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Making a tapered aluminum tube

laukejas

Plastic
Joined
Feb 27, 2022
Hi,

I am quite new to machining and metal work in general, and I'd like to ask for some advice about making a round tapered (cone shaped) aluminum tube. The tube in question is meant to be a mast for a sailboat, 5.5 meters in length, 70 mm in diameter at the bottom, gradually tapering to around half of that (35 mm) at the top. Tapering is done in order to save weight, which is important for sailboat stability - every gram counts. Wall thickness of said tube would probably be 3 mm.

Shape requirements are pretty relaxed. The tapered profile does not have to be perfectly round - it can have some teardrop shape, although the closer to the round, the better to resist sideway bending forces.

I am exploring options on how to manufacture such a tube, considering that there are no such tubes readily available for purchase (yes, I have looked up flag poles, street light poles, etc., but they are never the right dimensions).

I have considered making a "stepped" reduction in diameter (making this mast from sections of different diameter aluminum tubes that slide inside one another, like a fishing pole), but the required overlap between these tubes to have sufficient strength in the joints overcomes any weight savings of the tapering, making overall weight even higher. Additionally, these "steps" would create a lot of inconveniences when raising and lowering sail on the boat.

My idea - not sure how realistic - is to take a 70mm x 3mm tube, split it lengthwise with a V-shaped gap. This gap would be near-zero width at the point where the taper starts (around 1m above the bottom end of the tube),and then gradually get wider towards to the top. Then, the idea is to press the tube with clamps so that the edges of this gap would come into contact, and then weld them together again. The difficult part would be clamping this tube with sufficient force and precision.

Can anyone comment on this, do you believe this is a realistic approach to turn a round straight tube into a tapered tube? If not, can you suggest any alternatives to make tapered aluminum tube that are possible to do with typical DIY tools? I have angle grinders, welding equipment, 1 ton hydraulic press, a lathe (not large enough for 5.5m long tube, though!), and some other typical equipment, but no special machinery for this task.
 

laukejas

Plastic
Joined
Feb 27, 2022
I'd learn how to spiral wrap carbon fiber tape over a male plug if I was you. More expensive material, but with a little practice far better results if weight is your priority.

Carbon Fibre, Woven Tape, 232g/m2, Plain weave, 150mm (6") wide | Bucks Composites

Easy Composites Ltd - YouTube

Hi Milland, and thank you for your rapid answer! Yes, I am very well aware of this technology, and I am exploring various options including this one, but I'd like to focus on tapered aluminum option in this forum topic, since this is a machinist forum :)
 

guythatbrews

Cast Iron
Joined
Dec 14, 2017
Location
MO, USA
You got a tiger by the tail there bud. Maybe you should post in the fabrication section.

Pretty sure if you split a wedge out, smash together, and weld the thing it will curl up toward the weld something fierce. Also not a good idea to cold form heat treated aluminum. It will break sometime in the future.

You may be able to split two diametrically opposed wedges out and weld back together. Much easier than smashing and little warpage.

Somewhat OT here but check out ultimate restorations on amazon. A very interesting show showcasing giant, unusual, high dollar restorations. My fave was a turn of the century wooden ship the Coronet. Fans of Patrick O'Brian (I am) will love it! They made a hollow octagonal mast and it was LONG! The segents were keyed together with loose splines and glued. Then the glue-up was placed between centers and hand worked to round profile. Sadly you gotta pay to watch on amazon now but link below to site. These guys are WOODWORKERS! But maybe they would rather be called boatwrights.

Coronet1885
 

laukejas

Plastic
Joined
Feb 27, 2022
You got a tiger by the tail there bud. Maybe you should post in the fabrication section.

Pretty sure if you split a wedge out, smash together, and weld the thing it will curl up toward the weld something fierce. Also not a good idea to cold form heat treated aluminum. It will break sometime in the future.

You may be able to split two diametrically opposed wedges out and weld back together. Much easier than smashing and little warpage.

Somewhat OT here but check out ultimate restorations on amazon. A very interesting show showcasing giant, unusual, high dollar restorations. My fave was a turn of the century wooden ship the Coronet. Fans of Patrick O'Brian (I am) will love it! They made a hollow octagonal mast and it was LONG! The segents were keyed together with loose splines and glued. Then the glue-up was placed between centers and hand worked to round profile. Sadly you gotta pay to watch on amazon now but link below to site. These guys are WOODWORKERS! But maybe they would rather be called boatwrights.

