Roller Ways vs Linear Rails vs Linear Rods (Im searched out and still lost)
Hello All,
I have been slowly working up a plan for my home made CNC machine. I have converted a few manual mills but now I am trying to correct the deficiencies of the cheap mill converted to CNC by making my own design. Here is where am I just lost on why or how to make the selection.
I have been searching and reading for weeks, but I think I might be having a terminology issue so please excuse any terms that are incorrect.
Any advice, comments, suggestions would be greatly appreciated. What actual advantages do each offer? Which one should I chose?
I am sorry this is long, Thank you in advance for reading it.
When referring to direction load, it is assumed that the rail is laying down in front of you. X would be the left to right motion permitted by the rail, Y would forwards and backwards and Z would be pressing down on the rail from the top.
I am considering 3 different types of designs, I have included a picture to clarify what I am talking about.
1. Crossed/Ball Roller Ways
http://www.lulusoso.com/upload/20120...ball_slide.jpg
These seem to be similar to a traditional taper gib. Almost like it could replace a taper gib with these. They seem to be able to handle a much larger preload in the Z, and the V groove of the rails seem to resist Y movement due to the longer contact surface. They seem to be slightly cheaper than #2 Linear Slide Systems. But the design requires them to be set into the outer box and table.
2. Linear Slide System
http://www.ikont.co.jp/eg/product/ch...g/chc01-ph.jpg
These tend to be the most expensive. They resist motion in every direction except the sliding direction. Are they really that much better. It would seem that there is a design challenge of not placing to much moment on the narrow blocks. Is this actually a problem? Do they really maintain zero slop or backlash with significant loads on them?
I cannot decide if it is necessary to run multiple blocks and multiple rails. Such that there would be one rail on each side of the table, and one block on each corner. Or can you run two blocks on a single raid. Are they capable of handing the table loads on a single rail?
One big disadvantage seems to be ensuring a perfect mounting plane. As they resist motion in every direction except sliding the mount and table need to be parallel, in plane and square or there will be binding. In the end they need to be adjusted to this so the table is square so this may be a moot point.
What is the best direction to load them, should the design place preload force on the Z or on the Y? Such that they are mounted to the base with the table on top, or mounted to the side.
3. Linar Rods/ Ball Splines SBR12 LM12UU
http://image.made-in-china.com/2f0j0...BR16-TBR30.jpg
Cheapest solution which is nice. They do not resist rotational X motion but this could be advantageous by allowing eaiser adjustment. These do not seem as strong or as accurate as either of the above two solutions. Is this true?
Based on realistic concerns I can only afford a #2 linear slide with 2 rails with 2 blocks on each system for each axis. With the cheap cost of these it would not be unreasonable to run 3 or 4 bearings per rail and 2 or 3 rails per axis. Would this actually be advantageous over a linear slide?
With an unsupported rail there would seem to problems with deflection from Z loads along the axis as the length increases, but the supported rail seems to eliminate this problem is this correct?
Ok now I have pretty much spilled out everything I can seem to figure out here is the big question.
How do I chose between one or the other?
Thank you for your time and help.
Re: Roller Ways vs Linear Rails vs Linear Rods (Im searched out and still lost)
Is there any logical explanation, why are superhyper high load carrier 100% without play(slack?) 2 hiwin blocks needed. Has someone failed with 1 block design? If they are very 100% stiff(which i understand they are), what is the point of other block if it basically slides along then? i'm building 1,2m x 0,9m alum frame hiwin rail CNC. I have seen some 1 block designs but most ppl use 2. Is this because they just do as others do? Pointless to build tank if you need armored car. X axis(gantry i mean) can be balanced, so why 2? How great forces can there be, arent those blocks meant for something far more tougher job. I was thinking of 15mm rails. I will use 2 blocks if someone please can explain why is it necessary.
Re: Roller Ways vs Linear Rails vs Linear Rods (Im searched out and still lost)
Many multi-dimensional torques (moments) occur in a CNC machine, caused by both cutter forces and axis accelerations. Standard blocks are very stiff to carry a force, but relatively flexy to carry a torque. With 2 blocks (spread apart by some distance), torque is converted to a force at each block. With only 1 block, torque is handled entirely by that block, which it does not do very well. A rough analogy -- it's like standing on both feet vs 1 foot, or a bicycle vs a unicycle.
For full control of an axis, we need at least 3 blocks that are not all in a row -- 3 points define a plane (think of a 'chair' with 2 legs vs 3, or a bicycle vs a tricycle). 4 is almost always done for symmetry.
Also, realize that nothing is 100% stiff. Everything flexes. For a typical CNC router, Hiwin-type blocks (handling forces, not torques) are stiff enough that we can usually pretend they are completely rigid. But for a well-designed router cutting steel, even their small flex will matter.
Re: Roller Ways vs Linear Rails vs Linear Rods (Im searched out and still lost)
Thanks for an answer. i have had same same thoughts from time to time :D I overrated these blocks. Looks like it was necessary that someone brings me back to earth.
