[ljoe1969]

ok.

so my question is. If someone were to put some concrete in it would it stiffen it?

[HuFlungDung]

I would say yes, for so long as you can maintain the concrete column in one piece, ie., with no cross fractures. This could be difficult to achieve in small pipe diameters. Even making 95% sure that the cured concrete has not pulled away from the wall of the pipe as it hardens up, would be of some concern for the reliability of the method. Chances are good that the vibration dampening of the pipe would improve, but this has naught to do with stiffness.

[Hobbiest]

Why not just use a solid rod?

[chuckknigh]

I'm not sure about this combination, but composite materials often give the best of both worlds. Think of fiberglass or carbon fiber...or wood, for that matter. All composite materials.

-- Chuck Knight

[ljoe1969]

solid rod question.

would 3/4 dia. suffice to make rails for the x axis at 36" long one rail on each side of the bed. y axis being 24" long using 1/2" dia solid rod. with the building material being mdf.

[buscht]

The easiest way to get more strength out of a tube is to increase the diameter.

I have attached a zip file for an Excel spreadsheet that I use to calculate deflection in table legs.

Its not exactly the same as in a CNC with both ends captured. This spreadsheet assumes one end captured and a load applied at a certain distance out from the captured end.

You can vary the diameter, tube wall thickness, load, and distance of the applied load.

Simulate a solid tube by using 1/2 the diameter as the wall thickness.

You can see for example, that changing a 3/4", .125 wall steel tube to solid is only about 20% improvement, but changing the 3/4 diameter to 7/8" while keeping the wall at .125" almost doubles the strength of the tube.

I feel that you need a supported tube for 36" of travel if you want to use 3/4" dia. no matter what the material.

Don't use my math to 100% make your decision, but its a good place to get an idea on what might work.

[jeffs555]

buscht,
Thanks for the spreadsheet. I am designing a router with steel angle, and was just going to go way overboard on the angle sizes and hope for the best. Your spreadsheet got me to thinking there must be a program somewhere for calculating stress and deflection in angle iron. I searched the internet, and found a freeware program that is very versitile, allowing multiple loads and multiple supports for just about any simple beam type. It defaults to a modulus of elasticity of 30000000, but when I changed it to 29000000 like in your spreadsheet, and put in a round tube with a cantilever support at one end, the deflection agreed with your spreadsheet within less than 0.1 percent.

Here is the web site for the BeamBoy program. http://www.geocities.com/richgetze/

Jeff

PS ljoe1969,
Beamboy calculates that a 3/4 inch steel rod 36" long with simple supports on each end, and a 10 lb load in the center would deflect about 0.0209 inches, and like buscht said, just going to 7/8" rod changes the deflection to 0.0113 inches or almost half that of 3/4.

[Hobbiest]

Originally posted by ljoe1969
solid rod question.

would 3/4 dia. suffice to make rails for the x axis at 36" long one rail on each side of the bed. y axis being 24" long using 1/2" dia solid rod. with the building material being mdf.

if it is 3/4, it should be supported over that length. Half inch may be okay over 24 in, but I think I would use 3/4 if unsupported.

[Ryobiguy]

That's a great program, but I don't happen to know off the top of my head what the moment of inertia, or distance to furthest fiber of common materials. There's got to be some charts... first hit on google is a good tutorial of all this stuff: http://www.du.edu/~jcalvert/tech/elastic.htm
and http://www.csus.edu/indiv/j/johnstonj/SETRC/BEAM.html has a small chart of some common materials, but I don't see the huge super reference chart I'd expect to find somewhere on the Internet.

What a great tool to calculate slop BEFORE you go through all the trouble to discover it the hard way.

[ger21]

Half inch may be okay over 24 in, but I think I would use 3/4 if unsupported.

There is a link in this thread to a video showing how much 1/2" rod will flex, even at 24" or less.

http://www.cnczone.com/forums/showth...2&pagenumber=5

[ger21]

Here's the actual video link.

http://www.mikenchell.com/teardrop/rodflex.mov

[Dogsione]

Ryobiguy,

The program will calculate those things for you (Fortunately, since I failed Statics... ).

Under "Beam Properties" just click on the "Calculate" button and you can enter the dimensions of your pipe/rod/bar.

I, too, have found this program to be really helpful/assuring.

Thanks Jeffs555 for posting the link.