# Small mandrel bender



## keith5700 (May 29, 2013)

Whilst on a slow period with my V8 I have been designing my next engine, which will be a 1/3 scale V10.
Remembering what a nightmare the V8 exhaust manifolds were to make, I decided to have a go making a mandrel bender for the V10 exhaust.

The tube size is 14mm o/d x 1mm wall.

Hopefully the principle is self explanitory from the pics.
I cut the half diameters with a 14mm ball nosed end mill, after hogging as much steel away with plain end mills.

The mandrel bit, which goes inside the tube, is just hardened silver steel, sized to be as close a fit to the bore as possible.

I did 4 trial bends, the first without the mandrel inside the tube, and 3 more with the mandrel pushed a bit further past the tangent point of the bend each time.
Pushing the mandrel in any further than the last photo resulted in a lock up of the mandrel in the bend, so that dictated the limit of its forward position.

The dimensions written on the tubes is the thickness after bending, from the i/d to the o/d. I was hoping for the best one to be a bit closer to 14mm, but it's not too bad.

There is still a kink in the i/d at the start of the bend. The mandrel won't prevent this. I still need to see if there is any way of reducing this.

I have just got some ally tube to try. If that doesn't kink then it's just a consequence of the stainless being much harder to pull round the former than the ally.

I know full sized benders have a wiper to reduce the kinking here, but I doubt that's going to scale down very well.


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## ConductorX (May 29, 2013)

I am not a metal expert but I have bent and installed miles of stainless and copper tubing.  The commerical benders have a limited radius based on the size of the tubing. If you try to make a tighter bend you risk tubing failure or a bad bend.  I know the goal here is scale models but you have to work with the limitations of the material as well.

IMHO "G"


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## Lew Hartswick (May 29, 2013)

ConductorX said:


> I am not a metal expert but I have bent and installed miles of stainless and copper tubing.  The commerical benders have a limited radius based on the size of the tubing. If you try to make a tighter bend you risk tubing failure or a bad bend.  I know the goal here is scale models but you have to work with the limitations of the material as well.
> 
> IMHO "G"


  Some things don't scale well and some (like material properties) don't scale at all. 
   ...lew...


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## Septic (May 29, 2013)

ConductorX makes a good point... 

I too have spent decades working with tubing of various sizes and compositions and deviating from common practice often introduces weaknesses that aren't immediately obvious.


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## keith5700 (May 30, 2013)

I hear what you all say.
I know the bend radius is very tight, hence the idea of making the mandrel bender, as opposed to an ordinary type bender, or filling the tubes with low melting point alloy.

I know the bend radius I want, which will look the best, and am now trying to produce that radius.
Actually I'm not that far off what would be acceptable. At the moment there is 16 thou' of crush.  If I can get that down to 10 thou' then I'd be happy.

I am going to anneal the stainless, in case that makes a difference. 
Slightly tighter fit of the mandrel, ie. bigger diameter.
More grease in the tube so maybe I can push the mandrel further up the tube.
Cheers.


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## MachineTom (May 30, 2013)

In large diameter tubing bending, the inner mandrel is a cable with a series or Torus shaped disks, sort of like a doughnut strung on the cable. A hydraulic cylinder tensioned the cable and disks, this allowed the disks to pivot on the cable as the tube was bent.

It seems that you could make up a similar device for you small tubes.


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## johnny1320 (May 30, 2013)

I have often wondered if using a hydraulic hose under pressure inside the tubing would work for this problem.     P.S. Keith5700 awesome workmanship!


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## Lockstocknbarrel (May 31, 2013)

Hi Keith,
Impressed with you're craftsmanship, have you thought of filling the tube with fine sand and plugging the ends to limit the distortion, sorry somewhere in the old grey matter of my brain I remember the "Old Gunsmiths" used the method to reduce the problem that you are striking.
Please give it a go and let me know.
Kindest Regards
Beagles


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## jwcnc1911 (May 31, 2013)

Man that is ingenuity at it's finest there!  Necessity is the mother of invention.  I saw something similar to this in Modellbau, if you want I'll dig it up and send it to.  PM me if your interested.


