View Full Version : Truss style Gantry
Bearing in mind I'm neither an engineer or machinist but do have a bit of an epoxy fetish ;) I've been going through some designs of gantry and I'm a bit unsure about the feasibility of constructing something.
This is my inspiration and I'm aiming to end up with something similar.
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This is the basic gantry concept without any of the supporting struts in place:
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It is 60x40x4mm steel box, 1.4m long.
For the struts I can either use solid steel rods or steel angle, the problem I have is that using steel rod will mean welding the struts in place but if I use steel angle I will have to cut and bend the ends of it very precisely to get sufficent surface area for the epoxy joint to hold properly.
Question 1 - is the basic concept a good design of gantry?
Question 2 - is welding the steel rods in place really going to distort the steel, (they will only be small welds, not much more heat than a tack)
Question 3 - Do I have a snowball's chance in hell of bashing bits of angle iron into the right shape using only a few bits of wood and a big hammer?
Any thoughts?
Musht
12-12-2012, 12:35 PM
Bearing in mind I'm neither an engineer or machinist but do have a bit of an epoxy fetish ;) I've been going through some designs of gantry and I'm a bit unsure about the feasibility of constructing something.
This is my inspiration and I'm aiming to end up with something similar.
7604
Any thoughts?
Truss is strong with things hanging from it, not so sure about applying pressure other way round.
If you think about the design of that particular truss, it should deal well with forces in all sorts of directions I was hpoing it should perform better than an unsupported L shape gantry, big hollow box shape or a couple of pieces of extrusion bolted together.
Or am I missing something blindingly obvious?
Musht
12-12-2012, 01:12 PM
Lighting truss is designed to be like a hollow box with most of the weight taken out, the chords are thin walled typically 2mm, it`s very strong in compression. As shown apex up, lights would be hung below and the forces compress the top chord.
But as enough recent stage collapses have shown, push it in the middle and lose most of its strength.
That`s my very basic understanding, which may not be entirely accurate, not a rigger nor do play one on T.V. ;-)
As I understand it a truss works efficently because as force is transmitted through it one part is in compression and another in tension which leads to a stable shape. A triangle is a stable shape but a square isn't. When you diagonally brace a square you are in effect creating two triangles that form a compound truss which is why diagonal bracing works so effectively.
In the arrangement I posted, the struts should be able to cope with forces in any direction encountered by a cnc machine a lot more effectively than a design based on one or two beams that are fixed to a thick plate either end, or so I thought.
I've just had a quick look around the web for stage collapses and I can't find any where the investigation blamed a failure of the truss system for the accident, do you have any links to demonstrate what you mean?
For example if you read
Indiana State Fair stage collapse - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Indiana_State_Fair_stage_collapse)
The failure ... was due to the inadequate capacity of the lateral load resisting system, which was comprised of guy lines (http://en.wikipedia.org/wiki/Guy_wire) connected to concrete "Jersey barrier (http://en.wikipedia.org/wiki/Jersey_barrier)" ballast. The concrete barriers used as anchors for the guy lines were not fixed in place; they resisted loading only by friction (http://en.wikipedia.org/wiki/Friction) with the ground and through their own weight—about 4,200 pounds (1,900 kg).[6] (http://en.wikipedia.org/wiki/Indiana_State_Fair_stage_collapse#cite_note-TT-6) Just before the collapse, wind loading caused several of the barriers to slide or pivot from their original positions, allowing the top of the truss structure to lean toward the crowd. The subsequent bending (http://en.wikipedia.org/wiki/Bending) forces within the support columns were too large, and the structure collapsed under its own weight.[6] (http://en.wikipedia.org/wiki/Indiana_State_Fair_stage_collapse#cite_note-TT-6) Measurements indicated that the total weight was 70,000 pounds (32,000 kg).
Musht
12-12-2012, 06:53 PM
I can't find any where the investigation blamed a failure of the truss system for the accident, do you have any links to demonstrate what you mean?
****The subsequent bending (http://en.wikipedia.org/wiki/Bending) forces within the support columns were too large, ****and the structure collapsed under its own weight.[6] (http://en.wikipedia.org/wiki/Indiana_State_Fair_stage_collapse#cite_note-TT-6) Measurements indicated that the total weight was 70,000 pounds (32,000 kg).
Toronto the week before, there has been a few others.
The legs on self raising stage roof systems are truss on end, even with thin wall its very strong in compression, but tilted off axis , its a thin tube getting bent.
The saving is in weight, early lighting truss was steel antenna tri truss, its a lot of weight to manhandle and to hoist.
Mebbe 2D truss shape , as used by Steeldeck staging
Middlesex University steeldeck 8x4 by Rory - 3D Warehouse (http://sketchup.google.com/3dwarehouse/details?mid=98df51172c62ec2d634b3060582885e&prevstart=0)
again its used to reduce weight for a given strength, if weight is less of a concern apart from moving the mass of the gantry and possibly damping resonance?
