Author: BrianV

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Should you use shielded wire or not? I don’t use any shielded wire on my DIY CNC, and I haven’t had any noise or ‘phantom triggers’. I do notice interference (like a ‘fuzzyness’) on my computer monitor when I hit the power to my stepper motors, however, but that is only mildly annoying.

The input pins on the HobbyCNC boards are fairly high impedance, being pulled-up with a 10k resistor.  There is a potential for induced noise to creep into these signal lines. One of my customers was having a devil of a time with false triggers stopping his milling jobs. He added shielded wiring – no improvement. So we tried the solution I run on my machine – an opto isolator board (Fig 1).

But not just any cheap-ass opto board. I designed my opto board to have a very low impedance to minimize the impact of any induced voltages. I also designed each circuit to have it’s own 20mA current-source to allow the addition of LEDs in the wiring to add convenient indication of current flow without having to change any parts on the opto board (more on this in another post). We tested the board, and I’m pleased to say that, so far, there are zero phantom triggers.

The purpose of such a board is to electrically isolate delicate components (like your computer) from ugly, real-world possibilities, like putting a voltage on a wire that could fry your PC.  Better to blow an Opto board instead.

Properly powering the Opto board matters a great deal. Imagine the opto board has two ‘halves’ – one half is not isolated, and it connects to your sensitive computer.  The other half must have it’s own completely independent, fully isolated power supply, connected to nothing else. Using any power source from your PC or any other power supply in your system will completely invalidate the isolation. I have no idea how often this happens, but I’d guess too often.

Also, I made my own ‘twisted pair’ wiring for my limit and home switches. I take two wires, clamp one end in a vice and the other into the chuck of my hand drill. Pull taut, pull the trigger, and walk toward the vise as the wires twist, keeping gentle but steady pull on the wires.

Summary: skip the shielding (though it won’t hurt anything if applied correctly), use twisted-pair for your limit and home switch wiring, and if that’s not enough, add an opto board (which isn’t a bad idea on general principle).

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Fig 1. HobbyCNC prototype opto board (left) vs. cheap, basic opto board.

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Fig 2. HobbyCNC prototype opto board installed in customer system.

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I needed a quick-and-dirty 36VDC unregulated linear power supply to drive some HobbyCNC boards on my test system. I wanted the bridge rectifier to mount directly to the PC board.  The beefy bridge rectifier that I had in my stock was the type with the large spade-type terminals. I needed to mill slots in a PC board for mounting this bulky part.

Getting slots in the PC board took some digging. I designed the bridge rectifier footprint in KiCAD to have oval pads with oval holes, but this did not translate via the drill file into FlatCAM – only one round hole would be drilled.

To make the slots, I had to create a New Geometry ‘layer’ and add simple straight lines dead center of where I wanted the slots milled. FlatCAM would then just mill one pass, centered over that line. This required a third milling file – one for the bottom Cu, one for drilling and the third for the four slots.

Came out P E R F E C T!  Next board will have some pretty fine geometry, squeezing traces between DIP pads.  We’ll see. . .

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One thing I use my DIY CNC machine for is milling PC Boards (it’s technically called “isolation routing”). This is fairly fast, inexpensive, accurate and uses zero nasty chemicals.

I’d been using Eagle Schematic & PCB design software. Great product, works very well, I like it a lot.

Then Autodesk bought Eagle. I liked the idea of Eagle belonging to the Autodesk family (AutoCAD, Autodesk Inventor, Fusion 360 and many more). Autodesk is a great company, with equally great products.

But here’s the rub

Autodesk changed the licensing to a subscription model. So for the hobbyist, like me, I now have to shell out $100 USD per year. I don’t use it enough to justify this ongoing expense.

So I’m trying KiCAD. It’s free. This is good. It seems to have good community support. The core concepts are the same as any schematic capture software, but the implementation is different. KiCAD is different enough from Eagle that there is some learning involved. Nevertheless, I got a working schematic in and a PC Board designed. KiCAD has a built-in 3D viewer which I found to be super-cool.

Next I need to export the Gerber and drill files into FlatCAD which will in-turn produce the necessary G-Code for milling. I’ve got my new Tapered-stub End-mill PCB Traces-isolation Bits from Think-and-Tinker/Precise Bits.

I’ll provide an update once I get a board milled.

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A BreakOut Board – a BOB for short, takes an input, typically from a computer or a ethernet/USB-to-parallel converter and splits up the signals into individual pins. These pins are then wired to individual stepper driver boards.  Most stepper driver boards require two signals to make the stepper motor move – Step and Direction.