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## keith5700 (May 31, 2013)

Some general observations rather than answering specific points:-
The effort required to pull the tube round is far more than I had anticipated. I used a 30" overall length lever, and I had to brace my foot against the bench to stop me pulling it over. I think the forces involved are more than hydraulics or sand could withstand, by a factor of 10 at least. 
I have seen the torus shaped discs and they seem to be a good idea, but I think I'd struggle to scale this down into a 12 mm bore, although it may be possible. Maybe a 3mm cable would be enough to drag the ring through.
On a similar theme I wonder if I could force some 12mm ball bearings through after bending.

JW, I would certainly be interested in seeing that feature if you can find it.
Cheers all, hope to have some V8 news soon.


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## Scott_M (May 31, 2013)

Hi Keith5700

I remember seeing an episode of "How it's Made" where the were making Trombones and the method they used, now keep in mind this was thin wall brass tubing. They would fill the tube with melted pitch (I think) and let it solidify and then mandrel bend. They did not need to cap the ends. After bending they would melt the pitch out and clean the bent tubes. Here is the cool part , They would put the bent tubes , in this case a "U" shape, inside of a two piece mold with the proper O.D. dimensions of the bent tube. This mold then went under a lever operated press. They would press steel balls through it, all coated with an anti-seize lube. I think they used a series of 3 different size balls. And to get them back out they would press in the smaller sized balls to push out the last big ones. It got those mandrel dents out nicely.

I have no idea how this would work with 1mm wall S.S. tube.  Food for thought.

Scott


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## johnny1320 (May 31, 2013)

Scott_M said:


> Hi Keith5700
> 
> I remember seeing an episode of "How it's Made" where the were making Trombones and the method they used, now keep in mind this was thin wall brass tubing. They would fill the tube with melted pitch (I think) and let it solidify and then mandrel bend. They did not need to cap the ends. After bending they would melt the pitch out and clean the bent tubes. Here is the cool part , They would put the bent tubes , in this case a "U" shape, inside of a two piece mold with the proper O.D. dimensions of the bent tube. This mold then went under a lever operated press. They would press steel balls through it, all coated with an anti-seize lube. I think they used a series of 3 different size balls. And to get them back out they would press in the smaller sized balls to push out the last big ones. It got those mandrel dents out nicely.
> 
> ...


   Scott you read my mind, I was just coming here to post it and saw your post LOL :wall: they used one ball the diam of the inside and smaller ones behind, but it was also in a die the shape of the "u" bend so it would be hard to make a die the shape of the header pipe, it may still work but it might straighten out the bend.


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## Scott_M (May 31, 2013)

johnny1320 said:


> but it was also in a die the shape of the "u" bend so it would be hard to make a die the shape of the header pipe, it may still work but it might straighten out the bend.


 
I am not sure if Keith has CNC mill, If he does it shouldn't be a problem.
If he doesn't, It would be pretty simple to make a 2 piece mold/die with some fast drying epoxy or metal filled Devcon.

Scott


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## /// (May 31, 2013)

keith5700 said:


> I think the forces involved are more than hydraulics or sand could withstand



Hydraulic should be no problem, but I agree on the sand part.
I tried sand once and the grains just crushed, too course so too many voids between the individual grains.
I thought to try a fine grade of silicon carbide or aluminium oxide, say 1200 mesh or even 3 micron powder, but never got around to it.


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## gabby (Jun 1, 2013)

Hi guys, my 2 bobs worth, In the refrigeration trade and plumbing, we use heavy bending springs that allow for multiple bends in a short length of tube (mainly copper though) but this concept could be adapted to suit our needs as long as the spring used is strong enough and won't crush with the tube then you are guarantied to get a perfectly formed (round profile/ cross section) bend.
It works very well with hard drawn copper as well which would be comparable with light wall Ali.
Like I said, just my 2 bobs worth.
Cheers
 Graham :hDe:


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## gabby (Jun 3, 2013)

Sorry, the springs are an internal sizing and don't mark the tube at all.
Cheers
Graham


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## dr_kingston (Jun 3, 2013)

Hi Keith5700,
To remove the deformity in pipe bends, you can blow it out with CO2 or welding grade argon etc, set to about 500 psi, then capping the ends and gently heating the effected area until the deformity has disappeared. If you over heat the deformed area you can blow a bubble which is sure to get your attention. 
 Regards Dale


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## keith5700 (Jun 6, 2013)

I've just got sidetracked again so haven't tried anything to reduce the kinking.
I think the first thing will be to try and force some balls down the bore, but without the outer die. 
It would be a major project to make 10 different 3D outer dies, even with a CNC, which I haven't got.

