Thanks for the advice so far. Muchappreciated! Plan is still to build a base with aflat top and then bolt my machine on top of that. I'm basically done building the base and I'll post a few pics soon so its now time to start focusing on the actual machine.
I really had a hard time deciding onthe gantry design. I know the box section designs are proven and works verygreat but if possible I would like to try something a little different with a welded structure made from round tubes, angle section and flat cross supports. Tubes will be sand filled. I'm hoping to get away without epoxy leveling so the idea is to have the X and Y rails sit on 10mm thick ally plates bolted on top of the steel that I can hopefully machine flat and parallel. If that fails I'll go the epoxy route.
Below is what I have in mind. Pleaselet me know if you spot any fundamental issues or things I canimprove. Steel parts are blue and alu parts are grey
The core of the gantry looks like this consisting of 76mmx5mm round section, 50mmx5mm angle section , 100mmx10mm flat bar and several 10mm and 6mm thick braces
What are these 3 small diameter bars along the sides? mounting threaded bars with nuts to hold all together when welding?
What do you want to hear that may be you don't know-beautiful to look at but a waste of material with no further benefit, even worsening some strength points. Unnecessary heavy/plates/ where not needed / table structure/ but nevertheless pleasing to the eye .
What about the orange material, what is it made from? The sides are weaker than box section so if this is weak all will creep.
Basically the "fundamental" issue i see is this:
Instead of strong all supported rails supported by strong all supported steel box structure, supported by itself and gravity on a not so strong table, you are trying to do exactly the opposite- make very strong table, weaker structure and even its weaker semi supported just below the rails themselves. And that for at least 2 times the weight /$$$/ of the material needed.
That is why when i design something i design it like this- bench top, even if its 3x2m steel cnc. Why? Because that's the main structure that will do the job. All else is secondary-serves to lift it to some level or to put something between the floor and the "real structure".
So why not forget all and design a good strong benchtop CNC and then design separate table for it...
When i finished the yellow machine from my first build i said to my self one thing- on the next one i will do all possible to avoid laser cutting and massive laser cut part list and i see here lasercut steel that weights a lot and $$$ to cut.
Plus actually similar sized steel tubes are not stronger or more ridgid than box section:
100x100x4 moment of inertia 236cm4 , torsion inertia 353cm4 Shear modulus/rigidity 36.86cm3
100x4 tube 139cm4 278cm4 55.33cm3
Your mistake here is that yes, 200mm x4 steel tube is much more stronger and rigid than 2 x 100x100x4, but imitating it with 3 tubes and a lot of steel ribs does not do the same job . The questions with 200x4 tube is how to fix the rails and how to deal with overhang/the bulge of the tube/ mantaining at the same time the spread between the rails.
So, box profile structure beats all in simplicity and strength, especially if made stair like with short pieces in perpendicular direction. Sides and gantry.
Please excuse me about the colors,but it helps me better express my thoughts and organize them. I tried not to make all in red and bold
Last edited by Boyan Silyavski; 10-11-2014 at 12:03 AM.
Have you priced up laser cutting yet ?
One large dia tube is going to be much stiffer
Silyavski, thanks for the detailed reply, I do value your input. Like many others on here I'm no mechanical engineer and I go with the standard LAR approach which does not always yield scientific correct results, so please bear with me.
No,those thin parts are old fishing rods that I want to glue gun in there just to stiffen up those unsupported sides a little. Yep, its threaded rod just for for alignment when welding :-)
The orange part is 3 x layers of 18mm quality birch plywood glued together and bolted to the base.Considering the thickness of the wood and close spacing of the steel supports under the plywood, and big surface area of the X-axis sides, I think this will be more than strong enough to bolt my X sides to without, right? There's various good reasons why I 'd prefer NOT to weld the X sides directly to the base.
I totally agree, the base structure can be build stronger for less money, but its already build, so I guess that ship has sailed :-)
I sort of agree about the X sides that's not fully supported. I thought if the rail sits on top of 10mmalu on top of 10mm steel flat bar thats on top of the ribs spaced200mm apart it would be good enough? I'll fix that and make it stronger.
As drawn the steel parts of the gantry weighs +- 60kg if I remove the 750 x 100 x 10mm piece of flat bar which does not add much apart from weight. That gives me +-100kg including z and motors which is getting a bit on the heavy side. I can shave off weight by replacing some 10mm ribs with 6mm. I also agree, in this case the round tubes does nothing apart from cosmetics (which is also important to me) but does complicate some things a little bit. I'll get rid of them and go for something more practical. Would that be a step in the right direction or is the whole steel section and rib design just a bad idea?
I love welding but I hate cutting and grinding steel, and accurately marking and drilling big holes in steel plates is not much fun either, so the laser or hi-def plasma approach just seemed more suitable. If I may ask why are so against the laser approach apart from the additional cost?
I'm glad you did not do everything in bold red as that might have pushed me over the edge an got me to build the whole damn thing from polystirene and cold glue
Thanks again for helping me to get this right!
Last edited by mitchejc; 10-11-2014 at 08:49 PM. Reason: spelling
Yep, the big dia tube is much stiffer but there's just no practical way I've seen to make it work for a gantry router due to the reasons Silyavski listed above.
-IMO is very bad idea to lay the machine on anything else but steel. I will advice against it. I mean the orange part as you have drawn it is bad idea.
-I agree the design should please your eye. I wouldn't do it that way at all as my moto is "maximum efficiency, minimum effort" . I mean i advice against using steel tubes at all. Though as i see it the gantry could be left as it is if you insist on keeping the design, just take care the plate bellow the rails to be truly all supported, not hanging in the air.
-The sides definitely could not be as they are now
I mean, change everything, but if you insist keep the gantry
I haven't read this thread for some time now and I am surprised at the design of the router as it is now. It looks really nice.
Am I correct in that you will have pieces cut by laser as part of the build?
If so, why not build a steel torsion box from nothing but laser cut sections?
That would save a hUge amount of time, and be stiffer too.
Last edited by Sven; 11-11-2014 at 06:58 AM.
Yep Sven, the idea is to have the parts laser cut. Interesting idea. I'm sure what you are suggesting would be possible and EXTREMELY stiff. Its amazing how stiff some of the light torsion box or honeycomb composite structures are like on the photo below
I'm currently in contact with a guy who is building an extreme fixed gantry router. He can calculate this stuff in to extremes.
He suggested to me that a steel torsion box is nearly as stiff as a solid steel one and when designed right wil only need spot welding to keep it together, but can even be glued if you have the right technology available.
Ok, now its down to bartering. A friend needed some aluminium house numbers urgently and he had something I wanted, so I cut the numbers Wednesday night and swapped this
Great deal, don't you think? :-)
Back to plan A of using I-beam for the gantry sides. The beam I got was a bit higher (254x148) than I hoped for but I'll make it work. I guess for me DIY CNC is more about what I have/can get/can build rather than building the ultimate design from the best choice of material. I also listened to the advice and went with a slightly more practical approach and used 76x76 square tubing, and some 10mm and 5mm plate and also bolt or weld the X sides directly on the base. Gantry weight is +- 60kg excluding Z. I know Y rails top bottom is the better approach but I'm thinking its going to much easier for me to get them parallel and on the same plane if I put it on the front. I'm hoping the laser cutting is not going to be too expensive otherwise I'll have to go for plan B with the gantry. See pics below.
Again please comment if you see any obvious issues.
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