Also, like the board shown in Figure 1, there are opto isolators. These are to electrically isolate your computer from the stepper motor drivers. For most hobby installations, opto isolation for the stepper motor drivers is not necessary. There are cases where opti isolation makes great sense, like in more industrial environments where different machines at potentially different ground levels can cause havoc.  I’ve personally seen this destroy unprotected electronics. See Ground Loop. It would be highly unlikely for a large ground potential to exist within a single piece of equipment.

Some BOBs can include LEDs on some of the lines – and this can be very handy during installation and debugging.  BOBs can also include other features, but then they really aren’t BOBs anymore.

[one_half]Positives:

• Opto isolation (belt-and-suspenders)
• LED lites on some lines (maybe)
• Can possibly support more than 4 axis
• Looks impressive

[/one_half][one_half_last]Negatives:

• Opto isolation is not necessary
• Higher parts count – a whole extra board, components and LOTS of extra connections and wiring
• More complicated wiring
• Extra cost

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Compare to HobbyCNC boards:

A BOB is not required for any of our boards.  We put 3 or 4 drivers onto one board, and we route the wiring from the DB25 connector to the driver chips without the need of any additional circuitry. This provides a one-board solution with – zero extra wiring – for up to 4 axis – compared to 5 boards for a 4-axis BOB-based solution.

Figure 2 shows the wiring for a typical BOB and 4 driver modules. It’s not rocket science, but as my engineer friend says “a key role of an engineer is to take out as much as possible, but no more”.  If it can be done as good – or better – with fewer parts, less wiring, reduced complexity – then that is a win.

Of course, all that wiring and all those modules look really impressive, but that’s not why most of us are in this hobby!

As my daddy used to say “Keep It Simple, Stupid”.

Is cheap electronics a good value? It depends if you decide to play around with it using online tools like Upverter.com to arrange your schematic to ensure that the cheaper parts can deal with the electrical loads correctly. In this case, “Value” means the benefit you receive vs. the price you pay. The simplest form of this equation says, pay the lowest price you can find, and this will increase the value to you. However, the formula isn’t that simple. You need to factor in quality and support too. Unfortunately, you may find that you’re more likely to need Fanuc Servo Motor Repair if you decide to buy cheap electronics that are relatively low in quality.

Regarding the electronics to drive your Hobby CNC stepper motors, there are LOTS of cheap (and I mean that in the least-nice-way possible) Chinese controller boards from Alibaba. Figure 1 is just one of many for around $10 for one axis.

Right off the bat – something smells wrong. How can they do it so cheap? Well, here’s the answer: Use the cheapest-crap parts you can find from any bin you can. Parts that don’t meet spec, electrolytics that dry-out and fail, questionable assembly quality. Every quality corner that can be cut, is cut.

Proof Point: in Figure 1, everything is covered by a heatsink, but you can clearly see one electrolytic capacitor (enlarged in Figure 2).

Chengx. Ever heard of them? Me neither. Google “Chengx capacitor quality”. Not good news.

Here’s a great video ‘teardown’ of some knock-off Chinese crap electronics by Dave Jones of the EEVBlog. jaycar-dmx-rgb-led-lights-teardown/. You will see an excellent example of just how many corners can be cut, how much cost can be carved-out.

So, quality is highly questionable, “I’ll just buy 5 of them to have a spare-or-two – that’s still only $50!”

Besides low/no quality parts, dicey assembly practices and no/low quality service, consider who you are going to call if there are any problems? Nobody, that’s who. You can post on a forum and hope someone else has already figured out the problem. Your time ain’t free.

You will also need a breakout board (BOB). And a 5V power supply.

So where are we:

5 boards @ $10 each ($50) + BOB @ $10 + 5V power supply @$5. So you can have the lowest quality, cheapest, most unreliable, unsupported, poorly documented solution for $65.

HobbyCNC PRO 3 axis is $76. All quality parts, no knock-offs, no grey market, no ‘generic’ parts. No BOB required and no extra +5V supply – giving you simplified wiring as a bonus.

Think about it.

Additional Viewing

CapXcon capacitors (another ultra-cheap Chinese capacitor) EEVblog #347 – Bad Cap LCD Monitor Repair

EEVblog #365 – ESR Meter Bad Cap Monitor Repair

Toms Hardware, PSUs 101: A Detailed Look Into Power Supplies, Capacitors Manufacturer Tier List (Note: I only use Panasonic or Suncon)

“My X axis stopped working. What’s wrong?”

I was in pre-and post-sales technical support professionally for about 20 years of my career. I’ve provided support over-the-phone, in the shop and in the field. I consider myself to be pretty good at the process of systematic troubleshooting.

My daddy used to say “If you drop your car keys in the street at night, look under the streetlights first, because it’s easiest” and “The best place to start is at the beginning“. The takeaway – always check the easiest or most obvious things first.

