Rail Cnc
A look at what is currently available on eBay
 4pcs SHF12 12mm Linear Rail Shaft Support XYZ Table CNC US $24.99 
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 4pcs SHF13 13mm Linear Rail Shaft Support XYZ Table CNC US $24.99
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 4pcs SHF16 16mm Linear Rail Shaft Support XYZ Table CNC US $24.99
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 4pcs SHF20 20mm Linear Rail Shaft Support XYZ Table CNC US $29.99
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 4pcs SHF25 25mm Linear Rail Shaft Support XYZ Table CNC US $39.99
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 4pcs SHF30 30mm Linear Rail Shaft Support XYZ Table CNC US $49.99
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 4pcs SHF35 35mm Linear Rail Shaft Support XYZ Table CNC US $59.99
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 4pcs SHF40 40mm Linear Rail Shaft Support XYZ Table CNC US $89.99
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 4pcs SHF50 50mm Linear Rail Shaft Support XYZ Table CNC US $129.99
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Properties of concrete?
Could concrete be used to make a precision casting? If a highly accurate mold was made would the concrete cure and remain the same size or shrink? If it does shrink do the internal stresses act evenly so that bowing does not occur? I was thinking of using concrete in an application of constructing a large CNC router like the type you would expect to see in the marine industry. I was thinking of having the moving COMPONENTS mounted on a series of hard plastic (or other compatible material) wheels rather than use expensive linear rails. Any thoughts?
Thanks
David
Depends on your definition of precision. Yes, you will have some cure shrinkage dependent on the cross sectional thickness difference and different cure rates. You may be able to make a good estimate of the shrink. That's how pattern makers do it with plaster. You may have to do some experimenting first. I was just at the library the other day and saw a new book on making molds for concrete countertops. He made some pretty fancy designs and walked through making the patterns, mixing the concrete, molding, removing the molds, fixing surface cracks, sealing, and final finishing. I know it's not a precision application but he might be able to you started in the right direction. You might even be able to contact him with additional questions. Additional, reinforcing mesh or bars can help if you have large spans to bridge. You can place your mounting holes after cement cure to get higher precision placement because then it won't move (as much). Hard plastic wheels will creep over time under temperature and load and not give you good long term results. Good luck!
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July 20th, 2009 in
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