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Need beam engineering help
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Posted by: eightwgt

06/01/2003, 17:32:20

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I am building a CNC 3 Axis routing machine and using Cast aluminium tooling plate to build a 'beam' that will be the gantry - the element with spans the table and hold the moving tool (router or spindle)

here is what I need help with....

I am making the beam from cast aluminium tooling plate (stress relieved) It is type 6061. The problem is I want to keep it stiff, yet as light as possible. The beam is of a C channel configuration, and will be fastened together with M6 / M4 SCHS type bolts on aprox 3" centers or whatever is determined to be required.

The dimentions of the beam will be 8" in height, and 6" in width on both the top and bottom flanges.

I was going to use all 1" material but wanted to get reccomondations as to how to save weight. I wanted to try all 1/2" material or 3/4, or perhaps 1/2" for the flanges, and 3/4" for the verticle member... - By the way I cannot weld the assebly, as I am afraid the heat will distort things and I am working with a tolerance of .002"  The tool it supports weights 35 pounds including housing. The tool travels in X,Y,Z axis constantly, at speeds of 200 inches per minute while cutting,  350 with tool withdrawn rom work....

I hope someone can help

Thank you

Tom Mitzlaff


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Re: Need beam engineering help
Re: Need beam engineering help -- eightwgt Post Reply Top of thread Forum
Posted by: mektek30

06/09/2003, 12:13:28

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It sounds like you have a dynamic mechanical analysis problem. It can be fairly complicated, but...
If you just wanted to calculate static deflections, yield points of the aluminum and so forth, I would recommend going to www.beams.com and download their beam software. It costs 40.00 or so but it is well worth it because its an exceptional value. It will calculate deflections, moment, and shear for either concentrated or distributed loads.
You plug in the loads, beam geometry (length, length between spans, etc) and the moment of inertia of your cross-section.
If you have Solidworks or access to it, you can model all of your concepts for your gantry table and then let solidworks calculate the Moment of inertia for you! I am sure other CAD software will calculate section properties too. Just make sure the numbers make sense.
Well I hope this helps.

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gantry beam
Re: Need beam engineering help -- eightwgt Post Reply Top of thread Forum
Posted by: RKimball

06/02/2003, 20:35:18

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A veteran of CNC equipment for some years, (not willing to get involved at this time), I have comments to consider. You are actually asking for a professional consulatation, perhaps someone here is interested in offering their service.

Unless you plan to fill the beam or 'tube' with a dead mass as sand, you are very likely to get into a big harmonics problem. You would need T6 or at least T4 and can expect to have harmonics much larger than the .002 you plan to hold. Yes, aluminum is strong for the weight, but I think you will learn you need mass. This is why cast iron or steel is usually used. When the cutter flutes start to chip away they will produce a harmonic torque and load pattern, the light gantry (if it covers any sizeable span) will multiply that harmonic unless there is enough mass to overcome the harmonics by having a resonance frenquency consideably lower than the frequency of the cutter flutes. Secondly the cutter flute load frenquency will cause a secondary surge load resonance that will be many times less in frenquency, this will cause the spindle to wonder or climb the work in an uneven pattern, and is the one most likely to cause problems. It takes mass (weight) that is not condusive to resonance. Theory has it that the best gantry that could be built would be of granite. This is why inspection platforms are of granite. ... mass and thermo stability. I've often wondered how well a hollow gantry filled with sand would do. ... I figure quite well. The sand could be 'drained' to improve portability.

Also, aluminum has large thermal expansion properties, this translates into changes in day to day absolute positions, ... many times greater than .002, unless you maintain a tight thermal environment.

On the other hand, IF you are going to build a very small unit that cuts plastic or wood of thin gauges, you may have a winner. This is if the tool flute chip load is quite small compared to the gantry mass.

I'm not trying to talk you into scraping the idea, just to purify it!!

I wish you good luck and well earned success!

** The worst suggestion of your lifetime may be the catalyst to the grandest idea of the century, don't fail to listen to suggestions. -randy-

Modified by RKimball at Mon, Jun 02, 2003, 20:36:27

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