New CNC router build -- thoughts, comments, questions...

[twry84]

Hey all, i've been looking through the forum for a couple months now and decided to start a design of my own... i've never built anything like this before but being a cabinetmaker im confident in the build of the structure... i just have some questions about center of gravities of my components and hardware.

here are the specs

i want a cutting area of 26" x 74" (2' x 6' plus 2" on each axis to ensure a comfortable 2' x 6' cutting area)
i had initially wanted a 6" z axis but i think my design employs about a 5.25" z axis travel, which is fine with me.
im just going to be cutting mdf and wood with the machine but i would like to be able to cut aluminum without a problem.
im designing the machine to be built out of mdf with skate bearing/aluminum angle linear motion.

questions

im trying to design the machine such that when it milling there is minimal stress on the bearings. i assume you'd want to place the center of gravity of the entire gantry centered on the x-axis bearings??

i basically have a long and narrow cutting table (2' x 6'). i want to minimize the x-axis deflection as the gantry moves along the length of the table. ive been toying with the idea of utilizing a cabling system to correct the deflection. the cables arent drawn in my design but i would tie the front and back legs of the table together on each side... then tie each leg of the table to the overhanging table supports that run through the table ends. good idea? or too overkill?

attached are some pics of my design and the cabling system... any advice would be greatly appreciated.

also, ill attach a list of the parts ive semi-decided on

http://www.probotix.com/index.php?vi...&product_id=52
my gantry is looking like it will weigh about 50-70lbs and my router/router carriage will weigh about 10-12lbs will these motors suffice?

im thinking 3/8" 10tpi acme lead screws on all axis w/ with standard anti backlash nuts... i read some specs on the 3/8" lead screws and it looks like it'll handle the load just fine... is there a reason to upgrade to 1/2"?

thanks for the help!
-tim

[ger21]

One reason to use 1/2" screws is to minimize whipping of the screw at higher rpm's. 1/2" is much stiffer, but even 1/2" screws will whip. 10 tpi screws are not the best choice, as they will limit your speed. Most builders are using 1/2-10 or 1/2-8 2 start or even 1/2-10 5 start screws (5, 4 and 2 turns per inch). These will let the screws spin slower, which will minimize whipping, and also give you higher speeds.

[twry84]

does the cad/cam software or stepper driver boards allow you to run a 1/2" with one motor and 3/8" with the y and z axis? or should i not even bother and go all 1/2"?

[ger21]

It's the machine control software you need to worry about, and yes, most should let you mix and match. But just about every machine you'll see here uses 1/2", so I'd say stick to the 1/2" on all axis.

[twry84]

ok great, thanks gerry... ill check that off the list and just stick to 1/2"... what do you think of the motor choice for driving a fairly heavy setup but still wanting decent rapid speeds?

[ger21]

You probably should go with drives that can handle more than 24V, or you'll be disappointed with the performance. The Gecko G540 is quickly becoming the drive of choice among home builders.

And don't skimp on the Z motor, it should be the same torque as the others.

[twry84]

the gecko540 seems kind of expensive but ive read great things about it... im mostly only interested in 3 axis now... if i purchase the g540 can i disable the 4th axis? or do i just not hook up a motor to it? thanks

[abomb55076]

You dont have to even purchase the 4th motor if your not going to use it, Good luck!