2018-05-22

Building a rotacy CNC milling machine

Project history

  1. Previous project: TravelCNC
  2. current posting
  3. next step: Problem with optical input switches
  4. Next project: designing a chain link tool library

Before

At the moment I have a heavily modified Carving-CNC/OmioCNC 6040Z+S80 machine from 2012 with an automatic tool changer and an 80mm 4th axis.

However the 80mm chuck and the 150mm clearance below the gantry of the 600x400x150mm machine has proven to be quite a limitation.
Even if you could make your part just barely fit...the next largest standard size of stock material would not.
If the stock could fit, you would not be able to raise your cutter high enough to reach the outher diameter. At least not the electronic 3D touch probe (that is much longer then most cutters).

Idea

So the idea was to build a special purpose CNC mill just for parts on the rotary axis.
It does not need a Y axis at all but a chuck with a large diameter on the 4th axis.
I was inspired by the Rotary CNC (movable tailstock) and the Farm OX (horizontal spindle)

  • Use a chinese 130mm chuck with 1:20 gears as a 4th axis.
  • No automatic tool change needed because it doesn't do general purpose work.
  • Use Openbuilds C-Beam with trapezoidal spindles (you can't buy these with ball screws off te shelf).
  • paired with high powered stepper motors for acceleration.
  • Add some end-stops and an electric touch plate for tool-length.
  • Use the existing stepper drivers, Mach4 USB breakout board and power supply of my old, mobile 400x300mm CNC machine.
  • Use either the 2.2KW water cooled chinese spindle with RS485 controlled VFD (left over from the ATC upgrade) or a 200W DC spindle (lightweight and easy to accelerate).

Since the C-beam is trivial to assemble and all the other parts are already here,
this SHOULD be a fairly quick and straightforward build.

Photos


...more soon

Design


  1. The long X-axis uses a 1m C-beam ACME assembly with the XL gantry plate and a huge 4.2A stepper motor for maximum acceleration and thus direction-changes.
  2. There is no Y axis.
  3. The short Z-axis on top uses a 250mm C-beam ACME assembly with the extra-long gantry plate.
  4. A secondary X guide-rail in the front with a custom, 3D printed, extra-wide but not extra-long gantry plate stabilizes the Z axis and keeps it square to X.
  5. A 300W DC spindle with ER11 collets is enough for the soft materials this machine will encounter. It is also sufficiently silent. (a major consideration)
  6. Custom, 3d printed mounts for spindle, A-axis and tailstock complete the setup.

Lessons learned

  • Keep the oil-release screw accessible when designing a mount for a gearbox.
  • Openbuilds gantry plates can rotate slightly when presented with some force on a long, sideways lever. So a second, guiding rail and custom wide plates are needed to make the machine stiffer.
  • The Openbuilds ACME screws are performing surprisingly well compared to ballscrews.
  • The worm gear I used was of very bad casting quality and gets very loud in constant rotation. So instead of machining parallel to the A axis, I need to do it parallel to the X axis. This way X moves all the time but A only every few seconds for a short step.
  • Helical toolpathes in my version of Deskproto 7 seem to not render. The issue is being investigated and should be fixed soon.
  • I need to finish the custom mount for the tailstock quicker then I anticipated.
  • During an accidental plunge the modular Z axis proved surprisingly durable and very easy to dismantle, readjust and assemble again.
  • Always check for loose wires to your stepper drivers when things start to vibrate for the first time. ;)
  • My Z axis is too long but I could not get C-beam assemblies shorter then 250mm.

Next steps

I'm currently finishing the soundproof enclosure.

I'm currently adjusting my existing 4th axis, inductive homing sensor to the new 4th axis. At this time I only have home+limits on both linear axis.

I already ordered brushes to separate the axis from the machined parts and keep shavings out of the horizontal Z-axis.

I already ordered a proper guide-rail to replace that silver Openbuilds -contraption. This is fine for some first test-cuts but I don't want to re-tighten the wheels every few weeks.

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by The Editor on Dienstag, Mai 22, 2018 0 comments
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