Thick wall steel tubing, 4mm ID? Or a better idea?

[BTB 500]

I need to join two 86 cm lengths of 4 mm diameter carbon fibre rod end-to-end to make a hoop of 55 cm diameter. Carbon fibre rod is pretty tough stuff and my attempts so far (with the materials I had to hand) haven't been too successful!

I first tried brass tube - this simply snapped with a bang (failing in tension on the outside of the curve, as you'd expect).

I had some 4 mm ID carbon fibre tube, so I tried reinforcing that with several alternating layers of lengthwise carbon tow and wound carbon tow. That held but (much to my surprise) bent, to about a 25 degree angle.

So ... I'm thinking some kind of thicker wall seamless metal tubing, but I'm not an engineer so would welcome any advice on material and source. I only need a short length (maybe 8 cm total) ... Googling the interweb it seems that most suppliers are 'industrial' and minimum order quantities tend to be about 5 metres And what material would be best? Steel, stainless steel, titanium?

 

[wemorgan]

It sounds like you need a steel with a high yield stress. For reference mild steel yields ~200MPa, Boron steel after bake hardening ~1000MPa, some even higher.

Steel tube with a yield of ~500MPa shouldn't be too hard to find, but I suspect as you mention the minimum order will be an issue.

Are you able to find a local fabricator will will sell some off-cuts?

If you're in no rush I'll try asking some guys at work what method they would use.

 

[WDB124066]

You could try some UNS N06625 tubing the same way you tried the brass.

Very strong, very tough and will stay shiny - colour like stainless steel.

Tell them you'd like a sample, 80 mm is just within the "or go on, have it" range......

 

[LTD]

Am I right in thinking these are for internal joining pieces ?

Wouldn't you be best getting some 4mm stainless rod and fashioning it into the curve profile you need and then inserting it into the carbon fibre rather trying to bend it after it's inserted ?

 

[BTB 500]

Thanks for the suggestions so far guys.

It's not vital that the hoop is exactly circular, so short straight sections (3-4 cm) at the joins would be OK. Ideally I'd have done it with a single piece of rod and a single join but 1.5 metres is the longest you can easily get carbon rod in, and that was too short.

Just to clarify, it's solid 4 mm diameter rod, so the joiner needs to be external. I found some stainless steel tube that is 1 mm wall thickness but no idea whether that would be strong enough! Minimum order was 3 metres anyway

Good ideas about samples/fabricators.

 

[GordonTarling]

Bill - 4mm carbon rod is pretty tough stuff - could you not use 3mm? I've bought small pieces of metal tube/rod/sheet from fleabay in the past, but the problem with that is that postage costs can be quite high. Aluminium is likely to be too weak, unless you get some thick-walled stuff, so I'd go for steel or stainless steel. Model engineering suppliers also have small quantities of various metals - The College Engineering Supply or try Home, Noggin End Metals, Stoke-on-Trent both are useful and reliable.

PS I'll have a look in my workshop later and see if I can find anything suitable.

 

[balge]

How about some 4mm bore carbon fibre tubing?

 

[bolide]

My first thought was how will you anchor the tube to the CF rod? If one of the rods popped out the tube would become a projectile if it parted company with the other rod as it uncoiled

I would think an 8 mm diameter metal tube about 12 mm long, propelled by an uncoiling piece of carbon fibre would look and act exactly like a (slow) bullet

Nick Froome

 

[BTB 500]

Thanks again guys.

I have 4 mm bore CF tube. This would be useless as standard - the fibres run lengthways so it would split apart (like bamboo). As per the OP I tried reinforcing it with CF tow but it deformed under load.

No issue with 'anchoring'. The sideways load from the CF rod will lock it solid in any tube that's man enough to hold it Apart from that the hoop is going to be anchored in 6 places anyway.

Gordon - this is to form an arm brace for my F550 - the rod will be anchored under each motor mount. 3 mm rod would be better than nothing, but 4 mm is a fair bit stiffer for minimal weight penalty so I'd like to use that if I can.

 

[BTB 500]

The College Engineering Supply

They have the stainless steel tube I spotted earlier (6 mm OD, 1 mm wall ... so 4 mm ID) in 12" lengths

For £3.66 that's got to be worth a try, failing anything else.

 

[Bellow]

What is the maximum acceptable overall diameter of the metal joining pieces you intend using? Steel with thick enough wall should suffice. If the diameter has to kept low, you may struggle.
You will also have to watch for failure where whatever tube you use ends and the carbon rod is then on its own as there may be a concentration of stress at that point (read 4 points).

 

[BTB 500]

The OD of the connecting pieces isn't critical, but I'd like to keep the weight down.

Yes there will be stress points in the carbon rod at each end of the tube. I'm going to be careful to debur the tube, for sure!

 

[Bellow]

Just up the diameter of your connecting pieces.
Unless you go for something exotic such as titanium (very expensive and difficult to work) you will just have to accept whatever weight the required strength dictates.
I'd find a small machining shop to drill (steel or aluminium)bar with 4mm hole and get their advice re wall thickness. They will have the 'feel' for it.

 

[mattc]

I will have a look at work whether we have some small tube like that. I work for an American titanium producer

 

[Bellow]

Another possibility may be (if the rods are sufficiently long) to overlap the ends and clamp them. Bulldog grips as used for wire ropes (or a variation ) may get the job done. Not as neat as tubing though.

 

[balge]

Thanks again guys.

I have 4 mm bore CF tube. This would be useless as standard - the fibres run lengthways so it would split apart (like bamboo). As per the OP I tried reinforcing it with CF tow but it deformed under load.

No issue with 'anchoring'. The sideways load from the CF rod will lock it solid in any tube that's man enough to hold it Apart from that the hoop is going to be anchored in 6 places anyway.

Gordon - this is to form an arm brace for my F550 - the rod will be anchored under each motor mount. 3 mm rod would be better than nothing, but 4 mm is a fair bit stiffer for minimal weight penalty so I'd like to use that if I can.

You need wrapped tube, not pultruded ? - hate that word...

 

[GordonTarling]

Bill - I've PM'd you, but if some titanium tube of the right size is on offer, you might want to take it.

Balge - finding wrapped tube in those smaller sizes is almost impossible if you only want small quantities.

 

[BTB 500]

Thanks again guys.

CF rod is already cut (and I don't have any more), so it has to be a butt join really.

Yup I couldn't find any wrapped/wound carbon tube in that diameter. Hence my DIY attempt with the pultruded core

 

[balge]

I thought it would be available, it's used a lot for r/c helicopters, but as you say apparently not down quite that small...

Carbon Fibre Tube - Roll Wrapped - Easy Composites

 

[wemorgan]

I had some 4 mm ID carbon fibre tube, so I tried reinforcing that with several alternating layers of lengthwise carbon tow and wound carbon tow. That held but (much to my surprise) bent, to about a 25 degree angle.

Colleagues at work say you need to mount the carbon fibre joint in to a jig to hold its shape. Then apply compression to the joint to allow it to set - try tightly winding a bicycle inner type around it. Apply heat then leave to cure for several days.

The steel tube method sounds easier though......