube if it was soft? How to get it soft?

I just have a couple of Mapp gas torches.
How to heat the tube so that it can change state?
Rotate the tube while I run the torch down across the length?
Insert a metal rod into the tube and heat tube, would this distribute the heat more evenly?
Heating to a dull red glow and let cool?
I don't have access to a oven.

Rob F. · Jun 13, 2019

Just use the mapp gas and if you move the torch around a bit it will distribute the heat. Copper will distribute the heat even if you dont move the torch much so dont worry to much about it. If you are looking for a dull red be sure the room is dark you are working in. It dies not matter how fast or slow it cools, it will work harden again when you bend it, so it may need a second anneal after it bends a bit, no harm in this. That m tube may want to kink where the k tube would not, not as much wall to deform.

Bill D · Jun 13, 2019

Stuff some Al foil in both ends so the heat does not escape as fast.
Bill D.

Note that cooling rate has no effect on temper. Quench or air cool is equal softness.

moonlight machine · Jun 13, 2019

Bill D said:
Stuff some Al foil in both ends so the heat does not escape as fast.
Bill D.

Note that cooling rate has no effect on temper. Quench or air cool is equal softness.

Clearly you have never done this, as it is heated air pressure increases inside and pops the foil right out. You will heat the tube most evenly by putting the torch inside the end of the tube leaving the other end open.

POAx · Jun 13, 2019

If it would be critical to keep the inner aria constant trough the bend there was a method years ago. As I remember it was a metal alloy "Serro Bend". It had a melting temp of about 70 C°. You could melt the alloy, fill the tube, let it cool down then bend the tube and finally melt out the alloy from the bend tube. I have made it with thin wall aluminum tubes and stainless steel tubes with very good results.

moonlight machine · Jun 13, 2019

POAx said:
If it would be critical to keep the inner aria constant trough the bend there was a method years ago. As I remember it was a metal alloy "Serro Bend". It had a melting temp of about 70 C°. You could melt the alloy, fill the tube, let it cool down then bend the tube and finally melt out the alloy from the bend tube. I have made it with thin wall aluminum tubes and stainless steel tubes with very good results.

A mandrel bender is what the OP needs.

cyanidekid · Jun 13, 2019

POAx said:
If it would be critical to keep the inner aria constant trough the bend there was a method years ago. As I remember it was a metal alloy "Serro Bend". It had a melting temp of about 70 C°. You could melt the alloy, fill the tube, let it cool down then bend the tube and finally melt out the alloy from the bend tube. I have made it with thin wall aluminum tubes and stainless steel tubes with very good results.

Belmont Metals here in Brooklyn sells a Bismuth based alloy they call “Bend metal 158F” that is specifically for this application. It melts at that temp, (70C), so it is easily handled with just hot water. I have some, but havnt used it yet.

There are a number of other tricks to assist bending thin wall tube, “regular” tubing, and pipe.

packing with dry sand and plugging the ends securely with wood or welded on caps ( leave a hole for steam!!) is an old blacksmiths trick, as you can heat the bend area to a forging heat and the sand holds up.

Going the other way, An old jewelers trick is to freeze water inside a tube, but unless you are working in the sub freezing cold, you do have to work fast!

A tight coil spring that just slides inside the tube is sold for this purpose but I don’t know how well that works, and it needs to fit snug, but not too snug.

Mandrel bending is the least flexible of all requiring the machine set up for your tube dia and wall thickness. Easy if you have a racing header shop, but that still won’t help with 7/8 copper.

Back to the OP’s question though! You just need to get it to a dull red, which is really easy with a mapp torch. Even heat is good, thus the recommendation of a low light room to observe the heat. Keep it below a bright red heat or you will get a lot of oxidization and scale.

All the tricks for supporting during bending are really needed most for tight radius bends, if you aren’t going tighter than a “full sweep” bend (what is that, Radius 3-4x dia.?) You may not need any of this nonsense!

memphisjed · Jun 13, 2019

turn lights out (dark room or corner works, doesn't have to be black dark), heat areas you want to dark cherry color (why the dark room, you can go over temp and it will not be disaster- wasted heat and deformation happen tho), quench in water or pickle. If quenching in pickle have good forced air in and out of space, and note the fumes will rust everything they pass.
copper does distribute heat well, and really only need to anneal the areas being bent.
Mapp, air/acetylene, oxy acetylene all will work. Mapp is going to take you a little time comparatively.

rons · Jun 13, 2019

Thank you for info.

