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  1. #121
    Moving on to the mechanical side of things (at least for now) and essentially the gantry arm design. Am I right in assuming that it is the centre of gravity of the entire gantry assembly (incl. gantry arms, beam, z-axis, spindle etc.) that preferably should be half way between the X-axis (long axis) rail carriages?

    My thinking is that any force applied to the spindle in the X direction affects the balance of the entire gantry assembly. Now, the reason I'm double checking this is that when drawing the gantry assembly (as in the attached, albeit, unfinished drawing) I find that the COG is quite far back (the left side mark) if the beam is also included in the calculations. I know some parts are still missing from the drawing, but even when included, the end result will not change much (in fact, what's missing is mostly parts on the left hand side of the mark). Bottom line is, in this scenario my gantry arms would raise straight up from the X-axis carriages (hence, neither would the inclusion of the arms affect the cog) and, essentially, leaving the spindle quite far in front of the front carriage.

    Problem is that nearly all gantry arm designs I've looked at will have the spindle closer to the halfway point between the carriages or at least somewhere between the carriages. Now, when I calculate for the Z-axis assembly only (the mark to the right) the COG is naturally much closer to the spindle, and should I use this as my reference point my gantry arm design and more importantly the spindle position (relative to the X carriages) would end up looking much more like what I see others using. My intuition still says I should go with the COG of the entire gantry assembly as my reference point, but seeing that this would end up looking much different to the norm, it makes me wonder if I've overlooked something or simply using the wrong logic?

    Click image for larger version. 

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  2. #122
    Quote Originally Posted by Juranovich View Post
    I still cannot bring myself to do something without having built an understanding of my own of the matter. Hence, the occasional silly questions :)
    The only silly question is the one you don't ask...Keep firing away we understand.!
    What's I'm trying to say more than anything is don't let it slow you down from building because you'll still end up back at or around what's been suggested.

  3. #123
    Best would be if Spindle centre /the bit/ does not go further than than carriage, machine looked from side . But If not possible its not a problem . If square linear rail is used. Though for metal work i say better not, for a perfect finish i mean.

    Red line shows the ideal, blue line the normal. I would say 250-260mm span for me is the minimum of the gantry legs/ bearings


    Click image for larger version. 

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    project 1 , 2, Dust Shoe ...

  4. #124
    Quote Originally Posted by Juranovich View Post
    Moving on to the mechanical side of things (at least for now) and essentially the gantry arm design. Am I right in assuming that it is the centre of gravity of the entire gantry assembly (incl. gantry arms, beam, z-axis, spindle etc.) that preferably should be half way between the X-axis (long axis) rail carriages?
    Yes Ideally but again without wanting to sound like a broken record don't let these kinds of details bog you down from the building phase. The COG being off a little like what your showing isn't going to make one jot of difference to how the machine performs or how it affects component life in a DIY environment.

    Anyone who's built a machine and worried about these kinds of things will tell you that it was a pointless exercise and it's the smaller details that make a bigger difference to how the machine performs. Details like sturdy ball-nut brackets and adjustability, motor mounts, wire routing, Limit SW placement, access to grease nipples and lubing, etc, etc.
    If you build a Sturdy structure and pay attention to key areas like Z-axis then you won't go wrong or notice if COG is off a little, but you will notice if the little details are missed.

    My advice is to look around at other builds and pay more attention to the little details and pay less attention to if COG looks a little offset.!

  5. #125
    Quote Originally Posted by Boyan Silyavski View Post
    Best would be if Spindle centre /the bit/ does not go further than than carriage, machine looked from side . But If not possible its not a problem . If square linear rail is used. Though for metal work i say better not, for a perfect finish i mean.

    Red line shows the ideal, blue line the normal. I would say 250-260mm span for me is the minimum of the gantry legs/ bearings


    Click image for larger version. 

Name:	side view.PNG 
Views:	93 
Size:	98.2 KB 
ID:	27319
    I've planned on using a 250mm span between the outer ends of the square linear rail carriages and as it stands right now the arms would look like the blue lines in your sketch (though if cog is centered with centre of carriages the bit would fall outside or just on the outer edge of the front carriage). For reference the alu extrusions are 45x90.

  6. #126
    Quote Originally Posted by Juranovich View Post
    I've planned on using a 250mm span between the outer ends of the square linear rail carriages and as it stands right now the arms would look like the blue lines in your sketch (though if cog is centered with centre of carriages the bit would fall outside or just on the outer edge of the front carriage). For reference the alu extrusions are 45x90.
    It does not matter as far as you use Hiwin 20 size or comparable rails. You could offset al gantry rail to the back, to compensate a bit for this. As Dean says look at other build for inspiration and to figure the details.

    I will not tire to say that machine is as strong as its weakest point, so do not overbuild / did i just say that / just take care not to have weak details
    project 1 , 2, Dust Shoe ...

  7. #127
    Quote Originally Posted by Juranovich View Post
    I've planned on using a 250mm span between the outer ends of the square linear rail carriages and as it stands right now the arms would look like the blue lines in your sketch (though if cog is centered with centre of carriages the bit would fall outside or just on the outer edge of the front carriage). For reference the alu extrusions are 45x90.
    If it helps I use 300mm wide plates with this design.! Anywhere between 250 and 300 will work ok.
    However, keep in mind how you are going to fasten the profile to the plates and fasten the plates to the bearings if going with gantry on rails setup.! It's easy to forget things like access to bolts or one blocking another.!

  8. #128
    Quote Originally Posted by Boyan Silyavski View Post
    I will not tire to say that machine is as strong as its weakest point, so do not overbuild / did i just say that / just take care not to have weak details
    I DO NOT BELIEVE WHAT I'M READING... . . . FINALLY.........We agree on something...

  9. #129
    Quote Originally Posted by JAZZCNC View Post
    Anyone who's built a machine and worried about these kinds of things will tell you that it was a pointless exercise and it's the smaller details that make a bigger difference to how the machine performs. Details like sturdy ball-nut brackets and adjustability, motor mounts, wire routing, Limit SW placement, access to grease nipples and lubing, etc, etc.
    If you build a Sturdy structure and pay attention to key areas like Z-axis then you won't go wrong or notice if COG is off a little, but you will notice if the little details are missed.
    That's the paradox I guess! I keep reading all the theory but as I haven't done this sort of a project before it's hard to tell what's essential and what's not. And as you rightly point out, doing the details/assembly properly will matter quite a bit for the end result (as it would for any project really), maybe even more than getting all the theory right ;)

  10. #130
    Quote Originally Posted by Boyan Silyavski View Post
    It does not matter as far as you use Hiwin 20 size or comparable rails.
    20mm is the plan, but not sure yet about going full hiwin or not...

    Quote Originally Posted by JAZZCNC View Post
    If it helps I use 300mm wide plates with this design.! Anywhere between 250 and 300 will work ok.
    The idea crossed my mind. Would help bring the cog slightly forward + more stability.

    I could also get creative and hang a 5kg weight from the spindle :D that should balance it up!

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