1018 Steel vs 6061 Aluminum

[trmpyro]

Wanted to get thoughts on using 1018 steel vs 6061 aluminum for machine plates (gantry and x-axis end plates). I am getting ready to send drawings to a machine shop for a bid and was wondering how 1/4" 1018 matched up with 1/2" aluminum from a rigidity standpoint. I am not even sure if this is truly a cost savings but it appears to be at first glance; to use 1018 over 6061. I am planning a machine using 80/20 extrusions going between the plates in question. I realize there will be an increase in weight using steel over aluminum but they seem minimal. Thoughts? I realize I should know this or be able to put some numbers behind it being a structural engineer and all but I have been away from that field for way too long. I included a few PDF files of my machine in progress, table size will be around 39"x24".

Thanks

[LazyMan]

as far as how 1/4 inch steel will compare to 1/2 inch aluminum in rigidity I do not really know, but I would not use any of the two. On my machine I used Mic6 cast plate for all my aluminum. Mic6 is cast aluminum, which is then machined flat. The stuff is very nice and flat and actually a little bit cheaper than regular aluminum plate. It machines beautifully and does not distort.

You might wonder why it would be cheaper or even the same price, but the bottom line is its cast, so it wont handle high stresses the same way extruded aluminum will. It works very nice with CNC routers though.

I used 3/4 inch plates for my gantry sides on my machine and it is pretty nice and solid. Its thicker than what you are planning but my sides are fairly narrow.

http://cnczone.com/forums/attachment...6&d=1200863941

[jsheerin]

From a quick calculation (which I'd recommend you check me on), the 1/2" Al would deflect about 63% less than the 1/4" 1018. Modulus of elasticity is about 3 times higher for steel but area moment of inertia increases with the third power of thickness. So for a quick check, 1-(3/2^3)*100=62.5%.

[ahren]

Jsheerin's number is correct. For bending, stiffness is a function of the material modulus and the thickness cubed. So for a reduction of half in thickness, you get a factor of 8 reduction in stiffness. The modulus of steel is 3 times that of aluminum, however, so the actual stiffness of the 1/4" steel is 3/8 (or 37%) that of the 1/2" aluminum.

Extrusions are great, because they take even better advantage of geometry by putting material farther from the bending axis. That's why my part system is based around them

Best regards,

Ahren
www.cncrouterparts.com

[trmpyro]

Thanks for the information, I will plan to stay with the 1/2" aluminum plate. My Mechanics of Materials text book has been looking at me lately I guess I should crack it open again and do some calcs of my own. Jsheerin and Ahren, thanks for the direction.

LazyMan, do you have a good online source for the Mic6 cast aluminum? Everything I have found so far has been quite a bit more expensive than 6061. I may do better trying to find a local source of aluminum plate to save on shipping.

Ahren, I am for sure staying with the extrusions for all cross members shown in the drawings but felt the plate was a better fit for the gantry sides. I am slowly acquiring 8020 sections from Ebay and will hopefully start building soon. I plan to incorporate several of your parts into my machine and look forward to dealing with you. Thanks again for the info.