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  1. #1
    I really appreciate the sharing of experience of the community through this forum.
    I've been following the various projects exposed here for some time already and have decided finally to step in by myself. My english is not perfect (sorry for that) but I will make all my best to explain my project.
    For sure I will have many questions... but I'm here to learn :) from all your experience. So please don't hesitate to comment and give me advice to avoid letting me going in the wrong direction.

    First, I already own a lathe and a little milling machine (round column type) plus a big column drill (1.90m heigh with 30mm drilling capability) salvaged from srap... and totally refurbished. Let's say they are my basic tools.
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    I'm planning now to design and build a CNC router/milling machine able to machine light material but also aluminium. I already start from an existing welded frame (50x50x3 tubes) which needs some reinforcement; but however this gives me the overall size of the future machine: 1100mm x 650mm.

    My first investigations are giving the following axis travels: X 750mm Y 350mm Z 150-200mm
    The distance under the spindle attachment and the table will be adjustable to accomodage eventually the use of a milling wise or the use of a horizontal axis divider

    Here are some pictures of the project :

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    Screw ball types are 1605 for Y and 2005 for X (maybe 1605 sufficient ?)
    Next question is the use of simple or double separated screwball nuts or dual screwball nut (combined type). what should you recommend as I want to be able to machine large holes with H7 accuracy...

    The main beam of the steel gantry will be filled with concreat and same for X axis support beams.
    The steel tubes sections are 50x50x3, 50x50x4 and 100x50x3
    Currently the axes guides are round types (diam 20 and diam 16 respectively for X and Y axis)

    The Z axis will be fixed on the Y box (made of 20mm machined aluminium)

    Thank in advance you for all your future comments.

    Philippe

  2. #2
    Hi Philippe
    Welcome to the forum, Nice workshop.
    I am a noobe to the CNC field so can't offer much in the way of advice but jazz or one of the guys will be pitching in shortly with something more constructive,one thing is for sure you will probably have to up the budget to include profile rails and i would imagine you will require zero backlash ballnuts for the kind of accuracy you are looking for.
    Regards
    Mike

  3. #3
    Hi,
    i think the gantry ball screw that moves the Z left right is very high, it should be middle or lower, cause there will create a racking force.

    Same with the gantry ball screw nut housing. it should be in the middle of the bearings, not at the end, even better will be to be offset exactly where the gravity/force center will be, which is a bit further in front direction from that center.

  4. #4
    Thanks for those first comments.
    I agree with your statments and I am already considering changing the round chafts for Hiwin type rail. Would the type HGR15 be sufficient or should I move straight to larger type ? (don't plan to mill hard steel)

    The choice of the position of the y axis above the guides was dictated by the attempt to reduce dust pickup by the screw.ball
    For the X axis, I had in mind the possibility to mount 2 screwball nuts to be able to reduce backlash. Currently only one is shown on the drawing.
    However, I'll try to move the ballscrew upward to reduce the arm length.

    Thanks

  5. #5
    Hello Philippe,

    I see several things really.

    Yes change to profiled linear rails if you can afford them has they make much better and accurate machine. 15mm will be fine but 20mm often don't really cost much more and the larger bearing size helps.

    Ball-screws then I'd use 1610 not 1605 if your mainly cutting soft materials has they allow higher feed rates which you'll need. They also provide enough resolution for most Aluminium work unless working to fine tolerances. If you connect to screws with timing belts/pulleys, and I suggest you do, then it's very easy to change pulleys and increase resolution and torque but at the expense of speed,
    I would change the Position of the Long axis (X axis) ballnuts to mid point of gantry and have supporting braces at both sides of drop bracket not just one like you have drawn.
    While the Y axis nut being high up is not the ideal it will be fine there and better there than further back but lower down.

    Regards the rear plate of the moving carriage then you don't really need it has it's adds very little strength and more weight to the moving carriage reducing acceleration, also adds expense.
    It makes building the carriage more difficult has the connecting surfaces and edges must be perfectly 90deg to each other else it turns into a parallelogram, any error will increase binding on the bearings and it doesn't take much to bind them solid. It also interferes with other aspects of the machine which you haven't drawn and maybe not taken into account.? Like cable management. The rear of the gantry is perfect place for Cable chain and with the plate nothing can be fasten at the rear.!! . . Simple thing but can be annoying.
    Not having it takes nothing away from machine and makes building easier and less work. Just machine the top bearing plate so the ballnut mounts direct to this and is central, the bearing plate will extend back above the ballnut then you can bolt down direct into the ballnut mount from above so doing away with the drop plate and potential weak spot.

    Regards the Gantry then I'd consider having some Triangle gussets around the back strengthening the connection between sides and gantry cross piece, these braces are simple things which really help with resonance dampening.

