PDA

View Full Version : Which metal would flex more?



PaisleyPCdoctor
17-12-2017, 07:58 PM
What would be the most rigid of these flatbar?
Flexing as if you were using them as a bridge and standing in middle. I'm sure there is a tech term for it, but I don't know it.

25mm thick aluminium 500x100mm

10mm thick 500 x100mm mild steel

10mm thick 500x100mm stainless steel.

Thanks in advance.

magicniner
17-12-2017, 08:13 PM
That's easy -

https://en.wikipedia.org/wiki/Three-point_flexural_test

Unless you expect someone else to do the work for you?

PaisleyPCdoctor
17-12-2017, 08:34 PM
I was kinda hoping not to spend 250K+ on a test rig to find out. Lol

Just looking for advice for people that work with metals what they thought.

Zeeflyboy
17-12-2017, 08:39 PM
Would depend on the aluminium alloy to some extent but Youngs modulus of steel is roughly 3x that of Aluminium.

There is not really a particularly significant difference in modulus between stainless/mild.

Flexural modulus is usually fairly in line with Youngs modulus in metals. Polymers can vary more.

https://www.engineeringtoolbox.com/young-modulus-d_773.html

Edit - as it pertains to machine design, the general rule of thumb I've seen bandied around is that you need twice the thickness of aluminium to achieve similar stiffness to steel. No idea where that rule of thumb started though nor how accurate it is when considering all the different variables in real applications.

magicniner
17-12-2017, 08:44 PM
I was kinda hoping not to spend 250K+ on a test rig to find out. Lol

Just looking for advice for people that work with metals what they thought.

And in giving dimensions and materials you are asking them to perform standard calculations based on set formulae which are easily found.
It isn't Rocket Science and if yo want to pay my hourly rate I will look up the equations and do the calculations for you.

Around 8 years ago I wanted to know about material thicknesses in cylinders for a variety of diameters and pressures and looked up the required mechanical engineering equations, if you were to ask this on a mechanical engineering forum I'd expect an "off the cuff" answer but generally speaking by asking this here you are just expecting someone else to do your work for you, and someone might, but without seeing their working how will you know if the answer is any good?

Zeeflyboy
17-12-2017, 09:20 PM
Just out of interest, took your dimensions and applied a load of 200N to mild steel and 6061 alu:


23425


23426


10mm Steel deflection = 0.06861 mm

25mm Alu deflection = 0.01471 mm


So according to that simulation, the 25mm alu would deflect less.

PaisleyPCdoctor
17-12-2017, 09:41 PM
Just out of interest, took your dimensions and applied a load of 200N to mild steel and 6061 alu:


23425


23426


10mm Steel deflection = 0.06861 mm

25mm Alu deflection = 0.01471 mm


So according to that simulation, the 25mm alu would deflect less.

That is amazing! Totally opposite from what I thoughtModulus of Elasticity. I have the 25mm aluminium, but thought I'd be better buying steel. Excellent news. Thanks for you efforts- totally appreciated.

magicniner
17-12-2017, 09:41 PM
Good man!
Someone else was willing to do the basic maths for you!
You should do a degree course now! :D

PaisleyPCdoctor
17-12-2017, 09:45 PM
And in giving dimensions and materials you are asking them to perform standard calculations based on set formulae which are easily found.
It isn't Rocket Science and if yo want to pay my hourly rate I will look up the equations and do the calculations for you.

Around 8 years ago I wanted to know about material thicknesses in cylinders for a variety of diameters and pressures and looked up the required mechanical engineering equations, if you were to ask this on a mechanical engineering forum I'd expect an "off the cuff" answer but generally speaking by asking this here you are just expecting someone else to do your work for you, and someone might, but without seeing their working how will you know if the answer is any good?

