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  1. #31
    Quote Originally Posted by Iwant1 View Post
    Since this thread started, I've been thinking I got my machine drawings all wrong, and thinking about it day and night.
    Blame it on Jazz... lol... It was he who got me questioning that my drawings were all wrong, although I thought I had roughly copied my gantry design from your gantry design.

    But... I am extremely pleased that he sold me his simpler green design!

    LOL and night night.

  2. #32
    Quote Originally Posted by WandrinAndy View Post
    Blame it on Jazz... lol... It was he who got me questioning that my drawings were all wrong, although I thought I had roughly copied my gantry design from your gantry design.
    It's OK my shoulders are broad has well as my stomach.!! . . . .But never said wrong just could be better with wider profile.!!


    Quote Originally Posted by Iwant1 View Post
    Is there any chance we can see your machine's gantry or get a description, as I know you mostly work with aluminium.
    Well actually this will seem ironic in that my gantry is kind of like box section.? I'm a bit of "Womble" and hate waste. So It's rescued from scrap bin and started life has a Stanner stair lift real HD bespoke aluminium profile that the chair rails ran along.
    It's shaped like an "H" but imagine the bridge being box section with the sides sticking above and below about 30mm. Cant show pics because I'm not at home and my pics are on external drive.
    The void the top and bottom sticking up bits create have had 60x30mm profile inserted and bolted to gantry which the profiled linear rails fasten onto.
    The boxsection void is filled with sand to dampen resonance.! This was done afterwards because I wasn't happy with the tune it was Singing.!!

    I wouldn't recommend anyone copy it because it's a rubbish why to do it and I only did it because I had the profile for free. It took me along time "Shimming" to get it working to an acceptable level of accuracy and I'm still far from happy with it regards strength and vibration so while It's strong there's easier and better ways to do it.

    The secret to cutting aluminium successfully on a "Router" type machine is to limit tool and Z extension to the minimum possible and reduce resonance/vibration to minimum thru using dense gantry/machine materials.
    That's why box section is not ideal for gantry's cutting aluminium unless filled (Made denser) but for woods then it's not really a problem if a decent wall thickness is used and like I say the rail alignment/parallelism etc has to be dealt with thru shimming or surface adjustment regardless if rail mounting surfaces are on individual pieces of profile/B-section or one big boxsection/profile.!!

    If I was in the position lots are where they have to use off the shelf materials that can only be worked with hand tools then for a "Gantry" I'd choose Profile over box section every time.!

  3. Quote Originally Posted by JAZZCNC View Post
    Well actually this will seem ironic in that my gantry is kind of like box section.? I'm a bit of "Womble" and hate waste. So It's rescued from scrap bin and started life has a Stanner stair lift real HD bespoke aluminium profile that the chair rails ran along.
    It's shaped like an "H" but imagine the bridge being box section with the sides sticking above and below about 30mm. Cant show pics because I'm not at home and my pics are on external drive....
    This made me smile Jazz... I have nearly 7 metres of the stuff, in two 3.5m lengths, rescued from my late fathers house, had to cut it into two bits to get it in the car. I had plans to use it for 'something' but never did figure out how to get round the odd shape, I'd describe mine as like two round rails attached to a box section with fins (see the pic below, the end cap is roughly the shape of the profile). I keep thinking about chopping it up and turning it into ingots for future needs but its just looks too good as a profile. Also got the rack (that was already in 2 lengths) and pinion and the original drive motor and batteries still.


  4. #34
    Quote Originally Posted by irving2008 View Post
    This made me smile Jazz... I have nearly 7 metres of the stuff, in two 3.5m lengths,
    Yes I've seen this type on the newer models Irving but it's completely different to the stuff I had given, it was 3.5mtr thou and yes I cut it In half to get into car that's why my machines 1500mm wide. . Lol . .. . I actually didn't have a use for it when given just knew it was too good to go in scrap bin.!!
    My machine came about by watching A U-tube vid and thinking "I can do that.!!" then looking at this piece of profile and pile of spare box section and hey presto 6hrs later the frame was roughed together.!!

