Countering sag of extended X-axis

discussion of design changes / improvements / suggestions
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Countering sag of extended X-axis

Post by Philip » Tue May 06, 2014 9:05 pm

With people using long X-axes like 1800mm or longer it's obvious the gantry and spindle weight will make the X-axis sag. I've no idea how much and with 2D work it might not even be important, but I was thinking of a way to possibly counter the sag.

I came up with this:


There's a rectangular alu tube bolted or glued in between the double X-rails. Inside there are a number of metal pieces that loosely fit inside the tube and have M5(?) thread inside. The threads are tapped at different heights so that the rod that runs through them is bending downward. On either side of the alu tube is an end plate with a hole where the rod runs through. Then bolts are mounted on the ends of the rods. When the bolts are tightened the alu tube with both X-rails will slightly bend upward and counter the bending caused by the weight of the gantry and spindle.

What do you think, will it work?

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Re: Countering sag of extended X-axis

Post by Woodworker » Tue May 06, 2014 9:10 pm

What about a piece of the black extrusion bolted under the X rails with a spacer in the center. Measure the downward deflection and adjust the spacer to suit the current load. Both ideas are basically torsion bars.
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Improbable Construct
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Re: Countering sag of extended X-axis

Post by Improbable Construct » Tue May 06, 2014 10:09 pm

This idea has been gone over before. The main problem with it is the unintended consequences.
What is going to happen is that the tension will cause the span to move in unpredictable ways.
What you want it to do is to just remain rigid and flat, but in reality what it is going to do is bow out to the sides or twist.
If you get it sturdy enough to resist the bows and twists then you probably do not need the tensioning rod.
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Re: Countering sag of extended X-axis

Post by cvoinescu » Tue May 06, 2014 10:25 pm

IMHO, torsion is much worse than sag. At first glance, you could use your idea on the front rail and the same thing again, but upside down, on the back rail, and that would counteract the torsion caused by the offset weight of the spindle. However, there are variable loads too -- when milling in the Y+ direction, the force applied to the cutter tends to twist the X rail in the opposite direction.

If the goal was to correct sag, simply sandwiching a 40 mm x 2 mm steel plate between the two X rails, with the rails bolted securely to each other, would be better than M5 rod, and probably easier to do too. Steel plates on the outside of the rail (between the Vs) would work to counteract both torsion and sag.
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