Do you actually help anyone on this forum or just troll members with snipes, criticisms and general vinegar titted grumpiness?
#boring

magicniner
17-12-2017, 09:51 PM
Yes,
I'm glad you ask, actually I answer lots of direct relevant questions with well thought out technical answers from people who aren't lazy clueless wakners who can't be bothered to carry out a basic Google search to see if there is a good answer just sitting there for them on the net.
But I guess you are too lazy to search and discover that too!
#SLACKER!
;-)

PaisleyPCdoctor
17-12-2017, 10:04 PM
Yes,
I'm glad you ask, actually I answer lots of direct relevant questions with well thought out technical answers from people who aren't lazy clueless wakners who can't be bothered to carry out a basic Google search to see if there is a good answer just sitting there for them on the net.
But I guess you are too lazy to search and discover that too!
#SLACKER!
;-)

:beer:What you mean is that you didn't have a clue what the answer is so you snipe instead. Lol. Go troll someone else. :beer: For the record, any question I ask on this forum is directed SPECIFICALLY NOT at you, so please ignore it. I am directing it at technically knowledgeable people who may or may not choose to help. Goodbye.

magicniner
17-12-2017, 10:15 PM
����What you mean is that you didn't have a clue what the answer is so you snipe instead. Lol. Go troll someone else. ������ For the record, any question I ask on this forum is directed SPECIFICALLY NOT at you, so please ignore it. I am directing it at technically knowledgeable people who may or may not choose to help. Goodbye.

No,
I mean you come across as someone who can't be bothered to do even basic research for yourself, even though very good answers are available to a fairly basic Google search which could be expected of any "PC Doctor".

For the record you can use your preferences to ignore my responses, making a bit of an effort would have the same effect ;-)

P.S. use of multiple question marks is not a good personality stability sign :D

PaisleyPCdoctor
17-12-2017, 10:24 PM
Magicniner,
I honestly don't find any of your replies helpful at all,

I'm respectfully and hopefully asking for you to not respond to any of my posts. It's really easy- you literally don't have to do anything.

magicniner
17-12-2017, 10:29 PM
Please accept my apologies, I assumed that you had really basic access to Internet reference and the required brain power with which you could work out from the prolific use of Aluminium alloys in Aircraft that for a similar weight a suitable Aluminium Alloy can provide greater stiffness for a given weight than any Iron based Alloy.
From there it should be a basic mathematical job to compare size and strength, and hey, if you're lazy then someone else might do the work for you:D

magicniner
17-12-2017, 10:37 PM
Magicniner,
I honestly don't find any of your replies helpful at all,

I'm respectfully and hopefully asking for you to not respond to any of my posts. It's really easy- you literally don't have to do anything.

Be willing to do some work for yourself, making an effort, even if you fail abysmally in your efforts, reads far better than wanting an answer for bugger-all effort.

magicniner
17-12-2017, 10:41 PM
And block me, unless you are going to make an effort, I will point out everywhere that you (and anyone else) is making no effort to resolve their own issues.

PaisleyPCdoctor
17-12-2017, 10:45 PM
Please accept my apologies, I assumed that you had really basic access to Internet reference and the required brain power with which you could work out from the prolific use of Aluminium alloys in Aircraft that for a similar weight a suitable Aluminium Alloy can provide greater stiffness for a given weight than any Iron based Alloy.
From there it should be a basic mathematical job to compare size and strength, and hey, if you're lazy then someone else might do the work for you:D

Yes, I'm an uneducated, illiterate, retarded, deaf, blind, quadraplegic, emotionally unstable, little girly with serious mental health problems including hallucinations and psychosis. I have no Internet access and I'm banned from google for asking too many silly questions and overloading their servers (not easy as I have no Internet and my electricity was cut off when I was an infant and is still off).
I frequently soil my pants and I'm a total burden on society due to my asylum status and unwillingness to work but still letting the nhs replace my heart and lungs and my liver twice.

Does that qualify me as a forum member you could manage to ignore and possibly even better, not dignify with responses?

magicniner
17-12-2017, 10:53 PM
Yes, I'm an uneducated, illiterate, retarded, deaf, blind, quadraplegic, emotionally unstable, little girly with serious mental health problems including hallucinations and psychosis. I have no Internet access and I'm banned from google for asking too many silly questions and overloading their servers (not easy as I have no Internet and my electricity was cut off when I was an infant and is still off).
I frequently soil my pants and I'm a total burden on society due to my asylum status and unwillingness to work but still letting the nhs replace my heart and lungs and my liver twice.

Does that qualify me as a forum member you could manage to ignore and possibly even better, not dignify with responses?