This “obvious things first” approach can sometimes be offensive to the customer. Questions like “is it plugged in?” and “is the power switch on?” are good and realistic places to start, but it sometimes leaves the customer wondering if I think they are brain-dead.  Check the simple things first.

The problem of remote troubleshooting is compounded because it is over the phone. You can’t see it, touch it, smell it (yes, smell it – burnt parts have a unique and obvious scent). Often the customer is not an electronic technician, may not have the proper diagnostic equipment – or even know how to use a voltmeter.

There is a logical flow to troubleshooting in general, and this is even more important over the phone. Depending on the problem, the steps may be different, but they are all important. For example, if only one axis is ‘out’, then I would not suspect the power supply circuit – whereas if none of the motors were moving, the power supply is the first thing I would look into.  Regardless of the questions –  I need to know the answers in order to move to the next step.

“Yeah, I checked all those” is a too-typical answer to “what are the voltages at the 5V test point, the Fan Connector and the Input Connector?“. When I get real numbers as answers, it tells me A) things are good/not good and B) you really did check them.

The customer has to be my eyes and ears (and nose). Work has to be done to identify the failure. Read the troubleshooting steps (in the FAQs) and tell me the answers to each step. This will get us both to the solution, faster!

Why does HobbyCNC use unipolar stepper motors?

And what is the difference between bipolar and unipolar stepper motors?  Is one better than the other? Let’s start with the differences between the two types of motors*.

A unipolar motor works from a single polarity on each coil.  These require simpler electronics to drive.  However, they require two pairs of windings, wired in reverse of each other.  To oversimplify, power one coil and the magnet is North-South.  Power the other coil and the magnet is now South-North.  (or “suck” and “blow” as one of my college instructors used to say).  Unipolar motors have 5, 6 or 8 wires.  More wires = more complexity to connect.  Not substantial, but more complex compared to 4 wires.

A bipolar motor has just a single pair of windings, and the electronics to drive them must be able to switch polarity, typically with an “H bridge” driver circuit.  Since the motor has only one pair (two) windings rather than two pairs (4 windings), a bipolar motor of the same weight could theoretically have more power. Bipolar motors typically have 4 wires and are therefore less complex to connect.

Cost-wise both motors are similar.  The bulk of a motors weight comes from the body (stator assembly) and the rotor, not the wiring.

So, why does HobbyCNC use unipolar stepper motors?

Well, it’s not really about the motors.  It’s about the driver chip (Allegro Microsystems SLA7078MPR).  It is a mature, robust, easily ‘heat-sinkable’, durable driver chip. It has built-in short-circuit protection. It is a very reliable chip. It is priced very well for the hobby CNC market and helps us keep the cost down. It is also easily soldered, which is important since we only sell kits. We have been using this chip for many, many years, in 1000’s of boards, with outstanding success.

And, from a practical standpoint, 8 wires isn’t all that complex to handle.  Really.

To learn a bit more, there is an excellent article on Adafruit.com with drawings and great images.

* This is not meant to be a complete tutorial on stepper motors.  There are many types and designs of motors, many sizes and many price ranges. This description is kept basic for common, simple stepper motors to be used for hobby-level machines.

I have been thinking about my decision to use a 1/4-20 threaded rod to drive the axis instead of an acme screw. The answer is the typical engineering answer: “trade-offs”. Threaded rods can be a very valuable tool for a lot of building projects, and you might want to take a look at a page like https://tradefixdirect.com/threaded-rod-studding if you need one for a project you’re working on.

The primary goal of my HobbyCNC plans is to provide a functioning 3-axis router, with reasonable performance, using common materials, within the budget of a hobbyist. Whilst many people want to consider getting a router for router table (which is likely the smarter idea), this build will provide an excellent foundation for learning the spectrum from design through machining.

1/4-20 ‘all thread’ threaded rod

On the positive side – it is inexpensive and readily available (at least in the U.S.). I’m sure metric versions are easily available outside the US.

On the negative side – it takes 20 turns to move one inch. This means a big sacrifice in axis movement speed. I reliably get 30 ipm. I haven’t tried to push it, but I have reached 48 ips, but not in production. Also, if you get past 3 feet (approx 1 meter) or you get the speed up too high, this thin rod can ‘whip’ and create an unpleasant knocking sound.

FWIW, I use 1/4-20 drive screws on my machine. Works great for wood, plastic, PC board isolation routing and engraving aluminum.

Acme screws

On the positive side – typically larger diameter (less/no whipping), high accuracy, precision machined, fancy ball-bearing nuts available. Also fewer threads-per-inch you can get much faster axis movement!

The downside: Cost. These are at least an order of magnitude more expensive. Like $100 per axis instead of $10. (prices vary widely, don’t nit-pick me).

If you want to go with Acme screws, please do. If your primary focus is to learn and experience machining on a budget, stick with the 1/4-20.