    The materials for whole main frame including gantry of my machine (1500mm,sq) cost 77 and that includes Ali for sides and profile on gantry, Z axis Ali cost 25. The complete thing including Z axis but excluding Ali bed cost less than 102. The profiled linear rails cost another 175 second hand of ebay. The ballscrews from chai/ebay and for roughly 500-600 I'd got a working machine that after plenty of tweaking cut's Ali fine and doesn't even know it's cutting wood.

  5. #35
    Sorry bit off topic but JAZZ trying to get hold of you but pm's just bouncing back as your hoarding you must be collecting them lol

  6. #36
    Quote Originally Posted by D-man View Post
    Sorry bit off topic but JAZZ trying to get hold of you but pm's just bouncing back as your hoarding you must be collecting them lol
    Sorted mate.!!

  7. #37
    Quote Originally Posted by JAZZCNC View Post
    Depends which world you live in.? The theoretical or the real.!! . . . . . FFS wish folks would stop over complicating things.!!

    Here's the real world deal.!! . . . Irving is absolutly correct that unless you can surface the two sides parallel and flat then you can't be 100% sure every things correct.! . . . BUT . . . This is true in one way or another for ALL the surfaces the rails mount onto whether they be 1 rail on top & 1 on front, or both on front etc and unless you can put these surfaces complete and assembled onto a milling machine and accurately machine all surfaces flat and parallel or perpendicular to each other then you are going to have to accept they will need some adjustment or tweaking.!!

    How you do this will depend on the design But having the rails on 2 separate sections, no matter if that be profile, boxsection, ali plate will make very little difference and It's FACT that some shimming or filing or grinding will be needed to bring the rails into perfect alignment and parallelism either way.!

    Now I'm not going to blow smoke up your arse's saying this is easy and quick because it's not and doing this takes patience and a lot of measuring and masses of time if you want good accuracy.!. . . BUT . . . If your just cutting wood,foam and bit of plastics then even box section will be close and with slight bit of clean up and shimming you'll be there in no time.!

    I know it's hard if you haven't hands on experience of the forces cnc impart on frames and accuracy required but really for the application of mainly wood use then any of the designs shown here would be plenty strong enough.!! . . . For real meaningful Aluminium and steel work then all of them are not really up to it.!!

    Don't fry your noddles trying to create the perfect flex free gantry for cutting woods etc because your just wasting braincells and time when it's not needed.!

    thats a really great post and it goes someway to help people realise what they need. thank you :D

  8. The Following User Says Thank You to wilfy For This Useful Post:


  9. #38
    Bit late to the topic and most of it has already been discussed but here's my take on it

    Forces on a gantry are difficult to judge because they can change direction and magnitude and are mainly dynamic. To get an ideaof the problem of dynamic forces look in to why Marching Armys have to break step when going over bridges! At a basic level most beams only have to deal with a force in one direction and as such have a larger dimension in that axis, hence rectangular beams. however gantry's are also subjected to twisting so need to be able to resist torsion. circular sections are best for this and square the next best. In comparison rectangular beams are not so good especially if the are slender. Therefore a square beam would be the best option.

    To get an ideaof the problem of dynamic forces look in to why armys have to break step when going over bridges

    Box vs Profile

    A made up beam using tee slot extrusions will have an new 2nd moment of area but the calculation would assume that all the faces are bonded together to make one solid section. any movement in the bolted joints will reduce its torsional stability.

    Box sections are deemed the most efficient as they have all their mass at the extreme edges but can be affected by resonance. this can be damped or designed out.

    Extruded sections can have the same parallelism issues as box and I have seen builds that have milled the top of the extrusion to get the accuracy they need.


    So as Jazz and so many others have said its horses for courses but it does seem easy to get carried away and dismiss simple solutions. Just using linear rails an ballscrews wont result in a precision machine, they have to be correctly mounted and tweaked so they work correctly. so why not use an easy option and extend this tweaking to the mounting structure as well.

    I had a go at designing a composite beam a few years ago but not had time to progress it any further. Planned to use extruded Ali box and channel to made a sort of closed I beam that had a recess for the ball screw. Top and bottom sections filled with epoxy granite to provide damping and compressive strength.

    Click image for larger version. 

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