Coronet1885

Good point about warping! I thought that if I clamped that aluminum tube against flat floor it wouldn't warp while welding, but your suggestion - two opposed wedges - makes even more sense. But that would mean that at the top I wouldn't have a 35mm round section, I would have something like 35x70mm almond-shaped section. I suppose it wouldn't be possible to form it back into a near-circular profile without breaking, right?

I know of that woodworking technique, I've built quite a few masts like that in the past. But quality wood is becoming really difficult to get, and the amount of work required for a wooden mast like that is astronomical. Aluminum has pretty much the same strength for weight (I've done the maths), but is readily available for purchase in the dimensions and requires no treatment, no maintenance. Ideal in every aspect. I just need to put a taper on it...
 

steve45

Stainless
Joined
Jan 16, 2012
Location
Midland, Texas
I'm with Milland, if you must make your own mast, make it from carbon fiber or fiberglass.

There are many varieties of masts available that will be straight and strong. You can get 2-piece masts in aluminum, fiberglass, and carbon fiber. It would seem to me that a windsurfing mast would probably be suitable for your needs. You can also specify the stiffness.
 

laukejas

Plastic
Joined
Feb 27, 2022
I'm with Milland, if you must make your own mast, make it from carbon fiber or fiberglass.

There are many varieties of masts available that will be straight and strong. You can get 2-piece masts in aluminum, fiberglass, and carbon fiber. It would seem to me that a windsurfing mast would probably be suitable for your needs. You can also specify the stiffness.

Guys, look, I appreciate these suggestions, but I've been building boats for 8 years, and I wouldn't be here if I hadn't exhausted all other options :D Wood is very labor intensive, carbon fiber is extremely expensive, and fiberglass (like carbon) requires a very complicated, straight, smooth mold that is just not worth building for a single mast. I know because I have explored these options in great depth. Off-the-shelf masts like the one I need do exist, but they run for thousands of dollars/euros, and they still don't fit the exact specifications for my boats because they are designed for other boats. And I actually have tried windsurfing masts - even the stiffest ones are nowhere near the stiffness required for a boat mast, even with additional stiffening (fiberglass/carbon). So like I said, let's stick to aluminum here :)

Welded aluminum the full length....repeated reversal loading in bending....no fatigue limit....
What is Fatigue Limit - Fatigue Strength - Definition | Material Properties

What flavor of aluminum did you have in mind ?

Thanks for that link! As for the flavor - the only one that is available in my country, 6060. I would much prefer 6065 or 7075 T6 (aicraft grade), but there is no chance in hell to get anything like that shipped here, unless for the cost of several of my boats combined. Even 6061 is only available in small sheets - no tubing of any kind. So, 6060 it is.
 

Gordon Heaton

Stainless
Joined
Feb 19, 2007
Location
St. George, Utah
. . .Shape requirements are pretty relaxed. The tapered profile does not have to be perfectly round - it can have some teardrop shape, although the closer to the round, the better to resist sideway bending forces. . .

Make two identical flat tapered halves. Bend all long edges 'up' about 30 degrees, forming a shallow trough. Fit the halves together and weld the resulting V between halves. Weld plates on the ends, one with a fitting suitable for injecting water under pressure. Using water (NOT air) pressurize the assembly until you achieve the desired shape. The welds and HAZ will be soft for a while, and easily formed.

I've use this process on steel with good results. You can use a pressure-washer to supply the water. While aluminum warps less than steel during welding, I'd recommend stitch-welding, alternating between sides to keep it straight. The end plates, especially the large one, will deform too, so make the entire assembly a bit longer than necessary to allow trimming.
 

guythatbrews

Cast Iron
Joined
Dec 14, 2017
Location
MO, USA
Everybody is just throwing out ideas here. Welded mast no fatigue limit is very bad for sure.

