Fill H Shaped Aluminum Extrusion With Epoxy Granite?

Me and my daughter are building a small fixed gantry cnc router. Probably 600mm or smaller. I have some aluminum extrusion that I picked up for free. From what I can tell it was originally used for mailbox posts or deck railing. It's approx 3.5" square. My thoughts were to brace the bottom half of the 'H' with some pieces of 1/4"thick angle iron (free from work) and then fill the rest in with epoxy granite. I would then attach the linear rails to the extruded aluminum in the open side of the H. Would there be a substantial benefit to filling both side's of the aluminum with eg vs filling just half? I can get a lot of angle iron scrap from work. I was thinking about running two pieces of angle iron the entire length and then possibly welding some braces in between them. This would make a primitive frame that I would bolt to the aluminum before casting the eb. Would adding a significant amount of steel add any substantial benefits?
For the table. I have some 1/16 thick pieces of sheet metal. Some of them are already formed into "tubs". I attached some pictures to give a general idea. I was thinking about taking some 1/4" angle iron and building a square "ladder" shaped skeleton. I would then make a shallow "tub" out of the 1/16 steel sheet. I would attach the tub to the steel frame and then fill it in with eg. The steel sheet should be flat enough to mount the rails and then the eg would be the top of the table or vice versa. Would it be better to have the steel sheet for the top or use it to attach the rails on the bottom?

Attached are pictures of the aluminum extrusion and some of the random scrap to give you a idea.

What's the point of this exercise? To teach your daughter the virtues of recycling random scrap? Or to make a machine that functions well? I don't see that aluminum as having much rigidity at all. If you wanted to use it as table legs, that might work if you added some bracing. If you've got access to welding equipment and lots of free heavy angle iron, it seems a more promising approach would be to use that to construct your machine. But if you've got a lot of epoxy and granite lying around, you could alternatively build a monolithic bridge to attach rails and screws to. A hybrid approach doesn't seem likely to achieve anything better than either material used separately. Those "tubs" might be useful to put underneath your machine for capturing chips, but it seems unlikely that they'd be much use otherwise. Linear rails need something a lot flatter than that 1/16" sheet steel to mount to, whether or not you've put epoxy-granite on the other side.

Long story short... My daughter got to playing with a cheap Chinese 3018 cnc router I bought her and somehow in the process took it apart and ended up bolting the steppers to a cheap little aluminum cross feed table I had (proxxon copy). She rigged a Rotozip to a drill press and has been toying around with it all ever since. Unfortunately it's a pretty limited design so we talked about building a desktop cnc router. I landed some sbr rails for insanely cheap and have access to a lot of random scrap. I'm not overly concerned about the project considering she will be thrilled with about anything. With that said I want something usable without spending a fortune. What I like about the aluminum is the flat surface. It's also quite a bit sturdier than it looks. My concern with the steel is getting a flat/square surface to mount to. I suspect a lot of it would need to be ground and even then I worry about warpage from heat/welding. Even then the majority of steel I have is going to be angle iron. I think it would be beneficial to add some sort of filler to it either way. I like the idea of the eg because it will add weight and dampen vibration. In theory it should also give me a flat surface to mount to.

Why did she dismantle the 3018 router in the first place? Because it didn't work well? Or just because she likes tinkering with things? (A lot of us here can relate to that). It sounds like your simplest route towards a working desktop router would be to put it back together, maybe with different electronics, since that's usually their weakest feature.

Are those cheap SBR rails supported along their length? The unsupported ones are basically worthless, which might be why they were so cheap.

It sounds like your daughter might be looking for a manual machine, rather than a CNC, Are you and her really on the same page here? Did she ask Santa for a Bridgeport, but you showed her your pile of junk?

Hi Duke - Junkyard projects are always worthwhile. I'd steer clear of epoxy granite as it will cost lots of $$$ which conflicts with one of your statements and I don't think at this level you will achieve what you think may happen with the EG. Sure its damp and its heavy (so what?) but its many $$$ and not very stiff and once cast you can't drill a hole through it easily to get that wire where you want it to go or attach that bracket you forgot about.. If your keen on casting use concrete grout from the hardware. 1/10 the price and just as good as EG.

Its great to have a project with your daughter. I would use good quality plywood and the bits & bobs you have. Ply is easy to work with and modify as you go along. Its stiff and damp and light which suits your concept. If you don't know what your doing with EG or grout it will be the same stiffness as high grade plywood and much less friendly. I have spent a lot of time working and testing EG and have yet to be convinced of its stiffness. I'd choose good ply over EG unless the part HAS to be cast.

