Linear Rails/guides and Bearings - What size is best?

[Shawnusa]

Hello all,

First post. I have been looking for awhile and plan to build my first Gantry CNC router. I have been looking at all the various THK style guides and bearing blocks, but find it difficult to know which ones will work best for a 4X8 foot table with 5-8" z axis. This will be built using Jig plate aluminum as the X/Y/Z axis and table base. I plan to use 8020 for base frame. Since I do not have a engineers background all the specs are somewhat meaningless. I want to cut wood and at most aluminum on it. I desire .001 or better accuracy. So what is the most appropriate size to select without being to small or to big? IF you can assist that would be great. Also, cross refrence to various manufactures part numbers would be helpful in my searching for best price and possible Ebay options.

Thanks again.

Shawn

[Evodyne]

Shawnusa,

Welcome to CNCzone! Take a look at Swede's (one of our members) web site. In his mill build he details in laymans terms the differences between the different grades of THK rails. Some accept loads in certain directions, others in all directions. In general the parts are NOT interchangable: you can't swap the slides off of one type of rail onto another. Beyond that rail size is pretty much directly related to load. You can, with a little care, butt two or more sections end to end. This is handy to remember as long rails on eBay are somewhat rarer and command top dollar. I've got 25mm HSR rails for a similar sized table (not yet built) that can handle wood and probably aluminum. If your definately thinking aluminum it might pay to step up to 35mm. Deals are funny: I've got some mongo 80" rails (think 1/4 scale train track) that cost less than the 25mm. Every deal is different! Good luck.

Evodyne

[ger21]

Most 15mm THK's can handle close to 1000 lbs. 25mm double that. So 25mm rails is plenty big. Our router at work has a 1000 lb head hanging on 25mm THK's that are mounted to a vertical surface.

[JerryFlyGuy]

Anyone offer a ruff price of what the THK 25mm rails might be worth per meter [ or foot]Also how much each 'truck' is worth... I've looked at McMasters cataloge but was not to impressed w/ the pricing. I think I could build a track of my own w/ cold rolled steel and eccentric cam follower bearings for less than they have it listed.

Curious..

JerryFlyGuy

[DDM]

The THKs are kinda spendy, I'm looking at doing a larger table also, but I'd like to get by on 500-800 bucks on all my linear guides. I kinda like the look of the Igus but I've heard people have had problems with them binding, for what reason I'm not sure. I just can't afford to dish out 500 bucks a rail on thks 6 feet long. Anyone have any ideas?

Carl

[ViperTX]

ebay for linear rails.

[ahren]

The problem with the Igus is that as slippery as the "frelon" material is, the effective coefficient of friction at the shaft is still 10 times more than that in a rolling element bearing (don't know the exact numbers, but I remember doing this comparison in the past). This means that side loading will often make an Igus axis bind, whereas a rolling bearing will, well, "roll with it" (booo, hisss -- I know).

[ger21]

Originally Posted by DDM The THKs are kinda spendy, I'm looking at doing a larger table also, but I'd like to get by on 500-800 bucks on all my linear guides. I kinda like the look of the Igus but I've heard people have had problems with them binding, for what reason I'm not sure. I just can't afford to dish out 500 bucks a rail on thks 6 feet long. Anyone have any ideas? Carl

Bishop Wisecarver Dual V wheels and rails. www.bwc.com You can get cheaper chineses knock-offs at www.cadcamcadcam.com

[ESjaavik]

Find your rails at Ebay. Even the smallest THK (or similar) guides will bear much more load than BWC rails. Then build your machine to fit the rails you get. They have wiper pads built in, so will be less prone to ingress of dirt than any roller rail system. But you should have an outer protection also.

One problem with these (and BWC) is that the damping is almost zero. So you may have to add "brake pads" to avoid unwanted resonance phenomena. And you must use backlash free leadscrews, there is no way around that. If you have any backlash, the axis will rattle around freely inside your backlash like a marble on a dish.

[ger21]

Originally Posted by ESjaavik They have wiper pads built in, so will be less prone to ingress of dirt than any roller rail system.

The BWC roller systems have optional wipers, and they are designed to expel any dirt that gets in them. Once dirt get's in the THK's, it's probably not coming out. Bishop Wisecarver claims that their system excels in dirty environments.

I agree with you, though, that if you can get THK's on Ebay, get them. You have to be patient to get good deals, it may take weeks or months. And consider what the alternatives cost, and be willing to spend at least that. You'll be getting superior components, and you have to be wiling to spend a little.