Using Marlin’s auto leveling for PCB milling

As I’ve stated earlier, I designed my 3D Printer with light milling operations in mind. One of the activities I intended to tackle was the subtle art of  PCB milling. It seems pretty straightforward at first: get a new/raw PCB (printed circuit board), use something like FlatCAM to generate G-Code from your Gerber files, get yourself a good milling bit and you’re done. While this is true for coarser trace widths and packages, you’ll soon run into trouble when milling something as fine pitched as an HSSOP-28 (like I’m doing for this little fella). Where 3D printing is fairly robust to submilimeter bed misalignments, milling widths of under 14mils is an unforgiving process: cut depth variations of even 0.01mm can ruin a trace and a board. Naturally, trying to mechanically level the board is the first step. However, larger boards may be somewhat bent, which can be hard to compensate (or flatten).

I’m currently using Marlin as my printer’s firmware. After watching the advances on automated bed leveling (or tramming, as some call it), I figured I could profit from such feature to accurately mill PCBs. However, after some tests, I noticed that only a 3-point leveling wasn’t enough. While the plane of the PCB is identified, curvatures or slightly bent PCBs can’t be properly compensated. Luckily for me, the masterminds behind Marlin are now implementing a grid-based leveling, that does just what I needed. It took me a couple hours and some minor tweaks to the Marlin firmware to get it running. First, checkout the result:



To get all running, I started by printing some mounting clamps for my Dremel’s flex shaft. Carrying the Dremel itself was an option, but the flex shaft is much lighter and equally stiff.

Setup for PCB milling. Quite... ahm... Messy.

Setup for PCB milling. Quite… ahm… Messy.

Next step was to convert my Z endstop into a “dual crocodile clip” configuration, as in the picture:

Crocodile clips on the board and on the tool. When both touch, bingo: Z endstop was hit.

Crocodile clips on the board and on the tool. When both touch, bingo: Z endstop was hit.

This allows for an accurate on-the-spot Z endstop. After that, on the software side. First, in Marlin’s Configuration.h, I enabled and configured the leveling. Around line 407:

Another important thing to change: since the “extruder” (the milling tool) and the probe are the same, I also changed, in Configuration.h, line 457:

Last change. Since you want to mill a PCB, you usually will be cutting in the negative Z space. This means the tooltip has to be able to go to, for example, Z=-1. This won’t happen by default, because the printer “thinks” there is no space below Z=0 (and usually, there isn’t). So, in Configuration.h, give the printer some room in the -Z direction. Be careful! This modification can lead to several headaches once you forget it is in place.

This should suffice. Essentially, we’re tricking Marling into probing only “inside” the size of the board. The downside (for now) is having to change and recompile this for different board sizes. By default, however, Marlin needs to home the X and Y axis before allowing a G29 (auto-leveling) to be performed. Even though this makes sense in the grand-scheme of things, it was a nuisance in my case. I wanted to be able to place the probe in any arbitrary point on the bed/PCB and start the leveling. For that, all I had to do was remove the “homing verification” on Marlin_main.cpp:

Done! Now I can manually carry the mill/tool/extruder/probe to the desired origin (where I want the board to be milled). There, I just need to issue the following commands:

After the process ends, your preferred GUI/interface will spit out the surface’s correction matrix calculated by Marlin:

Pronterface, showing the calibraton results on the right.

Pronterface, showing the calibraton results on the right.

That’s it. Now just print normally – the compensations for the bed’s irregularities will happen automatically on the background. The calibration data is lost at each reboot (though an EEPROM save can be enabled).

’til next time.

Chronicles of my 3D printer, part 2: the electrical Pandora’s Box

Having finished the mechanical assembly of my printer, it was time to assemble the electronics to get everything moving.

