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  #1  
Old Mon 05 May 2008, 08:31
Robert M
Just call me: Robert
 
Lac-Brome, Qc
Canada
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Steel sizes & gauge substitute info! (can we make it lighter and cheaper?)

Gerald, not sure if it’s ok or worth to start another thread here, please relocate if needed !

At the rising cost of steel, I’ve call on some local steel distributors for quotes and one of them suggested me some alternative, which I’m not qualified to judge if I should go with !
For the 10 10 322 main beams, would it be to any detriment substituting to a less heavy gauge ?
Since I would like to go with 10’’ high C-channels, it calls for C10 – 15.3lb/ft = 1/4’’ thickness. I was suggested to consider 3/16’’ thickness for +/- 35% less cost !! ( 175$ Can. VS 275$ for the 1/4’’ >for 20ft lengths ). If the ¼’’ is not needed, I rather have this 100$ on some other goodies for the beast !!

Same approach for the X-bearers 10 10 302. Original plan calls for 76 x 38 x 7kg/m (transferred to C3-5.0lb = 3 x 1.5 x ¼’’ ) at 92$ - 20ft. For a less width & thickness ( C3-4.1lb = 3 x 1.3/8 x 3/16’’ ) it would cost 64$ / 20ft. A saving of +/- 28$ !! Another +/- 30% saving !!

I do not want to scarifies any structural integrity nor to be cheep, but why give out $ to others if there’s no detriment !! Is there ??
Thanks for your input, Robert
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  #2  
Old Mon 05 May 2008, 09:00
Robert M
Just call me: Robert
 
Lac-Brome, Qc
Canada
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MISTAKE… Doop
I miss judge the C10-8.4 at 175$. It’s only 1-1/2’’ wide >> Not good…sorry ! Will stick to original C10-15.3 at 2-1/2 wide !!
But I’m still interested on your thoughts & suggestion for substituting the 10 10 302 from C3-5.0 to C-3-4.1 ?
Robert
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  #3  
Old Mon 05 May 2008, 09:19
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
Have a look at this page for a reference:

http://www.engineersedge.com/standar...properties.htm

The Ixx column is the important one for the table cross-supports. The flex (stiffness) of the table top is directly related to the Ixx number.
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  #4  
Old Mon 05 May 2008, 09:22
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
If you want to save costs on steel, try to find recycled steel beams from a scrap dealer.
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  #5  
Old Mon 05 May 2008, 10:13
Greg J
Just call me: Greg #13
 
Hagerman, New Mexico
United States of America
For discussion only.
(disclaimer - I agree metric is better for calculations)

Assuming,

W = 100 lbs
l = 5 ft = 60 in
E = 29 x 10^6 lb*in^2

C3-5.0 - Ixx = 1.85 in^4
C3-4.1 - Ixx = 1.66 in^4

Using the formula for deflection,
C3-5.0 has a deflection of 5.24 x10^-4
C3-4.1 has a deflection of 5.84 x 10^-4

Wouldn't the C3-4.1 be acceptable?
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  #6  
Old Mon 05 May 2008, 10:19
Robert M
Just call me: Robert
 
Lac-Brome, Qc
Canada
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Thanks for this great link !
Still, I fell timid to say, but those values do not tell me much aside of being +/- 10% less for the C3 x 4.1 ?
How can I properly evaluate those
At the risk of assuming I can’t find recycled material I'd like, would C3 x 4.1 be ok or for the price difference I should stick to plan (C3 x 5.0) and pass to another mater ?!!
Robert
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  #7  
Old Mon 05 May 2008, 10:54
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
Robert, it is your choice if you want to deviate from the plans. If I thought it was ok, I would have put it on the plans.

Do you know what stress you will put on the table? If it is only foam, you can go much lighter and cheaper. If you want the table top to stay totally stable while you cut an aerfoil wing profile to micron tolerances over 3 days, you must cast a concrete table.

I really cannot give you a finite yes/no answer. The best I could do was to show you how to calculate the difference from that table.

If you have big sheet of thick MDF clamped to the table, and you cut the top skin off that MDF, it curves quite dramatically and fights with those cross-supports which are trying to keep it flat.

