Archive for September, 2019
I’m sure I’ll do this again some time …
Focus the camera at whatever distance needed to clear the longest tooling you’ll use or, at least, some convenient distance from the platform. You must touch off Z=0 at the surface before using bCNC’s probe camera alignment, because it will move the camera to the preset focus distance.
Align the camera’s optical axis perpendicular to the table by making it stare into a mirror flat on the platform, then tweaking the camera angles until the crosshair centers on the reflected lens image. This isn’t dead centered, but it’s pretty close:
The camera will be focused on the mirror, not the reflection, as you can tell by the in-focus crud on the mirror. Whenever you focus the lens, you’ll probably move the optical axis, so do the best you can with the fuzzy image.
You can adjust small misalignments with the Haircross (seems backwards to me) Offset values.
A cheap camera’s lens barrel may not be aligned with its optical axis, giving the lens a jaunty tilt when it’s correctly set up:
With the camera focus set correctly, calibrate the camera Offset from the tool (a.k.a. Spindle) axis:
- Put a pointy tool at XY=0
- Touch off Z=0 on a stack of masking tape
- Put a dent in the tape with the bit
- Move to the camera’s focused Z level
- Make the dent more conspicuous with a Sharpie, as needed
- Register the spindle location
- Jog to center the crosshair on the dent
- Register the camera location
Calibrate the Crosshair ring diameter thusly:
- Put an object with a known size on the platform
- Touch off Z=0 at its surface
- Move to the camera’s focused Z level
- Set the Crosshair diameter equal to the known object size
- Adjust the Scale value to make the Crosshair overlay reality
For example, calibrating the diameter to 10 mm against a shop scale:
At 10 mm above the CD, setting the camera’s resolution to 11.5 pixel/mm:
Makes the outer circle exactly 15.0 mm in diameter to match the CD hub ring ID:
I doubt anybody can find the pixel/mm value from first principles, so you must work backwards from an object’s actual size.
Two Funnel Weaver spiders spun their webs across diagonal corners of the garden tool rack and appear to be peacefully sharing the bounty attracted by nearby lights.
The one on the left vanishes instantly into its funnel, deep inside the corner post, nearly every time we step onto the patio:
The other spider worked around a stick emerging from its refuge:
But it’s doing all right:
Their less adventurous compadres build webs on the plaintains festooning what might be called our lawn, making me feel awful while mowing in these months. I hope the mower’s vibrations drive them deep into the grass before it roars overhead, but I’ll never know.
They don’t make tail fins like this any more:
It’s a 1962 Cadillac Fleetwood encountered on a walk around the block.
Nothing exceeds like excess!
The smaller and more rigid CNC 3018-Pro should be able to engrave text faster than the larger and rather springy MPCNC, which could engrave text at about 50 mm/min. This test pattern pushes both cutting depth and engraving speed to absurd values:
Compile the GCMC source to generate G-Code, lash a CD / DVD to the platform (masking tape works fine), touch off the XY coordinates in the center, touch off Z=0 on the surface, then see what happens:
The “engraving depth” translates directly into the force applied to the diamond point, because the spring converts displacement into force. Knowing the Z depth, you can calculate or guesstimate the force.
Early results from the 3018 suggest it can engrave good-looking text about 20 times faster than the MPCNC:
You must trade off speed with accuracy on your very own machine, as your mileage will certainly differ!
The GCMC source code as a GitHub Gist:
Having previously concluded running the CNC 3018-Pro steppers from 12 V would let the DRV8825 chips provide better current control in Fast Decay mode at reasonable speeds, I wondered what effect a 24 V supply would have at absurdly high speeds with the driver in 1:8 microstep mode to reduce the IRQ rate.
So, in what follows, the DRV8825 chip runs in 1:8 microstep mode with Fast Decay current control. You must apply some hardware hackage to the CAMTool V 3.3 board on the CNC 3018-Pro to use those modes.
In all the scope pix, horizontal sync comes from the DRV8825 Home pulse in the top trace, with the current in the two windings of the X axis motor in the lower traces at 1 A/div. Because only the X axis is moving, the actual axis speed matches the programmed feed rate.
Homework: figure out the equivalent two-axis-moving speed.
