Z axis THK flat or Round linear bearing

[flash319]

I am in the design stage of my router/plasma table. I am building it for the router but may plan down the road to use it for plasma. This will be a 3.5(travel) feet by 8 feet table. Anyway, what is the preferance with the Z axis? Much cheaper to use ground shaft and linear bearings but I see a lot of people using THK style rail bearings for the Z axis. I want to run 2 parallel linears (round or flat) say 5-6 inches apart with a ballscrew down the middle. I was thinking 3/4 inch shaft with bearings? Should I consider rail style bearings instead? What would the advantage be? Less flex? I am going for about 8" of travel so my shafts or rails will need to be around 14-16 inches long depending on my final design. What are some thoughts?

[ger21]

THK's are stronger, and easier to mount. Just bolt em on to a flat plate. But they cost more.

I used 3/4" round shaft on my router, with 8" of travel. 18" shafts, with 16-1/2" exposed. I have a set of 18" THK's for my next router. I actually paid less for the THK's on Ebay

[jans123]

Must it be two parallell linear bearings? It is almost impossible to get them perfectly parallel to avoid forcés built up between them. Using one good linear bearing and a simple flat iron guide with some ball bearings at some distancefro the linear bearing to prevent whatever you are guiding to rotate around the linear bearing is simple, cheeper and easier to align as the only precision device is a single guide (the linear bearing).
/jan

[flash319]

I guess a single would work but there is less rigidity with just the one. I see your point and will take a look at a design like that. I think it would be more work as well to do it with a single as the bearing supports for rotation would require some engineering. Think Think Think.................

[jans123]

Perhaps this link can help:
Diameter of shaft for linear-guide
(I don't know if the link works...)
I'm mot sure if it is much less rigid
/jan

[Zumba]

Must it be two parallell linear bearings? It is almost impossible to get them perfectly parallel to avoid forcés built up between them. Using one good linear bearing and a simple flat iron guide with some ball bearings at some distancefro the linear bearing to prevent whatever you are guiding to rotate around the linear bearing is simple, cheeper and easier to align as the only precision device is a single guide (the linear bearing).
/jan

The proper way to install the rails is to treat one as the master and the other as the slave. Should be self explanatory.... when done correctly, the bearings will not bind.

Adding a "flat iron guide with some ball bearings" to combat moment forces may prevent breakage but won't help you at all as far as accuracy. This is not the place to save money.

[Armstrong]

THK's is just the best. I hope your project will be very nice if you use it. Good luck with your work.

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