The process of locating holes with a CNC machine is copiously supplied with opportunities to totally mess everything up

Okay, here’s what happens:

You’ve got yourself a nifty robotic doodad called a “CNC milling machine” which cuts through hefty chunks of metal like butter… if you don’t mess up. If you do mess up, there are horrible grinding sounds, the smattering of “ping” and “tinkle” sounds as important bits and pieces shatter and fly into the walls (this is why safety glasses were invented), and long gouges in said metal chunks if you’re lucky, and in your nifty robotic metal-carving machine if you’re not.

We’ve learned that about 3:00 a.m. — at the latest — is when we start to make lots of mistakes. The other night we broke three tool bits in the space of ten minutes before it occurred to us that, hey, we were tired! and we were having difficulty concentrating!

Two of these bits were buried the in material we were trying to carve. One bit was embedded in the machine itself. Actually, it was embedded in aluminum plating which we’d placed between the work and the machine, because we just knew we were going to mess up badly at some point, and we wanted to protect the machine from ourselves. How prescient of us!

These ten minutes cost us about $30 in materials and tools, which is not much. It also ruined about eight hours of work, which was painful.

Now, we know that everybody does this when they learn how to use these machines, which is why we’ve been taking precautions to protect our investment, and why we’ve been taking notes to help us study and address the mistakes. Here’s what we’ve learned so far:

  • A nice big friendly red PANIC button must at all times be within arm’s reach. This is known as the “emergency stop” button. The purpose of this button is to make all the moving parts stop moving exactly two milliseconds before the machine eats itself according to your careful directions. The timing is important here: any later, and you’ll leave horrible scars on your work or your milling machine; any earlier, and you won’t almost have a heart attack, which means you simply won’t learn anything from the experience. The almost heart attack is very important.
  • Each degree of freedom of movement provides your milling machine more ways to damage itself. Supposing p_x, p_y, and p_z are the probabilities that the machine will jam in the next ten minutes on the X-, Y-, or Z-axis, respectively; the probability that the machine will jam on at least one axis in the next ten minutes is 1-(1-p_x)*(1-p_y)*(1-p_y). Add more axes to make the problem worse. This is a horribly complicated way of saying that sensors absolutely must be installed to monitor the machine’s movement in each degree of freedom. Otherwise the machine will become lost, and as it no longer knows where its cutting tools are, it will begin to cry and wail and throw an embarrassing tantrum, and then have a meltdown very like a lost child in a department store. And people will stare accusingly at you and your poor machine, and you will feel like a lousy parent.
  • Find a way to automate the measuring of each milling tool’s length immediately after you lock it into the machine’s spindle. Otherwise, when you run out of coffee, and you swap-in a new milling tool, and you tell the machine how long the tool is, you will probably be wrong, and the machine will either carve pretty patterns in the air above your work, or it will try to carve pretty patterns in the table under your work. In the latter case, the machine will happily puncture your work and shatter your milling tool on the way to the table.
  • Find a way to automate the process of precisely locating your workpiece in your machine’s coordinate systems. For pretty much the same reason as above, in order to avoid pretty carvings in the table, punctured workpieces, and shattered tools.
  • Most CNC machine controllers have a single “probe” input. You will probably want to use both a “touch probe” to locate workpieces in machine coordinates, and a “tool touch-off switch” to measure the lengths of your tool bits. Find a way to permanently and concurrently connect both sensors to the single probe input. Otherwise, you will eventually run out of coffee, and you will plug in the wrong sensor. This means that the sensor that should be plugged in will be destroyed in slow motion. If it is the touch probe, it will neatly fold and crumple. If it is the switch, it will be run-through with a sharp tool bit.
  • Take the time to carefully write and test programs to automate as much of your work as possible. If you cannot automate a particular step, if you must perform that step manually, you can at least write code that tells your machine to tell future-you what to do, step by step, when future-you runs out of coffee.
  • Place a protective layer of sacrificial material on machine surfaces which the machine might be able to attack if it decides to eat itself. For example, we bolted an aluminum tooling plate to our milling table, and we consider this tooling plate to be replaceable and disposable, although we hope to not have to replace it any time soon. Sadly, this hope is already dashed, but at least our milling machine isn’t damaged.
  • Finally, when you run out of coffee, stop operating the milling machinery. Even if it isn’t particularly heavy. Ours sits on a little table, and we can easily pick it up and move it to another table, so we can hardly call it heavy. Nevertheless, the good desktop milling machines are capable of many of the same heavy-duty operations performed on much bigger machines that weigh several tons, and which occasionally eat people. So treat your little machine with the same respect you’d give to heavy machinery.

Although none of our machines have tried to eat us, we have experienced the rest of these mishaps. We expect to make many more rookie mistakes as we learn. We try to plan for such mistakes. Taking notes makes this much easier.

Okay, then, back to work…

Good night,


a tiny collet


Today’s banner: a tiny ER 8 collet. For the purpose of improving rigidity for milling operations with a lathe, we tried securing an endmill in a four-jaw lathe chuck:

We wanted to secure the endmill base using an ER 8 collet, but alas, the 3/8″ shank wouldn’t fit in any of our collets. ER 8 collets are absurdly small. But then, so is the Taig lathe. We like tiny things.

In principle we could make a new collet to hold the end of the endmill. Obviously we had to put principle to practice. We often do this, if for no other reason than to see what happens. But making collets is hard. Tiny collets require a tiny slitting saw. We persevered:

Here’s the endmill’s base in the new collet:

And here’s the chucked endmill, capped with the new collet (photographed from the rear of the chuck):

We had fun. To give a sense of size, or lack thereof, here’s the collet next to a thumb and a penny:

Cheers — stochastic