    One last thing.? The Z axis rear plate doesn't look very tall so you may find you need to extend this from how you have it drawn now.? How do you plan to build Z axis.?
    With the short back plate I'm getting a feeling your thinking to have the motor travel with the front plate but I'd recommend you don't has it's harder on the motor and reduces acceleration. It makes using Belts to connect motors to screws harder and I suggest you do use timing belts/pulleys has it's better for few reasons. Mainly smoother working motors due to better resonance handling and flexibility regards fine tuning if needed.

    Oh and think about capping ends and using Dry sand rather than Concrete has it can be removed for when moving and dampens vibrations very well.
    Last edited by JAZZCNC; 02-11-2013 at 04:45 PM.

  6. #6
    Hello, Jazz
    Thank you for these valuable comments and suggestions. I have rebuilt the gantry with Hiwin style rails and blocs. size 15 for the Z axis and 20 for the Y axis and future X axis.
    You are definitely right, the size 20 is almost same cost than size 15 for a much better strength...
    I have tried my best to keep the Z moving plate as close as possible to the gantry beam (not so easy). Z axis travel is 170mm
    Following your advice I have plugged the beams with covers (but do not decide yet where to place the plugs to fill/remove the dry sand)
    To achieve correct geometry of the sitting planes for the rails, I can have access to a large milling machine at the company workshop (this will minimize the efforts of the alignment step)
    The ballscrew type has been changed to 1610 for Y and Z axis and will be driven by timing belt ratio 1:1 (with space left to replace for 2:1 ratio whenever necessary)

    Some pictures of the changes:
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    Again a few more questions :

    Do you think it would be better to design a cover on the Z sides (to limit dust and debris penetration) ?
    For lubrication, should I use grease (with the nipple) or should I already design oil pipes from now ?
    For the Z axis stepper motor; should it be facing downward or facing upward ? (would depend mainly on the position of the ballscrew...)
    Regarding the long X axis driven by two separate ballscrews and separate timing belts + motors, I plan to link the steppers on 2 different axis via mach3 sofware. Does its sound ok ?
    Last question related to ballscrews: does the chinese ones so called "antibacklash" realy fit the requirements ? or should I go for the coupling of 2 separate nuts as exposed in Jonathan thread (a sufficiently strong machine) ?

    Again thank you for your very valuable support.
    Last edited by idefix; 03-11-2013 at 11:46 PM.

  7. #7
    Hi

    Ok first my mistake and should have made clear, Only use 1610 on X & Y axis use 1605 on Z axis.

    Yes I would have cover over Z axis has it protects everything.
    I would only go to the trouble of oil system if access is difficult and really wouldn't go to all the trouble of Oil pipes when access is easy like yours would be.

    Motor position doesn't matter really other than ease of fitting and working on pulleys etc, I tend to always position them with motor down and thread the motor holes has it makes for easy adjustment and fitting. Either works fine thou and I've done both ways.

    Single or double Nut will depend on your requirements but for most router type work then single is plenty good enough and even for moderate accuracy milling work they are ok. If your wanting very fine tolerences, say for PCB or Engraving work then yes 2 nuts are best but then I'd say you've built the wrong machine anyway.!!

    Quote Originally Posted by idefix View Post
    Regarding the long X axis driven by two separate ballscrews and separate timing belts + motors, I plan to link the steppers on 2 different axis via mach3 sofware. Does its sound ok ?
    This I'm not 100% sure what your meaning.? . . . . If your asking is it ok to use 2 motors for one axis and slave in Mach3 then yes that's fine and is done all the time. You just have to be aware of it's limitations and tune the motors correctly other wise you can stall one motor while the other keeps going which can be very messy and destructive.!!

  8. #8
    Ok fine and clear ! Thank you so much Jazz for these directions.
    Will stay with the grease nipples and will correct the Z screwball type for 1605
    For the X axis with 2 motors yes I plan to slave them in Mach3.
    Let's finish the design work and will come back in a few days.

  9. #9
    Quote Originally Posted by idefix View Post
    Regarding the long X axis driven by two separate ballscrews and separate timing belts + motors, I plan to link the steppers on 2 different axis via mach3 sofware. Does its sound ok ?
    Philippe, It may not be obvious but if you use two motors you will need 2 drivers, a slightly bigger power supply, more wiring. In my case it added about 100 GBP to the cost. I'm not complaining it's just that I did not fully think about it at first but I still like the 2 motor idea best. The alternative of course would be to use one motor and longer drive belts to each ball screw.

    I see you have been following the frame construction threads closely
    Last edited by EddyCurrent; 04-11-2013 at 06:02 PM.

  10. #10
    Quote Originally Posted by JAZZCNC View Post
    Hi

    . . . . If your asking is it ok to use 2 motors for one axis and slave in Mach3 then yes that's fine and is done all the time. You just have to be aware of it's limitations and tune the motors correctly other wise you can stall one motor while the other keeps going which can be very messy and destructive.!!
    Dean,
    i have always wanted to ask that question, hope you know the answer- if the motors that drive the gantry don't drive it directly but with pulleys 1:1 ,if the gantry is racked by chance, wouldn't the belt give away and just start skipping instead of destructing something?

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