No, you can type, don't be lazy, learn how to block responses you don't want to see, this is a further demonstration of your lazy requirement for others to solve your problems! ;-)
Asylum status is a Red Light, you're actually obviously an Illegal!
If you're too lazy then you can ask other productive users on the forum to tell you how to block responses from someone you don't like!
To be honest you are becoming mildly endearing, make an effort and I may come to love you! :D

hanermo2
18-12-2017, 12:01 AM
The better A. is that it depends.

By mass the alu is 3x less rigid - and importantly it likes to ring and vibrate.
But when you get the same mass in alu, it is thicker, and thickness scales pwr 3 or pwr 4 by section size.

Steel is == 3x better than alu, in rigidity for the same mass and == 3x cheaper by mass.
Cast iron is cheaper than steel and nearly as rigid.
But cast iron is 3x better than steel in vibration dampening, 10x better than alu.

There are effectively zero commercial machines using alu in any modern machine tool bodies, if they cut something via contact (vs laser/plasma etc.).

All modern machine tool bodies are thin-skinned stressed-skin structures. All.
This means the frames are as big as possible, and very thin, and used ribs/preload/mass/anchors.
Typical thickness is around 10 mm on cast iron machine bodies, in the centres, give or take.

An extreme example are modern optical tables.
They are very very light, for what they do.
They are very very rigid, for their work volume or free span length.

Typically, an optical table might be 40 cm thick for a 1 m table, and == 20x more rigid than the best japanese mazak/integrex mill-turn of similar size, or 200-400k$ cost for the mill turn.
My numbers from tlar.

The optical table is two thin skins, and tubes inside.
When I researched them they seemed to be mostly vertical-only tubes, sometimes with various goops/ballast/members/unicorn snot/magic.
The guaranteed rigidity of optical tables is totally unbelievable.

My point.
No-one uses alu.

Here is a data point - I use Thorlabs stuff for some things.
They are a top-shelf supplier of excellent rep.

https://www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=9222

NOTE: !!!
1.7 um / 150 kg load deflection (constraints).
= 1500 N.
882 N / um rigidity.

The best machine tools might do 30-100 N/um, or 8-24x worse.
Iirc the industrial criteria for VMCs == 30N/um... on textbooks.

Zeeflyboy
18-12-2017, 12:35 AM
The better A. is that it depends.


A better answer indeed, but an answer to a different question.

Desertboy
18-12-2017, 12:49 AM
I was going bed but now I have to work out how to do simulation tests in fusion grrrr.

;)

Couldn't we apply antivibration (Like headphones) to our routers (This is something I've been thinking about for a while) seems like it would be simple enough to do.

I was thinking either hack a pair of headphones (Might not be able to get the frequencies you need) or a raspberry pi.

PaisleyPCdoctor
18-12-2017, 03:30 AM
I hadn't even considered vibration as a factor. Thanks.

For what I'm doing, I'll go with the alu, but when I get to the stage of building my own, I'll seriously consider a steel construction.

Thanks again!

routercnc
18-12-2017, 08:39 PM
If you want to work it out by hand it is not too tricky for a simple shape like that.

1. The geometry of the part (how the material is arranged, into what shape). This is the cross section property and is called the second moment of area:
Ixx = (bd^3)/12
where:
b is the width
d is the depth
For aluminium part this is (100x25^3)/12 = 130208 mm^4
For the steel part this is (100x10^10)/12 = 8333 mm^4

2. The end condition of the beam (how it is restrained at the ends)
Ec = 192 for fully welded
Ec = 48 for simply supported (e.g. pin joint)

3. The material properties for the material being analysed (Young' modulus)
For aluminium Ym = 69000 N/mm2
For steel Ym = 200000 N/mm2

4. The force being applied in the middle of the beam
For the example given this is 200 N

5. The calculation:

Deflection = (force (N) x length^3) / (Ec x Ym x Ixx)

For welded supported ends this is:
Deflection (Aluminium) = (200 x 500^3) / (192 x 69000 x 130208) = 0.01449 mm
Deflection (Steel) = (200 x 500 ^3) / (192 x 200000 x 8333) = 0.07813 mm

These are pretty close to the Fusion FEA results from Zeeflyboy (for fully supported end conditions):
25mm Alu deflection = 0.01471 mm
10mm Steel deflection = 0.06861 mm

If the ends are simply supported pin joints it makes a big difference:
Deflection (Aluminium) = (200 x 500^3) / (48 x 69000 x 130208) =0.0579 mm
Deflection (Steel) = (200 x 500 ^3) / (48 x 200000 x 8333) = 0.312 mm

So there you have it !

