L shape gantry

[jdlvfr]

Hello all,

I'm considering building a router to cut mostly aluminum, overall dimensions 800mm * 1000mm for 500mm * 750mm cutting area and 250mm Z travel.
So first question is about gantry, would a L shape gantry made with 45*180mm profiles be a bad idea?

31019

Eventually with angle brackets to add rigidity:

31020

The idea behind this is to have the rails parallels and to simplify the Z axis building while keeping a good distance between the rails.

Thanks
joël

[EddyCurrent]

I made an L shaped gantry and it works very well but uses aluminium box section with a flat plate on the front.
See here; http://www.mycncuk.com/threads/6565-Ready-Steady-Eddy?p=55076#post55076

[Voicecoil]

You'd certainly need those brackets to make something that's rigid, going for a thicker extrusion would help. I don't have any figures for 45x180, but looking at the moments of inertia of 40 x 160 they're 53x & 731y, whereas an 80 x 160 it's 362x & 1232y - 6x stiffer across the smaller dimension and nearly twice as stiff across the larger.

[JAZZCNC]

You'd certainly need those brackets to make something that's rigid, going for a thicker extrusion would help. I don't have any figures for 45x180, but looking at the moments of inertia of 40 x 160 they're 53x & 731y, whereas an 80 x 160 it's 362x & 1232y - 6x stiffer across the smaller dimension and nearly twice as stiff across the larger.

Don't even need 80 x 160 for a machine this size, 80 x 120 is massively stiff enough. I fit this size on medium-duty 8x4 machines and it makes for a very rigid gantry.

[jdlvfr]

On reflection it seems that the squares should not be very effective against torsional forces. Thicker profiles will certainly be more effective in this respect.
Thanks!

[jdlvfr]

After an extensive reading of this formidable mine of information, I still have a question as to the choice of the position of the rails on the gantry, both on the face, bottom and top, or on the face for the bottom and on the top for the top...
If I have understood correctly, the rigidity of the 3 solutions is roughly equivalent.

But I did not find any information about the perpendicularity adjustments of the Z plate in relation to the frame. The solution with the two rails on the face makes the adjustment around the X axis complicated but with aluminum profiles and a little care in the realization is this adjustment necessary?

Thanks

[JAZZCNC]

After an extensive reading of this formidable mine of information, I still have a question as to the choice of the position of the rails on the gantry, both on the face, bottom and top, or on the face for the bottom and on the top for the top...
If I have understood correctly, the rigidity of the 3 solutions is roughly equivalent.

But I did not find any information about the perpendicularity adjustments of the Z plate in relation to the frame. The solution with the two rails on the face makes the adjustment around the X axis complicated but with aluminum profiles and a little care in the realization is this adjustment necessary?

Thanks

Ok, I think I can say without anyone objecting that I must be by far the person with the most experience of the "L" shape gantry setup with probably well over 100 machines ranging from small to 10 x 5 machines using this profile design, which I believe was the first to use.

During this time I have located the rails in various positions ranging from both rails on the front face, both on the top, and bottom faces to 1 on the front and 1 on the top, and I can honestly say that it makes absolutely NO DIFFERENCE to how a normal machine performs or cuts in everyday use.

By normal machine, I mean a moving gantry design router used for cutting wood, plastics, and light cutting of soft metal like aluminum and brass. So anyone who tells you differently is talking bullshit and doesn't have any experience with this design.

Where it does make a difference is the setup and adjustment of the rails, OR, to help with the design and size of the machine.

The most space-saving design is the one with rails on the top and bottom faces, but this does take more setting up as you have to dial in 2 rails, that said it also allows the most adjustment for alignment and setup as it gives more freedom of movement of the rails.
This arrangement also has a few very minor advantages if you are looking for the absolute best strength and are happy to deal with the rail alignment.
#1 The distance between rails is greater so supports the Z-axis more.
#2 The distance between Z-axis and the gantry ball-screw/nut is shorter.
This said it also has a disadvantage in that the Z-axis hangs lower so you will need either taller gantry sides or taller frame sides if the gantry is sat directly on the bearings.

The least space-saving is both rails on the front face as this puts the spindle further forward, however depending on machine design regards adjustment, ie gantry sides, etc then it can be the easiest to set up as both rails are on the same plane.
This does mean the perpendicularity of the z-axis requires that the machine as adjustments elsewhere, ie: on the gantry bearing mounts or in the Z-axis.

The middle of the road and the one I use the most is 1 rail on the front face at the bottom and 1 rail on the top. I use this setup because it gives me a balance of easier setup and provides higher clearance under the gantry.

However, I wouldn't hesitate to use any of the setups if it suits the machine design best and in all 3 cases, performance is more or less the same.

You do however have to accept that because you don't have machined reference edges to fasten the rails against that there is a certain amount of tweaking and adjustment that goes into setting up the machine. This profile setup is NOT bolt-on and forgets regards rail alignment.!

So again I'll repeat what I say to new builders, ADJUSTMENT, ADJUSTMENT, ADJUSTMENT, is the secret to getting a good machine, so build in as much as possible to make this easier and be prepared to tweak for days, weeks, or even months if your chasing high tolerances.

BUT for an average wood router with users expecting sensible tolerances then it can be as easy as just bolting them on, and checking straightness and parallelism if the rest of the machine is built to a decent standard.

Hope this helps.

[jdlvfr]

for sure this helps, thanks a lot Dean!
Now back to the CAD...