My Humbling Week as a Machinist

A terrible ripping noise followed by a clang and a bang echoed through the metal shop.  Everyone turned to look at me.

The bang could have been my last heart beat as I destroyed a machine that costs over $10,000.  Or it could have been a razor sharp bit cracked in half spinning through air and deflecting off my skull.  

In reality it was my the sound of the machine making an emergency stop as I hit the big red button reserved for emergencies.

Developer to Machinist

I got to this point with a goal, a chunk of time, insufficient respect for a new field, and perhaps a touch of hubris.

The goal seemed simple enough: build an aluminum mold for an injection molding machine to help mass produce a part I needed for my hobby.

I was to accomplish this goal at Techshop, a maker group that provides access to tools, information, and an awesome community.  And I was to accomplish it in a week during a vacation.

The general process I used was:

  1. Design the part in Autodesk Inventor
  2. Prototype on a 3D printer
  3. Design the mold in Autodesk Inventor
  4. Design the machining (CAM) operations in an Inventor add-on called Inventor HSM
  5. Export the CAM operations to GCode and import them into a TorMach CNC Milling Machine
  6. Run the cutting operations on a couple of pieces of aluminum (a top and bottom)
  7. Run the injection molder
Autodesk Inventor

Autodesk Inventor is an amazing CAD package that normally costs over $7,000, but as a TechShop member, not only do you get access to a fully stocked metal shop, and just about every piece of machinery imaginable, but you get a copy of Autodesk Inventor.  Fortunately the help documentation and tutorials are excellent and designing a simple part wasn't too hard.

From there I used a Makerbot Replicator 2 to 3D print a prototype and iterate a couple of times on the design.  The part, in case you're wondering will be a cap for the led column on a Siren of Shame, but it's really just an excuse for me to fiddle around and learn machining.

Mold Design

With a TechShop class on mold design I learned the key pieces of information I needed to make the mold including how to:

  • Create a sprue that the molten plastic will be injected into
  • Multiply a single part multiple times (and maintain the link to your part so you can theoretically change it)
  • Create a runner system to distribute molten plastic to the parts
  • Build gates to inject the molten plastic into the parts
  • Design vents that allow air, but not molten plastic to escape
  • Add cold slug wells to keep gasses and sediment out of your mold parts
  • Add registration pins that keep the two halves of the mold (cavity and core) aligned
  • Design your part to be easily injectible (add draft angles and fillets to every surface)

Inventor HSM

Inventor HSM is an amazing plug in for Autodesk Inventor that allows creation of 2.5D and 3D CAM operations for CNC milling machines directly from your Inventor models.  Unfortunately it's pretty new, the documentation is a little weak, and there aren't any TechShop classes on it yet.  Worse, if you don't know what you're doing many of the default cuts won't be correct, to potentially dangerous effect.  


Everything is so nice and easy right up until you have razor sharp cylindrical blades spinning up to 5,000 RPM and automatically moving along a path that you may or may not have defined correctly.

I ran my first cuts in foam to validate my approach, gain practice with the TorMach CNC milling machine, and get experience with the wide range of bits I would need (including traditional end mills for surfacing, ball end mills for runners, an end mill with a taper for draft angles, and a couple of micro bits for air channels and such).

Unfortunately, my confidence increased after I ran through my cuts in foam.  Foam, it turns out, is completely different from aluminum.  Here's a video of my first disaster.

The surfacing was fine, but I was taking too aggressive of cuts.  After a kind "Dream Consultant" at TechShop explained my mistake I was able to correct it like so (notice the difference in how the machine sounds):


Unfortunately I didn't learn that lesson until after breaking a bit, lightly injuring a collet (bit holder), and embarrassing the heck out of myself in front of a small audience.

Lessons Learned

Throughout my milling time (30+ hours) I made numerous mistake.  Anyone with woodcutting experience (or machining common sense) will probably find these mistakes amusing.  I hope others may find this list constructive:

  • End mills only cut up through where the flutes end.  You can't run a mill into aluminum above that line without bad results.  And for goodness sake don't run the collet (the thing that holds the bit) into your aluminum or even worse things happen.
  • When bits break they go flying really fast and scare the crap out of you and it's loud and everyone in the room comes over to publicly shame you.
  • Even if you're cutting with the blades of your bit you should only mill a small amount of height of aluminum at a time (1/2 the bit width max) otherwise the drill makes a bad cut and terrible noises and you risk breaking the bit.
  • End mills (the ones with an L shape) aren't for drilling.  If you do this then miniscule aluminum shreds will bind to the bit and fling around and create an invisible shield that will block the flow of coolant generating smoke, risking overheating and damaging your bit and the aluminum.
  • Design everything in your part with specific bits in mind.  It's way too easy to design something on the computer that will be nearly impossible to machine.  Going back and modifying your part after you've designed your cuts in practice is extremely hard.
  • Getting your X, Y, and Z coordinates zero values correct on the machine is kinda important. If you get Z wrong, even a few hundredths of an inch too high for example, your cuts will be in air and you could waste a lot of machine time and not be able to tell because the area is flooded with coolant.

But as the kind staff reminded me, we are here to learn and we all have to start somewhere.

The good news is I never damaged the machine (it's actually quite forgiving), I only broke one bit, and most importantly I didn't (significantly) injure myself or anyone else.


In the end I successfully milled my pieces, finished them, ran a couple of runs on the injection molder (still some work to do there as you can see below), and ultimately accomplished my goal.

All in all trying on a completely different hat for a week was both fun and rewarding.  Building something real was a blast.  I learned a ton.  But to tell you the truth as thrilling as it was to accomplish my goal, I'm just as thrilled to be headed back to a job where a screw up won't injure anyone or damage a ten thousand dollar machine.  I'm happy to be headed back slightly smarter, happier and more humble.


Rajesh said…
Lee, That was awesome!!! Requires a lot of commitment and perseverance and you have both. I learned something new. Keep them coming....