For a 3 or 4 foot length, just run torch down the length at a pace to keep the surface red?

Rob F. · Jun 14, 2019

rons said:
Thank you for info.

For a 3 or 4 foot length, just run torch down the length at a pace to keep the surface red?

No, you are overthinking it. Once you get one part of it dull red move on, no need to waste time or fuel trying to get it all red at once. It does not need to be red to bend. It doesnt even need to get red to bend and is way better to bend if it is NOT red.

memphisjed · Jun 14, 2019

just work the heat from one end to the other. 4-6 inch red and walk it down. do not touch the black copper, it will brand you new vocabulary. work completely around pipe, not length as you walk the heat, evenness around is important to controlled bending.

Rob F. · Jun 14, 2019

Copper only needs to get to a dull red to aneal, it does not need to soak there for any length of time, this is m tube the thinest wall of plumbing copper. One it hits red move on.

rons · Jun 14, 2019

memphisjed said:
just work the heat from one end to the other. 4-6 inch red and walk it down. do not touch the black copper, it will brand you new vocabulary. work completely around pipe, not length as you walk the heat, evenness around is important to controlled bending.

Probably would mount it on a rotating fixture to do that. My last comment was trying to ask: Should I walk a red spot down the length? It seemed obvious to me that I would never be able to get the entire tube a consistent dull read all at once working with a couple of torches.

Whetstone · Jun 14, 2019

What type of bend are you trying to make? I think you are over thinking it. What bending tools do you have? You should be able to make the same 60" radius arc in the un-annealed state.

Rob F. · Jun 14, 2019

rons said:
Probably would mount it on a rotating fixture to do that. My last comment was trying to ask: Should I walk a red spot down the length? It seemed obvious to me that I would never be able to get the entire tube a consistent dull read all at once working with a couple of torches.

It does not matter how or in what direction you heat it to red, as long as you do it. Think of it like eating an ear of corn, do you go around or do typewriter method? Either way the corn will be eaten.
Heat some today and the rest next week, makes no difference, it will be soft.

MrStretch · Jun 14, 2019

It probably doesn't make any difference in this case, but quenching non-ferrous metals will make the metal slightly softer than letting it air cool. Heating the metal up past the annealing temp causes random alignment of the grains of the metal and quenching helps preserve it, whilst slower air cooling allows for a slight amount of realignment.

rons · Jun 15, 2019

Thank you for all the help.

I had a bad experience today. Was tightening a 1/2 to 1/4 NPT fitting down on a air tank 1-1/4 bushing. Heard a snap sound. The nut on the top of the bushing got a crack from the inside to the wrench side. A steel bushing. Got a hack saw (single sided hole saw) and sawed out wedges of the bushing. Used a magnet pen with a dowel and picked up all the metal saw shavings. Went to the industrial supply this morning and bought a brass 1-1/4 to 1/2 bushing.

I made this tube bending device. I offset the two wheels the correct distance for one revolution of tube. I was going to dismantle the wood and keep the hardware because I didn't want any memory of it ever existing. But I may keep it around. Only posting the picture because Whetstone asked.
I got the idea from the nice metal bender.

metalmagpie · Jun 16, 2019

cyanidekid said:
A tight coil spring that just slides inside the tube is sold for this purpose but I don’t know how well that works, and it needs to fit snug, but not too snug.

I have one or two such springs. It is my impression that the spring goes on the outside of the tube, not the inside.

metalmagpie

TheBigLebowski · Jun 17, 2019

Are you worried about the force needed to bend it or buckling the walls?
If you are trying to stop buckling you can fill it with water and freeze it before bending.

rons · Jun 19, 2019

I got the job done. The extra work I had to do was caused by some guy's big foot.

I bought a 50' roll of soft copper in a cardboard box but did not open it until I got ready to bend it. One of the ends was smashed down to reduce the diameter by one half. I sized a metal plug with a diameter slightly less than the tube. A length of all-thread screwed into the plug with a wooden handle held at the other end of the all-thread. I drilled a 7/8" hole through a block of maple and sawed it in half. Then I tapped the plug into the opening so it would enlarge the smashed area about 1 foot from the end of the tube. I used the two wood blocks to reform the outside shape. Repetitive operations of flame, tapping the plug further into the tube, and crimps with the wood blocks made the copper reform to circular shape again. Came out good.

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