Ninehigh
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 posted on 28/9/08 at 03:33 PM |
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Making it by a noobie
Through reading some of the other topics here I think I'll go with round tubing, but I have no experience with welding. Can anyone reccommend a
fish mouthing tool and what would the best kind of welding that I should learn?
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zilspeed
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| posted on 28/9/08 at 03:37 PM |
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For round tubing, look for a small app called tubemiter.exe
Input the data, print the sheet out, arap it around the tube and cut to the line.
Welding ?
Mig and tig 
Don't be tempted by gasless unless you absolutely must.
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clairetoo
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| posted on 28/9/08 at 03:39 PM |
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If this will be the first chassis you have built , then you're setting off on a very steep learning curve - mig is the easiest to learn , but
round tube is much harder to cut right and weld .
Good luck
Its cuz I is blond , innit
Claire xx
Will weld for food......
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mr henderson
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| posted on 28/9/08 at 04:22 PM |
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This is what you need for the welding
http://www.mig-welding.co.uk/
John
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mark chandler
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| posted on 28/9/08 at 08:31 PM |
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hacksaw and decent file and vice IMHO
I have got plenty of tools which could do this, an assortment of grinders, plasma cutter piller drill etc so have tried. But in the end just cut
accurately then file in a fishmouth.
Regards Mark
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Alan B
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| posted on 28/9/08 at 09:41 PM |
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I wouldn't dream of a round tubing chassis without a holesaw based tube notcher (or better)....waaaay too much work without...and they
aren't that expensive.
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martyn_16v
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| posted on 28/9/08 at 09:42 PM |
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quote:
Originally posted by mr henderson
This is what you need for the welding
http://www.mig-welding.co.uk/
Book yourself on a college course. I just have, and so far I've come further in two evenings than I would in a months of reading off
t'internet.
I must have used nearly as much in materials as I paid for the course already, and I've got 8 weeks to go yet
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Doug68
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| posted on 29/9/08 at 02:55 AM |
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You'll want something like this...
They come in either cheap Chinese or more expensive versions, the cheap one I have has stood up more than well enough.
IF you can accurately measure the required angle, then there'll be next to no filing required to fit.
If you're doing a car with non-exposed tubes though I'd go with square myself.
but there is something WEIRDLY satisfying about neatly fitting fish mouth that square tube can't match.
Doug. 1TG
Sports Car Builders WA
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Ninehigh
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| posted on 30/9/08 at 07:01 PM |
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Yeah I figured getting myself on a course, which I admit I haven't even looked into yet but at least I know what I want to learn now :-D
I've not got a real design at the mo but I wanted to use round tubing for the strength (don't want to orphan my baby by bending it round a
tree!) so to hell with the steep learning curve. I was going to build a trailer anyway so there's my welding practice :-)
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Doug68
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| posted on 1/10/08 at 01:16 AM |
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quote:
Originally posted by Ninehigh I wanted to use round tubing for the strength
Size for size assuming the same material, Circular tube is weaker in tension and compression than Square tube.
Doug. 1TG
Sports Car Builders WA
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Alan B
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| posted on 1/10/08 at 01:51 AM |
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quote:
Originally posted by Doug68
quote:
Originally posted by Ninehigh I wanted to use round tubing for the strength
Size for size assuming the same material, Circular tube is weaker in tension and compression than Square tube.
And lighter by the same factor....
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Doug68
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| posted on 1/10/08 at 08:32 AM |
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quote:
Originally posted by Alan B
And lighter by the same factor....
Only in tension, in tension only cross sectional area is important and not the shape of the cross section.
But in compression the cross section is important as it determines the second moment of area (I). And all other things being equal the greater
the I the greater the compressive force a column will take before collapsing.
For example a Dia 25 x 1.6 tube has an I of 8088mm^4 whereas a 25 x 1.6 box has a I of 13731mm^4 (roughly) so thats a 69% improvement in
compressive strength for a 27% gain in weight.
If you look at most cross section data sheets they quote I so there's no real need to work it out for yourself most times.
Buckling
Second Moment of Area
The other thing is looking at the buckling formula:
F is proportional to the square of the length L so reducing the unsupported length of a member has a disproportional effect on increasing its strength
in compression.
And of course if you trust my math and don't check it yourself you must be mad
[Edited on 1/10/08 by Doug68]
Doug. 1TG
Sports Car Builders WA
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mr henderson
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| posted on 1/10/08 at 09:44 AM |
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However, this discussion has a context, and that is spaceframe car chassis.
Now that there are various people on this thread who have informed opinions, I would like to know why so many skilled chassis designers choose round
tube
John
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Alan B
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| posted on 1/10/08 at 12:01 PM |
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Doug, I have a mechanical engineering degree, so I'm familiar with all that, but thanks anyway.
You are of course correct about buckling, but the usual arguement is to use the round tubes weight advantage and increase it's diameter to boost
it's I value.
Typically increasing the round diameter by 25% will make up for the loss of buckling strength and still be marginally lighter.
There are cases for both round and square...it just depends how you present your arguement....I use both round and square.
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Doug68
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| posted on 1/10/08 at 01:52 PM |
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Hi Alan,
I hope I didn't cause offense I was just trying to bring out some facts and hopefully cause a few people to get curious enough to go properly
research the subject. Rather than having it stuck in their head that X is better than Y.
