Add a second set of rails to X axis..need advice.

[bgolash]

Hi Group
My CNC is nearly done. I have noticed a problem with twisting of the Y axis.
I set a dial-indictor against the 4 x 4 uprights that support the Y axis. I pushed against the uprights in the X axis direction and got a movement of .010 + -. I then setup the thk rails on top of the machine frame with wood brackets clamped to the uprights. I tried my test again and I have .005+ -. This test helped to confirm I should add the second set of rails to the machine. My questions to the experts in the forum is should I mount the THK rails to the top of the machine frame. And two, what is the best bearing spacing for the sliding THK elements.
I like the top mount location because they will be very easy to mount. These THKs are very small so they don`t take up allot of the table surface. I plan to add a work surface to the top of the machine so I can hide THKs under the work surface. Its my idea this will help to shield them from dust and metal chips.
Another reason for mounting on top of the machine is I`ve noticed a oscillation of the X axis as it comes to a stop. By the looks of the test it seems to reduce the oscillation with the rails set at there current location..
At the moment for the test theTHKs are spaced at a overall measurement end to end of 5.5 inches. I`m wondering if anyone could confirm if the 5.5 spacing is a reasonable distance to eliminate the twisting that I`m experiencing with the Thomson round rails.

[jimbo]

are your thomson bearings adjustable for preload? I have seen them with a setscrew in the side of the bearing block to tighten the preload on the bearings. you might try that before installing the new slides.

[bgolash]

Hi
Thanks for the reply. I did fool with the setscrew. It seemed
no matter how tight the setscrew got...there was no change
in the fit of the rail. It seems the thomson rails were a bad
choice on my part. The Y axis uses IKO preloaded rails and that
axis is very ridgid.
Regards Barry

[jeffs555]

The main problem I see with using 4 rails is alignment. It is hard enough to get two rails aligned so they don't bind at some point in their travel.

[andy_ck87028]

Hope I am adding an opinion of value but y axis twisting is possibly caused by insufficent rigidity of 4*4 vertical columns. Could those columns be forced rigid through the construction of torsion boxes either side of both columns? Torsion boxes would be what I would do. Seeing your design, I can understand why flexing is happening.

I would have thought the further apart the bearings the better but that would limit the cutting length in the x plane so that question is an issue of optimisation.

Hope that helps

Andy

[bgolash]

Hi All

Work is moving along on the installation of the second set of THK rails. I have the rails mounted on one side with a bracket partially complete that will tie the sliding THK elements to the 4 x 4 uprights. So far the addition of the parts is making the Y axis tighter. I`m hoping that the second set of rails will also add additional rigidity to the machine. I am optimstic this will help the machine...too bad these THKs cost so much. Any advice is greatly appreciated.