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16th November 2014, 08:02 PM #1Member
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Table design - angle steel bends when plasma cut - what to do?
Hi everyone,
I've got a table design which I've started working on (render below). It has a solid timber top and a solid timber shelf underneath. I planned for the frame to be constructed from 55 x 55 x 5mm mild steel angle. Size is 1.6m long, 0.7m wide, 0.9m high.
As you can see from the renders I want to introduce slight curves on the inside of the legs and underside of the cross beams. This meant removing some material from the angle steel using a plasma cutter.
What I discovered when I did this was that the angle steel actually bent quite considerably. I guess much like wood, the stresses in the metal caused it to change shape when material was taken away?
So the question now is, how do I realise this design? I am thinking that I can start with 50 x 5mm flat bar, cut the curves out of that, and then make the angle using outside corner welds with the MIG? I am presuming that if I start with flat bar, I shouldn't see any deformations?
I am also thinking that 5mm thick steel is overkill, and I can probably get away with 3mm. Thoughts?
Thanks for your help,
Steven.
Manos_table_3.jpgManos_table_2.jpgManos_table_1.jpg
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16th November 2014, 08:18 PM #2SENIOR MEMBER
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So the angle was hot rolled, not duragal or rolled section?
The distortion could have also come from the heat of the plasma cutter? In which case one solution might be to use a water jet.
Maybe do a test with flat bar - I'd have thought 3mm would be OK for that application, depending how wide the span is and how narrow the steel gets in the middle.
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16th November 2014, 08:25 PM #3Member
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Hola..
No, it wasn't Duragal, it was simply listed as Black Angle AS3679/300 from here. What is the difference there between Duragal and this?
The steel doesn't get all that narrow. For example, in the widest span, it goes from 55mm to 35mm, so most of the steel is still there, so it should be OK.
Will definitely do a test. Plan to pick up some flat bar this week.
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16th November 2014, 09:22 PM #4
Have you tried bending it back into shape?
But yes, it's heat distortion - either using a cold cutting process (waterjet, jigsaw, bandsaw) or possibly tack welding it to another piece before cutting, and not cutting the tacks till it's cold again.
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16th November 2014, 09:36 PM #5SENIOR MEMBER
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How warm it was when it got bent into an angle. Black, unpainted steel sections are hot rolled - so they were red hot when put through the forming rollers. Duragal products and painted RHS are formed when cold. So Duragal angle has a much bigger outer radius on the bend compared to hot rolled angle.
The steel doesn't get all that narrow. For example, in the widest span, it goes from 55mm to 35mm, so most of the steel is still there, so it should be OK.
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16th November 2014, 09:38 PM #6Philomath in training
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Not sure it is all heat distortion. Plasma doesn't seem to heat things up as much as some cutting methods, so while there might be a little I would not have thought there was that much. I suspect that there is also stress that is trapped in the material from the hot rolling process. I think I've experienced something similar myself when cutting angle
As was suggested, if you got a piece of flat strip and bent it into an angle you could test whether it is the material or the process (flat strip should not have the stresses in it that hot rolled angle does).
Black steel is formed into shape when hot and so when it cools the stresses tend to get locked in. Duragal is cold formed, so the stresses are different and won't cause as much distortion if part of the section is removed.
Michael
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16th November 2014, 11:03 PM #7
What you are finding is that drawing something is, one thing, building it is another.
The first problem you have is that angle is fundamentally unstable, no matter how it is made.
it is an open, unbalanced section....when placed under stress it will twist and fail.
Yes I expect there will be heat stress in the rolled angle.
if you must persist with cutting from some sort of steel stock....think about U channel........it is at least a balanced section.
It is only because plasma cutters exist that peple would think about this type of profiling......prior to plasma it simply would not come to mind in steel.
A shape like that would be more or less trivial in timber, may be a bit less of a problem in aluminium.
In aluminium, you would rout the profile.
ever wondered why they don't make unequal angles in steel but they do in aluminium?
Now here are some thaughts.
Those sorts of shapes where fairly common in the 60s'...the tapered legs would have been made from custom rolled tapered tubes.....how it was done I'd be interested to know.
and the shape of the stretcher would have been fabbed from a straight section of something and the curve woudd have been bent from another
The other thing you may have to look at is the size of the angle and the amount of material you are removing.
In general working with angle it has to be very much larger and heavier than a similar closed section would be to achieve strength and stability.
A few thaughts.
maybe you could get away with using a larger angle and removing less material
Perhaps you would have sucess using a channel
My expectation for best sucess is to look for your strenght and stability from closed sections like RHS or tube and your profiles from flat material attached seperately.
or
make your shapes from a straight part and a seperate curved part.
Have a look at some of the older wraught iron work.....the very shapes you are looking for where produced form straight and curves sections of flat and square with no longditudinal cuts.
A final thaught......if you must persist with the angle..though I don't rate it for certainty of sucess is.
cutting the same shape out of both webs of the angle...it may equalise the stresses.
cheersAny thing with sharp teeth eats meat.
Most powertools have sharp teeth.
People are made of meat.
Abrasives can be just as dangerous as a blade.....and 10 times more painfull.
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16th November 2014, 11:11 PM #8
Now I come to think about it I have seen a table pretty much like that.
laminated chipboard top and steel frame.
