[magick_man]

I picked up a pair of 1.5" NB linear rails 58" long.
And 4 closed bearing blocks a while back for a great price.
Now I am wanting to replace my semi supported 20mm rails with these larger ones.
The unsupported distance is approximately 55" between the support blocks.
I anticipate my gantry will weigh about 65 pounds complete.
Does anyone have any experience with rails this large?
If I can't use them unsupported then I will just have to forget about them and go back to the 20mm rails.
But I really want to use them since the 20mm rails are being a pain to install straight.

Here are some pictures to compare the 1.5" with my .5" z rails.



The center ones are some 1" I got with the 1.5"




Thanks,
Joseph K.

[Horsedorf]

being a pain to install striaght? like.. .how? when I installed the side rails.. I started at the middle, and clamped it.. then shuttled teh carraige to an end and clamped it there, then went back to the middle, made sure it had not moved, and clamped it, then went to the oppoosite end with the carraige and clamped it there.. I measured a few times and then ran the carraige back and forth a few times to make sure it did not bind, then i drilled and screwed the rails down. Works like a champ!

Now Ijust have to mount motors, wire it, power it, get the x axis going, then do the......

[magick_man]

Well I am trying to install them onto my Y axis at the moment.
And every time I try and tighten the bolts down it shifts.
So I end up with a Y rail that is about as straight as a wet noodle.
I am setting the whole thing on a granite surface plate when i try and tighten them down.
and have tried everything I can think of to keep them from shifting.Short of spending $400+ for 4 nook support rails I just want to go another direction for now.

[Horsedorf]

Ah, I THINK I see.. when I built mine, I built the y axis on a box channel, then bolted it to one side of the x axis and used that to then secure the other side of the x axis.. the right side of the x axis floats.. not ideal, I know, but was built around what I had on hand. I am going to run it VERY slowly and use it to make parts for a second cnc router/engraver table that I have a flat plate for with about a zillion 10-24 holes drilled and tapped in it every square inch. THAT one will be driven on both ends with a belt drive with the gantry floating on side rails.

I really need to take some pictures of what I have so far. it looks.. goofy, and almost embarassingly poorly made, but, I'm doing it to learn and figure out how I want to design the next one so that I start with a design and not just a pile of parts.

Fun incarnate!
Rick

[Zumba]

Static loading will cause the rails to flex downward about 8-10 thousandths when the carriage is in the center of its travel.

[magick_man]

Is that based on about 35 pounds at the center of each rail (70 for the pair)
If so That doesn't sound very good.
one option I was thinking about is to mount a bearing or wheel to the bottom of the bearing blocks.
This would ride on the same face the rails are mounted to.
And hopefully take out most of the sagging.

[Zumba]

My mistake, forgot to count two rails. The deflection is proportional to the weight, so with 35 pounds each rail (Z-axis assembly in the center of the gantry), deflection will be more like .004-.005. When the Z-axis assembly moves all the way to one side, one rail will deflect more than the other.

I don't recommend adding a wheel to the bottom. First, your mounting surface is probably off by much more than .005". Second, it only compensates for deflection caused by gravity and does nothing for cutting forces.

[magick_man]

Well luckily I just scored big time on ebay.
And picked up 14 18" sections of THK SR20 rail and 14 bearing blocks to go with them.
All brand new.
So now I will be using that for my X and Y axis rails.
And the best part is I got it all for under $400.

Now I just need to find someone to buy all the other rails I have...

Thanks for the help anyway.
Hopefully these new rails will cooperate when I try and install them.

Joseph K.