As I stated in the previous post, my intention to build a large-volume printer/mill made me choose NEMA 23 stepper motors for the X and Y axis. Larger motors would be capable of handling greater forces and more aggressive acceleration profiles, that regular NEMA 17s would not. After some research, I purchased 2 Applied Motion’s HT23-601. With a holding torque a little below the 2N.m, I was (and still am) pretty sure that those motors would be overkill (which isn’t a bad thing). For the Z axis I got 2 Applied Motion’s HT17-275 – good quality NEMA 17s that would do the job.

The thing with the larger NEMA 23s is their current requirements. The maximum current for the HT23’s bipolar parallel arrangement is rated around the 4.3A. Regular 3D printer stepper drivers, such as the A498 or the DRV8825 handle up to 2.2A (with active cooling!), so they are way off the range. Also, keeping the ICs operating constantly at their max is guaranteed to greatly reduce their lifespan. Operating the NEMA 23s on lower currents also wasn’t an option, simply because I wouldn’t be able to make the most out of them. At that point, I thought I’d have to get industrial stepper drives that would leave my pocket echoing. Luckily enough, Google (oh, my dear) pointed me to the Powerlolu, a 10A stepper drive that still falls in the $50 range.

After getting the Powerlolus and my RAMPS 1.4 (the printer’s control board), the wiring-fun started. I quickly noticed that, even for small test setups, the amount of wires hanging all over the place was increasing rapidly. Besides being annoying, messy wiring is the best spice for cooking your components. In order to prevent that, I mounted everything in a proper enclosure box, and tried to keep the wires, buses and connectors as neatly organized as possible. The result, after dozens of boring hours, was worth it:

Nice.

Main electronics box. Bottom, the 12V/33A supply. Middle/Right: Stack of 3 Powerlolus. Top/Left: RAMPS.

For the record, there goes a somewhat simplified wiring diagram of the box’ contents. I’m using a 12V/33A power supply, so I added a 5A circuit breaker on the mains’ power input to avoid burning down my house if anything went wrong.

Colorful.

Simplified wiring diagram of the printer’s electronics. Some wires were omitted or labeled to keep the drawing understandable.

 

As the picture below displays, the components (motors, hot end, heat-bed, etc.) are attached via connectors mounted on the side of the box. This makes the connection more robust to vibration and eventual pulls. For the stepper motors, I used 3 4-pin circular connectors (the four, from the bottom). They resemble XLR connectors with extra pins and screw caps, and I’m not really sure how they’re correctly called. The heat-bed also uses such connector, in it’s 2-pin version. Such connectors handle up to 5A per pin. Though they can’t support the Powerlolu’s full 10A, they were (sadly) the most cost-effective solution I found. The bed’s thermistor and the endstops are connected to the RAMPS via P2 audio jacks. Lastly, a DB-25 plug handles all the wiring for the extruder and hot-end: stepper motor wiring, hot-end cartridge, thermistor and fans.

Connectors for the printer's parts.

Connectors for the printer’s parts.

Finishing up, a sideways view of the closed box, showing the USB plug, a fan, the power switch and the mains’ power input.

Everything has two sides.

Ready to rumble.

’til next time.

Printing something useful, for a change

After lots of bad prints, random parts, new extruders, a couple of bears and a Darth Vader, it was about time to print something with at least a bit of practical use. A bit. I’ll let you be the judge of that:

 

The hook... Holder... Thingy.

The hook… Holder… Thingy.

The making (and usage!)

 

The sad part is that I didn’t come up with that idea on my own. But then again, that’s what the internet is for. I guess.

’til next time.

Chronicles of my 3D printer

So, just a little throwback. I’ve always wanted to build a 3D printer (or any CNC machine, for that matter) on my own. Around two years ago, I’ve decided to take that desire seriously, and started doing some research on designs for 3D Printers and CNC mills. After some googlin’ I set myself with goals of building a machine that:

  1. Had a large working volume (around 35 x 35 x 30 cm)
  2. Enabled fast prints (80mm/s and faster)
  3. Was cheap-ish (I live in Brazil, and getting proper mechanical/electrical components here is insanely expensive)
  4. Enabled both 3D printing and milling (at least of softer woods and plastics)

So, pretty much a godmode machine – what every engineer desires, but only unexperienced newbies strive for. The last point there is specially tricky. Milling and printing are very different activities, that shape machines in very different ways.

