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  1. #21
    George please don't appologise and keeping asking the questions untill your satisfied you fully understand.

    The problem with long screws is whip at higher speeds. The only way to avoid it is to use large diameter screws but these bring there own set of problems, like needing BIG motors, drives, PSU Etc or moving over to servo's and gearing etc !!!. . . .all expensive.

    Personally I prefer ballscrews over R&P anyday and really it can't live with them in any department but they are not as simple to impliment with a long axis machine.

    If it was me then it would be a no brainer and it would be getting rotating ballnuts without a doubt.

    Regards parts machining then ask around several here will do them for you I'm sure. I would and will glady make them for you but unfortunatly I'm tied up for the next several weeks but if you can wait then I'd be happy to help.

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  3. #22
    i'll get there at some point-!!

    came across these badgers- bet they're worth a pretty penny-!

    good magazine if you get it? might also go to the trade show in april...

    thing is i have the skills to machine them just not the machines-!! i'll have a play with the drawings etc and see where i get too

    would you run 2, one at each side? or do you run into sync issues?


  4. #23
    Ye you'll have to sell your house and the wife to buy those mate. . Lol

    Yes absolutly run 2 screws and provided you don't over tune them they keep sync ok-ish.!! . . . I have my views on the sync issue and to be honest I won't go into it again in your thread but if you look around at some of the other post's on here I've gave them recently but I can't remember which thread.!!
    (This one amongest others I think.??!)

  5. #24
    When I started out with my machine I did have issues with motors stalling on the X-axis, but really that was just me trying to push it too much. Now I've been a bit more conservative (still 10m/min which is fast by most peoples standards) the machine hasn't stalled for several months so that's not an issue.

    I'm going to be making a couple of rotating ballnut mounts soon, so let me know if you're interested and I'll work out the price as it'll be more efficient to make several at the same time. I've updated the design to be fully enclosed with aluminium such that no swarf can get in. Something like this, can of course modify it to suit / aid mounting:

    Click image for larger version. 

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  6. #25
    Hi y'all, spent a bit of the weekend messing with my drawings-

    to see what i could come up with-! bit of a mash up of my lurking about here and there-!! to be fair mainly Jazz!

    so the plan is as pics below- however.....unsure of the Z as I've allowed for 150mm movement/ gantry clearance, might be over the top, but i get asked for odd things and it might be good to allow that tolerance for poly etc- had half a though of making the 20mm side plate adaptable with another set of holes for the carriages @ say 70mm up so you could drop the gantry-

    need to think about the rotating ball screw affair/ speak to Jonathan-(pm coming your way-!) current thinking is to have them below the 120x 80 edge frame connected to the 20 odd mm side plates- and whether the y ball screw is too far back off the spindle- seemed like a nice neat way at the time- feel free to comment-!

    also work out belt lengths for the y and z to make sure i can get something to 'fit' but will probably get the bits and 'measure' them before any plates are machined (for which i need a friendly person to quote on as theres some trenching/ holes to accurately cnc..chicken and egg- 'oh if i had a cnc mill i could.....!'

    the bed is currently long enough to have the gantry go to a dead space at the end which hopefully will help with loading etc- and is currently 25mm mdf which i hope to make into a vac bed of sorts when budget allows-!

    rest of the table frame/ cable management / extraction to draw up but cant see any probs with that but will draw them before i start building to make sure the 'problems' are thought about-!

    gantry will probs be a the same 120x 80 x 6mm RHS..... thoughts?Quite exciting really.... now to collate me list of parts/ electronics ( which scares me slightly!)

    Click image for larger version. 

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    hope that all makes sense?George
    Last edited by georgetheforge; 02-04-2012 at 08:40 AM. Reason: format

  7. #26
    Quote Originally Posted by georgetheforge View Post
    hope that all makes sense?George
    Makes perfect sense George.!

    Some will complain the Ballscrew is too far back from the Y axis carriage, which it is slightly, but because you are using profiled rails they will help absorb some of the effects. Thou be aware this will apply twisting forces to the bearings which over time could cause premature wear.? You may want to consider moving the screw forward and higher sitting directly on top of the upper bearing plate.?

    When you say 150mm of clearence between gantry is that from the Bed to the underside of gantry.? If so then again you may want to consider upping this if your wanting flexabilty.
    The things that tend to cause the most problems regards clearence are drills and clamping devices.!! Often drills have long length's so by the time you have material, sacrificial base material then clamps etc you don't much left for clearence.
    Also if you ever want to work with really small items you may want to add a vice to hold them and with only 150mm of clearence then again it's very tight even with normal endmills.