Toronto the week before, there has been a few others.
If you look at pictures of the Toronto collapse it is very clearly a scaffold structure that has collapsed and no report has been released so far, let alone one claiming truss beam failure. Just because stages collapse and truss systems are often to hang lights above a stage doesn't mean that all truss designs are fundamentaly flawed.
Anyway, given that the thread is about a cnc gantry built from multiple 4mm thick pieces of steel box section I doubt very much that any force is going to be generated that will come close to causing structural failure, death and destruction just so long as an invading column of tanks don't decide to take a short cut through my garage. :)
I was more interested in a comparison with the usual designs of gantry, like for example a couple pieces of alu extrusion bolted together or the unsupported L shape, bolted to plate supports at either end.
This is the gantry with the supports added, it makes it a little easier to understand what I'm suggesting (the view is from the rear side):
The red box section will be mounted to some SBR rails, the orange box section forms the main parts of the gantry, the yellow bits are struts made from steel angle and the grey bits are 10mm steel bar.
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Edit to add, click the piccy for a larger view.
Musht
12-12-2012, 10:12 PM
If you look at pictures of the Toronto collapse it is very clearly a scaffold structure that has collapsed and no report has been released so far, let alone one claiming truss beam failure. Just because stages collapse and truss systems are often to hang lights above a stage doesn't mean that all truss designs are fundamentaly flawed.
Sorry , where did you draw that i said all truss designs are fundamentally flawed?
As said not a rigger, not playing one on T.V. not playing one on the internet, haven`t said anything about the inherent safety or otherwise of trussing, of which many miles are hung above peoples heads very safely every day and night of the week.
Eaxmple of stage collapses, of which there has been multiple incidents in last 5 years, was used as an example of how aluminium truss or indeed steel tubular scaffolding will NOT:
"deal well with forces in all sorts of directions"
It is immensley strong for it`s weight but will fold up like a wet newspaper if forces are applied in the wrong direction.
Sorry you couldn`t read to end of the post before having to reply
" cnc gantry built from multiple 4mm thick pieces of steel box section"
a link was provided to a 3d model of a staging system that uses steel box section with triangulated reinforcement, more similar to your requirement than 3D alloy truss.
"comparison with the usual designs of gantry"
Cause you missed it last time:
"weight is less of a concern apart from moving the mass of the gantry and possibly damping resonance?"
Not questions can answer afraid.
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
12-12-2012, 10:50 PM
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/research/PrinciplesOfRapidMachineDesign/Principles%20of%20Rapid%20Machine%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
12-12-2012, 11:01 PM
I've considered using something similar to what you're suggesting on my next gantry:
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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.
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/research/PrinciplesOfRapidMachineDesign/Principles%20of%20Rapid%20Machine%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
13-12-2012, 12:22 AM
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 (http://www.mycncuk.com/forums/faqs-problems-solutions/1524-what-size-stepper-motor-do-i-need.html) 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.
Musht
13-12-2012, 01:12 AM
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....
Musht
13-12-2012, 01:30 AM
>>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.
Used an example of failure as an extreme example of how lighting trusses are only stiff in certain directions and almost without strength in others. It is made in a certain way to exploit weight savings for very specific reasons.
It may not be obvious that lighting truss has standard outside diameter chords , similar to aluminium scaffold pole, but the wall thickness is much thinner, the strength being compensated for by the solid rod welded in a triangular form between chords.
Not expecting catastrophic failure of a moving gantry on a router to be considered a high risk.
As said it may not be immediately apparent that trusses secret is generally thinner that would think walls on main tubes, like a staging system also linked to.
But then have already said all this.........
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 (http://www.mycncuk.com/forums/faqs-problems-solutions/1524-what-size-stepper-motor-do-i-need.html) 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.
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
13-12-2012, 02:38 AM
>>>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?
m.marino
13-12-2012, 12:01 PM
The only issue I can see is torsional flex under load might become an issue. I have seen That be a with this type of structure under loads that where not along their axis (which the Z plate and the resistance to cutting will not be at all times (other times it will be). As far as weight moving up to a 2:1 reduction in the pulley system you are going to using should more then take care of any mass issues. Also depending on what screws you use that can become a non issue rather quickly. Truss designs are fun to work with as I have helped build more then a few and done properly they can be extremely strong.
Michael
thats why there isn`t loads of commercial machines with lightweight truss gantrys
Oh look a truss.
DSG 1609 - YouTube (http://www.youtube.com/watch?v=8O4l0jg_qf4)
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)
7639
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. :(
Jonathan
15-12-2012, 01:52 PM
Oh look another truss (http://www.youtube.com/user/calvinoArchitect?feature=watch).
