I want it all

[MrWild]

Although the mechanicals of a CNC machine are no problem (retired tool and die maker/gearhead/motorhead), I'm having a conundrum regarding the driver board. A friend is tossing me four steppers rated at 300oz.in., 2.5v., and 4.6a.

The machine I'd like to build would have a 12" x 8" capability and would machine aluminum and some mild steel. Because I've read so much about the need for microstepping to reduce/eliminate lost steps, I want to run a microstepper driver for three of four axis. All the designs I've seen for microsteps only go to 3a and all warn 3a is pushing the limits. What I was thinking of doing was to use one of three boards and upgrade them if that is possible. The three are:

Linistepper
PMinmo PICstep
Hobby CNC
(I considered the 3977 board, but I no longer have the dexterity to solder a chip like that.)

I'd upgrade them with IRF250 MOSFETs, and overwired power traces. Is this possible?

If not, how many in.oz would I lose powering the steppers at 2/3 their capability (300 in.oz.).

Precautions I've seen so far are to procure a parrallel port board, optoisolated BOB, and to take the time to do it right.

The optoisolated open source board at Pminmo's site is only three axis. Is there an easy way to adapt it to four axis?

Should I just go with the non isolated four axis board and make sure to have a sacrificial LPT installed?

Do I really *need* microstepping capability? Would the L297 7a. uni-polar driver at Pminmo's site work well enough?

The power supply I have in mind will have two 12v 20a transformers in series to up the volts to 24v before being full bridge rectified to DC. I don't have the formula handy but that should be ~30v. Besides a large capacitor (bank) to smooth the DC, what else is needed to make dependable and clean DC?

I've spent a week of nights reading threads here and only ask because I haven't seen the exact answers to these. I understand that, "I can't use a 3a. (or less) controller to control 4.6a." I'm just curious if it is possible to upgrade the really nice microstepping controllers for a bit more power.

Thank you in advance for your time answering my questions.

[H500]

Microstepping is important because your system might resonate (vibrate) and lose steps without it. It would be very difficult to fix, so you might end up scrapping the controller.

None of the low cost designs can be beefed up. Everything is inside the chip. They cannot be modified to support external mosfets. If you want to maximize your performance, your best bet would probably be the PicStep board. (assuming you have bipolar capable motors) It can handle more voltage and current than the other low cost designs.

Other than fuses, you don't need anything else in your power supply.

[James Newton]

I know this is late, but I wanted to comment for those who may read this later...

The linistepper is an open source design. All the stuff "in the chip" is available to experienced users to modify as they will. Full source code, schematic, and all other documentation is at the web site. An experienced user could re-program it to work just about any way you wanted. It could even be made into a chopper with a few hardware mods (not that I'm suggesting that, just pointing out that it is totally OPEN).

You CAN replace the drivers in the Linistepper kit with higher current units, but based on the current rating of your motors, you would probably have to run the Lini in full step mode to avoid excessive heating since the Lini is a linear driver. It regulates the current being sent to the motors and dissapates the rest resistivly through the drivers.

And you could run your motors with a stock Linistepper, but not at the full rated current of the motor. So your motors would work, they just wouldn't put out the full 300 oz/in.

But to get the most out of those motors, I probably agree with H500, you need a high current, microstepping, chopper driver and that means money. It may be that those motors are free for a reason.