    150mm is a good Z axis travel amount but really I would look to more clearence if your wanting flexabilty. It's very easy to pack the work material up towards the cutter but not so easy to move the gantry up.!!!

  8. #27
    Quote Originally Posted by JAZZCNC View Post
    Some will complain the Ballscrew is too far back from the Y axis carriage, which it is slightly, but because you are using profiled rails they will help absorb some of the effects.
    I was about to complain... it's true that you'll probably get away with it because you're using profile rails, but its unwise to rely on one very strong component to compensate for suboptimal design elsewhere.

    20mm for the side plates for a machine this size sounds a bit small. To compensate I would make the 4 triangular brackets extend all the way down the sides as that will really stiffen things up.

    The alternative is to eliminate the gantry sides completely, like on mine (or Jazz's) machine:

    Click image for larger version. 

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    (Ignore the rotary table madness and yes my Y-ballscrew is badly positioned)

    It's close to the strongest format.

    Consider carefully what the profile rails on the X-axis are mounted to. Since you have orientated them with the mounting faces pointing outwards (i.e. in the YZ plane) the distance between the rails is critical, so you must have minimum one of the box sections bolted down, not welded, so that it can be adjusted. If you go for the gantry format in the picture above the rails are both facing up which would make this no longer significant.
    Same is true for the Y-axis. 120*80*6 box section is a good size, but by putting the rails on opposite faces you're relying on those faces of the box section being parallel. To the tolerances required for profile rails they wont be, so you'll probably end up needing to use shims and stuff to get it to run smoothly. With supported round rails the tolerances are lower, so you get away with it, and you want them in that configuration to even up the loading on each block. However for profile rails (with 4 rows of balls which are the only ones worth getting) the force rating is equal in all directions, so it wont make much difference if you mount them facing or in the same plane. Mounting them in the same plane will make life easier ... lends itself well to getting the Y-axis ballscrew in the correct position - right between the rails.

  9. #28
    Thanks chaps-

    What kinda clearance would you recommend?- obviously depends on what I'll be machining-!!

    mainly 18/ 25mm ply and mdf, some poly/ foams maybe and maybe some ali if possible....

    I wanted the side plates- rather than your solution- so i could keep the top surface clear for loading etc- i do have little triangular brackets which i forgot to make bigger when i was playing with the gantry height-

    was tepted to 'split' the gantry so i had 2 sections with the ball screw between them fitted to the back of the front/ z plate. obviously that makes it heigher and was worried about twist/ aligning them correctly.... i did look at getting some 100 x 200 x 10mm RHS and getting the front lasered out to make a so a proper c section rather than an channel- almost like a chunky Unistrut....? then reverting to plan a having one of the rails on the front- bottom and one on the top.....?
    Attached Thumbnails Attached Thumbnails Click image for larger version. 

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  10. #29
    so heres the result of some playing around with the drawing-

    main changes were thinking how to integrate the rotating ball nut- the idea is to sling it under the x rail ( which will bolt to the side of the table, so hopefully there will be some adjustment on the profiled rails. Ideally they'd sit on another tube to take the weight rather than just be on the bolts- any suggestions?!

    i split the y axis and am planning on using 80 x 80 x 8 with the ball screw between the two, fixed to the front and back of y carriage, but this stops any bracing between the 2 rails, do you think this will be a problem?

    i extended the z and moved the y upo to give more clearance- now 230mm travel which has meant massive side plates for the gantry-

    jonathan, do you think the roating nuts will sit nicely where i've got them or do you think it's hideous-!

    anyway- thoughts on this before i start ordering stuff would be awesome

  11. #30
    Splitting the gantry into two box sections as you have done is a great way to do it. I'd put an aluminium plate on the back to stiffen it further, although it's probably fine as it is.

    The gantry sides just look flimsy. Since you don't have an adjustable height bed most of the time the Z-axis will be operating near maximum travel, so the combined tool deflection from Z-axis and gantry bending will significantly limit the feed-rate and finish you can obtain. You should do what I put in the last post - eliminate the gantry sides and have the box sections for Y on the smallest plate, or box section, viable to link them to the X-axis bearings. Add to that an adjustable heigh bed and you'll have a really strong machine with as much clearance as you want for long tools/drill etc.

    You need to consider how to accurately align the X-axis profile rails. Currently you can get them both parallel in the X/Z plane, but the spacing of the rails (i.e. parallelism in the X/Y plane) is fixed. No frame will be accurate enough to allow that unless you build in some adjustment, or fix the rails in the same plane as the bed (like on mine).

    Correct me if I'm wrong but you've drawn the rotating ballnut mounts fixed to one end of the bed, not fixed to the gantry sides?

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