Fixed vs Moving gantry recommendations

[aarongough]

Hey all!
I'm working on the design for my first scratch-built machine. I already have some Thompson precision shafting and bushings so I am designing around these.

The shafts are:
1" x 36"
3/4" x 18"
1/2" x 9"

My question is whether anyone has recommendations about fixed vs moving gantry designs. I have been very impressed with Microcarve's A4 design, and I want to make sure that the new machine will be able to make deep cuts quickly in wood an plastics and be comfortable working in aluminium at lower feed rates.

My current machine is built from a kit and is flexy as hell, hence my hesitance to build another moving gantry design without considering the options.

I'm aware that I will give up a lot of work space with a fixed gantry design, but I expect my CNC machine to be able to make parts quickly, so in my mind the trade-off is worth it.

All opinions appreciated! I have attached a rough sketch of my proposed fixed gantry design, it's not a complete render by any means, mor of a brain-dump than anything.

[zool]

There are so many other factors than the gantry design in designing a CNC router to perform the operations you make highly generalized allusions to that it would be impossible to provide a reasonable response with any basis in fact or possibility.

Further, for anyone to make these voluminous design decisions for you would be an untoward and unnecessary expenditure of time on anyone's part, including yours, considering the time it would take you to interpret and correlate them into a unified whole.

There are more than enough builds detailed on CNC Zone for you to ascertain the design characteristics and performance potential that meet your initial hypothetical needs.

One clue though: Start at the tool tip.

Best of luck in your future endeavors.

[ger21]

Unsupported shafts are generally considered a bad idea, if you don't want any flex.

Other than that, though, it's really not that difficult to build a rigid moving gantry machine.

[aarongough]

Unsupported shafts are generally considered a bad idea, if you don't want any flex.

Other than that, though, it's really not that difficult to build a rigid moving gantry machine.

Thanks Gerry.
The unsupported shafts are one of the reasons I thought the fixed gantry design might be better. Longer space between the bushing centers in a moving table design would mean less leverage to flex the shafts.

If the general opinion is that for that size there's not going to be much difference between fixed and moving gantry then I'll likely go with the moving gantry, as the extra workspace is always good.

If the machine is not perfect that's still fine with me. Anything will be better than the machine I've got at the moment :-p Supported shafts can be the next version.

[cyclestart]

Longer space between the bushing centers in a moving table design would mean less leverage to flex the shafts.

Also there is a torque applied by a moving gantry, force x distance from y to x axis.
The drawing in this link explains better:
Do it yourself CNC router: Design Considerations, the Gantry
With 3 ft of unsupported rail this torque might be something to take into consideration.

In either design the rails have to support a weight, either gantry or table+work.
On the moving gantry the distance between cutter center line and x axis bearings is a constant which is some advantage. In theory this could also be accomplished on a moving table with stationary bearings and moving rails (easy to do but hard to do well).
Also a moving gantry allows a really well supported work table.
Otoh a fixed gantry is easier to build imho.

Either design is a compromise. Given the parts being used I'd go moving table. Keep in mind ger21 has built several routers whereas I've only built a work in progress that happens to be functional.

[epineh]

As somebody with a fixed gantry design, I would rather a moving gantry as the fixed one really gets in the way when you are trying to clamp a job in, I am forever leaning over and around the gantry to make parts. I know that a moving gantry will still get in the way but nowhere near as much as the fixed version.

Russell.

[cyclestart]

As somebody with a fixed gantry design, I would rather a moving gantry as the fixed one really gets in the way when you are trying to clamp a job in, I am forever leaning over and around the gantry to make parts. I know that a moving gantry will still get in the way but nowhere near as much as the fixed version.

Russell.

True.
I often hold down work with wood screws and to get access the table needs to be jogged around a fair amount. On some designs there may be points that are never fully accessible.

It may also be true that people who cut their teeth on vertical mills see every machine as an oversize C-clamp. When all you have is a hammer everything looks like a nail.

[epineh]

Lol yeah, I do know that as I had limited tooling at the time the fixed gantry was easier for me to build and make a nice solid gantry, have learned lots in the process and am now using the old beast to help make the new one Ironically with no gantry as such rather a cantilever design, so clamping work is going to be a piece of cake...if I ever finish it that is !

Cheers.

Russell.

[DonFrambach]

As somebody with a fixed gantry design, I would rather a moving gantry as the fixed one really gets in the way when you are trying to clamp a job in, I am forever leaning over and around the gantry to make parts. I know that a moving gantry will still get in the way but nowhere near as much as the fixed version.

Russell.

My fixed gantry router is very small (14x14) so I don't have this problem. However, I can see it being a pain to load and clamp material if the router were larger.

[epineh]

A big part of it is my own doing, as there is a servo encoder sitting right where you would lean against to load a job, and as I didn't have the right connector the wires are just kinda sticking out a bit...it is on the "to do" list