adding tension and adjusting roller bearing rails?

[animus_divinus]

hey everyone... ive nearly finished my router, i have the design all punched in... measured my maximum travel to be exactly 39x24x4 inches... however, i came into one small snag... the fact that i can assemble the parts in 3D with the 3D cad program i use has saved me a lot of headaches and mistakes when the real build comes... but anyway..

problem is, the bearings are not going to come into perfect contact without micrometer-like precision, and if they did, i dont think they would hold zero for too long unless there is some way to adjust the fit and tension on the bearings...

im using groups of four roller bearings per block, two of these assembled blocks per axis and i have the edges of these bearings perpendicular to each other at 90 degrees.. these will ride on a .125" thick 45 degree rail

for example, between the gantry legs, i have a .75" piece of MDF running between them to strengthen the legs.. to add support, and the edges of the MDF have been sharpened to fit a rail, which i will drill and countersink to fit to the MDF with screws, so they're replacable... ill show a photo of this...

so... i need some way to adjust either the distance between the bearings and the platform they attach to, or i need a means to adjust the distance between the rails they ride on so that i can "tighten" the fit between bearing and rail....

i did some search on this forum, and used a few key words but nothing immediately turned up....

if anyone has any ideas, or solutions, id be grateful to hear them

[animus_divinus]

heres a photo of an example of how my rails are so far.. so you guys can understand what i have.. and maybe help me figure out how to improve them...

[keebler303]

Most "skate bearing" type builds use eccentric bushings in the center of the bearing so that they can be adjusted radially and then tightened in the proper alignment.

Matt

[kithygh]

I too have build one with MDF as the structure, and I found that it wasn't important. I thought I was going to need an adjustment for wear or not cutting the parts perfectly, but the MDF turns out to be flexible enough that tightening the gantry sides to the bottom base snugged it up. I would design it to fit, and if its too loose, sand to fit and re-tighten. Attached is the sketchup pic of my now completed and working CNC.

[animus_divinus]

kithygh.. your design looks almost exactly like mine ive posted on here before.. your Y axis platform.. the Z axis.. at first i was actually going to sharpen the edges of the table for the angle rails too... but i decided to make a 48x30ish inch box of MDF about 6 inches tall... and have the rails mounted on this so they would be fully supported, and i would be able to remove and replace the table quickly and easily.. other than that.. nearly identicle...

what about this idea... on the gantry, the Y and Z axis assemblies, i drill about a 1/4 or 1/2 inch hole onto the platform where the rails will mount... then epoxy glue a threaded bushing into these holes to drive a bolt through..

then, on the block holding the bearings, i drill matching hole part way through, and inside it i epoxy-glue a thin metal washer, so that the bolt doesnt damage the mdf...

then... all i would need to do to tighten the bearings against the rails is is thread that bolt deeper, and fix it in place with a nut and a lock washer...

i could also use this for assembly, and disassembly, because with the bolt removed, i could seperate the bearings

question is.. will the forces be too much on the two bolts holding the ends of the bearing assemblies? or would a 1/2 inch bolt be sturdy enough?

[animus_divinus]

hey.. heres the idea i was talking about... about using a threaded bushing, a bolt, and a lock nut... question is.. will something like this be able to hold the weight of the CNC machine? i did have the idea of making the back side of my block.. the part that mates with the platform its going on a tad undersized... so i could sand it to get the right fit... im just wondering how long that fit would last before wear on the rails themselves, or just stress from its work load puts it all out of spec again?, am i simply foreseeing a problem that isnt there? or am i right to be concerned?

[animus_divinus]

i got it.. just dawned on me.. the simplest solution and i was overlooking it.. ok, so picture my last photo with the bolts threaded through a glued-in bushing... but instead of using lock nuts and lock washers and what not... couldnt i just tigthen the bolt until the bearings fit tightly against the rod... and use simple a simple thread-locking compound to prevent it from moving or backing out?... so simple, and the bearing blocks would be able to fit flush against the MDF if they needed to... wonder why it took me so long to figure that one out..

but my question remains... will this be strong enough to hold the forces of the machine?

[ger21]

Are the sides tied together under the table?

[animus_divinus]

yeah.. i have a piece under the table to add rigidity, also to screw my anti-backlash nut to and give a centralized locations for the leadscrew

[animus_divinus]

i think my design will be pretty rigid overall... using set screws to fix the bearing blocks to the platform, and being able to tighten them with an allen wrench i think will work nicely... with a little thread locker to hold them in place when its finished i think the whole design should do well, and if i use a thread locker i can break by hand.. i can always re-tighten the bearing blocks against the rails if neccessary... i think this is the route im going to go

[Walky]

What about pressure washers (or are they called spring washers)? I think they can be more reliable than thread locker under certain circumstances, and quick adjustment/maintenance would be easier and cleaner. Or you could probably use nylock nuts, too.

[PaulRowntree]

Originally Posted by Walky What about pressure washers (or are they called spring washers)?

Belleville washers?