How stiff is a single THK linear rail?

[MechanoMan]

I came up with a plan for machining long pieces of wood. It wouldn't be round so I don't plan to use a lathe for this.

What I did come up with was an idea for a long, long (~72") table and a ballscrew. I'd mount the linear rail's bearing blocks on the fixed platform about a foot apart, with the cutting head always in between. And a long table with the rail mounted on the bottom. Rotary table and tailstock here on top.

Now here's the thing. A LONG linear rail is frequently very expensive, but ones not in a matched pair can be cheap! But is it necessary to have 2 rails? I can make the table stiff so the moving table won't flex or twist. That's not the problem. The question is, how well will one rail itself resist torsion so the work doesn't rock front and back as the whole table and rail rock in the slider? The length of the table is actually irrelevant at that point, it's whether the single linear guide itself resists torsion.

If there's significant slack in that direction, the work's going to chatter and that'll be bad news!

The stock would be 2"x2" wood and about... 4"-5" above the linear rail. That's a critical factor since the torque created depends on the distance from the rail.

So, what's a linear rail like in this regard? I've got a Bosch I'm looking at, found a THK but the price was much higher.

[CarbonKevin]

Interesting concept. Can we get a diagram of what you're proposing?

Linear rails are quite stiff once mounted, I don't think the rail itself is your limiting factor. Most likely the bearing blocks will reach their load limit long before the rail.

Do a search for the exact bearing you're planning on using, and you can find PDF data sheets on THK bearings that specifically state what your limit of load is in each rotational axis.

I'm still a bit unsure how you would drive this system, as you would want to mount the ballscrew either directly above or below the rail - otherwise you'd be asking for all kinds of racking, methinks.

[Jesse B]

Some sketches would be nice.

I'd be worried about a 6' ballscrew though. It'd have an enormous amount of whip, and would severely limit your speeds. R&P would probably be a better choice.

[johnmac]

take a look in some computer scanners. You will notice they ride on one rail with an outrigger slide to stabilize the lens assembly. Maybe something like that with an inline skate roller rolling on a flat metal strip.

When you price a suitable ball screw you will probably look for another way. Rack and pinion would be a good choice. Just mount the rack gear with the teeth pointing down, so they don't catch saw dust.

John