[tjhj]

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

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

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

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.

[keebler303]

1.
The slides are quite long in relation to the available travel. (machine will be big but small travels)
Susceptible to contamination due to open design. (no seals)
Can only take loads in certain direction.

2.
Travel is almost the same as rail length.
Sealed bearings to reduce contamination.
Can take large loads in any direction.

3.
Lower loads, less life, less precise, less rigid than 2.


There are good reasons why a lot of hobbyists and almost all commercial manufacturers go with option 2. If you look around you will see it is by far the most common design, except where 3. is used due to budget constraints.

Good Luck
Matt

[cletero]

***EDIT: SEE POST NUMBER FIVE; AS I GOT CONFUSED WITH THE NUMBERS IN THIS ONE***

This post is really old but I will try to refresh it with my question:
I have #2 (I meant #3) on my newly built machine and to be honest, they do exhibit some movement with heavier loads, it seems the blocks tends to open up a little under loads in certain direction. Still, it's only about .04mm movement, so you be the judge if this works for you. Still they are nice since they are easy to adjust in preload and don't need super exact alignment.

#1s can be found nowadays with rollers instead of balls, which seems to be better, still they have the "open desing, no seals" issue.

I'm considering buying #3s (I meant #2) in the near future but wonder if preload is adjustable, or once they become loose they have to be thrown to the garbage can?? I have tryied googling this but had no luck finding information.

[keebler303]

LOL!

This post is so old, what like 7 hours?

If you buy decent quality (HIWIN Corporation | HIWIN Mikrosystems is a decent one without spending too much) and you buy the right part for your application, you will have ZERO play and they do not "open up under load". The preload is not adjustable in the #2 design either so I'm not sure what you are talking about to be honest.

[cletero]

Yeah, super old post! I think I got confused, I was looking at like 8 threads at the same time!!

Sorry for the confusion.

I also confused the numbers: I meant I have number 3s, the ones with the supported round shaft, to avoid confusions. Most of them at ebay have a small screw on one side to adjust play. But they still open up under load. I use two 16mm diameter rails (SBR16) and four bearing blocks per axis.

What I plan to buy are number 2s, the ones with a rectangular carriage, but was wondering if they were adjustable to eliminate play (as I see from your answer, they're not). I will definitely check out Hiwin, I came across some of them just a few minutes ago at ebay.

[keebler303]

You can buy them with various amounts of preload to suit the application. Something with high speeds and low loads might have 0 preload (perfect fit). For heavier loads, better stiffness and more accuracy, you can get a light, medium, heavy preload. For a router a light preload is probably good. If you are wanting to machine aluminum or steel, you're going to want a bit more preload. Hiwin should have some good application assistance notes or you can give them a call and they could help you get what you need.

Matt

[cletero]

Thanks for your help.

So once they wear out and loosen, there's no way of tightening them up again. However, I have seen they last for a very very long time. I have even seen some rated for "5 years maintenance free", and that's under heavy industrial use.

Aligning them shouldn't be so difficult depending on your machine design, in my case I know the beds have very good flatness, so I just to care about aligning them in one axis, so I just leave some play on the rail screws, mount the axis, table, or whatever and move it back and forth a few times, this makes the rails become parallel and then I tighten the screws. Voila!

My first machine was MDF, dremel, 1/4-20 screws, hose/hose clamp couplers, etc just to get familiar with software, design, usage. Check it out here:

[ame=http://www.youtube.com/watch?v=T9cyvM_6NY4]Low cost MDF CNC with Dremel - YouTube[/ame]

It's still precise enough so now I will use it to make its own upgrade parts like bearing mounts, proper couplers and upgrade to aluminium profile-Hiwin/THK style rail bearings to stiffen it up. I already have a new Ridgid 2401 spindle and will get precisebits collets for it.

[ger21]

So once they wear out and loosen, there's no way of tightening them up again.

As long as you keep them lubricated, they don't wear out.

[tjhj]

Thank you for all the good info so far. I think I am going to bite the bullet and buy the #2 linear rail system.

Now to figure out the load limits.

Any suggestions on the cage/ rail quantity per axis? 2 Carts and 2 Rails per axis?

What is the most common width rail for the Hiwin rails?

Thank you,

[dmalicky]

Definitely 2 rails per axis.
The stiff machines use 2 blocks per rail for all axes. AFAIK, all machines use 2 blocks/rail for the X (long) axis. Some machines (e.g., K2CNC) use a single extra-long bearing block per rail for the Y and Z, but 2 blocks per rail is definitely best.
Even the 15mm rails have plenty of load rating for most uses, but the 15mm use tiny screws to hold the rail down -- hard to tap those threads.

[victorofga]

automation4less site good prices
the 1500 mm length of 15 rail combo, one pillow block plus 1500 mm rail 130
AGW15CB1TxxxxZ0H, Combo, 15mm T Rail, 4 Bolt Flange Block, Specify Length, Hiwin AG 15 Combos - automation products from Automation 4 Less



vxb site fair prices
16mm 55" Rail Guideway System Linear Motion rail 55 inch for 108
CNC Bushing 16mm Linear Bearing Open Sliding Unit Linear Motion
one bushing or pillowblock for 20

you can see of aboves, from automation you get the hiwin rails for same price like the round rails..

i purchased from automation... they very helpful folks..
just check on prices brfore buy anything..

also you can gie a try ebay,
3 SBR16 Rails 3 ballscrews RM1605 3sets BK BF12 3 Couplers | eBay

this seller has plenty selection..

[dmalicky]

For supported round rail, if the bearings will see any "pull" (tension) forces, I wouldn't recommend the 16mm: the included angle in the open section is 80 degrees, rather than 60 deg for SBR20 or 50 deg for SBR25. The larger the included angle, the lower the stiffness and load rating in tension.
http://img801.imageshack.us/img801/5...555555555a.jpg

That said, the entire SBR series is not designed to resist much tension, as there's only 1 bearing track near the opening--and only on one side!
http://i01.i.aliimg.com/img/pb/439/4...493439_025.jpg
The nearest bearing track on the other side is oriented directly lateral to the shaft.

I think Thomson uses 1 track on each side (both near the opening) for some of their bearings, but their load charts still show low ratings in tension.

For a short travel z-axis, unsupported 25mm shaft with closed bearings might actually be stiffer than supported round. Especially if the shaft is supported at each end and also the middle... I.e., from top to bottom:
shaft support
clear shaft for upper z closed bearing
shaft support
clear shaft for lower z closed bearing
shaft support
That assembly takes up more height, though.

[victorofga]

the supported rail somewhere between the vrollers and hiwin type..
that supported rail would worth if it were half cost... but as i posted they are same cost..
so not really worth to buy.. if hiwin can be purchased for same price..

otherwise all true what you wrote..

[dmalicky]

Right, to clarify, my last post wasn't to make a case for SBR, rather to point out its limitations. Its only advantages (vs Hiwin) that I can think of are somewhat lower cost (ebay) and usually easier mounting than the "T" rail.

[tjhj]

Thanks for all the help, I am going to go with the Linear Slides. Now its time to start finalizing design specs and wrap up the other projects.

[smkuningas]

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.

[dmalicky]

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.

[smkuningas]

Thanks for an answer. i have had same same thoughts from time to time I overrated these blocks. Looks like it was necessary that someone brings me back to earth.