Can you confirm I got the maths right then?
I've been working for so long implementing software I think my Eng brain has gone soft occasionally.
Doug. 1TG
Sports Car Builders WA
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Alan B
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| posted on 1/10/08 at 03:04 PM |
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Doug,
No problem at all......no offence taken whatsoever..
The only minor point is that both compressive and tensile strength are directly related the cross-sectional area and not I value.
However, the chances of a compressive load causing a pure compressive failure in this scenario are very, very slim indeed, almost certainly a
compressive load would cause buckling failure, and consequently the I value does then come into it. So as far as it really matters you are correct
that I is important.
I completely agree that the X better than Y arguement is quite complex.
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Ninehigh
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| posted on 3/10/08 at 02:33 PM |
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Ok, which one would be less likely to bend, and therefore crush me, when some spanner decides they haven't seen me and rams into the side?
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Alan B
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| posted on 3/10/08 at 03:37 PM |
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On a size for size basis it would be square.....on a weight for weight basis, optimising the OD it would be round.
However, and it's a big however, neither will give you significant side impact protection. If the car is not restricted at the opposite side and
the impact is fairly widespread and even then more than likely you'll just get shunted sideways.
So many variables and unknowns really.....
Is side impact protection a major design consideration for you?
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Ninehigh
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| posted on 3/10/08 at 08:42 PM |
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I'm concerned about safety in general, I'm a fine driver so I'm not worried about ploughing head first into something I'm more
bothered about someone hitting me and the whole frame just bending or folding and doing me in. Like my girlfriend keeps saying I'm "not
insured yet!"
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MakeEverything
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| posted on 4/10/08 at 02:56 PM |
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Ninehigh,
I too have jsut started my build, and am starting my Square tube chassis today.
Personally, i feel that round tube looks better, and offers better strength / weight ratio's, however, you really wont notice the difference in
the type of build that were embarking upon.
Keep it simple is my advice. You could worry about the odd gram here or there, but square tube is good enough for a self build.
If youre worried about what other people are going to do on the road, then scrap it, and spend the money on a season rail ticket or bus pass. There
is only so far you can go to be safe, and to be honest, this type of car is quite a way from that if involved in an accident. The trick would be to
be "Motorcycle" vigilant when driving on the road, and save the thrashing around for the track.
By the way, youll probably blow your current speed record to bits in ths car if it gets finished. Mine is 185 on a bike, and my car wont get anywhere
near that i think.
Kindest Regards,
Richard.
...You can make it foolProof, but youll never make it Idiot Proof!...
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MakeEverything
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| posted on 4/10/08 at 03:04 PM |
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Incidentally, im going for TIG but have a MIG as well.
Tig is very tidy, but probably only once you know the basics on how to weld.
MIG is not so tidy, but is good for filling holes, getting onto hard to reach spots or bodywork repairs. I restored a mini with mine, but it is a
nightmare on clean sheet.
ARMOTO sells a pre-cut chassis metal pack (Square!) which ive bought and start today. He delivers really quick too. Ive yet to look and see that all
parts are there, and of the correct dimensions, bt he does say that if anythings missing, it would be replaced the next day. The cuts and service was
so good, to be honest im not that worried if there is anything missing. Im confident itll get sorted.
Kindest Regards,
Richard.
...You can make it foolProof, but youll never make it Idiot Proof!...
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Ninehigh
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| posted on 4/10/08 at 05:39 PM |
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Yeah good point, mind you I feel more uncomfortable on the train with all those people so close to me...
I'm gonna have to look for a welding course
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MakeEverything
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| posted on 4/10/08 at 08:03 PM |
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I would buy your welder (a good quality one) and visit your local engineering workshop to raid their scrap metal bin. Tell them your working on a
college project or something, and they might even give you a load of 25mm tube!! 
Get the welder and practice, practice, practice. Read as much as you can on the subject, which is all i did. Its quite daunting at first, but all of
a sudden, it will click, and youll have A1 welds 85% of the time. For the other 25%, just grind them back, and try again.
Im about to upload some pictures of the bottom part of my chassis, completed in about 30 minutes. Preparation is key. It helps to work from a datum,
and on a flat surface. For this, ive set up a work platform from 1" ply on stilts and drawn the book chassis dimensions onto it AS A GUIDE
only. Start at the square piece in the passenger compartment, and work forward. The back end will go on last.
Some may disagree, and you might think about it, and prefer to do it a different way. The thing is, there is no set way to build, (though there are
some chicken & egg stages). Its whichever way you feel comfortable. Youll learn quickly by mistakes!! I still am!
Kindest Regards,
Richard.
...You can make it foolProof, but youll never make it Idiot Proof!...
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Ninehigh
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| posted on 4/10/08 at 08:07 PM |
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Well I'm going to have to make a trailer to transport it until it's road legae, so there's some good practice :-)
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MakeEverything
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| posted on 4/10/08 at 08:08 PM |
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Oh, and if you get on a train, just keep scratching your head and slapping the back of your hand as if swatting something then occasionally say:
"Got you ya little B'stard". Theyll soon move!!
Kindest Regards,
Richard.
...You can make it foolProof, but youll never make it Idiot Proof!...
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