Question about Linear Slides

[bprzybyl]

I'm looking into supporting all 3 axes with THK linear bearings. Would I be able to support any with just 2 bearings and 2 rails (1 slide per rail)? I am not familiar with these at all. Also, is there any play in them in any direction other than the way they're designed for? I will be making a manual mill which will be converted to a CNC. It will basically look like a smaller bridgeport(compound XY table, motor will only move in Z).

Thanks,
Brooks

[Ken_Shea]

Brooks,
IMO, you have chosen among the best linear rails available. The HSR series are designed to carry equal loads from all direction.

For a mill, I would not consider only one truck per rail, I guess if you got the rail in the largest size this might be doable, but it likely would be less expensive and better suited for a mill to just get smaller rails with two truck each.

For a small to medium size DIY mill the HSR20 or HSR25 would likely be a good choice.

Here is a link to THK rail information.
It is very helpful in that it explainers the relationship of all those letters and numbers to the THK rails.
http://www.arsales.com/pages/thk/products/hsr.htm

Edit;
These are preloaded and have no perceivable play.

Ken

[ViperTX]

If the size of the trucks is huge...like those for the 45mm rails....then, yes.

[mxtras]

Personally, I would recommend using multiple points of support rather than one. Alignment becomes a bit trickier but the rigidity factor will go through the roof!

Look at the load capacity of the trucks - the smaller units have substantial load capacity - both static and dynamic. I think folks typically use much larger rails and bearings than are necessary. I am not saying this is a bad thing but if you have a tight budget it might interest you to look at the smaller series bearings.

Scott

[vacpress]

It would seem that multiple trucks must be better. However, I have seen some truley large THK bearing blocks that would probably work MUCH better as an axis slide alone.. The reason is that there are NO alighment issues. These 30-40mm wide rails have blocks that are 5 or 6" by 5-8"... This is pretty big for alot of small machines... The simplicity would be worth looking into..

Recently on ebay i saw a 15" THK rail with a single block that, i beleive, they claimed was something like 8 inches by 6 inches... A pretty good size for a Z axis.. These THK type rails... They are very strong. I beleive for trim-router or dremel size mills, a single HRW21 rail with 2 blocks would be fine. Even with 1 block it might work fine - and regardless, even with just 1 block, it would work far, far, far better than other thigns ive tried (gas pipe and roller bearings, mdf, igus plain bushings, etc)..

However, dont take my word for it - do some experimentation!

Robert

[dkowalcz]

You can get away with one block per rail, I did it for a while, but two is much, much better. The slop in these is not really measurable, the spec I have someplace around here states the slop and spring rate of the blocks, but it's so small that it's not really worth worrying about.

Good luck on your project!

[pstockley]

For a mill you really want medium to high preload on the bearings. Not many rails on ebay have this and most are generally light to no preload. With two trucks you can use shims to force the trucks horizontally against the rails to effect some preload. This will stiffen things up and avoid chatter when cutting metal.

[bprzybyl]

What about drill rod with ball bushings? Possibly open ball bushings w/ the drill rod supporded along it's length. I don't care if I have to watch ebay to be sure I get the best blocks/rail. But If I could get use Drill Rod, I'm pretty sure I'd save a mess of money.

[vacpress]

I beleive there is a fair amount of information about this. For a general idea what the accuracy tradeoff would be, check out mcmaster.com

i beleive they have deviation per foot or inch specs for most materials.

depending on what you are doing, i gather drill rod could indeed work fine and save you a load of money.

however, after seeing how well thk rail-style bearings work, you may find yourself stuck on it... one advantage to rods would be that you dont have to drill and tap the 20 to 100ish mounting holes!

[bprzybyl]

I'm gonna look into this now. I really don't mind drilling a mess of holes (sounds pleasant honestly) The only problem would be making sure they line up perfectly straight, as all I have is a Drill Press. I know I could buy pre-drilled from Mcmaster, but...eh. I'd rather do some work.

Does anyone know if Closed Ball Bushings can be converted to Open Ball Bushings? Like If I were to cut out one section of balls, would it still function comlpetely regularly?

Thanks,
Brooks

[jeffs555]

You need to run ball bushings on hardened rod. The tiny balls will quickly wear grooves in drill rod.

[andy_ck87028]

I know I might be the amateur talking to the professional but I am also using rod and bush for my Z assembly. I also only have a drill press.

I believe that because the drill press is inherently inaccurate it therefore should not be relied upon for accuracy.

My approach is to use the drill press to create enlarged holes on the Z assembly that are aligned whilst ensuring the holes are smaller than the outside diameter of the bushes (therefore ensuring the bushes are held in the vertical plane)

I am using four rods (one on each corner of a square) within the assembly with 4 bushes on each rod. Each bush obviously has to be aligned.

My plan is to align incrementally with an alignment jig that I am creating (basically two plates with 4 holes drilled and temporary bushes positioned / glued onto the plates). I will use the jigs to align the Z assembly and then after each of the Z assembly bushes are positioned I will lock them down with epoxy. (My bushes will be veconite)

Because I am using 4 rods, alignment absolutely has to be spot on but I will get heaps of rigidity benefits.

This will be a very cheap solution. A simple process but I think the best I can do using rudimentary tools.

Andy