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  1. #151
    James if your only 1.5mm out then thats only 0.75mm across the diagonal. If the machine is not bolted securely to the floor then the frame could very easily bend or move this amount just from moving the gantry back n forth.
    If you intend to move it around or it bothers you then put some diagonol adjustable tie rods braces so it can be adjusted and held square. I did this with my machine while building then re-adjusted when I eventually found the place in shop it was going to live then bolted it to the floor . Only then did I tighten and true the gantry/rails etc.

    Will it affect the accurecy of cut.? . . Well Yes if your working to high precision then this amount would be considered massive but for oridinery woodworking etc then it's acceptable to some degree.?
    I say some degree because there's another issue and IMO better far more important reason to find the inaccurecy and square the frame to near perfect as can be.!

    Like jonathan says you can effectively cheat and square the Gantry by forcing it square using the motors.!. . . Problem with this route is that it puts components under constant stress, bearings and even the screws are fractionly twisted this over time leads to premature wear or failing. . . . Much better to find the cause NOW than replace parts or worse still be constantly chasing inaccurecys or even missed steps etc from binding as parts slowly wear away losing performance.!

    Taking the time now will pay dividends later down, don't fall into the trap of rushing the machine into service only to pay a big price over the coming months.
    Promise you even if it takes a few days to sort this inaccurecy now it will save you weeks of hassle and unneccesary expense months further down the road.

    Pulling and holding the frame square with Diagonal tie rods (M12 Threaded rod works good) then bolting the frame down BEFORE finally trueing and bolting the Gantry and rails etc makes the process so much easier and accurate without stressing important expensive components like Ball-screws and linear bearings. This way Only frame is then under tension.!

    Edit: I know the T nuts did but have any of those Jigs turned up yet.?
    Last edited by JAZZCNC; 09-10-2011 at 02:33 PM.

  2. #152
    I'd only 'force it' square using the motors if it's not much out at all. Depends on the stiffness of the frame. If it's out by more then bolt the frame down as Jazz says and if it's still not good use the formulas in mach3 to compensate as that will completely eliminate any error.

    It's just occurred to be that you could purposefully have the gantry built a little out of square, force it with the motors/ballnuts to reduce or eliminate backlash. The pressure on the ballnuts from squaring the gantry would preload both X-ballnuts rather like if you have double nuts with a spring. Clearly if the cutting force is greater than that preload you will get backlash, but for light work it would work. Disadvantage is clearly extra load on the X rails and I suppose marginally more torque required, so feedrate will be negligibly less. I wonder if this is why I have practically no backlash in my X-axis...I'm not purposefully forcing the frame, but I guess it wouldn't have to be much out.

    Quote Originally Posted by JAZZCNC View Post
    Edit: I know the T nuts did but have any of those Jigs turned up yet.?
    Yes they have.

    If the bearings are not all the same way it wont affect this. However the force rating of the bearings must be different in each direction as the location of the rows of balls is not symmetrical. I doubt it will make much difference, but you might as well have them aligned so it's even.

  3. #153
    Quote Originally Posted by Jonathan View Post
    It's just occurred to be that you could purposefully have the gantry built a little out of square, force it with the motors/ballnuts to reduce or eliminate backlash. The pressure on the ballnuts from squaring the gantry would preload both X-ballnuts rather like if you have double nuts with a spring.
    That then would just be a bodge to hide poor workman ship.!!. . . . . . . Plus it wouldn't preload the nut it would twist the nut.!. . Yes the affect would appear the same as taking up backlash but what it's actually doing is twisting Nut body onto the balls which in turn apply force against the screw. Both the screw and Ballnut are not designed to run with forces in this direction so again your forcing the screw and nut to run out of design parameters.. . . This cause's premature wear.

    Much better to build right first time IMO.!!


    Quote Originally Posted by Jonathan View Post
    Yes they have.
    Aghh Yipee... Thanks for letting me know.!!

  4. #154
    Quote Originally Posted by JAZZCNC View Post
    That then would just be a bodge to hide poor workman ship.
    Or cheap ballscrews - hence why I think my measurement of the backlash might be misleading.

    Quote Originally Posted by JAZZCNC View Post
    Plus it wouldn't preload the nut it would twist the nut.!
    Maybe, maybe not. It depends what's twisting. If you assume the rails are rigid (not quite true for supported rails as they bend upon the support, but not much) then it must be the gantry in between twisting and any force not parallel to the X-axis is resisted by the X-axis linear bearings leaving only the axial force for the nut. On James' machine, clearly the weakest point on the gantry is the joint between the sides and 6" wide cross piece, so that and the cross piece will bend. The ballnut mount and gantry sides are solid so won't bend noticeably. .. etc it's obvious I don't need to tell you.

