Shafts lenght vs diameter 8mm x 500mm ?

It's time or very nearly time to cut metal and I have the option to expand my UM clone build up to 600 x 450 x 450 mm.

I'm concerned about the rods that for the X and Y axis, the original build uses 8mm rods but I am considering a length of 450mm between supports.

Note the print head assembly is always supported by all 4 of those rods and the Rods Rotate so their weight impacts the build.

Unfortunately the deflection calculations wouldn't mean much to me so I'm looking at what has been built and for comment.

I have found one build the CLM-300 that uses 8 x 500mm. rods and been told about a Stratasys printer using 8 x 500mm other people's comments say bigger would be better.

I would appreciate your thoughts and input.


Thanks. Aamcle

Edited 1 time(s). Last edit at 11/01/2019 05:28AM by aamcle.

For the application I don't think the rotational inertia is significantly different between 8 and 10 mm. The stiffness may be though. If an example already exists with long 8 mm shafts and produces quality prints at the speed you want to print at then there is probably no reason to go bigger. With that said if there is something to be gained with larger diameter you'll never know unless you try. I personally would go bigger. My Intamsys Funmat (260x260x260 UM kinametics) uses 8 mm all around. Seems flimsy but I don't print over 100mm/s. I'm assuming you're using case hardened shafts?

Not all shafts are the same. I have Chinesium 8mm rods that are chrome-hardened and they bend slightly under stress. At the other end I have the same type of rod that was manufactured in the Netherlands and these are way harder to cut and bend way less. So there are many factors at play here, such as the thickness/deepness of the hardened layer.

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http://www.marinusdebeer.nl/

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Ohmarinus
Not all shafts are the same. I have Chinesium 8mm rods that are chrome-hardened and they bend slightly under stress. At the other end I have the same type of rod that was manufactured in the Netherlands and these are way harder to cut and bend way less. So there are many factors at play here, such as the thickness/deepness of the hardened layer.

In fact not much less, the modulus of elasticity of low carbon is only a few % less than hardened steel. A common mistake.

If deflection calculations don't mean much to aamcle , he can simply measure it with weight close to what he plans to use as moving part.

But a 8mm X 500m is likely to bend just by glancing down at it

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"A comical prototype doesn't mean a dumb idea is possible" (Thunderf00t)

Quote
MKSA

Quote
Ohmarinus
Not all shafts are the same. I have Chinesium 8mm rods that are chrome-hardened and they bend slightly under stress. At the other end I have the same type of rod that was manufactured in the Netherlands and these are way harder to cut and bend way less. So there are many factors at play here, such as the thickness/deepness of the hardened layer.

In fact not much less, the modulus of elasticity of low carbon is only a few % less than hardened steel. A common mistake.

If deflection calculations don't mean much to aamcle , he can simply measure it with weight close to what he plans to use as moving part.

But a 8mm X 500m is likely to bend just by glancing down at it

Why is it a mistake if I have held a real world example in my hands? I think you should take into account the hardchromed part as well.

---

http://www.marinusdebeer.nl/

Quote
Ohmarinus

Quote
MKSA

Quote
Ohmarinus
Not all shafts are the same. I have Chinesium 8mm rods that are chrome-hardened and they bend slightly under stress. At the other end I have the same type of rod that was manufactured in the Netherlands and these are way harder to cut and bend way less. So there are many factors at play here, such as the thickness/deepness of the hardened layer.

In fact not much less, the modulus of elasticity of low carbon is only a few % less than hardened steel. A common mistake.

If deflection calculations don't mean much to aamcle , he can simply measure it with weight close to what he plans to use as moving part.

But a 8mm X 500m is likely to bend just by glancing down at it

Why is it a mistake if I have held a real world example in my hands? I think you should take into account the hardchromed part as well.

Because deflection is a function of the modulus of elasticity that varies by a mere 5% between low carbon steel and tempered alloyed steel ! Tensile strength may varies by 300% !

Show a picture of your "hardchromed" rods. I bet they are the polished standard hardened alloyed steel. In some countries it is called "silver" steel , no silver at all !

Note, these are the correct type to be used with ball bearings guides, unlike the undersized stainless steel rods included in the cheap chinese kits. These stainless steel rod are fine with Igus bearings BTW.

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"A comical prototype doesn't mean a dumb idea is possible" (Thunderf00t)