Calculating strength??

Not sure if anyone here can help, so please suggest better sites for
this question if I do have it wrong.
I have decided to build a stand from steel box section. The dimensions
were in Imperial measurements. Not a problem, I thought, just use the
other side of my rule, but no. I can only get a metric size.
The plan asks for 1 1/4" square box with a wall thickness of 1/82.
My steel is 30mm square, with a wall thickness of 3mm.
Now the exact metric conversion is
1 1/4" = 31.75mm ,so I'm .25mm down here
1/8" = 3.175mm, again down by 0.175mm.
There is quite a wait to go on this so I am worried about the strength
lost, but the next size up is just far too over the top.
Can anyone help here? Have I lost a huge amount of strength or not?
The dimension lost looks very small but when you calculate this as a
strength will it become a far greater problem, I just don't know?
Yours Hopefully
RV
Reply to
RVModeler
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The lightest of aircraft structures allow for a 1.5 times normal load before failure. Hopefully you are building something more conventional where design failure is five to ten times normal load. If you building something like a work bench then no worries. Your dimensions are only changing around 6 percent... Not a concern.
Randy
Not sure if anyone here can help, so please suggest better sites for this question if I do have it wrong.
I have decided to build a stand from steel box section. The dimensions were in Imperial measurements. Not a problem, I thought, just use the other side of my rule, but no. I can only get a metric size.
The plan asks for 1 1/4" square box with a wall thickness of 1/82.
My steel is 30mm square, with a wall thickness of 3mm.
Now the exact metric conversion is
1 1/4" = 31.75mm ,so I'm .25mm down here
1/8" = 3.175mm, again down by 0.175mm.
There is quite a wait to go on this so I am worried about the strength lost, but the next size up is just far too over the top.
Can anyone help here? Have I lost a huge amount of strength or not?
The dimension lost looks very small but when you calculate this as a strength will it become a far greater problem, I just don't know?
Yours Hopefully
RV
Reply to
R. Zimmerman
With some allowances for theoretical versus actual tube shapes: the metric tube is about 16 % weaker in bending, about 20% less stiff, and about 10% weaker in compression. If this is for a work bench, go right ahead. If this if for an aircraft part, better go back and recheck everything. Your application is probably inbetween, you will have to judge if the metric tube is oging to be good enough.
RVModeler wrote:
Reply to
RoyJ

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