Internal springs are good, but with this bend radius, and with stainless tube, I'm not convinced the spring would ever come out again,
unless the tube was overbent quite a lot, and then straightened up a bit. If I had some snug fitting 12mm O/D spring I'd give it a go though.

Heating with internal gas pressure sounds interesting, but I wouldn't have the nerve to try it.

I'll post back when I'm on it again.
Cheers.


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## dr_kingston (Jun 6, 2013)

Hi Keith5700,
                Don't let fear hold you back.
I have made tapered pipes by this method and repaires expansion chambers after removing all of the combustibles from inside the pipe.
Pressure and temperature control are the only skills that are required.
Basically the metal is drawn by the heat and the internal pressure so be carefull as it is easy to over do it.  
Cheers.


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## lohring (Jun 9, 2013)

It's a lot safer to use water rather than gas to expand out dents.  A pressure washer or plumber's pump can supply the pressure.  If something bursts there's a lot less energy to be released than with compressed gas.  By the way, that mandrel bender is a great idea.  I've not seen any nicer results in model race engine exhaust headers even from commercial benders.

Lohring Miller


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## etazelaar (Jun 9, 2013)

The melted pitch is a good idea. But, I remember seeing brass instrument brass tubes being bent after water was frozen inside. It can't can't compress like pitch. It can't move like large bearings or sand. I look forward to see your work and your answer to this problem.


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## dr_kingston (Jun 10, 2013)

lohring said:


> It's a lot safer to use water rather than gas to expand out dents. A pressure washer or plumber's pump can supply the pressure. If something bursts there's a lot less energy to be released than with compressed gas. By the way, that mandrel bender is a great idea. I've not seen any nicer results in model race engine exhaust headers even from commercial benders.
> 
> Lohring Miller


 
Water pressure will not remove the crush or kinks in the pipe as Keith has discribed. Anything hydraulic has the same pressure in all directions. Steel balls forced through the pipe with the effected area heated will work. 
Cheers Dale


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## johnny1320 (Jun 11, 2013)

dr_kingston said:


> Water pressure will not remove the crush or kinks in the pipe as Keith has discribed. Anything hydraulic has the same pressure in all directions. Steel balls forced through the pipe with the effected area heated will work.
> Cheers Dale


 

my intension was to use hydraulic pressure to stop the pipe from collapsing during bending, not to un kink after the fact, but I do think the ball method is the way to go, I would anneal the pipe and try that first. and heat if that didn't work.


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## walnotr (Jun 11, 2013)

I have bent aluminum, 304 stainless and titanium on machines with a mandrel, wiper die and follower. The mandrels all had between 1 and 4 "balls" that would allow the mandrel to extend past the tangent point of the radius.  Even with that it was at times pretty difficult to avoid the hump.  The primary hand-bending machine had a 4:1 mechanical advantage built into it which helped when bending larger diameters.  There was also a hydraulic extractor for the mandrel if needed.  I agree the wiper die would be a real challenge to make but would help smooth the bend.  I have never tried the various "packing" techniques but am intrigued by the frozen water idea.  It looks like you are almost there with your bender.  I look forward to seeing the finished product.


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## walnotr (Jun 11, 2013)

One other thing I just noticed.  It doesn't appear you are using a clamp block to hold the tubing.  Part of the cause for the "hump" is the tubing slipping during the bend and not being completely drawn along the mandrel. In other words, the tubing should be pulled along the mandrel and not just held against the radius.

Steve C.


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## johnny1320 (Jun 11, 2013)

etazelaar said:


> The melted pitch is a good idea. But, I remember seeing brass instrument brass tubes being bent after water was frozen inside. It can't can't compress like pitch. It can't move like large bearings or sand. I look forward to see your work and your answer to this problem.