The 12 V motor supply works well at 140 mm/min, with Fast Decay mode producing clean microstep current levels and transitions:
The sine waves deteriorate into triangles around 1400 mm/min, suggesting this is about as fast as you’d want to go with a 12 V supply:
Although the axis can reach 3000 mm/min, it’s obviously running well beyond its limits:
The back EMF fights the 12 V supply to a standstill during most of the waveform, leaving only brief 500 mA peaks, so there’s no torque worth mentioning and terrible position control.
Increasing the supply to 24 V, still with 1:8 microstepping and Fast Decay …
At a nose-pickin’ slow 14 mm/min, Fast Decay mode looks rough, albeit with no missteps:
At 140 mm/min, things look about the same:
For completeness, a detailed look at the PWM current control waveforms at 140 mm/min:
The dead-flat microstep in the middle trace happens when the current should be zero, which is comforting.
At 1400 mm/min, where the 12 V waveforms look triangular, the 24 V supply has enough mojo to control the current, with increasing roughness and slight undershoots after the zero crossings:
At 2000 mm/min, the DRV8825 is obviously starting to have trouble regulating the current against the increasing back EMF:
At 2500 mm/min, the back EMF is taking control away from the DRV8825:
The waveforms take on a distinct triangularity at 2700 mm/min:
They’re fully triangular at 3000 mm/min:
In round numbers, you’d expect twice the voltage to give you twice the speed for a given amount of triangularity, because the current rate-of-change varies directly with the net voltage. I love it when stuff works out!
At that pace, the X axis carrier traverses the 300 mm gantry in 6 s, which is downright peppy compared to the default settings.
Bottom lines: the CNC 3018-Pro arrives with a 24 V supply that’s too high for the DRV8825 drivers in Mixed Decay mode and the CAMTool V3.3 board’s hardwired 1:32 microstep mode limits the maximum axis speed. Correcting those gives you 3000 mm/min rapids with good-looking current waveforms.
I’m reasonably sure engraving plastic and metal disks at 3000 mm/min is a Bad Idea™, but having some headroom seems desirable.
The flanges around the door of our giant mailbox rusted through, leaving the door to bend along the embossed (debossed? Whatever) lines across the front. Eventually, the bend got bad enough to keep the door from latching closed, but reviews of the current crop of mailboxes suggest they’re even more prone to rusting after even fewer years.
Well, I can fix that:
Because the bottom third of the door, basically everything around and below that horizontal ridge, had corroded, the general idea was to stiffen it with an internal plate:
The array of small holes suggest the plate’s rich lived experience. Some are even tapped!
External angle brackets stiffen the sides along the corroded flanges and surround the equally corroded pivot holes:
The term “brick shithouse” springs unbidden to mind, doesn’t it? Those spare holes come from previous uses; I decided this application didn’t demand cosmetic perfection and, as a result, the remaining angle stock has no holes at all.
Also, the angle brackets are as long as they are because that’s the maximum throat depth for Tiny Bandsaw™. I splurged on a Proxxon 10-14 TPI blade (for future reference: PN 28172) that cuts aluminum like butter, much better than the stock 14 TPI blade.
The hinge pins used to be rivets. After careful consideration, I replaced them with 1/4-20 button-head cap screws:
Yes, the sheet metal now pivots on screw threads, rather than a nice smooth cylinder. The nyloc nut maintains the proper amount of looseness around the battered sheet metal.
While I had the door open, I slobbered hot melt glue over the flag anchor, which should keep it from spitting the ratchet pin into the roadside debris ever again:
A pleasant evening of Quality Shop Time, indeed!
The alert reader will note I’m securing aluminum plates with stainless steel hardware on a (nominally) galvanized steel box, thereby forming several batteries with a brine electrolyte from wintertime road salt. My engineering judgement determined this repair will last Long Enough™ and, most likely, succumb to somebody not quite making the curve while accelerating from the traffic signal.
Aaaaand those painted numbers still look pretty good after four years.
A young Coopers Hawk swooped across the yard, landed on a branch, and proceeded to dismantle something yummy, scattering little bits on the driveway below. One piece fluttered down like a feather, but, after the hawk flew off, we found this:
It wasn’t a feather, it was an entire wing!
A few feet away, we found another:
Not that there was any doubt, but these parts clinched the identification:
Some days earlier, we admired eight Praying Mantises on the decorative grasses and bushes out front. Perhaps it was this one:
Or this one, a few feet away:
We don’t know what, if any, the difference between brown and green wing covers might indicate. Age? Gender? Attitude? Skill level?
It’s a food chain out there!