LINEAR RAIL VIBRATION / STABILITY

Hi-
I'm considering modifying my Shoptask machine to linear rails. My plan was to bolt a 1/2" thick blanchard ground steel plate to the top of the main bed casting to attach the longitudinal rails, making the mounting points wider than the standard 6" width of the casting. Then use another steel plate on top of the blocks to attach the cross slide rails and table. I have no experience with linear rails and want to know if they will be as solid as the traditional method of the iron carriages sliding atop one another. Was looking at the 20 MM Chinese rails that are for sale on E-Bay. Also, I'm wondering about the ability of the stepper motors to lock and hold the rails in place against cutting loads since there is no physical clamp. My CNC system uses 640 OZ/IN motors and I have the 5 pitch Chinese 20 MM ball screws.
Thanks in advance for any help.

If you are talking about round shaft rails, then don;t do it. They are not great for metalwork.

dovetail ways have been used on cnc machines for a long time, I think the only downside is friction, and low speeds.

Can you say why you are considering the switch?

Alan,
Thanks- I have seen people advertising mills with linear rails. To be honest, I am a bit of a fiddler- my machine works just fine, but I got this idea to modify it mostly for something to do. I'm guessing the problem with those round rails is the small contact area that supports all the machine weight.

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Originally Posted by smallblock

Alan,
Thanks- I have seen people advertising mills with linear rails. To be honest, I am a bit of a fiddler- my machine works just fine, but I got this idea to modify it mostly for something to do. I'm guessing the problem with those round rails is the small contact area that supports all the machine weight.

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The linear rails used for metalworking are profile rails with some preload. You'd want the ones with 4 ball bearing races. Examples would be THK HSR and SHS series...

First you have to realize that people on this site often refer to there machines as "milling machines". More often then not they are referring to there router and don't know that there is a implied difference between the two. Compounding the issue is all of the kickstarter's that list there machines as milling machines when there really flimsy routers. This often causes people who are interested in cnc but have no knowledge or experience in the trade to believe that because they are called milling machines they are as capable as the real thing for cutting metal. Most of these machines use round rails of some sort or another. Those round rails are fine for wood and sometimes even some non ferrous metals on machines that are properly designed for those loads. Most industrial machines built for the metal industry use linear rails as well but not those low cost round ones. Instead they use larger square rails like thk and hiwin that are more then capable of loads seen machining metals including steel and exotics at high speeds. If you really decide to build a z axis use square rails of at least 20mms with 2 trucks per rails. 25 mm or more would be better. They can be had fairly cheap on eBay and are designed for what your thinking of doing.

Ben