 
I have seen this method as well but they add soap to the water to make it flexable or else the ice just cracks.


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## lohring (Jun 12, 2013)

dr_kingston said:


> Water pressure will not remove the crush or kinks in the pipe as Keith has discribed. Anything hydraulic has the same pressure in all directions. Steel balls forced through the pipe with the effected area heated will work.
> Cheers Dale



True, but using gases to inflate pipes is dangerous.  There's a reason boilers are pressure tested with water.  Look at the Mythbusters episode where a cylinder containing gas under pressure penetrates two cement block walls when the valve is sheered off.

Lohring Miller


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## dr_kingston (Jun 12, 2013)

lohring said:


> True, but using gases to inflate pipes is dangerous. There's a reason boilers are pressure tested with water. Look at the Mythbusters episode where a cylinder containing gas under pressure penetrates two cement block walls when the valve is sheered off.
> 
> Lohring Miller


Hi Lohring,
I use CO2 gas so that I can *heat* the effected area to remove the distortion. Hydraulic testing of boilers has always been the norm but this process will not do the job we are asking. I do not use 3000 psi, I have done jobs with as little as 50 psi to blow out 1mm thick steel tube and pipe diameters up to 50mm.
The Mythbusters get it right sometimes.
Cheers Dale


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## Scott_M (Jun 12, 2013)

In this thread      http://www.homemodelenginemachinist.com/f31/1-4-scale-v8-first-project-13548/index3.html  Keith uses  Cerrosafe  "Woods Metal" low temp alloy to fill his pipes before bending.  And judging from his results it worked pretty good !!  

Scott


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## johnny1320 (Jun 12, 2013)

Scott_M said:


> In this thread      http://www.homemodelenginemachinist.com/f31/1-4-scale-v8-first-project-13548/index3.html  Keith uses  Cerrosafe  "Woods Metal" low temp alloy to fill his pipes before bending.  And judging from his results it worked pretty good !!
> 
> Scott



Yes they did turn out nice, but Keith5700 was the one that started this thread to improve on the method.


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## Scott_M (Jun 12, 2013)

johnny1320 said:


> Yes they did turn out nice, but Keith5700 was the one that started this thread to improve on the method.


 

Doooh  !   oh:

Scott


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## keith5700 (Jun 13, 2013)

I've not had chance to try any new ideas out yet, as some of you may have noticed , I'm back on the V8 for the time being.
The discussion on this thread is very interesting however, and there are some things I want to try out when I get the time. The first thing will be to check if the tube is sliding back when pulling round the die, and causing the distortion, as has been suggested.

It would probably need a redesign and a new die to get a clamp on the tube in this area, but would be worth it if this was the cause.

The Cerrobend idea gave good results, but was very time consuming and extremely messy.
Cheers.


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## Gedeon Spilett (Jun 13, 2013)

You may have a look at the way music instrument maker now repair dents in brass saxs, trumpets or others instruments : with a strong neo magnet outside and a polished steel ball inside (many videos on the Net). It looks really magic with thin brass, but with steel  the magnet would not work so well...

 ie this video 
[ame]http://www.youtube.com/watch?v=LJi_QzGY--k[/ame]


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## Bob_P (Jun 16, 2013)

Hi Keith
I've been reading up pre-bending some small copper pipe. Suggestions are filling the tube with salt and plugging it. Perhaps better thanks sand as its finer, as long as its flushed well. Also have a look at "woods metal", I haven't used this but perhaps others could comment?
Bob..


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## keith5700 (Jun 16, 2013)

Hi Bob,
Woods metal is another name for Cerrobend, or low melting point metal, which is what I have been using before the mandrel bender.
As for sand or salt or ice,etc. which could be packed into the tube before bending, I just can't believe that this method would do anything positive, when I take into account the massive forces involved when pulling the tube round the die.
The way I look at it in my head is that the thing that's inside the pipe would need to be rigid enough to prevent kinking the tube if I was to give ita good clout with a steel hammer.

The steel mandrel would, of course, but I can't imagine anything deformable would.
Cheers.


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