My CNC

[Boomslang]

Hi All

I've been busy drawing the CNC I want to build, and want to hear what you guys think.

Bed Size: 1400x1000mm
X-Axis Movement: 900mm
Y-Axis Movement: 750mm
Z-Axis Movement: 250mm
Need accuracy of +-0.2mm.

Still have to finish the Y-car and Z-Axis. The bearing blocks also need some supports, and I already have a few changes in mind to what I have drawn so far. The bed weight is 70kg, and the gantry with Z will be about the same weight . I might also add a sliding knot setup on X if racking is a problem.

X and Y will run on ballscrews, not sure about Z or any of the stepper sizes. Plan is to cut rifle stocks on it, and maybe use it later on to build a CNC out of 10mm alluminium plate.

Pics:







Thanks,
Darius

[Boomslang]

A few more views for clarity. My main concern is the bearings. With 16 per axis the load should be carried by at least 8 of them at a time, giving around 15kg static load per bearing on the gantry. Do you guys think I am asking too much from the skate bearings?

[The J]

Are you planning on making all of this out of MDF? If so, your gantry is going to flex too much in the middle from front to back.

[RedskinsJBS]

I have to admit that this is a neat take on a moving table design. However, I see one fatal flaw in the design.

The primary draw to fixed gantry / moving bed designs is that they produce better accuracy than a moving gantry design could for the same money.

The draw to moving gantry designs is that they maximize the cutting area that a certain table size can provide.

You have designed a machine that uses the support structure from a moving gantry machine and the machine volume from a fixed gantry machine... kind of the worst of both worlds.

If you are set on maximizing your cutting area for a given space, I would re-design your machine into a moving gantry style machine. This can come with a loss of accuracy, and it may be a challenge to make a gantry that is stiff enough to provide the .2mm precision that you are looking for.

However, if you can sacrifice the added space, go with a moving bed design where the entire bed is supported all the way around. Larger distances between the bearings will keep everything in line better. And removing the cantilever from your table can only help. The table you have designed is made out of wood, not 3" steel. Placing wood on a cantilever is only asking for trouble.

I'm subscribing to this thread because I really would like to see how this turns out.

Good Luck!

Jay

[Boomslang]

Hi

Thanks for the feedback guys.

J: 4x alluminium angles + 4x16mm thick mdf boards + torsion box on the rear Y rail... thought that would be enough to prevent flex in the middle. I can always add another torsion box on the front rail, or will that not be enough?

RedskinsJSB: It's actually a moving gantry design ..... just did not get around to give the bed some legs. Do you still feel the same with it the other way around? What do you mean by removing the cantilever on the table?

Thanks for your time.

Cheers,
Darius

[The J]

Darius,

Yes, over time...even with 2 inches of MDF, it will give out. Is there any particular reason you are wanting to do a central mounted router instead of the traditional?

I would go with a torsion box with 3/4" MDF outsides and 1/2 to 3/4" plywood on the inside. I would cut the ribs at 3/4" thick, notch them 5/8" deep, then sandwich that together.

[Boomslang]

Maybe when I have this one built, I'll be able to cut the wood like your drawing. Nice idea.
I think a torsion box is stiffer... might be wrong though.

[RedskinsJBS]

Darius,

In the renderings that you posted, the table is floating and the gantry is on the ground. But, if the table becomes the fixed part and the gantry begins to move, this is a whole different ballgame. I had led myself to believe that the table floating way out yonder with the pictured cantilever would harm your precision. But if the table is going to be supported around its base, I say go right ahead with your design... be sure to cut replacement parts as soon as you get the thing working...

[Boomslang]

Darius,

In the renderings that you posted, the table is floating and the gantry is on the ground. But, if the table becomes the fixed part and the gantry begins to move, this is a whole different ballgame. I had led myself to believe that the table floating way out yonder with the pictured cantilever would harm your precision. But if the table is going to be supported around its base, I say go right ahead with your design... be sure to cut replacement parts as soon as you get the thing working...

When I read your first post, I immediately saw why you thought it was moving table, my fault.

The table leg structure will be designed to further stiffen the bed.... it's just that the gantry that is very heavy. I don't see a way to make it lighter without losing stiffness.

[beermkr]

I looks like you are using 1/2 MDF for the structure. I think that you may have some strength issues with that. You might look at using MDO in 3/4 inch. Stiffer and a little lighter than straight MDF.

R/

[RomanLini]

... My main concern is the bearings. With 16 per axis the load should be carried by at least 8 of them at a time, giving around 15kg static load per bearing on the gantry. Do you guys think I am asking too much from the skate bearings?
...

As a best case the gantry weight is resting on the top 8 skate bearings, so the weight is /8 or (like you said) about 10 to 15kg per bearing, which is already high for skate bearings if you want long life.

But once you factor in cutting and acceleration force the weight is shifted and that weight per bearing could double or add in some binding from an imperfect alignment and it can go very high.

Maybe you could double up the bearings at least on the top 8 or look at other options?

[Boomslang]

Thanks for the input guys. I'm working on v1.1 and will post some renderings when done.