mistergrumpy
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 posted on 8/9/06 at 04:46 PM |
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Compound Mitre Joint
Am trying to build the front of the chassis LA, LB, LC, LD? and a compound mitre joint has come up. How the f**king bejesus is this done? I've
been at it since about this time yesterday and still can't get my head around it. I've done the first cuts to end up with a piece like
this /__/ sorta and now I've two more angles to cut, how? Do I rotate the piece through 90 degrees then what. I've somehow managed to cut
some in wood and cocked one of my pieces up as well but they're the wrong angle and too short now. Its really winding me up this. Have done a
search and seen jigs and folded bits of paper stencils but I wanna understand and crack this without. There must be a science to it. Please help as
I've already started losing me hair and I can't afford to pull any of it out. 
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JB
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| posted on 8/9/06 at 05:16 PM |
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Difficult Tubes
I am unsure of the tube you are trying to cut but when ever I shape tubes I get them close with the saw and then angle grind them close and finally
file. It is very rare I can get a tube to fit just by hacksawing unless it is very simple.
Most times there will be at least 30 trial fits, file, try, see where it touches and file a bit more. It may sound long winded but it would be quicker
than what you have done!
I sometimes use cardboard templates to get close.
John
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mistergrumpy
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| posted on 8/9/06 at 05:28 PM |
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The bits I am trying to cut are the side tubes that go on the front triangle shaped nose. They lean outwards but also backwards (or forwards? Depends
which side you look at them from) I'll see if I can get a picture somewhere.
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James
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| posted on 8/9/06 at 05:34 PM |
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Easiest way with these really is to cut each face of the tube individually with a hacksaw.
I tried 3/4 times cutting it in one go before giving up and doing each face individually.
Then just flatten it off with a good file.
Oh, and by the way... you may well want to lean the whole 'L' assembly back a further inch so as to get the bracket sitting on it better.
So compensate for the extra inch off set when you calculate the angle! 
Hope that helps,
James
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"The fight is won or lost far away from witnesses, behind the lines, in the gym and out there on the road, long before I dance under those lights."
- Muhammad Ali
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mistergrumpy
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| posted on 8/9/06 at 05:38 PM |
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This is it. I think that I'm not describing this too well. I have been marking and cutting each face as you say but with this mitre joint
I've absolutely no idea where to start. I've an idea of what it should look like but I can't mark owt as I just can't get my
head around this.
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KENLUDE97
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| posted on 8/9/06 at 06:01 PM |
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Granted i have not tried this but this is how i think that i will try it when i make my frame.
Set up LC and LD at the correct distance apart. Then take a piece of steel the approx length ( and the first angle cut in it (SPLAYING THE LA LB
OUT)) and lay it in the correct orientation on top of both LC and LD. Now project your lines down/across and this "SHOULD" give you the
2nd cut. This the easest way that i used to get the cut, when Modeling it in 3D.
Good LUCK! I hope to be attempting this fall/winter.
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t.j.
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| posted on 8/9/06 at 06:30 PM |
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must of lenghts i use are just cut to 90 degr.
A gap of 2-3 mm can easly welded.
So don't worry, be happy you can build one!
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Mark Allanson
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| posted on 8/9/06 at 06:52 PM |
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quote:
Originally posted by t.j.
must of lenghts i use are just cut to 90 degr.
A gap of 2-3 mm can easly welded.
So don't worry, be happy you can build one!
If you can keep you head, whilst all others around you are losing theirs, you are not fully aware of the situation
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joneh
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| posted on 8/9/06 at 06:56 PM |
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If your mitres do not fit - fill the gap with glue and shit.
It a mitre fit, it a mitre not a fit.
Arf.
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t.j.
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| posted on 8/9/06 at 07:04 PM |
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Hi Mark,
Why not?
Welding the parts 1-2 mm apart result in definitly in a good connection.
It's the same effect as 45 degr grinding the parts.
