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  1. #1
    Join Date
    Sep 2004
    Posts
    21

    Why make the gantry so high?

    I am in the middle of both researching 3 axis and 5 axis machines for work and designing a 3 axis machine for home. All the designs seem to raise the gantry off the rails to the height so the z-axis then travels down to meet the cutting table. Raising the gantry to gain this z travel would necesitate a heavier duty block and rail system because of the twisting of the high gantry. Why not keep the gantry low beaf up just the z axis mount, and lower the cutting table the distance of your z travel? On huge cnc machines I have seen the rail be 8' above the floor. Just thinking out load.

    Trent Taylor

  2. #2
    Join Date
    Mar 2003
    Posts
    6855
    You can but you will be limited to the length of tool you can use and the thickness of material you can cut. That's the reason they are so I so as not to limit yourself to thin material.

  3. #3
    Join Date
    Nov 2003
    Posts
    634
    Trent, on commercial machines the Z axis clearance height is set based upon what 1000's of potential customers might need. If you are cutting 1/4" plywood that might not be much, but if you are modeling a full size car on a 5 axis machine, it would be quite high. They try and hit a happy medium. I suppose if you want to cut the edges of a molded spa on a 5 axis machine, you'd need about a 3 to 4' Z axis.

    On your DIY machine you can set it as low as you want. But if you cut 1/4" thick parts and only make the Z 1/2" you won't have any room to get the router bit out of the chuck.

    These are extreme examples, to try and illustrate my murky points.
    T

  4. #4
    Join Date
    Sep 2004
    Posts
    21
    I will try to post a drawing of what I am thinking about. Basically the gantry rides on rails that are 10" above the cutting table. The Z axis could still travel down the 10". You just wouldn't have the gantry rise so much higher than the rails. Thanks for the ideas.

    Trent Taylor

  5. #5
    Join Date
    May 2004
    Posts
    290
    Trent,

    Some commercial machines use this approach. Shop bots are designed this way. Uptime.ca also makes router/mills where the rails are elevated above the table (see attached file).

    Carlo
    Attached Thumbnails Attached Thumbnails left-side Uptime CNC #F88DA1.GIF  

  6. #6
    Join Date
    Sep 2003
    Posts
    226
    My thinking was the same as yours, why not work below the rails...
    This is a not so good image of what I did.
    Attached Thumbnails Attached Thumbnails Imag0026.jpg  

  7. #7
    Join Date
    Sep 2004
    Posts
    21
    Sol,
    That is pretty much what I was thinking. Great site showig how you did it. What size motors run everything? Feel free not to say, but how much did the project run you?

    Trent

  8. #8
    Join Date
    Jun 2004
    Posts
    487
    That's how I have mine http://www.cnczone.com/forums/showth...02&page=5&pp=5

    I thought the same thing. The torsion/lever factor is affected by the total length between the Z point of contact on the material and the supporting rails. The shorter this is, the stronger it will be. I better not say much more as I really don't know what I'm talking about but this design makes a whole lot of sense to me.

    Later,
    JR

  9. #9
    Join Date
    Dec 2004
    Posts
    5
    I can see what you guys mean, it just seems logical to do this design. One could even say it feels right!

    As was said, the limitation was the ease of use. If you want to use a big workpiece, you will have to slide it into the open end. So the machine would have to go along the wall of a long wall of a garage to give sufficent access.

    Oh, well, would you look at that! My first post!

  10. #10
    Join Date
    Mar 2003
    Posts
    35494
    The downside of that design is that with large pieces, it can be difficult to load and unload the work into the machine. Maybe not so bad with small pieces, but especially so with large 4 x8 size. If you can load from the side, one person can manage a 4 x8 sheet, but loading a 4x8 sheet from the end by yourself is not something I'd want to do often.
    Gerry

    UCCNC 2017 Screenset
    http://www.thecncwoodworker.com/2017.html

    Mach3 2010 Screenset
    http://www.thecncwoodworker.com/2010.html

    JointCAM - CNC Dovetails & Box Joints
    http://www.g-forcecnc.com/jointcam.html

    (Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)

  11. #11
    Join Date
    Sep 2003
    Posts
    226
    Yeah like Gerry says!
    Notice that my table's end is right at the double door. Sheets slide right off the truck onto, or into, the table; it would be a really big pain otherwise... and according to my back and knees wood gets heavier every year.

  12. #12
    Join Date
    May 2003
    Posts
    550
    CBass - do you have a url for uptime.ca ? www.uptime.ca doesn't work..

    Sol - big machine! how are you driving the X (long) axis? belts? rack?

    thanks..

    Andrew

  13. #13
    Join Date
    Nov 2004
    Posts
    260
    Quote Originally Posted by tltaylor
    I will try to post a drawing of what I am thinking about. Basically the gantry rides on rails that are 10" above the cutting table. The Z axis could still travel down the 10". You just wouldn't have the gantry rise so much higher than the rails. Thanks for the ideas.

    Trent Taylor
    I see what you mean. Yes I think it would be more ridgid to have
    the x,y and z axses closer together you would reduce the
    leverage that is active on some gantry designs.
    It would be a challange to design a ajustable table that could be
    raised and lowered and still maintain alignment the ajustments
    would also need to be ridgid.
    Hard to come up with something that would not be more of a mechanical
    hassle then the machine itself.
    Good Luck

  14. #14
    Join Date
    Sep 2003
    Posts
    226
    Answering questions:
    X axis is rack and pinion. After building the machine there were posts here recommending 20 degree rack rather than older standard 14.5 degree that I used. Live and learn.
    While on the subject ( but I do not want to thread jack) belts have been highly recommended by those in the know, one reason was rack & pinion's tendency to skip teeth when heavily loaded. Yes, I learned the hard way this is true. Click my name for a link to further info on my homemade solution..Second machine, page 4.
    Machine cost...$2500 would cover it, including servos and control box ...but the rails and lead screws are pipe and threaded rod. After pricing 10 ft. rails and bearings it became clear upgrading would have to come later.
    The motors are 430 oz in servos.

    Back on topic...Regarding the height of the Z, With a solid push it is still possible to twist the gantry because of the leverage of the Z. Other machines use a large box style gantry to address this. Ninewgt and Dean of Axxus have some good pics on their sites. It seems that system keeps reappearing because it is so simple and solid.

  15. #15
    Join Date
    May 2004
    Posts
    290
    Quote Originally Posted by fyffe555
    CBass - do you have a url for uptime.ca ? www.uptime.ca doesn't work..


    Andrew
    Sorry but I don't. Nicholas Hughes is the owner and is a member here (try contacting him if you'd like). When I spoke to him last, he was working on a new site. He sent me some pdfs on his machines which I can post if you're interested.

    I hope they didn't go under because I'm thinking of buying one myself. It's one of the only "low cost" routers that can machine steel. Hardcore stuff...

    Carlo

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