Bench-top or welded frame

[Doddy]

Eddy - aha, I was preoccupied with rigidity, not mass - something of an oversight of mine.

[Doddy]

So, I've been playing with Fusion 360, as much to try to get some familiarity with the software, using the idea of a frame as a starter project...



Design based around 50x50x4 and 100x50x4 (shamelessly stolen from Neale's AVOR design), external dimensions of 1200 x 700 x 600(h). I'm not afraid of cutting / welding steel (or at least gluing steel together with a Tig torch), but there must come a point where you say enough is enough. I'm looking at this in cold light of day and thinking that I'm creating a design for a somewhat top-heavy frame, be a bit of a git to weld up (and use a hell of a lot of argon up in the process). I've found myself randomly adding bits to the design - the braced bed, the bracing for the internal space to house a small compressor, and maybe a suds sump, partial bracing for the frame - but it's at this point I decided to take a step back and question my sensibilities.

(To do - add the top rails - similar to Neale's, as a raised set of rails on the top of the frame)

Basically... when do you stop adding steel? Is a design like this remotely sensible?, or is it steel-work for steel-work's sake? Are there any pointers or directions into what the requirement for a frame is, and how to best approach a design?

...but it needs to be designed correctly.

I suppose that's really my question - are there any sensible guidelines out there that can be used as a frame-bible?

[Neale]

(shamelessly stolen from Neale's AVOR design)

Come on, now - you should at least feel a little bit of shame

Personally, I reckon that's a bit over-built. Mine uses 3mm steel, not 4mm, and although my pictures don't really show all the bracing/triangulation that well, I've used a lot less than yours. You could probably do without all that triangulation under the bed, although it does depend on what the bed is going to look like. Have you thought about how you are going to sort the bed? That's the kind of thing that it's worth planning at this stage, just to make sure that you don't paint yourself into a corner. Also, where are the X rails going to be mounted, and what's the gantry going to look like?

Unfortunately, there doesn't seem to be any magic formula, and the best thing to go on is other people's designs and their experience with them. Mine could be a little heavier, although I really haven't seen any structural weakness in it that worries me, even when doing machine-shaking detailed 3D work.

[Doddy]

Come on, now - you should at least feel a little bit of shame

Nah, not a bit :p

My preference for 4mm over 3mm was partly due to deflection calcs (which I don't understand but can sense that one number is better than another), cost (not much in it) and more chance to hold a thread (although nothing at this stage is tapped). The bit I hadn't done was add the final two horizontal members on top of the bed - very much in keeping with your design to mount the rail. My intent was to measure and order the steel, and spend the next few weeks welding on warm days, and designing the gantry on cold days... but you might guess that much of the gantry would be somewhat familiar (I do like your design).

The bed, and the bracing beneath the bed... the bracing from the maxim that more is good (but you can see that it's at this point I take a step back and wonder if I need to buy shares in a welding or steel supplier) and providing some bed rigidity and also some support for the bed itself. The bed - I haven't thought too much above but imagine a 15mm Ali sheet bolted to the bed frame, shimmed or skimmed, or both.

Plans for more bracing under the centre - top to bottom but I have a strong desire to keep the right bay open for storage - controllers/computer/services and just tool storage.

Painting into a corner?, that's kind of where I was losing confidence with the number of acute angles for bracing - I can see it being a bit of a git to weld all that and it's starting to feel heavy just on screen. And, at the end of the day how much is really necessary. That's when I tried to google about the effects of triangulation and bracing, but I think I'm using the wrong search strings.

Yours, I think you are targeting hard woods, from memory?, or have you done any aluminium? My areas of interest are light alloys and laminate, plastics etc. I'd be interested if you have had success with your design on similar.

[Neale]

Sorry - I just haven't done anything with aluminium apart from one very small job where the problems were more to do with using a small cutter and getting clogged cutter flutes than machine loading. Most of my work to date has been with ply/mdf/hardwood/plastic - that's just what happens to have come along!

I want to do a bit of work on steel, but it might just be limited to spotting hole centres accurately to finish on a drilling machine. One day I shall have time to experiment with these things!

One thing to bear in mind, perhaps, is that because the tubes you are using are fairly chunky, there is quite a lot of stiffening in those welded joints without explicit diagonal braces. My own experience (again, anecdotal not rigorous!) is that most of the "trying to shake itself apart" loads are trying to move the whole bed plane fore and aft, needing the side frame triangulation. There are much smaller "lozenging" loads in the plane of the bed. I doubt if you need quite so much bracing across the bed, therefore. And it's difficult enough to get the bed support components flat as it is.

[Doddy]

All good points

[Neale]

Thinking about it, I cannot remember seeing a single aluminium extrusion-based machine using diagonal bracing under the bed (although that might be a faulty memory!). And some of those machines have certainly been capable of machining aluminium. Additionally, the biggest "shaking" loads are involved in accelerating and decelerating the gantry, followed by accelerating and decelerating the Z assembly along the gantry. That is much less than the first, obviously - the mass of the gantry is the biggest load. I've reduced the effect of the second load by raising the side rails so that the Z assembly is not as high above the X rails - moving mass is more in line with the support points - although that means that the rails need good support. I think I've achieved that. I instinctively feel concern about the designs that use a single tall plate each end of the gantry, which I can imagine bending under dynamic loads. Clearly I'm wrong there as there are plenty of successful machines that are built like that although they need pretty heavy plates to take the bending loads. But engineering is all swings and roundabouts and there are always trade-offs to be made. My gantry design is more fiddly to make, for example, and probably needs a vertical mill as a minimum (which I have available, hence the design).

I do have a single diagonal corner to corner across the end (shorter) sides of the main frame. I have two more-or-less diagonals across each long side, but these are arranged to take the load of the intermediate X rail tube supports down to the bottom of the legs and do not run corner-to-corner. That does leave space to get the control cabinet in (although to be honest I had forgotten that when I built the frame, and I was lucky that the cabinet fitted the space available with a few millimetres to spare). Occasionally the gods smile on us.

I don't have a support leg in the middle of the bed, and I happily crawled over the bed structure when I was building the machine. My bed is all 50x50x3; if you used your 100x50x4 as cross-rails and braced it with intermediate 50x50 segments between them, I'm sure that would be stiff enough.

I would advise having a suitable device for cutting the box section to length accurately and squarely; I used an angle grinder in a small pivoting stand and struggled to achieve accuracy. This was a bad decision on my part, born out of lack of experience. One of those cut-off saws, maybe, with a decent size blade? Personally I would go for a metal-cutting bandsaw but that's because I have other work I do that would make use of it.