Wanting to build a CNC

tlhiv wrote:

(I'm thinking 2 of the 3 motors will be running in parallel to one another on each side of the table as hopefully can be controlled by the same driver. I'm not sure if that's doable or not,

It would be very unconventional. I think you could make it work with the motors wired so that the A-phase of one motor is wired in series with the A phase of the other, and similarly for the B-phase. Wiring them in parallel (which would seem more natural) will probably lead to uneven motor heating and will probably really confuse the driver current feedback electronics.
I wouldn't recommend it.

A better solution (and as cheap) would be a cross-shaft to mechanically couple the two sides of the gantry as suggested by JT.

In this photo you can see the shaft that I tested with...



John

Here is the back view with the Y slide on the opposite side of the drive shaft.



BTW, I replaced the drive shaft solid bar with a thin walled tube to stop wind up on the shaft. I also replaced the lovejoy couplings with real CNC type of couplings.

John

tlhiv wrote:

My brother-in-law will be fabricating the table from carbon steel, but we were certainly wanting to use aluminum for the gantry (if possible because of the weight issues like you suggested). If you run a shaft across the table, what will keep it from getting "cut" during the cutting process. Should the shaft be outside of the cutting area? Sorry for the elementary questions.

It rather depends on your drive mechanism. I think JT has a belt running up each side of the table with the gantry bolted to it, then at one end of the table there is the motor, pulleys and cross-shaft, below the working surface level.

If the motors move with the gantry then the shaft is still in no danger of being cut by the plasma, is it will move with the cutting head. The only problem is that the location of the shaft will limit the maximum work thickness. You could always step the shaft up higher and then down again with toothed belts.

As there are no cutting forces with plasma (other than tube/cable stiffness) you might not even need a 2-sided drive. Flatbed plotters (much smaller, in general) use a wide bearing span on one end where the drive is and just have a simple wheel at the other end to take the weight.

andypugh wrote:

It rather depends on your drive mechanism. I think JT has a belt running up each side of the table with the gantry bolted to it, then at one end of the table there is the motor, pulleys and cross-shaft, below the working surface level.

If the motors move with the gantry then the shaft is still in no danger of being cut by the plasma, is it will move with the cutting head. The only problem is that the location of the shaft will limit the maximum work thickness. You could always step the shaft up higher and then down again with toothed belts.

As there are no cutting forces with plasma (other than tube/cable stiffness) you might not even need a 2-sided drive. Flatbed plotters (much smaller, in general) use a wide bearing span on one end where the drive is and just have a simple wheel at the other end to take the weight.

The X and Y motors are on the gantry on mine and the belt is attached to the rails at each end. The jack shaft is out of the way all the time because it travels with the gantry. I examined the plotter style and for me it would added too much to the Y rails to fit in my shop...

This is my Y axis drive



John

tlhiv wrote:

Not that I'm promoting buying eBay (or any product on there), but here is a package consisting of 3 of these Nema 23's with a 3-axis driver board and a 350W power supply. I know that this is not the Gecko G203 (or even 3 of them), but I'm wondering what one pays for when you get a more expensive driver board (or even 3 individual driver boards as opposed to a single 3-axis driver board).

I started off with one of the eBay kits and it is in many ways not a bad starting point.

I found the motors to be fine (I have spent money since on upgrade motors, but they are not actually a great deal better). The driver board I found to be a bit fragile. You certainly need to be extra careful with the power wiring side, and to not even think about connecting or disconnecting a motor with the drive powered up.

It is said that the more advanced drivers do clever things with mid-band resonance and switching to single-step at high speed as that works better than microstep. I can't say I noticed any difference with my drives in that respect, but then mine are not especially advanced.

One big difference is that the eBay driver boards are only good to 24V. JT said he was using 64V I think. More volts translated directly into more speed (and, perhaps more importantly, more torque at medium speed). The motor _current_ is limited by the driver, but at high speeds the drivers simply can't "push" the current past the motor back-emf. The more volts you have to push with, the later this problem appears.

For the price of one of those kits you can be up and running fairly quickly. It might even be all you need. There is an argument that you won't really know what you need until you have had the machine running for a while.

One drawback of the Chinese kits is that the documentation can be dreadful, and is occasionally for a completely different driver board.
see this thread, for example. www.linuxcnc.org/component/option,com_ku...0/lang,english/#4602

tlhiv wrote:

BigJohnT wrote:

The steppers from Automation Direct was the lowest cost item of the build at about $30 each. Looks like they are $39 now for a Nema 23 triple stack 276 oz-in holding torque stepper. At the bare minimum you need an X and Y axis. I have used a drag tip on mine once or twice on the hand torch. What you see today on my Z axis is not what I made my first cut with, but evolved to that after some experience.
John

Not that I'm promoting buying eBay (or any product on there), but here is a package consisting of 3 of these Nema 23's with a 3-axis driver board and a 350W power supply. I know that this is not the Gecko G203 (or even 3 of them), but I'm wondering what one pays for when you get a more expensive driver board (or even 3 individual driver boards as opposed to a single 3-axis driver board).

As Andy pointed out cheap is not always the best way to go. Do you have a link to the cheap drive? Nema 23 is the bolt pattern and shaft size not how much power capacity the motor has. Automation Direct has 3 different Nema 23 steppers with different power capacities. Keep in mind that holding torque is a rather useless number as it only reflects how much resistance it has to not moving as opposed to actually moving and moving something. Gecko has lower power drives that are the same as the G203v but won't handle as much voltage. Voltage means acceleration and speed which you need for plasma. A G251 is $69 and can handle 3.5 amps and 50 volts max for 175 watts of power per axis. It comes complete with terminal blocks and a heat sink.

The G203v will require a heat sink for higher voltages. Here is how I mounted mine... I know it is over kill but it was what I could scrounge up from the rubbage bin from some old computers.





I use flea bay all the time for certain things.

John