October 28, 2020

Another batch of furuta pendulums

 I did another batch of Furuta pendulums and took a timelapse of the assembly:

These were done in a bit of a hurry, and nothing was changed from last time.  At some point I'll probably do a refresh of the design, though.  After building 16 of these, I have a bunch of ideas about things I'd like to do differently, and I've gotten a few feature requests from the recipient as well.  

And now, back to the other things in the pipeline.

October 6, 2020

Benchtop 5-Axis Mill

This was inevitable, but the pandemic has accelerated my home-shop setup.  What with leaving the lab almost a year ago (I miss you Lab Haas) and no MITERS for the foreseeable future, I haven't had access to any machine tools other than the tiny lathe for a while.  Time to fix that.

I've been following Pocket NC since 2012, after seeing them at the NY Maker Faire - they make a tiny but surprisingly capable 5-axis CNC mill.  It has basically the same layout as the big GROB machines, with a horizontal milling spindle moving in Z and X, and a cantilevered trunnion on the Y axis.   I was tempted to get one of their machines, but I think they're just too small and underpowered (and pretty expensive) for the sort of parts I make.  In my search for a small-enough-to-carry-upstairs-into-my-living-room-but-not-too-small CNC mill, I ran across a scaled-up Pocket NC clone made by a Chinese company, Xinshan Tech.  I couldn't find any examples of real people outside of China owning the machine, but after some emails back and forth with the company, including a bunch of videos, I was convinced that the machine was real and not a scam.  This machine was appealing, since it's a fair bit bigger than the Pocket NC, with ~50% more travel in each axis, all steel, servo (rather than stepper) driven, and much heavier (~70 kg).  I've gotten along just fine with 3-axis machines up until now, but I couldn't actually find any 3-axis mills that fit what I was looking for any better, and 5-axis opens up some interesting opportunities.

I decided to go for it, and a month later DHL showed up with a heavy crate:

Out of the crate and on the bench.  The machine is deceptively heavy given how small it looks, and is very awkward to hold onto.  Rigol scope in the background for scale.  

Back side of the spindle visible:  It's a generic 800W ER-11 water cooled spindles.  Maybe something to upgrade in the future:

The three linear axes are driven by brushless servos with integrated controllers:

A peek under the bellows way covers at the (allegedly Hiwin) X-axis ballscrew and one of the linear guides:

The linear axis ballscrews are coupled to the servos with disc couplings, which is nice to see

The backside of the machine, where you can see the X,Y, and A axis servos.  The rotary axes are driven with harmonic drive reductions and supported by crossed roller bearings, which I'm surprised was possible given the cost.

When I unpacked the electronics box, a few screws had worked loose in shipping and were rattling around.  The inside of the electronics box was not confidence-inspiring.  There's a generic 36V DC supply to run the servos, a 1.5 kW VFD, and some pretty sketchy wiring.   The machine seems good mechanically, so I'm letting this slide for now....

I've been using the shipping crate it came in as an enclosure.  If I fold the door up it keeps the chips in and the noise down a little, at the cost of seeing what's going on.  Eventually I'll build a proper enclosure.  I set the crate on top of a Harbor Freight rolling tool cart, which stores the power supply box, water cooling, and related tools:

Screenshot of the the control software below.  The software isn't anything fancy, but does the job.  There are amusingly mis-translated buttons like "Knife", which actually runs the tool probing macro, "overrate", which is the feed override, and "Cold Fog", which presumably would turn on mist coolant if the machine had it.

Not particularly exciting, but here was my first working attempt at a multi-axis toolpath (video at 16x):

Here's one of the first "real" parts I've made - the front side of a motor housing.  Most of the material removal was done with a Datron 3mm single-flute end mill.  Thanks to the rotary table I could do this part in one setup, rather than machining one side, machining a fixture, and machining the other side like I would on a 3-axis machine.  Video at 8x speed.

Here's pretending the mill is a lathe and machining a shaft for said motor, including a lock ring thread cut with a 60 degree chamfer mill.  HSMWorks, which I've been using for CAM since the beginning, is pretty terrible for this style of toolpath, so the surface finish is a little weird.  It buffed out easily though.  In this picture the part is done and being parted off:

And here's that half-a-motor-housing and shaft with a rotor for a frameless motor installed (held on by a threaded lockring also made on the mill), and bearings and commutation encoder magnet pressed in.  

I haven't pushed the machine too hard yet, in the interest of keeping noise down.  It's in my 2nd floor living room, and not very well enclosed, so I've been making the toolpaths conservative to avoid complaints from my neighbors.

Amusing setup from today - a part in the tiny lathe 3-jaw chuck, held in an ER-40 collet chuck, bolted to the table.  Milling a circular dovetail in the part (which is a fixture for yet another part), to match the dovetails milled into the lathe chuck soft jaws:  No dovetail cutter required.

Summary of thoughts/impressions so far:
  • The core mechanical pieces of the machine seem pretty good.  Brushless servos, linear guides, ballscrews, and harmonic drives, on a steel structure is impressive at this price point.
  • The less-critical hardware is a little janky - the bellows way covers, the sheet metal spindle cover, the electronics box, the water cooling loop.  But those are all relatively straightforward to re-do or repair if needed.
  • With good tools (I've been having great luck with Datron single flute cutters for roughing), it performs pretty well cutting aluminum.  I've been roughing with a 3mm endmill at 20k RPM, 2.5mm stepover, .5mm stepdown, and 2000mm/min feed with no problem.  It could definitely be pushed harder but it gets loud.  I haven't tried harder materials yet.
  • I haven't done a ton of experimenting yet, but "conventional" style toolpaths with large stepover and small stepdown seem to perform better than "adaptive" or "trochoidal" style toolpaths with a large stepdown and small stepover.  Needs more testing though.
  • I wish I had a way to pre-set tool lengths with the ER-11 spindle.  The machine has a built-in tool probe, but swapping collets and probing takes a while.  I'm thinking about making a bunch of shrink-fit holders that all have the same shank diameter and bottom out in the spindle, so the collet and collet nut never have to come all the way off. 
  • There are a few vendors of the machine (It's even on Amazon, but also RobotDigg and a few other sites), but it's cheaper to get directly from Xinshan.  
  • Support has been really excellent so far.  I've had tons of questions about how things work, and I always get a response the next day.  When I've had g-code that behaves confusingly, they'll run the code on their machine and send me a video.
  • CAM-wise, I've been using HSMWorks.  I think the kids these days are using Fusion 360 (Autodesk bought HSMWorks and I think has ported it into Fusion), but I've never tried it.  Solidworks integration is super nice since your CAM updates when your model updates, and I don't really want to switch to Fusion for CAD.  HSMWorks/Fusion 4/5-axis toolpaths are kind of a joke, but 3+2 (position the rotary axes then do a 3-axis toolpath) work great.  The "real" answer for good 5-axis toolpaths is probably a more legit stand-alone CAM software, but I'm going to stick to HSMWorks for everything I can.  
  • Cool feature, the machine can do TCP, Tool Point Center control - this means that the CAM origin does not need to be at the machine origin (the intersection of the two rotary axes).  You can put the CAM origin wherever, set the work coordinate system offsets by touching off to the part, and the machine will figure out the kinematics.  This makes CAM much simpler, since you don't have to know where the part/stock are on the machine when you're doing the CAM.  That's what you'd expect for 3-axis stuff, but with the two rotary axes, it's not just an offset in X, Y and Z anymore.  I haven't been to adventurous with this, but offsetting the Z at least seem to work just fine.