Machining /CNC 6

[IM TechMech]

thread404-446842

Hello,

I have a question similar to the one in the referenced post:

I need to make a tube (will be used as the top tube for a bicycle) out of steel or aluminium or both that have varying diameters.

It needs to be one piece (or as strong and as stiff as one piece).

The difference between my question and the one in the referenced post is that the tube I need to make has to be hollow. It is for a bicycle.

Would machining work ? Is there any other way to do it than welding together two tubes and a washer ?

Thanks...

 

[BrianPetersen]

Machining something of that shape with such thin and flimsy walls will at best be wasteful of material and will certainly be difficult - it will be like machining a wet noodle.

Hydroforming comes to mind, particularly if you are wanting to make a nice-looking continuously-varying profile along its length, and if you do it that way, the cross-section doesn't even have to be round.

 

[3DDave]

https://www.reynoldstechnology.biz/materials/how-butted-tubing-is-made/

 

[mfgenggear]

IM TechMech

a sketch is a thousand words, can you supply a drawing and quantity

 

[hendersdc]

The typical bicycle top tube with a complex shape is made using hydroforming.

 

[mfgenggear]

wikipedia
Quote: Tube hydroforming
In tube hydroforming there are two major practices: high pressure and low pressure. With the high pressure process the tube is fully enclosed in a die prior to pressurization of the tube. In low pressure the tube is slightly pressurized to a fixed volume during the closing of the die (this used to be called the Variform process). Historically, the process was patented in the '50s,[7] but it was industrially spread in the 1970s for the production of large T-shaped joints for the oil and gas industry. Today it is mostly used in the automotive sector, where many industrial applications can be found.[8][9] It is also a method of choice for several tubular members of bicycles. In tube hydroforming pressure is applied to the inside of a tube that is held by dies with the desired cross sections and forms. When the dies are closed, the tube ends are sealed by axial punches and the tube is filled with hydraulic fluid. The internal pressure can go up to a few thousand bars and it causes the tube to calibrate against the dies. The fluid is injected into the tube through one of the two axial punches. Axial punches are movable and their action is required to provide axial compression and to feed material towards the center of the bulging tube. Transverse counterpunches may also be incorporated in the forming die in order to form protrusions with small diameter/length ratio. Transverse counter punches may also be used to punch holes in the work piece at the end of the forming process.

Designing the process has in the past been a challenging task, since initial analytical modeling is possible only for limited cases.[10] Advances in FEA and FEM in recent years has enabled hydroform processes to be more widely engineered for varieties of parts and materials. Often FEM simulations must be performed in order to find a feasible process solution and to define the correct loading curves: pressure vs. time and axial feed vs. time.[11] In the case of more complex tube hydroformed parts the tube must be pre-bent prior to loading into the hydroforming die. Bending is done sequentially along the length of the tube, with the tube being bent around bending discs (or dies) as the tube length is fed in. Bending can be done with or without mandrels. This additional complexity of process further increases the reliance on FEM for designing and evaluating manufacturing processes. The feasibility of a hydroforming process must take into consideration the initial tube material properties and its potential for variation, along with the bending process, hydraulic pressure throughout the forming process, in inclusion of axial feed or not, in order to predict metal formability.

 

[mfgenggear]

for round tubes , and for a single parts cnc tube bending

 

[IM TechMech]

Thank you guys a lot. Sorry for my late login.

I guess hydroforming would be the way to go. I wonder if that is possible to do for quantities like mine (around 50 tubes in one run).

Below is a very modest sketch of the tube. It is a top tube of the bicycle. I want it to be particularly thick from the front (to lay a battery inside and for aesthetics - like a cannon) and it has be thinner towards the seat because the legs of the rider need to clear out from it while pedalling and also for ergonomic aesthetics - (like a rocket)

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From the internet, I found Phonenix 3D metaal in the Netherlands. I will contact them to see.

Do you recommend anyone to do it for me ? Preferably in the Netherlands, Germany or Belgium (So that I can go driving and easily transport tubes) ?

 

[IM TechMech]

Drawing

 

[kjoiner]

There is a video on Youtube where someone hydroforms a part using a pressure washer as the source. Another possible way is to fill the tube with water and put it in a freezer. That could be difficult to control, however.

Kyle

 

[hendersdc]

For simple, axis-symmetric diameter changes like what is shown in your sketch you could look at starting with the large diameter pipe and reducing it. You would be looking at a larger scale version of what is occurring in this video: [URL unfurl="true"]https://www.youtube.com/watch?v=h_AD78aPZIY[/url].

You would need to get rid of the sharp corners and right angle transition but this could be significantly cheaper than hydroforming in small runs if you find a shop with the machine and dies on hand already.

 

[BrianPetersen]

The step in diameter (and the magnitude of that step) is not good for the strength of the completed workpiece, nor is it good for practical hydroforming or "shrinking/expanding" operations. Redesign the part.

 

[Tmoose]

IF I understand the uploaded image correctly the small diameter tube is just stuck on the flat disk end of the larger tube. As drawn, The bending stiffness and the axial stiffness of the top tube will depend 100% on the thickness of the flat disk.
As other said it will likely be VERY flexible.

Some experiments with a bolt with washers and nuts and a tin can will make this obvious.

When bearing journal stub shafts installed on the ends of process roll some serious internal details are required. Double bulkheads at each end or some form of VERY thick end plates are common

 

[durablack2]

Couldn't you cast it and machine the outside down? Does the inside surface roughness matter to you?