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  1. #11
    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.
    Old router build log here. New router build log here. Lathe build log here.
    Electric motorbike project here.

  2. #12
    I've considered using something similar to what you're suggesting on my next gantry:

    Click image for larger version. 

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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.
    Attached Thumbnails Attached Thumbnails Click image for larger version. 

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    Last edited by Jonathan; 13-12-2012 at 01:39 AM. Reason: grammar
    Old router build log here. New router build log here. Lathe build log here.
    Electric motorbike project here.

  3. #13
    D.C.'s Avatar
    Lives in Birmingham, United Kingdom. Last Activity: 05-01-2016 Has been a member for 4-5 years. Has a total post count of 326. Received thanks 30 times, giving thanks to others 24 times.
    Quote Originally Posted by Jonathan View Post
    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.

  4. #14
    Quote Originally Posted by D.C. View Post
    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.
    Old router build log here. New router build log here. Lathe build log here.
    Electric motorbike project here.

  5. #15
    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....

  6. #16
    >>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.........

  7. #17
    D.C.'s Avatar
    Lives in Birmingham, United Kingdom. Last Activity: 05-01-2016 Has been a member for 4-5 years. Has a total post count of 326. Received thanks 30 times, giving thanks to others 24 times.
    Quote Originally Posted by Jonathan View Post
    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.

  8. #18
    D.C.'s Avatar
    Lives in Birmingham, United Kingdom. Last Activity: 05-01-2016 Has been a member for 4-5 years. Has a total post count of 326. Received thanks 30 times, giving thanks to others 24 times.
    Quote Originally Posted by Musht View Post
    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.

  9. #19
    >>>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?
    Last edited by Musht; 13-12-2012 at 02:48 AM.

  10. 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

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