[John.R.]

Alright, so here's the story. I am looking to create a PCB mill and I am having a bit of trouble in a few areas. I have on hand a set of three NEMA 17 bipolar 4 wire stepper motors http://www.adafruit.com/datasheets/12vstepper.jpg and I am looking to create a control board for a three axis mill. I have some L293D and ULN2803 chips laying around and I am wondering if it is possible to create a control board for the motors I have or if it would be better to use a different type of stepper. I have been trawling the internet for the past few days and have ended up with more headaches than answers to my questions, so I thought I would give this a shot. Also, any information on where a good place to acquire 8mm outside diameter threaded rod for linear motion control would be appreciated, along with info on where to acquire bits suited to PCB milling.

Thanks in advance,

John R.

[CitizenOfDreams]

A common trend in small DIY CNC machines today is the Toshiba TB6560 chip. There are several varieties of TB6560-based boards on the market; some work better than others. I'm currently using one from Sain Smart, $40 on eBay.

[HPbyGD]

CitizenOfDreams
Can you give some more info on your system ? I was wondering what other parts you needed to complete your system ? Do you have a blog in your build ?

Thanks GD

[CitizenOfDreams]

I didn't really make my machine from scratch, just bought it "well used" on eBay and rebuilt it. It is probably a Zen Toolworks 7x7" kit, or something very similar:

Zen Toolworks: Zen Toolworks CNC DIY KIT 7x7

Unless you already have access to a machine shop, you may be better off buying a kit like that instead of making everything from scratch. Even with a pre-packaged kit you will have plenty of DIY fun assembling the machine, tuning, modifying, experimenting...

As far as parts, here are the basic components of my machine...

- The frame. It's made of PVC plastic which is not the strongest material. As a result, the machine is not very rigid and cannot cut hard materials at a high feed rate or high depth of cut. It will cut aluminum if you use the right tools and take your time.

- The leadscrews. They come with a different thread pitch. Choosing leadscrews is a compromise between speed, precision and torque. My machine uses 1.25mm pitch screws, which allows - with the right motors - to move the toolhead as fast as 40ipm.

- The stepper motors. They come in several common sizes; many small DIY machines use NEMA 17 size. The motors I have are Altronics ALTN1728S56.

- The motor driver board. It's the interface between the control computer and the motors. It also passes signals from limit switches and handheld controllers ("pendants").

- The computer. If you want to use an inexpensive driver board, you will need an older computer with a parallel printer port.

- The software. For Windows, a common CAM program is Mach3. You can download a feature-limited version (500 lines of code limit) for free. There are many choices for CAD software; my personal favorite is CamBam. For PCB design I use FreePCB. I don't have a special program for isolation routing; I do it semi-manually in FreePCB and CamBam.

- The spindle. Some people use a Dremel or a similar rotary tool as a spindle. I personally could not make them work, they had too much runout. My spindle only goes as high as 8,000RPM which is not ideal for PCB milling. You will probably want something faster, but it still needs to be a real spindle with real bearings, a real metal body and a real tool chuck.

- The tools. The simplest trace isolation tool is a V-cutter (as cheap as 2-3 dollars each). There are more sophisticated and more expensive bits too. All you can do is try them and see which one works best for you.

I don't do blogging, just lurk here on the forum occasionally. Ask your questions, I will be glad to share whatever little experience I may have.

[doorknob]

Do you have a link to the specific spindle that you are using?

[CitizenOfDreams]

It's Electrocraft E240 motor with ER11 chuck. Permanent magnet DC motor with brushes. 90 volts, 200 watts, 8000RPM.

https://sites.google.com/site/cncdiyorg/e240-spindle
Attachment 183702

If I was to buy a spindle now, I would probably try this Chinese one from eBay (search for "ER11 spindle"):

Attachment 183704

It's supposedly rated at 12,000RPM.

[CitizenOfDreams]

Oh, and a couple of essential parts I forgot to mention...

- Power supply for the steppers. Three NEMA 17 motors will need a hefty 12V 12A power supply.

- Variable speed drive for the spindle (and, possibly, a power supply for it).

Here is my old stack: stepper drive board, spindle drive, 12V power supply. The blue board in the photo has since been replaced with the red one I posted earlier.

[blinkenlight]

Just a note on the "ER11" spindle - I'm pretty sure I'm using that exact model (even the support is the same) which is not that surprising considering I have a Chinese mill, and it works fine for PCB jobs...

[Jay C]

Is the intent to "roll your own" or are you more interested in having a PCB mill that you can use to make your own PCBs? For Nema17 I would look at the the Pololu stepper motor drivers and work on a backplane interface for them. Look at how repraps have leveraged these as the plug-n-play drivers for 1000's of machines.

As for the 8mm rods, any place that sells reprap parts will have these, but so does Amazon.com via Smallparts.com. Fastensal also carries metric and are in most metros.

[CitizenOfDreams]

Blinkenlight, have you disassembled your "Chinese spindle" yet? I'm wondering what size of bearings and brushes it uses, since they will need to be replaced sooner or later. How is the chuck attached - interference fit or set screws? Is the fan cage well balanced (the one on my Reliance spindle was way out of balance)?

[John.R.]

I guess what I was trying to get across is that I do not wish to buy any sort of premade driver or board. I am looking to create a control board for a four wire bipolar stepper driven 3 axis CNC PCB mill from separate components. Ideally the interface is going to be a parallel port and the software is LinuxCNC. Is it at all possible to take a chip such as the L293D or a similar IC and interface it more or less directly with a parallel port?

