Definately a scrapyard challenge

[motoxy]

Well that about it. I now have the touch plate working and from here on in its just tidying and tweaking. I have cut some signs and other test pieces and I am very happy with the final product. This probably is the end of the build log so a big thanks to all of you who helped me complete this project.
Any further updates or info I will just make separate posts.

Bruce

[jonbabbz]

Congrats. Looks awesome and I love that floating dust shoe.

[JoeHarris]

Awesome...

[Jonathan]

The main problem with the 150x10mm plate is that that cross section has very poor torsional stiffness. The deflection neglecting twisting is not too bad, though it can easily be improved. Torsional stiffness is very important since the Z-axis acts as a lever applying a large moment to the gantry, so poor torsional stiffness leads to high tool deflection = chatter, poor finish, lower loot life and not being able to cut aluminium well.

The best cross section for torsional stiffness is one where as much material as possible if far from the centroid, that's clearly a tube, and since the stiffness constant is proportional to the radius to the power 4, a small increase in wall thickness or diameter has a big effect. Unfortunately it's rather difficult to attach rails to a curved surface! So the next best thing is to use box section. As a reference, the torsional stiffness of 80x80mm aluminium box section, with just 3mm wall thickness, which is what I use on my machine is almost 40 times greater than 150x10 plate. For forces parallel to X (i.e. orthogonal to the gantry) the stiffness is also significantly greater for that size box section.

If you add two pieces of 50x10mm aluminium to the plate, attaching 10mm surface to the top and bottom of the 150mm surface, the deflection parallel to X is massively reduced (to the point where it's about equal to that for the box section) however the torsional stiffness is still 16 times less than my box section. Add a thin plate on the back (3mm or whatever) and it's a whole lot better. But then you've effectively just make a box section! Adding aluminium profile could work to, but bear in mind the same rules as in this little example apply (look at the cross section of the aluminium profile).

Perhaps you could instead use the 150x10 for the Z-axis (as torsional stiffness there is irrelevant) and add the plates as above to the Z-axis as 10mm is probably not enough on its own.

Hope that helps / gives you some ideas.

Another important factor is the rigidity of the joints between the piece(s) between the gantry, so bolt it well. Doesn't matter how rigid the beam is, if the ends are not connected firmly it will bend and the joints. At the moment you want some more bolts nearer the edge of the aluminium angle, but I guess you've just not got round to putting those in yet.

(Note in the calculations above I didn't take into account the rails themselves. These will add to the strength, but I think relying on the strength of the rails to make the gantry sufficiently rigid is going about it the wrong way. You should get it rigid enough to start with.)

What's the width of the gantry between the supports?