I know dick about boatbuilding and fiberglass layup. But maybe use an internal jointed foam core. You could turn the segments, join together, and leave in place after layup. Probably be hard to keep straight but you've got a tough job any way you go.
 

laukejas

Plastic
Joined
Feb 27, 2022
Make two identical flat tapered halves. Bend all long edges 'up' about 30 degrees, forming a shallow trough. Fit the halves together and weld the resulting V between halves. Weld plates on the ends, one with a fitting suitable for injecting water under pressure. Using water (NOT air) pressurize the assembly until you achieve the desired shape. The welds and HAZ will be soft for a while, and easily formed.

I've use this process on steel with good results. You can use a pressure-washer to supply the water. While aluminum warps less than steel during welding, I'd recommend stitch-welding, alternating between sides to keep it straight. The end plates, especially the large one, will deform too, so make the entire assembly a bit longer than necessary to allow trimming.

That sounds like a very promising option! I'm just not exactly sure about that first part: why do the halves have to be flat? Wouldn't it be easier to start with a round tube, cut in half with tapering? Starting off with flat plate and making 30 degree lengthwise bend sounds like a very tough challenge...

Also, what kind of water pressure am I looking at for the dimensions I mentioned?
 

Gordon Heaton

Stainless
Joined
Feb 19, 2007
Location
St. George, Utah
. . . Welded mast no fatigue limit is very bad for sure. . .

That's too broad of a brush. Countless critical aluminum parts are fabricated by welding. Material attributes, especially aluminum, change within relatively short periods of time. A proper weld with a suitable filler should be adequate for the job. That's ultimately up to the builder who seems to have considerable experience.
 

Gordon Heaton

Stainless
Joined
Feb 19, 2007
Location
St. George, Utah
. . .why do the halves have to be flat? Wouldn't it be easier to start with a round tube, cut in half with tapering? Starting off with flat plate and making 30 degree lengthwise bend sounds like a very tough challenge...

Also, what kind of water pressure am I looking at for the dimensions I mentioned?

The flat-panel method is just for simplicity. Accurately splitting and tapering a round tube is, in my experience, a very difficult task without specialized equipment. I've not had good results using a bandsaw, and cutting aluminum with an abrasive disc isn't much fun either! Also, bending up edges on flat panels automatically leaves a nice V for a through-weld, although forming that V with a flap wheel after the fact isn't a problem.

Pressure is hard to determine (by me) beforehand. There are many variables beyond dimension such as alloy, heat-treat and weld effects. I'd expect it to require several hundred psi. Also, due to the taper the forming or 'rounding' rates along the structure will vary. I would expect the bottom to round out fairly well, and the small end to remain somewhat oval. Might be a functional and attractive result! If I were you, I would make a small test piece and try to determine maximum safe pressure so that you don't split a seam.

If you are able to split and taper the tube, that would help a lot to achieve a more round and uniform result.
 

Gordon Heaton

Stainless
Joined
Feb 19, 2007
Location
St. George, Utah
One more thought. If you were to use something like a pressure washer for the pump, I suggest injecting the water directly into the part and using a 'dump valve' to limit pressure and flow by simply dumping excess water out of the cavity. This generally works much better than trying to throttle the output of a positive-displacement pump.

edit: I should make it clear that I've not done this with aluminum, but in principle it should work.
 

Ries

Diamond
Joined
Mar 15, 2004
Location
Edison Washington USA
This is the classic "how do I make a working space shuttle with a hammer and a cordless drill, for ten dollars?" kind of question.

If it was easy, everybody would be doing it.

The two classic ways this type of tapered tube would be made are both equipment intensive- not hand processes.

I have worked with a major US manufacturer of tapered poles, in my case steel, for outdoor lighting. And mine were not only tapered, but curved, too.
They have perfected a way to do this with 20 to 40 foot powered rolls, very small diameter for that length, with some pre pressing on a 20 or 40 foot press brake (several hundred tons) with specially made curved dies. They use a series of thin (1" to 2") phenolic donuts, each smaller in diameter than the last, to do the final forming on a press brake, then pull the donuts out the big end with a winch.
Then they weld them with a motorized mig gun.
This gives a relatively low per piece cost, because typical quantities are in the hundreds or thousands.
But the setup price would be in the millions today.

The other way they frequently do this is with a tapered steel inner die, and a cnc swaging machine, that has six or eight "hammers" which simultaneously swage from all sides at once.
Again, millions in setup.

