Quote Originally Posted by Musht View Post
thats why there isn`t loads of commercial machines with lightweight truss gantrys

Oh look a truss.
DSG 1609 - YouTube

I guess Dean Smith and Grace know sweet FA about designing machines, bunch of cowboys...


The main reason why I started exploring down this route was by looking at the design decision taken by other builders, my budget and my desires for the machines performance.

I need at least 6 inches of travel as I want to 4 axis work, the wood lathe I'm planning to canibalise will turn 12" wood and I have a few design ideas that need hefty chunks of wood.

I'll stick another picture in with axis labels: (This is missing a piece of box section either end)

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Lets compare this design of gantry against some of the others bearing in mind this going on 'my gut' and I have no actual engineering experience and I have never built a cnc machine in my life and I have not yet been able to find an open source FEA package that I can get to grips with. (I've been trying to learn Z88 but it keeps speaking German to me :( )

Leaving aside gravity, the gantry momentum and motor problems etc I assume all the forces that this gantry will encounter will be coming from the tip of the cutting and will be transmitted to the gantry via the four bearing blocks on the rail. (Is this right?)

If the machine is moving forward in a straight line on along the x-axis, the forces it will generate will want to push back against the front bottom beam and pull the front top beam forward.

Another assumption of mine, the distortion of steel is does not have a linear relationship to the force it is put under so if there is a small amount of force the deflection is negligible, increasing slowly until it approaches breaking point and then the deflection increases extremely quickly. (The graphs I was looking at for this seemed to be exponential or worse, is this about right?)

In this design the point forces that are acting on the gantry through the bearing blocks get spread out and transmitted through the rear struts to the back beam. What musht was saying about a plate being stronger is true, but this design can come out of £17 worth of steel angle, to plate three sides of this gantry in 10mm alu would mean buying a 8x4 sheet of the stuff and a quick check online puts that at about £500 + VAT. Which is just taking the piss cost wise, a single offcut piece of alu plate this length will cost more than the entire gantry...

For twisting forces or forces parallel to the y-axis the same should apply, because the truss system isn't in one plane but is in three planes this style of truss creates a very stable form in all directions.

If I had the ability and software to model dozens of different designs and do FEA on them I would be doing it right now, but I don't so I'm fudging it. I do have a hard time believing this design (which is cheap) is going to distort more under load than a couple of bits of alu extrusion bolted to an alu plate or the L style gantries that I've seen.

That said, I'm absolutely clueless about vibration and resonance. :(

Just one extra question about the gantry, for mounting SBR rails they don't have to be 'that' flat do they? Would mounting onto some cold rolled bright steel bar be accurate enough? I'm working without access to a good straight edge or surface plate, the closest I can get is sheet glass or MDF, neither of which seem really flat enough for a good cheap epoxy transfer, the price for the high viscosity self leveling specialist epoxy is also very high. :(