Super Strengthened Axis?

Hi all, first post here. I have been reading up on this exciting open source product for a few weeks now since I have discovered the capabilities. I am going to be working on a design here soon based on the awesome designs that I have seen (my hopes probably exceed what is "cheaply" possible, but I can expand on that later). First I had a couple questions about strengthened beams.

I saw some people using 2 c-beams back to back to create an I-beam to vastly strengthen unsupported spans which seems great for the Z axis where the force is more of a cantilever and the forces from the router are more linear in the X and Y directions. Vertical deflection is just as important on the X&Y axes as well, but there are also more torsional forces involved from the router, the gantry in my head would be the biggest one since the (I think we are calling the bottom axis Y) axis rails are sort of tied together and the cutting forces would be turned into more of a linear horiztonal force.

For these cases, wouldn't it make more sense to tie the C-beams together to make a box beam? This may be done elsewhere, but I have not been able to find anyone else doing it this way in my searches. Even for lateral forces, the moment of inertia is 46% higher than a dual C-Beam I-Beam (Sketchup is telling me 703,646.9 mm^4 vs 1,031,956.1 mm^4). Granted, this does assume full rigidity between the pieces in both cases. Unfortunately, it has been a few years since I have graduated and have not been using my engineering degree enough to remember how to calculate the torsional constant, but from "eye-balling it" calculations it definitely would be stronger torsionally. I wish I could put a number to this though.

Granted, the downside is you can't use the recess of the C-beam for v-slot wheels, so it makes the machine a bit bigger.

And adding onto this tangent, to add even more strength to the dual C-Beam Box-Beam, why not slide a 40x40 channel in the useless void in the middle to stiffen up the beam even more? Granted, Sketchup is telling me the moment of inertia increase is only 7-8% in each direction since all of the added support is so close to the centroid, but added stiffness is added stiffness provided that it doesn't increase the weight too much for the motors to push! At this point, I think we are approaching the size of a 3030 piece of 8020, and that is used on aluminum cutting routers with 48" gantries.

Anyways, these are just some questions/thoughts I have had trying to implement the design I am looking to create. I look forward to your comments!

 

I was under the impression that the C-beam is the largest beam. Wheel strength and wear on aluminum extrusion is next on my list to figure out.

 

Ahh, that is what I meant. The biggest available for the V-wheel system. What kind of system would you use for a larger setup if you aren't using wheels?

Before I was turned onto this system, I was set on going with one of the CNCRouterParts setups, but they are a bit too expensive for me for my first go at a CNC.

 

There are almost an overwhelming amount of machines and ideas on this site!

The base plan for my machine is to cut foam and bondo for plugs to make fiberglass molds from. In looking at efficiencies of layering and piecing milled parts vs cost of a larger machine, it looks like the sweet spot for me is a cutting area of a 25"x25"x6" cutting area. I know full cutting depth of 6" will be impacted by the spindle and the mount, but there wont ever be just a full 6" plunge cut, it will be more of slopes over the 24". In looking at models of completed builds, it looks like I can position the cutting pieces to adequately clear the mounts.

Due to lack of budget and room, I also would like the machine to be able to mill some wood/MDF/small bits of aluminum at times. A way out of left field goal here would be to have a gantry width that can clear a sheet of MDF. This is a secondary goal though, and probably not really achievable with this system.

I know that cutting harder pieces and a long Z axis dont mix. My thought that I have not been able to model out yet, is a recessed bed to allow the z-axis to extend down 6" to the spoil board cutting area of about 24"x24", then have a sort of a drop in secondary bed of 48"x24" to bring the bed level up to the same level as say the Ox CNC for cutting harder materials. I have seen builds with 48" and even 1500mm gantries, but have not seen any real conclusion on them to see how stiff they actually are, so I am guessing that I will loose quite a bit of rigidity going with that large of a gantry.

 

That is similar to what I am thinking of, just a lot simpler! I took a quick look at your build. Looks like an awesome machine! Have you been able to cut aluminum with it with the bed raised up? It looks like you used dual 20x80s and 838mm long, a bit of an upgrade from the Ox! How rigid is it for you?

 

Nice! I may steal a thing or two from your build if that is okay.

I like the connector pieces and bolting for the X axis. I haven't gone all the way through the Ox build, but it didn't look like they are connected in that build, I know its not shown in the model. It really makes sense to tie them together so they have to twist as a single beam instead of being able to twist independently and rack.