For our own table, I would certainly not go any thinner - maybe even go thicker - but our table is 1900mm wide [6'+]
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  #8  
Old Mon 05 May 2008, 10:56
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
Realise that recycled steel is no weaker than new steel.
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  #9  
Old Mon 05 May 2008, 11:58
Robert M
Just call me: Robert
 
Lac-Brome, Qc
Canada
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Quote:
Realise that recycled steel is no weaker than new steel.
I realize this to well… nothing against use & recycle, au contraire I’m an advocate !!
It’s just that some sizes & specs are not always available. I like to scavenge around, but... so much to do, the show must go on !
Quote:
I really cannot give you a finite yes/no answer
I wouldn’t ask & was not expecting for definite yes/no, just trying / hopping to understand these structural & other values so I can give a better evaluation & direction for my intentions. Kind of less guess more understanding, but hate to push & ask around! Since I’m trying to design my version with an integrated indexer valley, it’s subject to have some little mods from original plans. I’ll see if I can consult a local engineer or some one more knowledge than I !
Thanks for the assistance
Robert
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  #10  
Old Mon 05 May 2008, 12:17
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
Oof, when you mention indexer valley, then EVERYTHING changes! Those valleys are very nasty for the strength of the table top. If you are thinking of this valley, then you should not be considering a thinner structure.
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  #11  
Old Mon 05 May 2008, 13:27
Robert M
Just call me: Robert
 
Lac-Brome, Qc
Canada
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Thanks for your input
Robert
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  #12  
Old Mon 05 May 2008, 16:37
dmoore
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Quote:
Originally Posted by Robert M View Post
Since I would like to go with 10’’ high C-channels, it calls for C10 – 15.3lb/ft = 1/4’’ thickness. I was suggested to consider 3/16’’ thickness for +/- 35% less cost !! ( 175$ Can. VS 275$ for the 1/4’’ >for 20ft lengths ). If the ¼’’ is not needed, I rather have this 100$ on some other goodies for the beast !!

Same approach for the X-bearers 10 10 302. Original plan calls for 76 x 38 x 7kg/m (transferred to C3-5.0lb = 3 x 1.5 x ¼’’ ) at 92$ - 20ft. For a less width & thickness ( C3-4.1lb = 3 x 1.3/8 x 3/16’’ ) it would cost 64$ / 20ft. A saving of +/- 28$ !! Another +/- 30% saving !!
Imagine that an expensive material (1" thich Acrylic, SignFoam, large solid wood blank, etc) is ruined due to vibration, deflection, etc at which the root cause is due to the lack of good structural support. That $100 saved will be worthless and could even require a complete rebuild of your MechMate, with the right materials, plus additional materials destroyed in the rebuild. When you are spending over 6,000$ USD on a CNC machine, 100$ is chump change.

You'd be better off skimping on the motors and going ebay recycled instead of the $1000+ geared motors - that saves you $800. Or you could buy a cheap set of drivers - that could save you $250. Both options which are still cheaper to replace (and you could sell to recoup some of your inital investment).
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  #13  
Old Mon 05 May 2008, 18:28
Robert M
Just call me: Robert
 
Lac-Brome, Qc
Canada
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Thanks David, but my goal was not to skim off the budget.
I was curious & intrigued by an lower cost alternative witch I could not approved or discard.
Thanks to Gerald & some other sources I've call to, If anything I’m going with thicker material. I jump again too quickly by asking…

As for motors, it’s debatable but for me, I’m not sure about Ebay motors !
Drives & other stuff, I’m just not rich enough to buy cheep !
Thanks again for the concern & input
Robert
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  #14  
Old Mon 05 May 2008, 23:07
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
The round pipe I used on my first table came from the borehole in my garden!

It always strikes me when I visit our running MechMates, how much they vibrate, even though they are so heavy.........

My logic is also that one can more easily cut costs on the motors because they can be changed in an hour if it doesn't work out. But, when the monster table is in production earning $500 plus per day, it is too late to think of making the table heavier.

However, there is nothing wrong with someone wanting a lighter table because they are doing lighter work. My risk will be to "approve" a lighter table and then I get blamed because the machine cannot earn $500+ per day.
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  #15  
Old Tue 06 May 2008, 07:40
domino11
Just call me: Heath
 
Cornwall, Ontario
Canada
Quote:
Originally Posted by Gerald D View Post
The round pipe I used on my first table came from the borehole in my garden!

Gerald,
What is a borehole?
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  #16  
Old Tue 06 May 2008, 08:19
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
Hole bored in the ground, with a pipe put down inside it, through which ground water is sucked up for irrigation. About 6m [20ft] deep. You might call it a wellpoint?
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  #17  
Old Tue 06 May 2008, 09:03
domino11
Just call me: Heath
 
Cornwall, Ontario
Canada
Gerald,
Ahh, yes we call them a well. Never heard that one before. If you used the pipe from that for the mechmate, what do you water your garden with now? You are certainly not going to make good points with your wife for that one.

Last edited by domino11; Tue 06 May 2008 at 09:07..
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  #18  
Old Tue 06 May 2008, 09:51
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
Now we use grey/gray water because the "well" was blocking up with a fine kaolin (clay like substance). The garden is thriving on the runoff water from my shower!
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