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I think you should make ends of the diagonal pieces meet, or at least close.
Lets compare this design of gantry against some of the others
Most of the gantry designs posted on this site are essentially two beams with something in between. You've added a third beam to the back and braced it, so it's bound to have better stifness, both linear and torsional, to a design with only two beams, for the same amount of material. Where you gain is you can use less material to get the required stiffness since this is an inherently more rigid structure, so you save money. Often people discount this idea due to the apparent complexity and time required to build it, but evidently that's not a problem here.
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?)
Yes, let:
Fx=forces due to gantry parallel to X=force on X ballnut.
Fy=forces due to gantry parallel to Y=force on Y ballnut, total radial force on X bearings.
Fz=forces due to gantry parallel to Z=vertical component of force on X bearings.
mg=mass of gantry
my=mass of Y-axis carriage
mz=mass of Z-axis
fcn=cutting force parallel to axis n, where n=x,y,z
an= acceleration of axis n, where n=x,y,z
Hence the forces on the gantry are:
Fx=(mg+my+mz)ax+fcx
Fy=(my+mz)ay+fcy
Fz=mzaz+fcz+(mg+my+mz)g
There are also forces on the gantry due to the moments in each plane, but it doesn't really achieve much to list them. There's an old thread where I mentioned it here (http://www.mycncuk.com/forums/linear-rotary-motion/2739-calculating-forces-linear-bearings.html). There are some other forces involved, but they're generally negligible.
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.
Yes...although it's not clear in what direction the force on the front top beam is since in addition to the force you describe from cutting, there is a force in the opposite direction due to the beam's mass, which will partially cancel. Either way there is a force on the top beam in the X direction, so having the material there to resist it is necessary.
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?)
It depends on how the 'steel' you're analysing is supported. If you're just squashing or stretching a lump of steel, then the relationship between force and deflection is linear, like a spring (F=kx). That is until you reach the elastic limit (which clearly isn't going to happen here) and the material fails which is probably the graph you were thinking of?
If the beam is supported at both ends, then the deflection formula has a different exponent depending on the position of the force:
Euler (http://en.wikipedia.org/wiki/Euler-Bernoulli_beam_equation#Three-point_bending)
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.
Agreed...
That said, I'm absolutely clueless about vibration and resonance. :(
You've said you're putting expoxy between the joints in the steel box section, presumably this is to damp vibrations in which case resonance should be less of a problem? Also note how in the .pdf I linked to earlier their analysis was based on finding the minimum number of beams to achieve the required stiffness, since by reducing the mass you increase the resonant frequency, ideally making that resonant frequency above the frequency of any forces imposed on the gantry.
Just one extra question about the gantry, for mounting SBR rails they don't have to be 'that' flat do they?
Not compared to linear guides, but what if you decide to change to linear guides at a later date? You should still aim to get it close.
Would mounting onto some cold rolled bright steel bar be accurate enough?
It depends how you mount the bar. If the bar is just bolted to the box section it will simply bend and conform to the box section, so you haven't gained anything except for maybe a smooth surface. You'd need to fix it with adjustment - for instance use lots of bolts with nuts to sandwich the steel flat bar between nuts, so you can then adjust them to make it flat. Ideally you then need an accurate reference...but for mounting SBR rails a 1m steel ruler is probably good enough.
the price for the high viscosity self leveling specialist epoxy is also very high. :(
The epoxy is cheap, i.e. west system 105, but the hardener (west system 209) isn't. If you did use it the surface would be accurate enough for either type of rail, and you wont increase the overhang as much as with other methods. The interesting bit will be making sure the surface doesn't get twisted when you fix the gantry to the X-bearings/rails, so you would ideally cast a resin surface on both.
The model at the moment is just a rough, I will join up the supports it was just pain in the butt trying to rotate things in to odd angles in sketchup. :)
I was going to epoxy to avoid any welding distortions, the rear struts I will weld onto plates and then epoxy all the rear strut plates on to the beams in one go.
My maths still isn't good enough to confidently sit down and work it out, so thanks for that!
I'm almost positive I won't upgrade to guides, if I upgrade this machine it will be to extend the x-axis to cater for 8x4 wood sheets and I can't really see anything I'll be doing at size needing the accuracy of guides.
If resonance does become an issue I always have the option of filling the beams with vermicrete or something similar, a poster over on practicalmachinist reckons truck driveshafts just use thick cardboard tubes inside to dampen vibrations.