Milling usually requires more robust and rigid machines that can handle the high (read: tremendous) opposing forces applied on the end-effector (the cutting tool). Cutting speeds are relatively low, and depend on the material that’s being milled. Arrangements with a moving portal carrying the Z gantry are quite common, since they allow the workpiece to remain stationary, and provide a stable frame. Such configuration, on the other hand, means that A LOT of mass has to be constantly moved around: the motors for the Y and Z axis, the portal, all mechanical components attached, and so on – which constraints aggressive acceleration profiles. Partially because of that, CNC mills are normally leadscrew driven. Most of such leadscrews present pitches in the 2 to 5 mm range, allowing increased torque transmission and movement precision, but reducing the overall speeds. From the beginning of this project, I knew that I would be working with a 12V power supply and NEMA 23 steppers – the most cost-effective solution available to me. From the graph below it’s easy to see that, even with such a 5mm pitch leadscrew, achieving speeds of around 80-100mm/s on that configuration is hard/impossible. Leadscrews were a no-go.

Typical NEMA 23 stepper motor (torque vs. rotation speed)

Typical NEMA 23 stepper motor (torque vs. rotation speed)

3D Printing behaves quite differently. During a print, the end-effector (hot-end) moves practically freely, with almost no opposing forces whatsoever. Machines are built with a minimum rigidity to prevent vibration or backlash in the moving parts, but the goal usually is to reduce the inertia of the moving elements. Classical setups include floating Y axis, and are often built out of off-the-shelf screws and 3D printed parts, MDF sheets, etc. Timing belts are the default means for movement transmission, enabling higher speeds. This comes at the cost of reducing torque, precision and general frame rigidity – but good print qualities are generally attainable.  Note: I’m clearly addressing low-cost, desktop hobbyist printers in this description.

My faith on joining those two worlds in a single machine grew after I saw the Shapeoko 2 milling aluminum. The Shapeoko 2 is a desktop-ish CNC mill that is built out of many elements that figure on a regular 3D printer: NEMA 17 motors, GT2 belts, makerslides, and so on. With all that in mind, I started designing my machine. After a couple weeks, I came up with a design that made me happy:

main_assemb_render_2

Everything looks cooler raytraced.

Yeah, it ended up looking an awful lot like a Prusa i3 on steroids. But I took it as a good sign, that a lot of thinking took me to the same tested-and-proven design. Though I could’ve spared the time. And now the CoreXY looks darn interesting. Whatever. Anyway, I had no 3D printer available at the time, so I struggled to keep all the custom parts “laser-cuttable”.

After purchasing all the couplers, screws, belts, pulleys, aluminum profiles and other bits, I sent the designs to be laser cut on 3mm aluminum sheets.  All parts came in; the build started.

Starting to build. And to complain.

Starting to build. And to complain.

As the pic above shows, I had originally chosen a different arrangement for the Z axis: each screw was coplanar to only one linear rail (instead of being coplanar to both simultaneously). Soon enough I saw that the smallest wobble of the screw would cause the whole gantry to wobble along, rendering even the coarsest prints impossible. Couple more days in front of SolidWorks and the mistake was fixed [the rendering above is the fixed version]. There, no one saw it. Move along.

A couple parts for the fix had to be recut. They came in, and the result soon popped out:

printer_nice

Assembled printer. Disregard the extruder, that one never worked.

Everything looked OK. Rigid enough for me. Next up; assembling the electronics. I’ll leave that for a next post. I’ll also put the CAD files up anytime soon.

Ah, I’ve obviously ‘Instagramed’ it. “The experience is not complete if you don’t post it on Instagram”, said someone once.
 

From concept to finish (almost). 🙂

Uma foto publicada por Martin Vincent Bloedorn (@martinbloedorn) em

’til next time.