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Old Jun 26, 2015, 01:48 PM
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Building a CNC router?

I'd like to have a cnc router machine, and would prefer to build one out of wood, if that is possible. I've seen the "buildyourcnc" and the Wetlands build, but not sure either would do what I'm trying to do. I'd like a machine at least 24'X48", and would prefer something 48"X 48" so I can cut several wing ribs for large scale gliders (wing chord 30+") without having to cut a few, and load in another sheet of ply. I've been told that gantry mass (and attendant backlash) can be an issue with getting rib notches (for spars, etc.) cut accurately, so not sure a wooden machine is even possible. Has anyone used such a machine to cut large wing ribs, etc. using a CNC router? I'd like to build because I don't think I can buy one for under $3000.00. I'm open to any recommendations.

Thanks,
Larry Fitch
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Old Jun 27, 2015, 02:41 AM
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I would think that one could be made to do a reasonable job. The ones I have seen had serious issues from using substandard guide rails. So hard to judge. Even with the issues they did a reasonable job, as long as the cuts where light.
I looked at the Wetlands build. I think its gantry issue had more to do with the design, than the material.
I haven't seen in person a well implemented wood design. To bad you didn't ask last month. I passed up a couple of decent sized routers in auctions recently. Both because I didn't want to deal with shipping.

Mass in a machine can be good, or bad.
Good: it helps reduce vibration, for a smother cut.
-It is usually more rigid. Often allowing for a more aggressive cut, or the ability to handle harder materials.
Bad: it has more momentum, so takes more power to accelerate, and stop. So may end up being a slower machine.
-Can require more expensive slides, and electronics to deal with the weight.
-Might not be as portable.
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Old Jun 27, 2015, 03:41 AM
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i have used sum wood on my cnc mostly ply steel for the frame and liner rails
the money is in the liner rails but there is a new system that offer a good rail at a good price V slot if i rebuild my cnc that is what i would go for i have built a 3D printer with it and am thinking abut building a new foam wing cuter with it

http://ooznest.co.uk/blog-section/C-Beam-Now-Available

pdf of my old cnc router
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Old Jun 28, 2015, 10:21 AM
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Thanks guys

Rob, can you share what the gantry issue was with the Wetland design/build? Not sure I saw that. Dpot, thanks for the insight. I've seen the V-groove rails and agree they look rather good. I currently think I would go for CRS rails and steel rollers as well, like on the Wetlands build. I agree that mass can/is both "good and bad". Everything is in most mechanical designs.

Thanks,
Larry Fitch
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Old Jun 28, 2015, 02:04 PM
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Sure, the router had all the ply running the same direction. The flex he had in the middle of the gantry was forward, and aft. That happens to be the easiest direction for the ply to flex. If he had left the outer ply as is that would resist vertical flex. The interior ply could have been rotated 90 degree's to resist forward, and aft flex.
It might take some testing to figure the right mix, to limit flex in each direction while keeping a light weight assembly. The machines I have worked with usually have a higher load to the side, and less vertical while routing. That side force on the router also tries to twist the gantry. The buildyourcnc machine has spacers between the gantry ply with the ply running fore, and aft. That helps prevent twist. Would take some figuring to calculate the best configuration, for the amount of wood used.
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Old Jun 28, 2015, 07:18 PM
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Thanks

Rob, after reading your statement above, I went back to YouTube, and found his video about the deflection of the gantry. What I saw seems to say he measured the deflection with the dial indicator (appears to be about 2-4 thousands of an inch). I guess my question is if this amount of deflection is an issue in machine accuracy. I agree with your comment about having all the plys running in one direction. I think a better use of the plywood's potential could improve performance. Any information about how other materials (80/20 forms) would perform in this type of measurements. How much better would the aluminum be in similar structures? I wonder how "stiff" these structures have to be to be usable. I'm not sure.

Larry
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Old Jun 28, 2015, 09:26 PM
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The deflection he had is not that much. The force he used was rather vague, but gave the impression he was using a good deal.
Rigidity allows it to work harder, and faster. I have seen machines not nearly as nice do work that is better than many would do by hand.
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Old Jun 29, 2015, 08:27 AM
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When I set up both of my machines, I measured using 50lbs of cutting force at the tool. My old machine was MDF and had a long extrusion for a gantry. I measured almost 15 thousandths of deflection at 50lbs at the center of the gantry. My new machine is about 2 thousandths.

My old machine was very capable. Knowing what and where the flex was helped a lot. Keep the speeds down on harder materials and cut them near the end of the gantry travel if possible to minimize deflection. It's really pretty easy. I eventually (6 years) built a stiffer machine with a much larger and shorter gantry but my old machine served me very well in the past.

Building a machine and knowing how to use a machine are two distinctly different skills. Once you have a CNC router that you can use on a daily basis, you quickly get an understanding of how to work WITH the tool to overcome it's weaknesses.
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Old Jun 29, 2015, 10:32 AM
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Thanks,

Rob, Tom,
Thanks for the insights. I think I will venture down the wooden road, designing a stiff machine using the attrubutes of the plywwod as best I can, and go from there. Tom, you are very right, most machines have a weakness or two, and you learn to get work done dispite them.
Thanks again,
Larry Fitch
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Old Jun 29, 2015, 11:49 AM
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Depends

I think it depends on what you want to cut or work with. I“ve made mine with mdf, and i can work Mdf correctly, as well as cutting plywood with no problems.

It also depends on how much precision you want.
I built mine with very little money (for a CNC) and performs well.

If +/- 0.25mm is fine then yeah you can do it cheaply. Precision increases cost.
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Old Jul 01, 2015, 09:43 AM
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Plans?

Gonzalito,

Any detailed information (plans, pictures, etc) of your MDF machine available? Inquiring minds want to know.
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Old Jul 01, 2015, 08:21 PM
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Hey LR.

I have the plans i think, but many thing were done on the fly, for example holes for the wheels or tensors. But anyway everything works very well.


this one was one of the first test. As you can see it still didn“t have the router on.
Cnc casero 2 (1 min 56 sec)



Video of some days ago. Lower speeed for carving so surface finish was better.
1 de julio de 2015 (1 min 26 sec)


I“ll have to check for plans, but you will need to improvise also (speacially for rails, and transmission), and i am very busy until saturday. I can give you more info on sunday (photos and such). I used geared belts for transmission. Good torque, good speed, and acceptable resolution, for something under 0.2mm/step.

Anyway this is not a factor to take into a lot of account because, rigidity of the frames are not the same as in a metal one, and because my bearings, are just regular bearings (the cheapest i could find, some of them like 6 pesos, which is like 60 dollar cents). Though deflections might be countermeasured through mach3 options.
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