The epoxy path is starting to get expensive, £120 quid just for a flatish surface which would mean I would have to ditch the shiny leadshine drives. I think I'm going to have to see if I can find some where local with a decent sized surface table and use the cheap stuff & clingfilm method...
mocha
15-12-2012, 05:07 PM
DC, you'll love this one then! :-)
MOAT (http://www.yangtong.com/UploadFiles/20071117161959342.jpg)
DC, you'll love this one then! :-)
MOAT (http://www.yangtong.com/UploadFiles/20071117161959342.jpg)
Pass me the tissue paper, I may be some time...
Ricardoco
15-12-2012, 05:44 PM
Hi DC, ive been reading this thread with interest and i was wondering, what is the maximum tool size you will be using and what forces would be required to break it, Im not sure but isnt there a relationship with that information and the gantry stiffness required? I would be interested to know..
Rick
routercnc
15-12-2012, 06:39 PM
Hi D.C.
If it helps answer your first questions about the suitability of this design for a cnc router gantry, then my answer would also be yes. Trying to avoid getting to technical:
Although your individual box sections are slightly smaller than a typical size used for a 'single piece gantry', they are located along way from the 'neutral axis' (region of ZERO stress which for your shape is the exact centre of the triangular shape). This makes it much stiffer than the 'single gantry' designs under all loading conditions because the 'distance of material away' factor in stiffness is to the power 4 for torsion and to the power 3 for bending. Doubling the distance away from the neutral axis makes the structure 2^3 or 2x2x2 or 8 times stiffer in bending.
There are 3 loading conditions you need to consider:
X axis cutting loads put bending into the frame are resisted by both front beams in compression and the rear lower beam in tension. Because the beams are spread well apart (and linked by the connection pieces) this is what mainly makes it very stiff. Cutting loads in the other X direction reverse the compression and tension.
X axis cutting loads also put torsion loads into the frame because the tool cutting forces are offset from the centre of the beam. This tries to bend the beams which in themselves are not that stiff, but are prevented from bending by the bracing connection pieces. A structure under torsion has maximum stress at a 45 degree angle to the central axis, therefore angling your bracing pieces at about 45 degrees will be optimal.
Z axis cutting/plunging loads are resisted by both bottom beams in compression and the top beam in tension. Weight of the spindle is resisted with these forces reversed. Again wide beam spacing is the dominant factor to give good stiffness.
So the only thing 'wrong' with this idea is the work involved.
All simply supported beams (of any shape) will suddenly fail when they get to the yield stress, but the loads are massive. You are at the other end of the graph so don't worry about that.
Hi DC, ive been reading this thread with interest and i was wondering, what is the maximum tool size you will be using and what forces would be required to break it, Im not sure but isnt there a relationship with that information and the gantry stiffness required? I would be interested to know..
Rick
To be honest I can not answer your questions, I'm approaching things from the exact opposite direction.
I don't have any calculations for minimum acceptable tolerances, I'm just trying to make the best that I can knowing the skills and budget that I have. This is a hobby cnc, more one off multirole artsy kind off stuff and not industrial high tolerance required scenario.
Despite all the fussing about the arrangement of bits of steel, I will initially be using MDF plates for many parts of the machine, horrible I know but I can cut the alu parts I require and destroy the MDF in the process of doing so.
For what I want to do I need to 4 axis woodwork, I need to cut acrylic, I need to do PCB work and any alu past upgrading the machine is strictly artistic so the tolerances are not very high, as long as a curve is smooth enough for the human eye, I'm a happy bunny, most of that smoothness can be achieved by a good walnut finish, exterme accuracy not required.
Despite that it just is not in my nature not to study, investigate and attempt to the best job possible, if I can make a better machine than I need I would be disappointed in myself to settle for anything less.
Ricardoco
15-12-2012, 07:23 PM
To be honest I can not answer your questions, I'm approaching things from the exact opposite direction.
I don't have any calculations for minimum acceptable tolerances, I'm just trying to make the best that I can knowing the skills and budget that I have. This is a hobby cnc, more one off multirole artsy kind off stuff and not industrial high tolerance required scenario.
Despite all the fussing about the arrangement of bits of steel, I will initially be using MDF plates for many parts of the machine, horrible I know but I can cut the alu parts I require and destroy the MDF in the process of doing so.
For what I want to do I need to 4 axis woodwork, I need to cut acrylic, I need to do PCB work and any alu past upgrading the machine is strictly artistic so the tolerances are not very high, as long as a curve is smooth enough for the human eye, I'm a happy bunny, most of that smoothness can be achieved by a good walnut finish, exterme accuracy not required.
Despite that it just is not in my nature not to study, investigate and attempt to the best job possible, if I can make a better machine than I need I would be disappointed in myself to settle for anything less.
Hey DC thanks for Answering anyhow, i guess i will have to wait till someone else knows the answer, i do look forward to seeing your machine come together as ive not seen one like this before, and to be fair MDF machines are a lot more usefull than may be first imagined, when i had Mine it surprised me.
Rick
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