Tailfins!
They don’t make tail fins like this any more:
It’s a 1962 Cadillac Fleetwood encountered on a walk around the block.
Those are a mere shadow of their former selves, though, as per Wikipedia’s photo of a 59 Caddy:
Nothing exceeds like excess!
CNC 3018-Pro: Diamond Drag Engraving Test Disk
Posted by Ed in Machine Shop, Software on 2019-09-20
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:
CNC 3018-Pro: DRV8825 Drivers at the Edge of Madness
Posted by Ed in Electronics Workbench, Machine Shop, Science on 2019-09-19
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.
Mailbox Door Rebuild
Posted by Ed in Home Ec, Machine Shop on 2019-09-18
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.
Hawk vs. Praying Mantis
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!
Walnut Husk Fly Damage
A recent Amazon purchase of three 3 lb bags of walnuts from a known-good seller arrived with many damaged nuts:
The damage matches what I read about Walnut Husk Fly infestations: shriveled kernels and terrible taste.
In round numbers, I found 8 oz of damaged nuts in each 3 lb bag, enough to ruin the entire batch. The seller immediately refunded the purchase price for all three bags, so there’s that.
It’s definitely not one of the counterfeit products plaguing Amazon, but I wonder why that lot didn’t fail incoming inspection.
I’m loathe to buy more walnuts for a while, though.
Memo to Self: Always inspect incoming purchases, even from reputable sellers!
Monthly Image: Praying Mantis vs. Monarch Butterfly
Posted by Ed in Oddities, Photography & Images, Science on 2019-09-15
The Butterfly Bush in front of the house attracts all kinds of insects, including Monarch Butterflies (shown here on the Goldenrod planted in the garden):
This year, the bush also attracted a Praying Mantis:
Then lunchtime happened:
A closer look:
Now, if that isn’t enough nightmare fuel for you, find some in your own neighborhood.
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