    Not suggesting one should actually do this intentionally. The force ratings of the ballnuts is huge, so I think you'd get away with it. I'd be most worried about the linear bearings.

  5. #155
    Quote Originally Posted by Jonathan View Post
    Maybe, maybe not. It depends what's twisting. If you assume the rails are rigid (not quite true for supported rails as they bend upon the support, but not much) then it must be the gantry in between twisting and any force not parallel to the X-axis is resisted by the X-axis linear bearings leaving only the axial force for the nut. On James' machine, clearly the weakest point on the gantry is the joint between the sides and 6" wide cross piece, so that and the cross piece will bend. The ballnut mount and gantry sides are solid so won't bend noticeably. .. etc it's obvious I don't need to tell you.
    Yes the Gantry will more than likely twist at the point where the sides meet cross brace at one side but the Ballnut is attched to the side so will twist against the screw unless the screw roates the same amount as the gantry twists. Plus like you say the bearings will suffer the worst.

    Either way it's wrong and not recommended.!!

  6. #156
    Quote Originally Posted by JAZZCNC View Post
    Yes the Gantry will more than likely twist at the point where the sides meet cross brace at one side but the Ballnut is attched to the side so will twist against the screw unless the screw roates the same amount as the gantry twists.
    No, as the gantry sides will not twist at the point where the ballnut is mounted. Looking at the machine facing the Y/Z plane from the top it's cross brace, gantry side, X bearings, X ballnut. The gantry sides will twist between cross brace and X-bearings, but not much at all if any between the X bearings and ballnut as the bearings fix it. The twist will be tiny as the sides are very thick/strong aluminium plate. The same is true for machines configured like ours - the gantry 'ends' have the ballnuts mounted roughly at the same level as the gantry, and the X bearings provide support between so there's nothing to twist the screw. If the X-axis linear rails were not there then yes the screw will definitely be twisted, but that's just silly anyway. Imagine resolving each of the forces, any force not parallel to X that could cause twisting must appear as a contact force on the X-bearings and thus prevent twisting as there should be no clearance in the bearings.
    In either case if there is any twist on the screw it's going to be a tiny negligible amount and the extra force that would apply is surely small compared to say the force from screw whipping.
    Last edited by Jonathan; 09-10-2011 at 10:59 PM.

  7. #157
    Don't matter how you try to flog it Jonathan it's simply wrong and not a good way to build a machine. . . Sooner or later the price of inaccurecy and miss alignment of precision components will have to be payed for.!! . . The degree of inaccurecy will be the determining factor to how long it takes . . . . BUT Like Death & Tax's there will be no escaping it.!

  8. #158
    Quote Originally Posted by JAZZCNC View Post
    Don't matter how you try to flog it Jonathan it's simply wrong and not a good way to build a machine
    I'm not trying to flog it at all. I was just pointing out then that it is wrong to say there is any significant twisting force on the screw, and therefore you're only increasing the axial force on the ball-screw which is the same as what would happen if you did it properly with double nuts and a spring. If it wasn't for the large force it places on the rails I see no reason not to do it. It is obvious that there are far better ways to attain low backlash. I mentioned it originally more to point out how it could lead to a false reading of backlash.
    Last edited by Jonathan; 09-10-2011 at 11:43 PM.

  9. #159
    Quote Originally Posted by Jonathan View Post
    I'm not trying to flog it at all. I was just pointing out then that it is wrong to say there is any significant twisting force on the screw, and therefore you're only increasing the axial force on the ball-screw which is the same as what would happen if you did it properly with double nuts and a spring. If it wasn't for the large force it places on the rails I see no reason not to do it. It is obvious that there are far better ways to attain low backlash. I mentioned it originally more to point out how it could lead to a false reading of backlash.
    Well It wouldn't be acceptable to me or a practice I'd employ on any machine with my name to it.!! . . . . So again we'll have to agree to disagree.

    Oh you might want to checkout how proper double nuts are connected and preloaded.? Dont think you'll find meny springs around.?

  10. #160
    Quote Originally Posted by JAZZCNC View Post
    Well It wouldn't be acceptable to me ... on any machine with my name to it.!!
    Same

    Quote Originally Posted by JAZZCNC View Post
    Oh you might want to checkout how proper double nuts are connected and preloaded.? Dont think you'll find meny springs around.?
    I know, but proper double nuts aren't as good as two nuts and springs / belville washers are they.
    When I referred to 'double nuts and a spring' earlier I was thinking of doing something like this as it's a simple way of implementing it:

    http://www.mycncuk.com/forums/showth...ull=1#post8924

    Especially with only C7 grade screws where the pitch error limits how far you can go with double nuts.

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