Yet another 3 axis CNC Router Design/Build

[daveysapps]

Well I thought that I would try my hand at building a CNC router, and would like to post my current design status to see if anyone has feedback on my current machine design.

Specs:
X axis;
26" travel, 2525 rolled ball screw, 1200 ozin nema 34 stepper, direct drive stepper, 20mm guide rails

Y axis:
50" travel, 2525 rolled ball screw, 1700 ozin nema 34 stepper, direct drive stepper, 20mm guide rails

Z axis:
sourced z axis assembly 5 to 8 inches of travel.

All axis driven by Gecko G203V drivers using Mach3.

Welded steel 11ga 2.5" sq tube base (depicted in blue image below). Approximately 400lbs.



Machine table (depicted in blue in below image) comprised of 1.5"x3" 80/20 Al with 11ga galvaneal steel skin. Table assembly is entirely bolted and weighs approx 350 lbs. Transparent picture below as well.

Gantry is built of 1" thick 6061 plate for the vertical members with 1.5" x 3" 80/20 cross members. The bottom "drive" cross member is 1.5" x 4.5" 80/20. Approx gantry weight is 150 lbs with a Velox 5" z axis and PC892 router.


Let me know if you have any feedback, design concerns or questions.

I am very open to constructive criticism.

Thanks,
Dave

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[ger21]

1) The gantry is too tall. Drop is down a bit. The bottom of the gantry should be even with the bottom of your Z axis.
2) I wouldn't go any larger than 960 oz steppers, as those larger ones will actually be slower. Especially the 1700oz.
3) That Z axis with round rails will be the weak spot.

[daveysapps]

1) The gantry is too tall. Drop is down a bit. The bottom of the gantry should be even with the bottom of your Z axis.
2) I wouldn't go any larger than 960 oz steppers, as those larger ones will actually be slower. Especially the 1700oz.
3) That Z axis with round rails will be the weak spot.

1) Good eye on the gantry height, as there is an unnesssacary drop between the z axis and the bottom of gantry. I will address this.
2) I wouldn’t have guessed that the 960 steppers would have been faster, I will keep this in mind as the design progresses.
3) Another good observation on the unsupported rails. The z axis assembly is an off the shelf design from velox cnc. I am not a fan of the unsupported rails and will be looking for a different solution. I might have to build the z assembly.

I appreciate your observations and comments.

Thanks

[ger21]

2) I wouldn’t have guessed that the 960 steppers would have been slower. I wil keep this in mind as the design progresses.

I'm saying that the 960oz motors will be faster.
Generally, the larger the stepper, the lower it's usable top speed will be.

[daveysapps]

I'm saying that the 960oz motors will be faster.
Generally, the larger the stepper, the lower it's usable top speed will be.

Understood, I had a typo. I meant to say faster. Thanks

[fretman_2]

Wouldn't you want to move the gantry with two screws instead of one to avoid racking??

[millhouse_]

I wouldn't go for steppers anymore. 2010 ballscresw with 400watt iHSV60-30-40-48 Nema24 integrated servos. If you use a gearbox or belt drive you could possibly use 180 Watt iHSV57-30-18-36.

20mm ballscrews should be fine and you could replace the round rails on the z-axis with linear guides too.

For more rigidity you could add some vertical "wings" on the sideplates.

PS: I bought a 4Axis set 425oz-in nema 23 steppers for my build which will only run a short time until I upgrade to servos. Someone said that the 180W iHSV57 have more power than his 5.5nm closed loop nema 23 steppers. If you look on Youtube there are some heavy builds direct driven by the 180W version and it seems to work fine.




For inspiration you can lookup grabcad: https://grabcad.com/library?page=1&t...y=cnc%20router

[daveysapps]

I was hoping that with a 35" wide gantry and a bearing block span of 11.88", that I would get minimal racking. Perhaps I should increase the span of the bearing blocks to reduce the likelihood of racking. I would prefer to stick with a single ball screw to drive the gantry, but maybe that isn't realistic...

