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  #1  
Old Wed 13 May 2009, 04:20
glintid
Just call me: David
 
Back in Toronto
Canada
Extended Z input please-a mock up of altered spider

My intention is to cut foam for theming projects. I altered the spider assembly in the sketchup model I found on the site by adding 150mm +/- in length (z) to the spider and the car. Hopefully it will alleviate the lateral forces by moving the Dualvees further apart. The Z axis will be constructed of an aluminium I beam like Sean's. Any comments welcome. I Intend to have about 1100mm vertical travel.
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Last edited by glintid; Wed 13 May 2009 at 04:26..
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  #2  
Old Wed 13 May 2009, 05:08
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
In principle, the structure it looks okay, but I think the wheelbases of the y-car (and gantry maybe) are too small to handle that length of Z.

Imagine in your last picture that the y-motor wants to make a quick movement and has a fast acceleration......the inertia/mass of the z-motor high up there is going to resist that acceleration and it wants to stay static. So the y-car "falls over". Also the same if the x-motors accelerate.
You will either have to accept very low accel values, or you must increase the wheelbases in the x and y directions.
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  #3  
Old Wed 13 May 2009, 12:01
glintid
Just call me: David
 
Back in Toronto
Canada
Thanks Gerald

Thanks for the input, I didn't think of the moment arm of the motor, and the router. I'll look at doing what you suggest and post it for input.
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  #4  
Old Wed 13 May 2009, 17:04
smreish
Just call me: Sean - #5, 28, 58 and others
 
Orlando, Florida
United States of America
Gerald.
Keen observation. I actually noticed this slight tilting when at full extension in the X direction - which the tool bit tip is at Z-50" or so from the y-car. The machine was not in motion, but I was changing the tool with the slide all the way down. Long story short - I am now writing the parallel finishing tool paths to coincide with the y-axis movement to counteract any possible moment arm induced forces. Again, I haven't noticed any tipping during cutting, but I am also running around 180-220 ipm in foam. No real resistance - yet!

David,
If you choose to use an I beam for the slide - I highly recommend you get Aluminum ASSOCIATION W shapes, NOT the standard structural variant. The Parallel flanges in the association shape is much easier to work with, albeit harder to find.
If I was to do it all over again, I would actually use 3/8" Aluminum flat bar and a tube section to mimic the actually z-slide that come with the MM design package. OF COURSE, the extra bearings on the bigger spider plate is a great asset. I will upload the dxf's of the spider I modified. The Ibeam doesn't really gain you anything in the long run. I just happened to have it in the building. If you run the structural calc's, the section modulus and bending moment is a little better with a made up shape! Funny math stuff!

Good luck,
Sean
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  #5  
Old Wed 13 May 2009, 23:28
glintid
Just call me: David
 
Back in Toronto
Canada
Sean,
Yes, your suggestion about the plate and tube assembly makes sense, although I expect that mounting of the router becomes a bit more complex in trying to retain its stability. Although, I don't expect large lateral forces with foam, but with the 1100 mm extention, it may become significant. All my (limited) knowledge is about static structures and I'm really out of my depth with dynamics. So any of that "funny math stuff" observations are certainly welcome.

Gerald,
I'm thinking of increasing the car dimensions by 150mm in each direction. This is a top-of-my head dimension based on nothing but a guess. Do you think that will do anything significant to help the stability issue?
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  #6  
Old Thu 14 May 2009, 00:13
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
I think that 150mm will be significant and certainly not an overkill.
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