[LeeWay]

Right.
That was static weight too. Add the weight o the Z axis as well. Then when you add in the driving forces of the weight of the whole gantry, it uses that weight against itself when it changes direction abruptly. Even though it is relatively light, the span makes it susceptible to many different types of error at the tool. I would venture to say the tool itself would not be rigid enough to get any kind of tool life from carbide tooling in aluminum. Anything less than rigid is going to quickly be fatal for carbide.

[tkms002]

Right.
That was static weight too. Add the weight o the Z axis as well. Then when you add in the driving forces of the weight of the whole gantry, it uses that weight against itself when it changes direction abruptly. Even though it is relatively light, the span makes it susceptible to many different types of error at the tool. I would venture to say the tool itself would not be rigid enough to get any kind of tool life from carbide tooling in aluminum. Anything less than rigid is going to quickly be fatal for carbide.

OK so how do we define rigid?

[LeeWay]

I think it may be easier to describe what is not rigid.
And again it is about what the machine is designed to do and it's size as well as speed and accuracy while doing that.
On the gantry in question, it might be able to machine aluminum up next to one of the gantry supports rather well if the spindle is strong, fast and we have very low backlash and run out.
If cutting full size material, you will not see the same cut quality across the entire span. Cut quality will diminish toward the center and get better when approaching the opposite support.

[wizard]

OK so how do we define rigid?

That is an interesting question, rigidity is acceptable when the machine can get the job done. That is probably not extremely helpful but in the end you the user needs to decide what sort of quality you want to achieve. Even quality has an individuals definition that might vary with respect to what a third party would call quality.

Now I may be jumping to conclusions here but I suspect that most people want a Machine rigid enough to produce good surface finishes. That would be followed up by a machine that is highly repeatable. In most cases absolute precision comes In last, at least after the first two.
The reason here is that routers by definition are not replacements for milling machines so you can’t really compete with a milling machine precision wise. However that do ant mean surface finish isn’t important. So people aim to make the machines as rigid as possible given the context of a “router”.

[tkms002]

That is an interesting question, rigidity is acceptable when the machine can get the job done. That is probably not extremely helpful but in the end you the user needs to decide what sort of quality you want to achieve. Even quality has an individuals definition that might vary with respect to what a third party would call quality.

Now I may be jumping to conclusions here but I suspect that most people want a Machine rigid enough to produce good surface finishes. That would be followed up by a machine that is highly repeatable. In most cases absolute precision comes In last, at least after the first two.
The reason here is that routers by definition are not replacements for milling machines so you can’t really compete with a milling machine precision wise. However that do ant mean surface finish isn’t important. So people aim to make the machines as rigid as possible given the context of a “router”.

OK Here is what I hope to do with it. I want to make non-ferrous sheet metal inlays( Aluminum, copper, brass) in wood(table tops, bench backs, etc). I don't need speed(within reason) but I do need the inlays to fit "perfectly" in the recess I cut.
That being said, how big of a gantry beam will "get er dun"?
Thanks