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  1. #1
    Join Date
    Oct 2005
    Posts
    32

    Moving gantry.....or moving stage

    I am a newbie who is looking to build a table top router CNC. Before I start buying parts I need some help in the design. I want to my unit to be about 2 foot by 2 foot. (size also depends on what parts I find on Ebay)

    However, I have a design question....Moving gantry or moving stage? Does anyone have any opinions on this for a small unit like this? I've also seen a small unit..(couldn't find the picture again) that has an open ended gantry. Is this a design problem?

    Thanks

  2. #2
    Join Date
    Apr 2005
    Posts
    629
    My opinion is that a bridge style machine is about the easiest to make at home. A bridge machine looks like a gantry but is far more simple. The table on a bride machine moves - this is the X axis and you have to allow space for it travel beyond the ends of the machine.

    The Carriage, which is the Y axis moves along the bridge and the Z axis, is mounted to the carriage and moves up and down. This allows you to build the bridge rigid and solid and eliminates the problems with racking associated with gantry designs. The X axis way system is also simplified as is the motor mounting and location of where to run the ball screw for the X axis.

    However, at some point in size, the gantry makes more sense as the table is stationary and you don't have any overhang to deal with. This also makes for a more compact, large scale machine. Each person has their own definition for "it's too big for my shop", but in my rough opinion on this, once the X axis travel exceeds 36", then it's time to consider a gantry machine.

    Chris

  3. #3
    Join Date
    Jun 2005
    Posts
    1425
    If you move the table, which I am doing in my m/c, you need double the space in x and y than if you move the gantry.
    I'm fixing my spindles because the weight of my table + workpiece is 4.5 lbs.

  4. #4
    Join Date
    Oct 2005
    Posts
    32
    Ok...that makes sense. Do you consider the picture I've attached a "bridge" type? I found this in the DIY section. I like the simplicity of it.

    I've also seen some first time CNC routers made with wood. I like the looks of the steel or aluminum better, but is there any difference functionally if I would choose to make the frame and bridge/gantry out of wood?

    Thanks for your help guys!!
    Attached Thumbnails Attached Thumbnails cnc2.jpg  

  5. #5
    Join Date
    May 2005
    Posts
    3
    No, that is a gantry style machine. The disadvantage to using wood or mdf is that it is less rigid and more prone to vibration than aluminum or steel. Of course its alot cheaper and easyer to work with.

  6. #6
    Join Date
    Apr 2005
    Posts
    629
    Quote Originally Posted by envcons
    No, that is a gantry style machine. The disadvantage to using wood or mdf is that it is less rigid and more prone to vibration than aluminum or steel. Of course its alot cheaper and easyer to work with.
    Well, I wouldn't agree that it is more prone to vibration - rather, MDF (Wood) is chosen for speaker enclosures because it is "dead" and dampens vibration. However, it is very weak and is not an ideal choice for framing anything.

    Workng aluminum isn't much more difficult than wood and yields a much better machine, more compact, however, the cost is greater.

    Regarding the picture above, you can quickly see that the "Bridge" would have to move along the long axis (X axis), thus, it would make it a gantry machine.

    Chris

  7. #7
    Join Date
    Oct 2005
    Posts
    32
    Thanks Chris...and Envcons and Grey beard too.

    I leaning toward the gantry style right now. It looks like something I could build. I also so a unit someone built on CNCZONE that had a 4th axis. That is too cool!! I would like to add something like that at a later time. Is this truely a 4th axis or is this kinda a lathe set up with the lathe spindle substited with a stepper/turntable? Whatever, it looks like fun!

    Ok another question. If I build a gantry style like the picture I submited, should I find a x-axis ball screw first in order to determine the length of my table? Can screws be cut...or is that something to avoid? Hmmm.....where do I start?

    Thanks for your help guys!!!

    Cutloose

  8. #8
    Join Date
    Jun 2004
    Posts
    463
    Ball screws generally come with the ends machined for bearings. You would have to get the right length screw, unless you have the facilities to cut and machine the ends.

    However, I wouldn't waste my money on ball screws for your first design. Start out with all-thread or cheap acme rod, and tapped delrin for the nuts. If the design turns out to be solid enough you can always upgrade to ball screws later. You might consider building your first machine from wood and treat it as a prototype and a learning experience. Unless you start with commercial plans, your first machine is going to have a lot of problems.

    I would suggest that you read as many of the threads on CNCZONE as you can stand, and then read some more. There is a lot of good information here.

    Jeff

  9. #9
    Join Date
    Oct 2005
    Posts
    32
    What are the "all-thread" and "acme rod"? How are they different from ballscrews?