There are cnc tube tapering spinning lathes, too. Still out of your price range. https://youtu.be/idHAxXANdHA

I woud consider a square or hexagon shape, rather than a round- that, you could either just cut flat tapered pieces, and make an inner buck from plywood to hold it in place while welding.

Or, you could bend halves or thirds or quarters, if you had a 6 meter press brake.

The reason that you cant buy one of these off the shelf for a hundred dollars is because its HARD.
Just because you want it, doesnt mean it can be cheap or easy to make.

And, "machining"?
Yes, theoretically you could machine one of these- with a 6 meter gun drill, step drilling. And a 6 meter lathe, turning the outside, from a solid blank.
Again, hideously expensive, and probably would only be done by a goverment if it somehow was needed for a weapon or a space vehicle.

This is fabricating, no matter how you look at it, not machining.
And carbon fiber will be cheaper.
 

Milland

Diamond
Joined
Jul 6, 2006
Location
Hillsboro, New Hampshire
I like the idea of pressure shaping the raw part, however it's done. I'd suggest welding the inlet to the small end as a machined solid, manually "hammer shaping" the tube end round first and sizing the water entry plug to fit tightly before welding it in.

For the large end, unless it's already round due to starting with tubing, I'd use a hemispherical shape rather than a flat plate to close it off. This can be adjusted with a hammer to match the end for welding, and will handle distortion from pressure better than a flat plate would.

Consider having the tube annealed before shaping, and age hardened afterwards to give it better mechanical properties.
 

laukejas

Plastic
Joined
Feb 27, 2022
The flat-panel method is just for simplicity. Accurately splitting and tapering a round tube is, in my experience, a very difficult task without specialized equipment. I've not had good results using a bandsaw, and cutting aluminum with an abrasive disc isn't much fun either! Also, bending up edges on flat panels automatically leaves a nice V for a through-weld, although forming that V with a flap wheel after the fact isn't a problem.

Pressure is hard to determine (by me) beforehand. There are many variables beyond dimension such as alloy, heat-treat and weld effects. I'd expect it to require several hundred psi. Also, due to the taper the forming or 'rounding' rates along the structure will vary. I would expect the bottom to round out fairly well, and the small end to remain somewhat oval. Might be a functional and attractive result! If I were you, I would make a small test piece and try to determine maximum safe pressure so that you don't split a seam.

If you are able to split and taper the tube, that would help a lot to achieve a more round and uniform result.

Thank you for this additional info. But by now I am wondering: wouldn't it be easier to take make all of this from square stock rather than two flat panels? I mean, split square stock into two, taper, form bevel on edges to allow through welding, then weld back together, and finally expand into a cylinder with water pressure? My assumption here is that square stock should be far easier to cut and taper lengthwise than round stock, but wouldn't require doing that 30 degree lengthwise bend.

I like the idea of pressure shaping the raw part, however it's done. I'd suggest welding the inlet to the small end as a machined solid, manually "hammer shaping" the tube end round first and sizing the water entry plug to fit tightly before welding it in.

For the large end, unless it's already round due to starting with tubing, I'd use a hemispherical shape rather than a flat plate to close it off. This can be adjusted with a hammer to match the end for welding, and will handle distortion from pressure better than a flat plate would.

Consider having the tube annealed before shaping, and age hardened afterwards to give it better mechanical properties.

Interesting ideas, thank you! But annealing requires heating up aluminum to ~300°C for hours. How would you suggest I do that on a 5.5 meter long piece? I can't imagine building a heating chamber that big. And how would I perform age hardening?
 

deltap

Cast Iron
Joined
Sep 3, 2004
Location
Wisconsin, USA
Do you build racers or cruisers? High tech materials are more expensive. Metal masts are available from marine suppliers in a wide variety of stock sizes. You can't build one as good. If you are a sailor, a big advantage of metal is lightening protection and it becomes apparent once you experience storm conditions.
 

Milland

Diamond
Joined
Jul 6, 2006
Location
Hillsboro, New Hampshire
Interesting ideas, thank you! But annealing requires heating up aluminum to ~300°C for hours. How would you suggest I do that on a 5.5 meter long piece? I can't imagine building a heating chamber that big. And how would I perform age hardening?

I started a description for how to do it, then decided my method might lead to collapse of the tube in the middle. I think I'd leave this aspect to a professional, or trust your luck to just go with as-welded.
 








 
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