Truss style Gantry

[D.C.]

Sorry about this Musht when threads get like this I normally apologise and walk but as this is my thread and I was actually asking for help and so far all I have had is an argument about stage collapses that is completely and utterly unhelpful to me.

Would you mind walking away from the thread please so that someone else could try to help me without derailing the thread with an argument?

I am really not interested in having an argument but I would like some relevant and helpful feedback from people to assist me.

[Jonathan]

That concept is excellent since it will have good stiffness for forces parallel to X and excellent torsional stiffness.

In case you've not already spotted it:
http://www.mech.utah.edu/~bamberg/re...e%20Design.pdf

I'd probably stick to using one cross section throughout, most likely box section. Your main concern will be making the surfaces on which the Y-axis rails mount sufficiently accurate, i.e. parallel. It's much easier if you mount both the rails in the same plane (i.e. on the 'front') since you can then just use epoxy granite if necessary to obtain the required flat surface. Using that orientation does however increase the overhang of Z, but I think that's not a big problem given how strong the gantry will be.

Any discussion of structures failing is hardly relevant, since in a CNC machine we are aiming to obtain a certain level of stiffness, not just aiming to ensure the material does not fail. That is why to the beginner the designs can look over engineered, since the size of material used is comparable to much larger structures. The critical difference is in civil engineering the allowable deflection is substantially greater - nobody cares if a stage truss deflects by a couple of millimetres, but if your gantry deflects by just 0.1mm you could be in trouble.

[Jonathan]

I've considered using something similar to what you're suggesting on my next gantry:



I drew the above to test the frame using FEA to determine whether I could obtain the required stiffness from this concept, hence some bits have not been included, and vice versa.

[D.C.]

That concept is excellent since it will have good stiffness for forces parallel to X and excellent torsional stiffness.

In case you've not already spotted it:
http://www.mech.utah.edu/~bamberg/re...e%20Design.pdf

Yup seen that a while ago, it is probably that which stuck in mind when I had the idea.

I'd probably stick to using one cross section throughout, most likely box section.

I had thought about that but I was getting concerned with the weight, the design as it is will probably go 80kgs with spindle&Z etc on it. Using all big box section would probably add another 20kgs to that. I was starting to chicken out running two screws to move a 100kg plus gantry on a single 3NM motor. :(

Am I misunderstanding how trusses work? I thought that the struts functioned by effectively transfering force to the main beams by being in a combination of compressed/tensioned members when it is loaded.

Do the struts really need the mass of the main beams or were you suggesting that for ease of construction?

Your main concern will be making the surfaces on which the Y-axis rails mount sufficiently accurate, i.e. parallel. It's much easier if you mount both the rails in the same plane (i.e. on the 'front') since you can then just use epoxy granite if necessary to obtain the required flat surface. Using that orientation does however increase the overhang of Z, but I think that's not a big problem given how strong the gantry will be.

It isn't shown on the design but the beams that the SBR is going on I was planning to attach using a bracket and bolts. I was hoping an eccentric bolt would be sufficent to give me fine adjustment and get the two pieces parallel when fitted. The steel angle seemed like a good choice as I could get the fine adjustment finished and then epoxy the steel angle in place knowing that it wouldn't move out of alignment. The angle also allows me just enough room to run the ballscrew inside the gantry.

Any discussion of structures failing is hardly relevant, since in a CNC machine we are aiming to obtain a certain level of stiffness, not just aiming to ensure the material does not fail. That is why to the beginner the designs can look over engineered, since the size of material used is comparable to much larger structures. The critical difference is in civil engineering the allowable deflection is substantially greater - nobody cares if a stage truss deflects by a couple of millimetres, but if your gantry deflects by just 0.1mm you could be in trouble.

[Jonathan]

I had thought about that but I was getting concerned with the weight
...
Do the struts really need the mass of the main beams or were you suggesting that for ease of construction?

I wouldn't worry about the weight too much from the point of view of the stepper motor(s). Try using irving's motor calculation spreadsheet and you'll see how little difference an extra 20kg out of 80kg will make. I was just suggesting it for ease of construction but there is no reason to assume I meant make all the box section the same size.

[D.C.]

I wouldn't worry about the weight too much from the point of view of the stepper motor(s). Try using irving's motor calculation spreadsheet and you'll see how little difference an extra 20kg out of 80kg will make. I was just suggesting it for ease of construction but there is no reason to assume I meant make all the box section the same size.

I sort of ended up with the odd mix based on what I knew I could do, the bar is there because I have no way of accurately bending steel and I don't think just bashing it with a hammer would get a good enough epoxy joint.

That said I could weld the supports to a mounting plate off the gantry and then epoxy the plates...

The biggest flaw I think is with the rail location, I'm losing travel either side and also won't be able to get the bottom bearings off the rail without removing the whole rail. Hmmmm, back to drawing board.

[Musht]

Usually just mark you as an idiot incapable of basic reading and understanding and move on but since you provoked me and waiting for some email.

Perhaps the point wasn`t clear enough, I`m not involved in your imaginary argument with an invisible opponent about stage collapses.

Sorry that brought that up as an example of how truss is not able to withstand forces in all directions, but it is a very clear example.

Your just not able to maintain concentration for more than two sentences and your unable to understand what is written.

You then blame those attempting to help you for your lack of understanding, you don`t think, you expect to be spoon fed every item.

The question was posed what advantage do you expect to acheive with a open webbed truss gantry?

If lower mass than say a steel I beam, to what advantage?

Lowering mass may result in more possible resonance ,that would most certainly may have an effect of machining.

You haven`t read this far though, you think you have invented a revolution in moving gantry routers, anything that makes your idea less than the invention of the wheel is to be discarded and insulted, thats why there isn`t loads of commercial machines with lightweight truss gantrys, sure Boeing will be on the phone to you shortly....

[D.C.]

A load of bullshit

Musht, wtf?

What side of bed of who's bed did you get out of to put in such an obnoxious mood?

Ok I'll play.

You have said several times that you are not a rigger, well here is a bit news for you, up until I wrecked my legs I was a professional abseiler and rigger, fully ticketed and doing it for a living. I've taken plenty of 15-20ft falls on to alu trusses and know for a fact all your talk of 'folding like wet paper' is total rubbish.

One of us is looking like a complete prat here and I'm pretty sure it isn't me feel like editing all the personal abuse of your posts?

PS I did read all of your posts, I did understand them they were just in no way of any use to me, sorry if that pissed you off.

[Musht]

>>>feel like editing all the personal abuse of your posts?

Its fair comment , no.

>>> I did read all of your posts, I did understand them they were just in no way of any use to me, sorry if that pissed you off.

Didn`t piss me off, just sorry wasted the points to be honest, dosen`t make for interesting debate or a useful learning experience.

Will have to hope Jonathan or others has some answers on why the mass saving would be helpful and wether he regards resonance as a possible issue?

[D.C.]

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)




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. :(