  10. #10
    Join Date
    Sep 2003
    Posts
    226
    Allthread is cheap threaded rod that can be found at home centers and hardware stores. 5/16-18 costs less than $1 /ft.
    Acme rod costs a bit more but is still inexpensive. It is more efficient than Allthread. Its threads are "squared-off" . A quick Google search gave this image.
    http://www.keystonethreaded.com/prec.htm

    With Ballscrews the threaded rod is designed to be a track for balls which ride in a ball-nut. It is very efficient, tight and costly. Another Google. http://www.negribossi.co.uk/newslett...ballscrews.htm

    I agree with Jeff completely, go cheap first.
    David
    www.solsylva.com

  11. #11
    Join Date
    Oct 2005
    Posts
    32
    Very good comments! What specifically would I be sacrificing by going with the cheaper screws....accuracy of the cut? What could I reasonably expect to cut and with what kind of detail? You said that the acme screw will probably run a little more that a dollar a foot....what would I be looking at cost wise for ball screws? Should the acme screws be used for every access to reduce cost or should the Z axis be a ballscrew?

    Thanks for your help....!

  12. #12
    Join Date
    Apr 2005
    Posts
    1778

    ACME vs Ballscrew

    Cutloose,

    You can buy 5/8x5tpi ballscews for $1.20/inch or $14.40/ft. However, that is with unmachined ends. The cost really is in the ballnuts and mounting (and machining of the ends if you can't do it). Precision rolled acme threaded rod and rolled ballscrews have almost the same thread precision 0.003"/ft. The difference is ultimately in the friction required to turn the screw hence moving the axis.

    A single ballnut (read backlash) costs about $25, a double ballnut (read no backlash) costs about $166. Plans are available from HomeCNC for $7.50 to buy 2 single ballnuts and modify them to produce a single backlash free nut. Mounting threads on the nut are probably 15/16x18tpi, so you will either have to buy a mounting flange or have the capability to bore a hole and cut internal threads on a lathe.

    With 1/2" acme screws (e.g. 10tpi) you can use R8 bearings (1/2") and acme nuts (probably not suitable for drive nuts) for mounting. Some people have also clamped their motor drive piece directly to the acme rod.

    You can produce an acme tap from a length of Acme threaded rod, good enough to tap a piece of Delrin to produce drive nuts (read almost backlash free) for the cost of that length of rod. You can also buy 1/2x10tpi acme taps but you probably get a better fitting nut with a tap made from the rod that you are actually using.

    Peace,
    Alan

    Quote Originally Posted by Cutloose
    Very good comments! What specifically would I be sacrificing by going with the cheaper screws....accuracy of the cut? What could I reasonably expect to cut and with what kind of detail? You said that the acme screw will probably run a little more that a dollar a foot....what would I be looking at cost wise for ball screws? Should the acme screws be used for every access to reduce cost or should the Z axis be a ballscrew?

    Thanks for your help....!

  13. #13
    Join Date
    May 2005
    Posts
    3
    Quote Originally Posted by acondit
    Cutloose,
    You can also buy 1/2x10tpi acme taps but you probably get a better fitting nut with a tap made from the rod that you are actually using.
    I'm not sure thats true, taps are usually just slightly oversized compaired to the threads that will be running through them to reduce friction and binding. But if you made a tap from the actual thread the delrin might spring back a bit and cause an opening just slightly too small. I know alot of people do it, and it's very cheap, but its going to give you at best an equivalent thread to a professional tap.

  14. #14
    Join Date
    Jun 2003
    Posts
    2
    Hi,
    Here is one for an amateur machine builder. All parts ( well almost) salvaged from other equipments. I plan to use a Xylotex stepper drive. More details to follow...
    Attached Thumbnails Attached Thumbnails Router 1.jpg  

  15. #15
    Join Date
    Apr 2005
    Posts
    1778
    evcons,

    The problem with a professional tap is that there are slight variations in form for ACME threaded rod. If the tap cuts to loose you have backlash. Some people end up splitting the drive nut and using a adjusting mechanism to tighten or loosen the split and adjust the fit.

    I guess it largely depends on the amount of money someone wants to spend, how much they want to do themselves and what tool resources that they have available to them.

    Alan

    Quote Originally Posted by envcons
    I'm not sure thats true, taps are usually just slightly oversized compaired to the threads that will be running through them to reduce friction and binding. But if you made a tap from the actual thread the delrin might spring back a bit and cause an opening just slightly too small. I know alot of people do it, and it's very cheap, but its going to give you at best an equivalent thread to a professional tap.

  16. #16
    Join Date
    Mar 2003
    Posts
    35538
    From my experience, using a homemade tap left a snug fit. I'd imagine a "real" tap would have been looser.
    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)

  17. #17
    Join Date
    Oct 2005
    Posts
    32
    Thanks Alan! What is backlash and how concerned should I be about it on a beginners project?

  18. #18
    Join Date
    Oct 2005
    Posts
    32
    Maybe it's the picture...but your y-axis doesn't look like it has any threads. Explain what kind a couple that is on your y-motor. Keep the updates coming....I want to see more.

    Tim

  19. #19
    Join Date
    Aug 2004
    Posts
    2849
    Cutloose....backlash is when you hold the table or gantry stationary and you turn the leadscrew.....however much the leadscrew turns before it stops is your backlash in the leadscrew.....you will also have additional error based on how much your table or gantry twists when cutting/moving.

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