Weld Changes unrelated faces

[orbiter]

I have only a little experence of Wings3d but have made two or 3 models.

I am making a model for the Orbiter Space Sim and it requires that there are only a few objects in the model. To this end I have started with a cube and by means of connecting faces and extruding them, I have built up a model of a gantry ( like scaffolding you see outside building being built)

I now wish to add another object and so made two matching faces on each model. Using "absolute move" lined them up and was able to weld the two objects to form one. So far so Good.

When I come to weld another copy to the new bigger model, it will only work if I play with the tolerences of the weld, (yet I know its lined up as good as the first!). It also removes faces from elsewhere in the model i.e. nowhere near the site of the weld

I have put before and after screenshot JPG's and before and after Wings files on the following web page If anyone has the time to help.

http://www.orbiter.comeze.com/Gantry.html

I know I am only a new kid on the block but if I can make this model work I can write up a tutorial as a form of payback

Thanks in advance
paddy