[cyclestart]

Wouldn't you want to move the gantry with two screws instead of one to avoid racking??

And the other advantage would be allowing a fully supported frame instead of being supported in 4 corners in benchtop format.
That said... with the materials listed this thing should be pretty rigid as is.
I used 16mm round rail on the Z and you can feel it twist when the collet is tightened, the Z is not a good place to compromise.

[daveysapps]

And the other advantage would be allowing a fully supported frame instead of being supported in 4 corners in benchtop format.
That said... with the materials listed this thing should be pretty rigid as is.
I used 16mm round rail on the Z and you can feel it twist when the collet is tightened, the Z is not a good place to compromise.

I am not too concerned about table deflection from the unsupported nature of the design. I did an FEA analysis on the table with a 300lb static load on the center of the table and there was only .0006" of deflection, so I am feeling pretty comfortable from that stand point.

That is x2 on the z axis rails, so I will certainly circle back on the z for a more rigid solution.

Thanks!

[ger21]

I was hoping that with a 35" wide gantry and a bearing block span of 11.88", that I would get minimal racking.

The rigidity of the total assembly will have more to do with it than the bearing spacing. If the entire bolted assembly is capable of flexing at all, then you'll likely see some racking.

I am not too concerned about table deflection from the unsupported nature of the design. I did an FEA analysis on the table with a 300lb static load on the center of the table and there was only .0006" of deflection, so I am feeling pretty comfortable from that stand point.

While deflection may not be an issue, you might see vibration during cutting, which may have an effect on cut quality.

[millhouse_]

I don't really like the classic old gantry style cncs with the large sidemounts and can't understand why people still keep building them. You could take a look at some overhead gantrys. Or something like these:





A straight forward keep it simple solution:


With z-axis rails mounted to the frontplate of the z assembly it is possible to get large travels.







PS: Sorry for spamming you topic.

[millhouse_]

PS: This one is by far one of the coolest minimalistic looking gantrys available.



On the same channel is another video of it with a hexapod. Alien...

[daveysapps]

The rigidity of the total assembly will have more to do with it than the bearing spacing. If the entire bolted assembly is capable of flexing at all, then you'll likely see some racking.


While deflection may not be an issue, you might see vibration during cutting, which may have an effect on cut quality.

Ok, I am thinking that I will start with a single centered ball screw drive for the gantry, but will make provisions to easily add a second ball screw (one on LH side and one on RH side) if the bolted assembly proves to not be rigid enough.

Good point on vibration and chatter, yet another oversight. I think that I am going to start with what I have, I can add additional stiffening members under the table, as I do have extra room.

Thanks

[daveysapps]

millhouse,

I understand the advantages of the designs that you are suggesting, but I am thinking that the tall gantry plate design will suffice for my needs. Perhaps there will be a second build in my future with this type of design.

The last video that you posted is one sweet looking machine. Thanks for sharing.

[daveysapps]

Update;

Reduced gantry side plate height by 2.25".
Designed z axis assembly with supported profile rails (6.5" of travel).

What are people's thoughts on a counterbalance for the z axis?

[millhouse_]

Maybe you should switch to 25mm linear rails for the long axis. With the rails mounted at the sides the whole weight of the gantry pushes against only 1 row of the bearingballs so using bigger ones would be better.

Your builds size is similar to my one (see attachment framesize 1000mm x 800m) and I used 25mm rails on all axis:

I don't think that you "need" a counterweight but if you want to implement one you could use gas springs like the ones in the video of the minimalistic gantry or just a spring, mounted from the z backplate to the frontplate.

[ger21]

20mm rails can handle several thousand pound loads. You won't be using 10% of their capacity.

Yeah, no need for a counterweight on a little Z axis like that.

[daveysapps]

Looking at the specs, I thought that 20mm rails would be adequate.

Thats x2 on not having a counterbalance for the z... good deal, one less component to design around.

thanks folks!

[pippin88]

I don't understand the benefit of using aluminum extrusion with steel bolted on the outside.

Just use steel.