[Oxyd]

Hello

I have been searching about this one specific issue. I have some used THK linear rails and bearings, so i am thinking about building another cnc machine, using these. Pretty much every problem, that i need to solve to start designing is solved, except one: how far should two bearings be from each other on one rail, in order to reduce chances of frame racking (excluding other mechanical parts, that also reduce racking chances), while using one screw in the middle of X axis? Also the same goes for other Y and Z axis to reduce bending, that occurs while cutting or simply pushing Z axis by hand.

Is there any practical optional rating for eg. increasing distance between two rails, proportionally increase distance between bearings on each rail, or increasing the height of Y frame, therefore increase distance between bearings, to maintain a strong base for Y frame to sit on. I can make a huge distance, but that will simply waste my working area, distance too smal - not good also.

I had this racking problem with my first machine, though i used simple, cheap, home made slides, until i put two screws on sides, which solved my problem.

Here is the photo of rails.





Also i wonder if this racking is an issue with these kind of rails? I suppose, that these rails are designed to hold loads not only from top, but also from sides, turning momentum along rail, perpendicular to rail and etc.

[Perfect Circle]

I clicked on one of your pics and it gave me a Virus!!(flame2)

[Oxyd]

Hmm, everything looks fine for me. Weird. But the pictures are not very important, just wanted to show these rails.

Maybe direct link images will be better?
http://img717.imageshack.us/img717/9170/1003371r.jpg
http://img522.imageshack.us/img522/2034/1003370i.jpg
http://img511.imageshack.us/img511/6643/1003373i.jpg

[Perfect Circle]

Ok the direct link is fine...Thanks for posting.:cheers:

[Mogal]

I would have to say the further apart the better...
of course being practical with the distances...

[Mike Everman]

Any reasonable spacing rail to rail and block to block, and you will never see racking with those bearings. It's almost impossible to make it happen on purpose with recirculating linear blocks.

[Oxyd]

Thanks for replies!

I do believe also, that these bearings are designed so that there won't be any unwanted movements.

So i suppose that everything leads to good frame and alignment between rails, rather than optional distance between bearings?

I saw one guy made cnc router, very similar to what i want to make and he made 30mm distance between the same model (i think) bearings, which looks quite wide to me. Probably gonna test various distances by attaching long metal bar on two bearings to see if there is any unwanted movement or something like that.

[RICHARD ZASTROW]

I clicked on the pics 1/27/11 and a virus then prevented access to the internet by crippling my browser after start-up. AVG gone, McAfee in.

Dick Z

[handlewanker]

Hey, it's june 2011 and those photos are still up there.

If'n the photos had been porno shots they would have been deleted immediately by the Admin....wake up they're infected, get 'em off.
Ian.

[wildwestpat]

The photos in question pass as free from any infection by Norton NIS and no adverse effects noticed having viewed them.

On the question of how far apart to mount the trucks the manufacturer should have the data on the amount they will twist under a given load in all three axis. These figures will help determine the number of trucks you need per slide as well as how far apart to mount the rails for a given ridgidity as seen from the tools cutting edge.

Good luck as the rails should make a very nice machine.

Regards - Pat

[bassman2914]

This post is getting a bit old but i will comment anyway. As far as the pictures go, who cares? That's not what this post is about. With the racking, look at the manufacturing specs for two trucks at a given distance apart and it should have data for flexing amounts in mrad's and microns(hopefully the units won't be bigger). You could calculate the force exerted on the trucks with the cutting tool on the far side of the table using this link(End Milling Horsepower Calculations For Kennametal End Milling Application) which will give you force in pound to make the cut and you'd convert that to newton meters and that will get the the maximum flex since you can't account for the fact that there's rails on the opposite side(at least not easily). you would then convert the angle of deflection into distance using this(Right Triangle Angle And Side Calculator). To help minimize the deflection make sure you have a rigid undercarriage that won't warp allowing the the trucks to shift. My rails have 1 truck on each but they are 7" long and the y carriage is 23" long so i'm pretty sure it won't have any racking. just my 2 cents.