5th axis getting up to speed

After a little bit of calibration and tinkering in the fusion360 cps output filter files the cnc is starting to take advantage of the new 5th axis. Below is a "Flow" multiaxis toolpath finishing a sphere with a flat endmill.

There are still some issues that I have to address. It has been a surprisingly good experience so far. Starting out cutting 5 sides of a block and moving on to cutting the edges at an angle. This is the third test where I rough out the sphere using a 3d adaptive path and then use a flow multiaxis path to clean things up. Things look better in video and there is some of that to come.

CNC made close up lens filter holder

Close up filters attach to the end of a camera lens and allow you to take photos closer to the subject than you normally would have been able to do. This is very handy for electronics and other work as you can get clear images of circuit boards and other small detail. I recently got a collection of 3 such filters which didn't come with any sort of real holder, the container they shipped in was not really designed for longer term use.

The above is the starting design for a filter holder cut in layers from walnut and stacked together to create the enclosure. The inside is shown below where the outer diameter can hold the 80mm black ring and the inner circles are 70mm and are there to keep the filters from touching each other. Close up filters can be quite fish eyed looking with a substantial curve to the lens on the filter, so a gap is needed to keep each filter away from the next one. A little felt is used to cushion the filter from the walnut itself which adds roughly 1.5mm to the design so the felt layer all have space to live as well.

The bottom has little feet which extend slightly beyond the tangent of the circle so they both make good contact with the ground and there is no rocking. Using two very cheap hinges works well in this design to try to minimize the sideways movement (slop) in the hinges themselves. A small leather strap will finish the enclosure off allowing it to be secured closed.

It is wonderful to be able to turn something like this around. I can only imagine what the world looks like from the perspective of somebody who is used to machining with 5 axis CNC.

LinuxCNC and latency

When you enter the 4th and 5th axis world for cnc things start getting interesting. I have started looking at how LinuxCNC works with a plan to use it as the primary gcode runner on a higher axis machine.

Of the many options that LinuxCNC offers, I might start out using a parallel port and small breakout board to software signal some stepper controllers. These controllers have an enable, step, and direction pin and can be wired up in a few ways. This involves if the signal goes to the A+ pin or if you run high voltage to those A+ pins and run the signal to the A- pin. See for example about half way down this smoothieboard page.

Anyway, back to the LinuxCNC topic. The main thing that controls how well you can control stepper motors over the parallel port is how accurately the LinuxCNC process can schedule itself. if there is a big delay or jitter between calls that are supposed to be at a fixed distance in time apart then the machine control will not be as you might like. The latency-test program can be used to see the jitter.

The LinuxCNC images are based on Debian wheezy. This distribution includes a Linux kernel designed for supporting real time operation. There are a few real time Linux options floating around and the kernel on stretch (rt preempt) that is installed by installing linuxcnc-uspace is different to the one used by the wheezy image (RTAI). IIRC.

For controlling the machine I put together a combination for around $200 including an Intel Pentium G4560 on a Asus H110M-C2 with 4gb of 2400Mhz RAM. This relies on having cases, psu, drives etc already. This has an Intel NIC and onboard prt header (no pci bracket). The CPU has 2 cores and can do two HT.

Booting the current LinuxCNC iso image and running the latency-test gave about 3200 idle and 5100 under load. The two downsides are that the kernel was old enough that M.2 didn't work and the NIC was not detected.

Installing Debian Stretch on the machine the M2 and nic were both detected and all was well. I then installed the LinuxCNC packages which brought in the preempt rt kernel:

Linux bitbreaker 4.9.0-6-rt-amd64 #1 SMP PREEMPT RT Debian 4.9.88-1+deb9u1 (2018-05-07) x86_64 GNU/Linux

The latency-test came out at about 40,000 idle and up to 70,000 under load. Not quite what I had hoped for. Adding "isolcpus=2,3 idle=poll intel_idle.max_cstate=0 processor.max_cstate=0" to the kernel cmdline and using taskset -cp 2 $pid moved the numbers to 22,000 and 40,000 respectively. Better, but not what I had seen on the official recommended ISO bootup.

To cut a long story short(er) doing a telinit 3 and going non graphical on the control machine and using ssh -Y to login and run the latency-test I was able to get down to 4000 under load. This is again by forcing the rtapi_app to use a reserved CPU core.

Maybe using a different graphics card or some other graphics options would allow the latency to be lower on the display of the machine. But I'm fairly happy to run the GUI to display on the laptop to harvest the better latency numbers.

It is handy to be able to get closer to the jitter numbers that are given when using the official ISO for LinuxCNC as it gives the feeling that I'm not loosing out on good machine control because I'm on stretch and a customized setup. Ironically, using the nice new hardware for the build actually made initial setup harder. Though I imagine that is a temporary situation.

Hopefully those better jitter numbers can be had for real machining over the network too.