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  1. #11
    Quote Originally Posted by JAZZCNC View Post
    25mm ballscrew is also not recommended for a machine this size, you've probably gone with 25mm because of the 10mm pitch not being available in 20mm.
    Maybe 16mm screws with 10mm pitch - RM1610.

    Quote Originally Posted by JAZZCNC View Post
    25mm screws are large and they produce a lot more inertia which has to be stopped and accelerated this means big strong motors needed amongst over things which all make it not ideal.!
    I was about to suggest looking into using rotating ballnuts...

    Quote Originally Posted by JAZZCNC View Post
    You could go with rotating the nut but it gets a bit more involved, thou does work well if done right.
    But you beat me to it! You might be interested in how I did the rotating ballnuts on my machine:

    http://www.mycncuk.com/forums/showth...t-design-ideas

    They work well, so I guess I did it right.

    I agree with using belts and two ballscrews on the X-axis. Spacing the bearings further apart will help prevent the gantry racking a bit, however it's still not ideal.

    Quote Originally Posted by chip View Post
    i wouldnt look at my build for any design inspiration,its a bad build and a lot of good materials wasted,im on the verge of scapping most of it and starting again:tup:,
    Oh don't say that ... you're doing yourself a disservice. There's still room for improvement on mine and there always will be. The distance parallel to the X-axis from the Y ballscrew and cutter is far too great on my machine for instance.
    Last edited by Jonathan; 31-08-2011 at 07:20 PM.

  2. #12
    GTJim's Avatar
    Lives in Coventry, United Kingdom. Last Activity: 2 Weeks Ago Has been a member for 8-9 years. Has a total post count of 110. Received thanks 3 times, giving thanks to others 20 times.
    Quote Originally Posted by Ricardoco View Post
    Ok ive just done enough tinkering to give you an idea, of where bracing could do with looking at.
    It may also be an idea to widen up underneath the table on the gantry just enough to stop the diagonal flex when the spindle is working at the far reaches of the gantry axis.

    All the best Rick
    If I was to mount a plate (as shown in pic) to the back of the axis with many fixings would that stiffen up the gantry sufficiently?

    Click image for larger version. 

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    The current design supported round rail, but this was down to cost and if available I would use profiled rail.
    I also think the 25mm ballscrew is OTT but now that it has been mentioned I'll look at 20mm with 5mm pitch with pulleys and belts. I am concerned about whip in the 16mm screws, but again it could be a saving.
    I am hoping to use a single ballscrew on each axis but if I find problems I think it would be easy enough to add an additional one, so I'll look out for a 4 motor and driver package.
    Never thought about resonance, but kiln dry'd sand could be a good solution.

    Thanks for all your input and please feel free to continue.
    Jim


    I can see this becoming a bit of a monster and it has to fit in my garage with all my junk.
    Last edited by GTJim; 01-09-2011 at 11:34 AM.

  3. #13
    GTJim's Avatar
    Lives in Coventry, United Kingdom. Last Activity: 2 Weeks Ago Has been a member for 8-9 years. Has a total post count of 110. Received thanks 3 times, giving thanks to others 20 times.
    Just a couple of more sketchs showing potential positions for ball screws and motors.

    Click image for larger version. 

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    Click image for larger version. 

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    Keep the comments comming as I will be buying materials soon.

    Jim

  4. #14
    :confused:
    Quote Originally Posted by GTJim View Post
    If I was to mount a plate (as shown in pic) to the back of the axis with many fixings would that stiffen up the gantry sufficiently?

    Click image for larger version. 

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    The current design supported round rail, but this was down to cost and if available I would use profiled rail.
    I also think the 25mm ballscrew is OTT but now that it has been mentioned I'll look at 20mm with 5mm pitch with pulleys and belts. I am concerned about whip in the 16mm screws, but again it could be a saving.
    I am hoping to use a single ballscrew on each axis but if I find problems I think it would be easy enough to add an additional one, so I'll look out for a 4 motor and driver package.
    Never thought about resonance, but kiln dry'd sand could be a good solution.

    Thanks for all your input and please feel free to continue.
    Jim


    I can see this becoming a bit of a monster and it has to fit in my garage with all my junk.

    Ok let me show some diagrams..

    Click image for larger version. 

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    With no load and Spindle central this is the look of the machine from above.

    Click image for larger version. 

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    Now cutting brings in to play these forces and consequences.