[doorknob]

I guess what I was trying to get across is that I do not wish to buy any sort of premade driver or board. I am looking to create a control board for a four wire bipolar stepper driven 3 axis CNC PCB mill from separate components. Ideally the interface is going to be a parallel port and the software is LinuxCNC. Is it at all possible to take a chip such as the L293D or a similar IC and interface it more or less directly with a parallel port?

Although I have not done it, I believe that the answer is "yes", you can drive a bipolar stepper via an L293D from LinuxCNC via the parallel port. It will not be a particularly good or fast driver (for example, no microstepping, no high-voltage, current-limited driving), but you should be able to get it working. You will need to configure either stepgen type 7 or type 8 (I'm not sure what the difference is, see STEPGEN for starters) to drive the H-bridges, rather than the more common step-and-direction interface.

[CitizenOfDreams]

Is it at all possible to take a chip such as the L293D or a similar IC and interface it more or less directly with a parallel port?

That is precisely how popular drive boards are made. The driver chips are connected straight to the paraller port output signals (well, not counting the optocouplers that protect the parallel port from mishaps).

You can use pretty much any driver chip in that manner. But I suggest that you choose something more advanced like TB6560. It will move the steppers much smoother, with higher speed and higher resolution - which is essential for precision work such as PCB milling.

P.S. Just looked at the L293 datasheet... It's just a primitive H-bridge. You will need some "glue logic" to convert STEP and DIR signals from the CAM software to quadrature signals for the H-bridge.

[John.R.]

Although I have not done it, I believe that the answer is "yes", you can drive a bipolar stepper via an L293D from LinuxCNC via the parallel port. It will not be a particularly good or fast driver (for example, no microstepping, no high-voltage, current-limited driving), but you should be able to get it working. You will need to configure either stepgen type 7 or type 8 (I'm not sure what the difference is, see STEPGEN for starters) to drive the H-bridges, rather than the more common step-and-direction interface.

Thank you. This is very helpful to me. So now that I know it is possible, the next question is how to work with HAL and stepgen, because I have absolutely no idea, haha. At least I know where to start looking.

[John.R.]

So I did a bit of looking around and found the A4988 ChipA4988SETTR-T Allegro MicroSystems, LLC | 620-1343-1-ND | DigiKey Which looks like a better solution to the L293D. It looks to me like I would be able to interface that directly with the step/direction pins of the LinuxCNC parallel port configuration. Has anyone else worked with this chip that could confirm or disprove my thoughts?

[doorknob]

So I did a bit of looking around and found the A4988 ChipA4988SETTR-T Allegro MicroSystems, LLC | 620-1343-1-ND | DigiKey Which looks like a better solution to the L293D. It looks to me like I would be able to interface that directly with the step/direction pins of the LinuxCNC parallel port configuration. Has anyone else worked with this chip that could confirm or disprove my thoughts?

That would be a much better choice than the L293D.

They are used by many folks to drive the stepper motors in small 3D printers, for example.

See Pololu - A4988 Stepper Motor Driver Carrier for a driver board based on the A4988 that is commonly used.

The IC package is a bit of a challenge to solder if you don't have experience with surface-mount devices, though.

[John.R.]

I think that will probably more or less take care of the driver part. Now I just need to figure out where to get and what type of bits to use for PCB milling.

[CitizenOfDreams]

I think that will probably more or less take care of the driver part. Now I just need to figure out where to get and what type of bits to use for PCB milling.

My personal choices...
For trace isolation:
- 30-degree V-cutters from eBay (dollar a piece shipped from China);
- 45-degree PreciseBIT cutters from Think and Tinker (15 times more expensive);
For large area copper removal:
- 1mm and 2mm "copper rubout" stub endmill from Think and Tinker;
For PCB cutting:
- any "chipbreaker" carbide router, 2-3mm in diameter.

[CitizenOfDreams]

So I did a bit of looking around and found the A4988 ChipA4988SETTR-T Allegro MicroSystems, LLC | 620-1343-1-ND | DigiKey Which looks like a better solution to the L293D. It looks to me like I would be able to interface that directly with the step/direction pins of the LinuxCNC parallel port configuration. Has anyone else worked with this chip that could confirm or disprove my thoughts?

I haven't tried that chip, but judging by the technical specs it would be a good choice for a small stepper. But the board thermal design might be a bit tricky due to the small package size, high currents and high power dissipation involved. If this is your first project of this magnitude, I would stick with SIP chips with external heatsinks.

[blinkenlight]

Blinkenlight, have you disassembled your "Chinese spindle" yet? I'm wondering what size of bearings and brushes it uses, since they will need to be replaced sooner or later. How is the chuck attached - interference fit or set screws? Is the fan cage well balanced (the one on my Reliance spindle was way out of balance)?

Disassembled, no, I haven't. Sorry but I don't intend either any time soon, as long as it works fine... The funny thing is, I even received mine with a spare set of brushes - but to be fair, mine came installed on the mill I've bought (at least it's a clue that spare brushes might actually be available somewhere). I don't do enough milling to be able to tell you how long one set lasts. The ER11 chuck is attached with a single set screw, it didn't give me any trouble yet; and the fan shows no visible signs of imbalance - the whole thing runs really nicely and quietly, no significant vibration or anything. All in all, it's what I would get again if I needed something a bit more pro than a simple dremel.