skate bearings versus linear bearings

Hi. I'm a newbie here and have just started the well trodden path of cnc router design. There are some great designs and engineering skills on show on this site.So here's my first (of many) question. I have come across lots of designs using skate bearings running on round rails for the linear motions.These bearings are currently just over £1 each on ebay and typically 6 are needed per bearing unit. I have found a supplier of open linear bearings (20mm) for about £6.50 each. So given that there is not much price difference and that linear bearings only require a few accurate holes to be drilled in a block of aluminium to mount them, why use skate bearings? The complexity of the mounting arrangement alone would seem to be a good argument against.Can anyone throw any light on this?

Thanks Paul

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Originally Posted by paul langwade

.... linear bearings only require a few accurate holes to be drilled in a block of aluminium to mount them....

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Hi Paul.

From my point of view as a cnc wanabee, that is the crucial problem.
Since started down the cnc path, I've collected quite a few bits and pieces, including linear bearings from ebay.
I've found that with my lack of machine tools and engineering experience, the "skate board" bearing approach allows me to build each trial rig with adjustments built in to overcome these shortcomings.

Needless to say each failed rig has taught me quite a lot :)

John

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Originally Posted by paul langwade

I have come across lots of designs using skate bearings running on round rails for the linear motions.These bearings are currently just over £1 each on ebay and typically 6 are needed per bearing unit.

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Look harder. ;)

http://cgi.ebay.com/100-Skate-Wheel-...QQcmdZViewItem

As the other posts suggest the skate bearings probably can be found cheaper and they are easier to experiment with. Another difference which could be a killer because of cost is that linear bearings need hardened and ground shaft skate bearings can run on anything (almost).

I'm currently building a machine using the skate bearing route. One thing to note in particular is that it is immune to contamination. A wheel will roll over just about anything, whereas a linear bearing can get clogged with crap.

However, rolling over crap can cause inaccuracy in the movement of the machine, so it's smart to use some sort of scraper, at least on the upper bearings. Spring steel is a good material to use. Unfortunately, it might scratch up your rails unless you get hardened shafting, which kind of goes against what Geof said.

Lots to consider. :)

Thanks for the quick resposes everyone

It seems the concensus is that skate bearings are used purely because they are cheap and offer some adjustment. Does this mean that if price were no object linear bearings running on ground rails would be prefered?

Paul

As long as they are not unsupported round shafts, then yes. I'd use THK type everywhere if price is not an issue. It's what all big commercial routers use.

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Originally Posted by Zumba

I'm currently building a machine using the skate bearing route. One thing to note in particular is that it is immune to contamination.

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I disagree a little. I've seen dust start to stick to the bearings, which would make it "bump" each time around.