    As the others have touched on instead of using 1 screw in the center if you use 1 on each side that will sort it out but at the end of the day it is all down to cost and accuracy. if you take the above examples and apply them to the other two axis you will see the weaknesses in your structure at the time of design not when you have spent the money, most of us on here have been through the building process and made the mistakes for you...:whistling:

    BUT. my first attempt at building a machine was flawed and at the end it was not accurate but it was cool looking and i played for hours with it just amazed ide built it and to me for that split second it was PERFECT!!

    then i took it to pieces and joined this forum... the rest is history...

    ""MOST"" of these guys on here really do know their stuff, but dont just settle for one opinion get lots of opinions and you will soon see the way or be extreamly confused :confused:


    Rick
    Always bear in mind that your own resolution to succeed is more important than any other - Abe Lincoln

  5. #15
    The plate on the gantry to stiffen it is an excellent plan. I think you need to consider more how you are joining the box section, and gantry sides to bearings. Currently you have drawn the box section attached on just one face which is a bit weak.

    I have done a drawing for you to illustrate how I think it can be made stronger:

    Click image for larger version. 

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    Click image for larger version. 

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    That way the sheet reinforces the joints in addition to the general structure. I would be inclined to weld it for good measure. If not put plates (say 20mm thick) in the ends of the box section and bolt into them.

  6. #16
    Jonathans suggestion is a good one not just for strength but also it protects the ballscrew from flying debrie.
    If you must go with 1 screw done the middle then I suggest you make the cross piece that the ballnut fastens to wide as possible. you need strong bracing and width to counteract racking with a single screw.
    Something like the pics below will handle just about most jobs at this width with a single screw. A 16mm 10mm pitch would work nicely.

    Edit: The sketchs are rough quick knock up's and I would add some more corner braces to the cross piece but it should give you an idea of what I mean.

    It probably looks a bit extreme but honestly it's not and the extra work and slightly more expense would be worth every minute and penny spent belive me.
    Dont under estimate the forces's even with steppers.!! . . The forces can be extreme when cutting Ali and hard materials and the torque produced by steppers and ballscrews can twist and mangle a gantry built like this in the blink of an eye when the stick stuff it's the fan. :surprised:
    Attached Thumbnails Attached Thumbnails Click image for larger version. 

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    Last edited by JAZZCNC; 01-09-2011 at 09:52 PM.

  7. #17
    Quote Originally Posted by JAZZCNC View Post
    torque produced by steppers and ballscrews can twist and mangle a gantry built like this in the blink of an eye when the stick stuff it's the fan. :surprised:
    Yep, mine has done that once or twice. It's quite surprising. Once nice test I can do is if I disengage the motors and rotate one ballnut by hand the gantry is stiff enough to make the other one backdrive and rotate too... but not much.

    I wonder how much difference it would make if you have two ballnuts on the single screw? One on each of the two cross pieces on the gantry.

  8. #18
    Quote Originally Posted by Jonathan View Post
    I wonder how much difference it would make if you have two ballnuts on the single screw? One on each of the two cross pieces on the gantry.
    Dont think it would add much to extra torque but it would certainly add extra precision due to removing backlash and compensating for wear also help with stablizing whip on longer screws.
    I have some lovely 25mm Comtop ground Class 3 preload 2 double nut ballscrews for my next project and the difference between the cheap chinese screws and these is night and day. . . . Cant wait to get on with it.

  9. #19
    GTJim's Avatar
    Lives in Coventry, United Kingdom. Last Activity: 2 Weeks Ago Has been a member for 8-9 years. Has a total post count of 110. Received thanks 3 times, giving thanks to others 20 times.
    Thank you all for your very good comments.

    Rick - I will redesign the frame and go with 2 ball screws as suggested, possibly 16mm 10pitch.

    Jonathan – In my initial sketches the axis was constructed in a similar manor to your drawings but I found the spindle did not cover the whole of the table. So I either extended the frame of the base or took the rails to the extreme of the axis allowing the spindle to go past the work surface while keeping the foot print of the machine as small as possible. But as you have pointed out it has created a weakness and I will take this on board.

    JAZZCNC – I also agree Jonathans suggestion is a good one, but as for the protection of the ball screw I was hoping to use brush seals fixed to the gantry to prevent wood chips causing damage. I’m sure I’ve seen this being used on a machine on this site.
    I also like your design for the gantry, so I’m back off to the drawing board. I’ll keep you posted.

    What are your thoughts on these Chinese screws as I don’t want to work to microns?

    As you are all well aware of, everything is a compromise.
    Jim

  10. #20
    Quote Originally Posted by GTJim View Post
    What are your thoughts on these Chinese screws as I don’t want to work to microns?
    The ones I have (C7) are all smooth with next to no backlash. The backlash is less than the resolution I get so it's not significant.

    Apparently they do vary...

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