Now that I made a cut in wax, my next step with the PocketNC was doing basically the same thing but in aluminum. There is of course less room for error with the harder (but I suppose by no means actually hard) material. It seems that I managed to use up a bit more luck than I expected, but still not too terribly costly so far. My “learning moments” errr… goofs, so far:
Ensuring the part is contained within the stock
When I first made the toolpath, I programmed the stock a few mm larger than the actual stock to handle any mis-registration between the vise and the machine origin. This was a good idea, as I still have about a 2 or 3mm mis-registration in the Y axis that I have yet to resolve. However, when I did that, I managed to get the actual part about 0.5mm outside the actual stock. This was easy to fix and for this part geometry, I even kept using the same stock for subsequent runs.
Here’s my very first aluminum chips running the first version:
Inconsistent assumptions between tool paths
As I was developing the tool paths for this part, I started out using a 3D adaptive path to cut away most of the outside material using a 3mm Datron single flute mill.
That was then followed by a series of contour paths or circular paths to finish up each of the outside edges. However, in one incremental stage I had tweaked that first adaptive clearing to stop at the top edge of the wide flange, but still had a contour that traced it out. Watching the simulation at high speed, I didn’t notice the problem, but when the resulting contour was run, it plunged the mill straight into unmachined material.
When running this, needless to stay the PocketNC, while tough, was in no way able to pull that off and the spindle stalled. Fortunately, I was watching and actually got the machine safed before the spindle had completely stopped. My camera had shut off before that point though, so I got no footage.
Next, I wanted to use a second longer reach tool to handle the central cavity, which goes relatively deep. I dutifully entered the tool geometry into Fusion 360, including the fact that it was a 3 flute mill, but then managed to neglect to even look at what the chip load was and left all the feeds and speeds the same as for the 3mm single flute Datron.
Watching this live, it certainly sounded bad when it was doing the helical ramp in, but I chalked it up to chatter with the longer reach tool. However, as soon as it started the adaptive clearing inside, it sounded much worse, but seemed to be making forward progress. However, on the second plunge and adaptive clear, it managed to completely stall out the spindle. I jogged the Z axis back out, fixed the feed rate, but when I spun up the spindle again, boom. That was the end of that end-mill.
3rd (or 4th time) is the charm?
At this point, I reworked this first program to do what I could with the 3mm Datron end mill, which just meant I didn’t go all the way deep into the central cavity. This completed all the way through with no big problems. The last contour I did on the internal cavity also removed 0.001″ of material axially. When that first plunged in, it did sound terrible, but only for a fraction of a second. Still, I probably wouldn’t do it again that way.
After all that I had this first half-part done.
The tolerances on the surfaces actually seemed to be nearly identical to the wax version, in that it was plus or minus a thousandth. This was without any actual work to get the size I wanted, so is more of a baseline. Presumably I can use tool width compensation or just tweak the contouring paths by a thousandth to get it where it needs to be. That said, it was close enough out of the box that my bearing kinda fits on.
Future versions of this part
Not too long into programming this first part, I realized that with slightly different stock, I can do the entire part in a single setup with a breakaway tab by orienting the part off by 90 degrees. That’s what I’ll try next.
I guess these lessons are what I’m trying to get out of these exercises, and while they are certainly obvious to anyone who actually knows machining and CNC, I’ll write them out here as much for my reference as anything.
- Watch at least the beginning of each tool path in simulation at real-time speed
- Actually look at the computed chip load when configuring a new tool
- Don’t rely on contour paths to remove any axial material
- When it sounds bad, stop and rethink!
- Run everything on the machine at 30 or 40% speed at least until nothing new happens before upping it to full speed
That said, I’m not trying to create a production environment here. I’d like to be able to quickly turn my CAD models into tool paths and get parts made, and as long as the machining time is the same order of magnitude as the programming time I’ll be happy. I am OK with breaking things now and then in service of that goal, so I don’t need to double and triple check of tool paths.
Finally, I want to thank Ed Kramer https://www.instagram.com/ekramer3/ for posting his results with the Pocket NC V2-50. All the mistakes here were mine, but he has provided a wealth of information on what feeds and speeds are possible in a range of materials with this machine.