Your expectations and aims are different to your childs. So spend as little $$$ as possible (as one of the project rules) , use plywood and the bits you have, get a machine working on a shoestring and see how it works out.... have fun most of all, another important rule. This build is not about creating a serious machine but a fun functional one. Once your through that exercise you can build a better one with this one etc.... Peter

She likes to tinker which is the reasoning for taking it apart. She hooked my rotozip up to a hand drill press holder. One of the ones you can strap a hand drill in and use it as a drill press. She was using it along with the cross slide table and a end mill bit to mill on some aluminum and even steel. It actually worked way better than I would have expected. Her little 3018 is pretty jankity and rattles all over. It's fun to play with but she got bored with it. That's when she decided to take the steppers off and put them on the cross slide table. She was successfully able to cnc some aluminum but it's pretty limited (she only has 2 axis and has to manually raise/lower it). She had all sorts of plans to "upgrade it" but really it's pretty much as good as it will get. She can successfully use it to do some "hand milling" and basic cnc but she likes to build things and I definitely encourage it and enjoy the projects. I don't think she would have any interest in a manual mill. We have a Bridgeport copy at work and it never really got her attention. I think she likes the idea of creating something but is just as much intrigued by the programming and electronics. I sell, service, install and repair commercial automotive equipment (frame machine's, air compressors, car lifts, tire balancers etc). Most of this goes to repair shops and dealerships but we work with a variety of different shops and manufacturers. Because of this I can usually get plenty of free metal, motors, plastics and so on. For example I got the sbr rail and blocks for very little along with some lead screws, bearings and so on.

For railing I currently have sbr16 and sbr12 but have already been offered some sbr20 if need be. We are talking about building a small machine and I am sure either the sbr12 or sbr16 would be fine. Really my plan was to use the aluminum as a "housing", add a steel skeleton/frame inside it and then fill the rest with eg. Being that I would probably be looking at less than 600mm at its widest point I don't think I would be looking at a major expense as far as the eg goes. The aluminum would obviously add some structural integrity but really it would be just as much for containing the eg and maintaining a flat, uniformed surface. This would apply to the sheet metal tub as well. A steel skeleton would be added for strength and the sheet metal would be mostly for a permanent mold and flat surface.

I'm definitely not set or committed to any of these ideas. We were going to work on it this weekend but plans changed with her mom so that's currently on hold. I would definitely be open to design ideas or suggestions.

Peteeng... Pretty sure we were both typing at the same time. I didn't read your post until after I posted mine.
Your advice is very very very helpful. I have actually been debating concrete or plywood but kept having 2nd thoughts. I read all sorts of posts about concrete cracking, shrinking or not working with different materials. I was looking at the countertop stuff but will definitely look at the concrete grout. With that said I have a bunch of plywood bed slats collecting dust and I'm starting to think they might work perfect. I will still throw in some steel and aluminum but for added "bulk" the plywood sounds promising. I could always go back with grout later.
You hit the nail on the head as far as my goals and agenda. I'm not overly concerned about the little details. Its mostly a fun project to get her brain thinking and to spend time in the shop with pops. She likes watching all the YouTube videos of people building things and that has been some of our motivation.
Really half the fun is her being able to rummage through the scrap piles and kind of day dream about all the different things we could do. I remember as a kid wanting to take everything apart or constantly trying to make something out of leftover wood and metal (I'm 40 and still do this lol)

Hi Duke - You have to be very careful and knowledgeable to mix materials to get a good result. If its just for fun that's fine.... If you put steel and aluminium beside each other in a structure the steel is doing the work as it is 3x stiffer, so the loadpath is through the steel not the aluminium. Same with using EG as a filler the EG is 20GPa maybe 30GPa at best so its less then half the stiffness of aluminium. So you can create very inefficient structures by using mixed materials.

On a creativity level just use what you have and see what comes out. Can't go wrong if its free and fun. Don't weld as this will create unhappiness, bolt or glue things together, When I was a kid I pissed off people pulling things apart as well so know about that but this was all good learnings. Added bulk does not help, the trick is to use the right material in the right place at the right thickness. This is a lifelong pursuit of mine designing/building machines, cars, planes, ships, yachts, stuff (I'm 60 +) still at it... most of all have fun with daughter that time is special and limited and will run out. Peter

Again... Excellent post. Your explanation about the different materials makes perfect sense. I know the primitive basics of the process but that's about it. So my thoughts are that I have plenty of the extruded aluminum, scrap steel and plywood. I know the steel would probably do most of the "grunt work" but unfortunately most of the pieces I have are pretty rough. It's used to crate up equipment and the manufacturers aren't really concerned about straight edges or cosmetic. It's strictly structural for shipping. There's a lot of warpage from welding and cranking down bolts. It's sturdy and strong but not precise. That's why I feel the need to have the mixed materials. The steel alone would probably take to much work to use alone.
When I refer to the plywood as bulk or filler part of what I mean is that I could see it being handy to use for anchoring and joining. Instead of trying to attach steel, aluminum and rails to each other I could hopefully get by screwing everything into the plywood and using it as a common base to build on. The aluminum would be mostly for the flat surfaces, steel for strength and wood to help hold it all together. Either way I suspect the mix would help with vibration. The steel angle iron doest take much to make it "ring". The plywood and even aluminum are more of a thump.