BUT the steel frame was made out of custom sections...the horisontals where U channels with the U upright
One thaught on how it could be made is.
take a section of rectangular RHS, bend it the hard way into a curve, then cut the top straight...leaving a curved U channel....still not a metod I would chose.
cheersAny thing with sharp teeth eats meat.
Most powertools have sharp teeth.
People are made of meat.
Abrasives can be just as dangerous as a blade.....and 10 times more painfull.
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17th November 2014, 09:35 AM #9
How I would do this if I really had to.
Get 50x50x6, yes 6 not 5 not 3
Spot weld against another backing angle.
Weld a 25mm flat bar diagonally inside the angle with alternating 30mm long welds every 50 or even 100mm
Let cool down.
Cut with plasma, (Who on earth has a water jet?)
Goes without saying that you will cut both sides not only one right?
if you send for cad cutting to a third party, choose one that cuts under water to minimise distortion.
Let cool down
Grind the backing angle off.
Like I said that is how I would do it, you can try 3mm. The diagonal flat bar will add lots of stiffness to it.“We often contradict an opinion for no other reason
than that we do not like the tone in which it is expressed.”
Friedrich Nietzsche
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17th November 2014, 10:05 AM #10
Hi Steven
The bending is typical on what you may expect from cutting the one edge an angle section in that thickness /length/ configuration.
The fact that the angle section has the stress released from one side only causes the bend. While plasma is most certainly the best way to minimise thermal distortion , its not what is happening here.
Flat back black steel with the curve cut first and then tacked together form a fabricated angle should be the way to go.
Distortion, if any from the cutting process is much easier to rectify with the curved cut piece to be done as a flat section cut. Distortion can be induced in welding the of the assembly if you aren't careful and plan it all out.
In fact the whole assembly will need to be a series of short tacks to minimize heat distortion caused by the weld process. Over welding - by this I mean fully welding - will create distortion.
I would aim to have each flat of the angle placed in a simple jig and tacked along with a spacing of - tack 10 mms, space 100 mms using a 3mm fillet size. In terms of the assembly, bugger all strength is required and a sequenced tacking should cover that more than adequately.
I take it that the plasma cutting is hand guided and if so, the tendency might be to cut slow to maintain adherence to the curve. Slower than optimum travel speed could induce more heat input than is necessary- ie distortion.
I am not sure of the curve type - whether it is a chord from a circle or a parabolic curve, but if a chord ,a radius bar and a support wheel may be worthy of consideration.The advantage will be a faster and consistent speed, good finish on your cut surface and minimized heat input. To answer the question on suitability of 3mm flat section , I would say yes, as the rendering looks as thought the dimension from top of the arc to the straight side of the flat would be about 25mm.That should be quite ok for a vertical deflection on the average dining(?) type table. The table top itself though will be to be flat .
I understand it is all a bit of a rubics cube of number of the requirements to be satisfied, time, money ,effort, available tools and equipment but as a metal worker I would consider the above necessary to achieve a decent result.
Hope it helps
Grahame
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17th November 2014, 10:39 AM #11Member
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17th November 2014, 10:41 AM #12Member
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Michael, Soundman & Mark..
Thanks all for your responses as well. This has been a huge learning experience.
I think the approach I'm going to take is to get flat pieces cut out using a water cutter and then weld them together (carefully) into the necessary angles for final assembly. Now, to chase down some quotes and see if I can get this done at a reasonable price.
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17th November 2014, 02:59 PM #13GOLD MEMBER
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Hi Steven,
Going from experience, seeing that you are getting the pieces water cut, why not get the legs and sides cut, so that all that is required is to get someone with a brake press to fold them into the angle. Eliminates any distortion from welding, no clean up, saves time and you get a nice radius on the legs also. When you make the drawings, at the points where you want it folded, mark a small V about 2mm deep with the centre of the V deleted, so that the person bending the legs can find the centre without having to measure, again saving time/money.
Laser cutting will be OK also, as the cutter does not cut in a continuous line but a series of cuts, culminating in a piece cut out that I defy anyone to pick the stop/starts. All screw holes can be done at the same time also.
If you can get/do a CAD drawing of the parts required, it will be a lot cheaper, as all they will have to do is download it from a flash drive/Disc and convert it into the program for the cutter. If you could find out what type of file that they use jpeg; bitmap etc. and give it to them in the preferred file it will be easier still. Make sure that all intersections connect, as the cutter will not cut at those points, thinking that it is supposed to be that way.
If you have problems drawing them out, let me know by PM, I might be able to help. If you decide to go 3mm, I would put a 30x30 folded angle inside to help stiffen the legs, 4 tacks about 20 mm long equally spaced would be all that is required.
Regards
Kryn
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17th November 2014, 03:54 PM #14Member
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Kryn,
Thanks a lot for your offer of assistance. The bending is a great idea and I'm going to looking into the cost of that as well as the cutting. The V-tip is also awesome. I'm doing all my graphics in Google Sketchup and Layout so I can just export the lineart to scale in Autocad format.
S.
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17th November 2014, 08:40 PM #15GOLD MEMBER
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Why not just use your 50 x 50 x 5 hot rolled, cut your curve and the straighten the section. It shouldn't be that hard. Either using contra heating or mechanical means will work.
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