Does bolting a THK rail to 8020 alum add rigidity?

[HereinCS]

Just curious, I plan to bold 2 THK sr25 rails to my 25 series 50100 extrusion. Does this add or take away rigidity to the beam? Thx.

[cnctoolman]

It should add to it. How much, I cannot say. But its like adding a stiffening rib or a brace.

[dcb007]

Originally Posted by HereinCS Just curious, I plan to bold 2 THK sr25 rails to my 25 series 50100 extrusion. Does this add or take away rigidity to the beam? Thx.

With my machine it helped. I did mill a slight recess in the extrusion (somewhere around 0.05 deep) to seat the rail. You can see it on my site

You Can Build A Homemade DIY CNC Machine

and specifically this picture

http://www.neo7cnc.com/images/large/large014.jpg

In hind site I would have used 15 series 80/20 just to add to the rigidity but I don't have any issues with the 10 series I used.

Take care

[HereinCS]

Thanks. Cool machine btw. I'm using 50100(basically 2" by 4") with a span of 48". Hope it's rigid enough.

[giz]

Yes, it will stiffen the beam. This is called a composite beam because of the dual materials.

What you are doing is basically adding cross-sectional area to the beam. But since this added area is made up of steel which has a higher Young's Modulus, you have to take this into account with what is called a transformation factor. This factor increases the width of your steel beam to give you an equivalent aluminum beam.

If your rail is 0.91" wide for example, you would multiply this by n=Esteel/Ealum and this would give you an equivalent aluminum width. I'm too lazy to look up the Modulus values for the rails and aluminum right now, but I imagine you are adding quite a bit of solid "equivalent aluminum" to your 50x100 extrusion.

More beam = more rigidity.

[LeeWay]

You might consider bolting the rail to some steel first, then bolt that to the 80/20.
This is what I did on my lathe. It made a more solid base for them. The 80/20 rails are actually bumped up a bit around the tracks. When you tighten down on it though, it pulls whatever you are attaching down to the face of it. It might not be worth the effort to you, but that will also help with rigidity as well as give a more solid base for the rails should you need any shimming.

[cliffy]

Aluminium and steel don’t expand at the same rate,
bolt it nice and tight when it’s cold, then when it’s warm; it’ll be bent.

That is how a bimetal temperature sensor works.