[samsagaz]

Hi guys, im planning to build a machine using Steel Frames, i think that the Y/Z will be too heavy so i want to move the table piece instead the Y.

i finished some drawings, think that if use 200mm support will be rigid enough.

What do you think about this two images? will be OK to place the Y/Z at center or will be better to place it like in the img 2?

diameter of rails will be 20mm rigid and as i told you will use 2 x 100mm support together under the table.

img1



img2



of course design will need to be changed if use img2 instead img1

Thanks in advance,

[Walky]

That support scheme won't work ok, the bearing blocks must have a greater separation or the table will bounce (it will worsen the farther you cut from the blocks).

Once the table design is done you can check the movement range and use it as a reference for the gantry position so the bit is always over the table itself. This will be close to your first image, as in the second image almost half the table is out of reach, and you have 50% table and 50% empty space under the bit depending on the position of the table.

[samsagaz]

Hola Walky, i know what you say, so if i want 1000x1000 table i will need like 2000mm guides to be the most rigid!




thats amazing :/

[samsagaz]

Is possible to calculate wich is the safe distance between the bearing blocks to build an good precision machine?

[Walky]

Maybe a little less, but close to 2 mts (the bearings don't need to be at the absolute extremes of the table, the thing is not to leave much of the table hanging). Depending on the table material and thickness, it can bounce at the center too.

That's one of the reasons most CNC Routers of this size are of the moving gantry type, most of the time the fixed gantry system takes just too much space on big machines.

[Walky]

Is possible to calculate wich is the safe distance between the bearing blocks to build an good precision machine?

To be honest, it would be much better to use a moving gantry design with nicely spaced bearings on the gantry sides. For a long gantry like that it's very important to reduce racking, unless you can use dual screws (one on each side) which would completely eliminate racking.

[samsagaz]

Thanks again Walky.

First design was made using moving gantry, but i im scared about the weight of the gantry as i will use 2510Ballscrew, SBR20, the Spindle, plus the steel frame think that will be very heavy.

Maybe can purchase a bigger stepper with more torque.

[Walky]

What size steppers do you have?

[samsagaz]

i have

3 x nema23 5.1V - 3A - 7mH - 382oz-in

and purchased

4 x nema23 2.5v - 3.5A - 2.8mH - 381oz-in


maybe can use two screws as you recommend.

[samsagaz]

Dual motor in the X axis will be great i think

Problem will be if one of the motors fail :S

[Walky]

As long as you use good drivers and cables the chances of one of them failing should be minimal (otherwise nobody would use dual drive).

You should be able to use those motors, I'm pretty sure even a single 381 oz-in motor would be able to move that gantry pretty easily. Your max acceleration will suffer with a heavier gantry, but it will probably still be ok.

Of course, two motors will accelerate that weight more easily.

[samsagaz]

As long as you use good drivers and cables the chances of one of them failing should be minimal (otherwise nobody would use dual drive).

You should be able to use those motors, I'm pretty sure even a single 381 oz-in motor would be able to move that gantry pretty easily. Your max acceleration will suffer with a heavier gantry, but it will probably still be ok.

Of course, two motors will accelerate that weight more easily.

Ok drawing the two motors design now

[samsagaz]

well, looks too expensive use two motors +4 sliders + 2 ballscrews :/

Think that using 2 sliders and 2 ballscrews will be not precise enough.





will try to find other way to design it.

[Walky]

You can use smaller blocks (the price of 2 SBRXXUU blocks is similar to one SBRXXLUU), and keep a good separation between them.

I'd also try lowering the gantry, it will increase rigidity and you'll most likely not need so much Z axis clearance anyway. A diagonal bar at the gantry sides will help stiffen things a lot (so each side looks like a right angle triangle).

[samsagaz]

but use SBRXXLUU will not give me more rigidity?

how much Z Axis clearance will be OK, i was thinking about a little less than 10cm, my ballscrews are just 230mm ones, something like 66mm machined + 109mm Travel + 56mm of the Screw

[samsagaz]

for example in the same distance, first pic will be better than pic2? or will be the same and will be spending money for no improve rigidity¿

[Walky]

In pic 1 the weight is distributed among twice as many balls, this should help reduce wearing on the rails/bearings over time. It might also give a better "fit" over the rail (if you modify them to be adjustable, very important!, otherwise they're worse than the smaller blocks). BUT, it doesn't mean you need to use them: 2 short blocks are enough, and rigidity depends on many, many more things. You could even be using $2000 rails but they would be no good if the overall design is flawed. Good block spacing, not excessive Z clearance, not excessive overhang (ideally your gantry should be close to balance in the hypothetical case of it being put over a pivot right between the blocks, not fall forwards of backwards), and a lot of other considerations, some specific to each particular design.

How thick is the Z plate?

[samsagaz]

Thanks a lot Walky, dont know yet about the Z plate, was thinking about use 6,3mm steel.

But now i changed the design, Z Plate 6,3mmthink that will be ok, coz max length plate is 220mm

can purchase 6,3 - 7,9 - 9,5mm

[samsagaz]

Im using 2 LUU Rails, but will order some single ones to check wich works better.

Now looks better the design, right?

and i get less weight now. using 90x50x2mm structural steel

[samsagaz]

upgraded the Z