Place two 90 degr tubes to one another like this == how would you weld it?
By placing them a little apart you get easly a deeper weld, then sticking them to getter and weld them. Correct me if i'm wrong.
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flying_coffin
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| posted on 8/9/06 at 07:23 PM |
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When I drew it in CAD I got the two individual angles to be 26DEG and 15DEG.
So what I was going to do is put a 26DEG angle on the chop saw and incline the job by 15DEG with a wedge.
So if you put the work piece on a flat surface and rotate it 26DEG to your saw, then incline the workpiece 15DEG that should give you the cut. I
think! I've yet to try it myself.
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MikeRJ
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| posted on 8/9/06 at 08:54 PM |
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quote:
Originally posted by t.j.
Hi Mark,
Why not?
You get a weaker join and far more distortion than if the tubes are close fitting.
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Mark Allanson
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| posted on 8/9/06 at 09:34 PM |
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quote:
Originally posted by t.j.
Hi Mark,
Why not?
Welding the parts 1-2 mm apart result in definitly in a good connection.
It's the same effect as 45 degr grinding the parts.
Place two 90 degr tubes to one another like this == how would you weld it?
By placing them a little apart you get easly a deeper weld, then sticking them to getter and weld them. Correct me if i'm wrong.
You are laying in molten metal into the gap, lets say 2.5mm wide, weld falls in loverly, but as it cools from white hot to 16°, it will reduce in size
from 2.5mm to approx 1.7mm, if you do a few welds like this, the distortion you build in is massive, even if you match the welds side to side, you
will be adding stresses into the chassis that Uncle Ron had not invisaged.
If you can keep you head, whilst all others around you are losing theirs, you are not fully aware of the situation
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leto
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| posted on 8/9/06 at 10:58 PM |
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mistergrumpy
Some time ago there were paper templates available here, they might still be around somewhere.
“I'm gonna ride around in style
I'm gonna drive everybody wild
'Cause I'll have the only one there is a round”. (J. Cash)
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kb58
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| posted on 9/9/06 at 12:14 AM |
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Because your two tubes aren't touching, when you fill the void with molten metal, you'll get tremendous contracting forces on the joint as
it cools. It will very easily warp whatever's being assembled. About the angle-ground ends being welded, the catch is the two pieces are still
touching each other before being welded.
Mid-engine Locost - http://www.midlana.com
And the book - http://www.lulu.com/shop/kurt-bilinski/midlana/paperback/product-21330662.html
Kimini - a tube-frame, carbon shell, Honda Prelude VTEC mid-engine Mini: http://www.kimini.com
And its book -
http://www.lulu.com/shop/kurt-bilinski/kimini-how-to-design-and-build-a-mid-engine-sports-car-from-scratch/paperback/product-4858803.html
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t.j.
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| posted on 9/9/06 at 05:38 AM |
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quote:
Originally posted by kb58
Because your two tubes aren't touching, when you fill the void with molten metal, you'll get tremendous contracting forces on the joint as
it cools. It will very easily warp whatever's being assembled. About the angle-ground ends being welded, the catch is the two pieces are still
touching each other before being welded.
Hi,
Ok sounds fair to me.
I din't know this. i've always thought that a gap wasn't a problem. So I tacked every joint at 4 corners to get non warp.
Guess I have to make a new frame now?
I'm not planning tough. I use 2 mm box 25mm. measuring my chassis cross less than 1mm warp.
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JB
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| posted on 9/9/06 at 06:07 AM |
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Gaps In Welding
If you were welding 6mm plate then a good weld prep (ie tapered gap) would be a good idea to ensure complete penetration.
With the thin wall thicknesses we deal with you should get good penetration with no prep.
John
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mistergrumpy
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| posted on 9/9/06 at 07:53 AM |
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Thanks lads. Got what I needed now, just have to get out and try it. (Always the trickiest part!) 
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