Your Y axis looks like a good upgrade and a great way to drop the spoiler board down a bit lower. As rigid as your supported X rail and spoiler board table looks, without securing it to a torsion box, I bet it is as solid as an Ox router bolted to a torsion box. I may actually stick with around an 850mm granty for now. While a 1250mm cutting width gantry would be awesome to be able to push full sheets through, it may be overkill for now, and honestly to expand the X later I really only need a wider table and gantry.

I guess my next step is looking into cutting forces. Is there any data out there on a typical cutting force of aluminum, vs mdf vs foam?

 

Awesome! That's good to know. Haha fair enough. A smaller machine will tuck away easier in my garage too! Are you able to tell me about what your machine cost all in, just so I have an idea of what I am in for?

 

Hahaha. Much appreciated even though it was painful! I'll have to look at this in more detail, but I was actually expecting worse. The Ox kits I've seen are about $1,100 base for the larger foot print, and that doesn't include the spoilerboard, mobile work station or torsion box. Your design has a larger work area and looks much stiffer! I'd say well worth the extra coin.

 

I definitely agree about the belts. I think that for my purposes, screws are the minimum. I am a fan of the rack and pinion setup even moreso, but I believe that is because if I want to extend an axis in the future, I can just get some more rack and add on instead of having to buy a new screw and have a junk one sitting around. I don't feel like I am wasting parts, or at least wasting less so. Even still, the more I think about it, if I were to expand the machine, I would probably be more likely to build a completely second machine in tandem.

Haha, I like that idea. One thing that would be beneficial in my garage is a plasma cutting table. It would be nice to be able to combine those machines, but without some heavy work I don't see getting a two for one special like that!

Could you elaborate more on the 20x80 vs. C beam a bit more? You guys mentioned the price jumping up, but to get the same footprint of the C-beam requires 2 20x80 beams.

The printer/router combo sounds pretty cool. I would be interested to see how that would be used. In my limited experience, it seems like 3d printing is great for rapid prototyping to make sure that the part fits before committing to cutting it in the final material. I have been wanting one for years, but when it is time to break out the checkbook, I just can't come up with a great list of things that I would make.

 

Haha, well that money is an "investment". After a few jobs it will begin to pay for itself. Well that's the hope anyways!

Actually the dual 2080 beam has a higher moment of interia vertically, so from direct sagging due to weight it would be stronger. In civil engineering, we can put two C beams together to create an equivalent I beam (I have one in my living room, 2 150lb beams were easier to lift and bolt to each other than one 300lb beam). They need to be bolted together at a certain spacing, which I can't remember off the top of my head. The only concern I'd have would be the potential twisting of each beam independently between bolts, but it would be tiny, tiny, tiny twisting. I like your stiffner idea, I'm trying to figure out a way to use something like that to be able to raise the Y rails up another 3-4 inches or so to get some clearance to the table and still be rigid.

 

This may be a dumb question, but what is the typical length of the bit sticking out of the collet on something like the Makita RT0701C? I searched quickly and couldn't find much, most of the models I have looked at show about 1" under the collet.

I am not concerned with the cutting depth so much, just looking at clearances with the tip of the bit, the Z axis and clearing the material below. Is it 1"? Are there "cheap" options for longer bits for say 2-3"? It would be just for cutting foam and other soft materials, so strength isn't much of a concern with longer bits. Thanks!

 

Thanks guys! So I started playing around with trying to get 6" of usable travel (the model is just under 7" of clearance) without the vslot interfering with the model too much, and came up with the below. Looks like if I use a bit that sticks out about 3" from the collet, and extend the z-carriage down about 40mm past the wheel, I can get the bit to touch the spoil board below.

I have a few things here kind of extended to their max cantilever-wise (a 4" bit, the router is slid all the way down in the mount, the mount is attached below the bottom wheel), but are any of these deal breakers for cutting foam?

I am hoping with this design for a rare aluminum or hard wood job, I can slide in a reinforced spoil board on that sort of shelf you can see behind the gantry, swap in a much shorter bit, slide the router up a bit in the mount for less of a moment arm, and be pretty much the same dimensions and strength as a regular ox.

 

Here are a couple renders of what I am trying to convey. The blue is a sample of a foam block that I would be cutting. Granted, I could turn the piece around and there wouldn't be a clearance issue due to the vslot, but I believe that there will be a few pieces where there may not be that option, like a void in the middle, so if possible I would like to build for the flexibility!