Desertboy
18-12-2017, 08:49 PM
If you want to work it out by hand it is not too tricky for a simple shape like that.

1. The geometry of the part (how the material is arranged, into what shape). This is the cross section property and is called the second moment of area:
Ixx = (bd^3)/12
where:
b is the width
d is the depth
For aluminium part this is (100x25^3)/12 = 130208 mm^4
For the steel part this is (100x10^10)/12 = 8333 mm^4

2. The end condition of the beam (how it is restrained at the ends)
Ec = 192 for fully welded
Ec = 48 for simply supported (e.g. pin joint)

3. The material properties for the material being analysed (Young' modulus)
For aluminium Ym = 69000 N/mm2
For steel Ym = 200000 N/mm2

4. The force being applied in the middle of the beam
For the example given this is 200 N

5. The calculation:

Deflection = (force (N) x length^3) / (Ec x Ym x Ixx)

For welded supported ends this is:
Deflection (Aluminium) = (200 x 500^3) / (192 x 69000 x 130208) = 0.01449 mm
Deflection (Steel) = (200 x 500 ^3) / (192 x 200000 x 8333) = 0.07813 mm

These are pretty close to the Fusion FEA results from Zeeflyboy (for fully supported end conditions):
25mm Alu deflection = 0.01471 mm
10mm Steel deflection = 0.06861 mm

If the ends are simply supported pin joints it makes a big difference:
Deflection (Aluminium) = (200 x 500^3) / (48 x 69000 x 130208) =0.0579 mm
Deflection (Steel) = (200 x 500 ^3) / (48 x 200000 x 8333) = 0.312 mm

So there you have it !

Nice one! I always want work it out by hand if I can! I normally do my trig on paper then check it in cad so I already know what numbers I should be getting back.

Zeeflyboy
18-12-2017, 09:21 PM
Nice one! I always want work it out by hand if I can! I normally do my trig on paper then check it in cad so I already know what numbers I should be getting back.

By hand gets super complicated super fast when considering more complex shapes and even more so when you are talking complex shapes with complex assemblies... That said always good to have a rough idea as a gross error check for computer simulations (even if nothing more than "hmm... that doesn't sound right"), after all garbage in = garbage out.

Desertboy
18-12-2017, 09:23 PM
By hand gets super complicated super fast when considering more complex shapes and even more so when you are talking complex shapes with complex assemblies... That said always good to have a rough idea as a gross error check for computer simulations (even if nothing more than "hmm... that doesn't sound right"), after all garbage in = garbage out.

garbage in garbage out is why I haven't watched TV in 12 years lol ;) the only thing I've watched this year is Rick and Morty and South Park and I feel better for it ;) That said I have never learnt as much since I was a child as I learnt this year lol so so much reading and the brain is better for it.

I've found the router changes you someone along the way nothing scares you any more and you can solve any problem. Drooling over yours and routercnc's work and thinking you know what I can do that is also a healthy thing.

It's good to aspire to better things, although I do wish you'd stop upping the ante every 5 seconds I can't keep up.

Can't you do a bit of crap workmanship for once so us mere mortals can feel good about ourselves for 10 seconds lol.

Zeeflyboy
18-12-2017, 09:25 PM
Watching TV right now... perhaps that explains a lot :distrust:

PaisleyPCdoctor
19-12-2017, 01:26 AM
Thanks for all the shared knowledge. Fascinating stuff.

I was handling the 500x100x25mm bar last night and it really does feel solid. I certainly couldn't noticeably deflect the middle without tools. Also the bar hasnt been touched for 5 years (surplus purchase for an old project) and it just happens to be exactly 1mm longer than required. I'll have the sides milled nicely and it'll be perfect.

The alu was destined for its new role.

Thanks again!