Reason for lower bar on gantry?

Hi guys,

Random question time :biggrin:

I have seen in a few builds that a bar was used to connect the gantry sides together, the bar was placed underneath the bed and usually below the runners. I'm trying to get my head around what it's purpose is?

I'm not meaning to pick on anyone's work in particular so forgive me for using an example.

The red arrow points to the bar in question (in purple).
http://i.imgur.com/346INT1.jpg




As far as I can see, it shouldn't really contribute anything to the overall rigidity of the structure.

If it's to add more stiffness to prevent the gantry frame from lozenging (losing it's square shape and going all trapeziody <-new word right there), surely a better way would be to add a triangle somewhere instead? Like this:

http://i.imgur.com/o3O6VxN.png



What are other peoples thoughts? Personally I like cheese toasties.

Husky :)

You generally see that bar added underneath when people use only one ballscrew to drive the gantry, since with just one screw you need to attach it to the centre. For anything but the smallest machines that's a silly idea anyway, since then you should use two screws and the connecting bar underneath is no longer needed. In fact it's good to get rid of it since then you can support the bed and rails better.