The Smell of Molten Projects in the Morning

Ed Nisley's Blog: Shop notes, electronics, firmware, machinery, 3D printing, laser cuttery, and curiosities. Contents: 100% human thinking, 0% AI slop.

Category: Machine Shop

Mechanical widgetry

MPCNC: Rail Height Measurements and Plot Effects
After once again figuring out how to read a vernier height gage, I measured the height of each end of the MPCNC rails:

Brown and Sharpe 585 Height Gage

The process:
- Position the gage near the end of the gantry’s travel
- Twiddle the knurled ring to lower the probe (a.k.a. lathe bit) until …
- It firmly captures the paper slip, then …
- Twiddle the ring the other way until …
- The paper barely moves
- Read the vernier and take a picture
So the numbers come out one paper thickness higher than the actual rail height; subtract 0.1 mm = 4 mil to get the true height:

MPCNC Rail Height – 2017-12-23

In round numbers, the difference is under 0.3 mm along each rail.

The outer numbers on the lower sketch show the difference between each reading and the lowest value along that axis: the left rear corner is (roughly) 0.5 mm higher than the right front. The numbers inside the square give the additional height, rounded to sensible values, required to raise the low corners.

Which means you can’t plot at, say, Z=-0.2 mm to reduce the pen loading, because the pen doesn’t uniformly touch the paper across the entire plot:

MPCNC – Unlevel Z -0.2 plot

These images have been perspective & aspect ratio corrected, then ruthlessly contrast-stretched to make the traces visible; the lighting isn’t that awful in person!

With the plot at Z=-0.2, the legends toward the front came out OK, but they’re missing along the far edge. The Spirograph traces go completely missing toward the left rear as the pen rises away from the paper, although I think we’re also seeing some ripples in the paper sheet.

Although such a small error probably makes no difference to a wood router, let’s see what we can do.

Manually editing the G-Code to put successive traces at 0.1 mm increments from Z=-0.3 to Z=-0.6 mm, then replotting on the same piece of paper, shows the problem a bit better:

MPCNC – Unlevel plot – multiple Z

All of the legends remain at Z=-0.2, because I wasn’t up for editing every pen-down command.

Even at Z=-0.6 mm, the pen doesn’t quite touch in the left rear corner. Previously, I’d been plotting at a nice, round Z=-1.0 mm, which worked fine. I didn’t run any tests below Z=-0.6, but I think Z=-0.8 would draw a complete plot.

That agrees reasonably well with the height gage measurements.

It’s obviously impossible to re-level the rails by dinking around with the corner post lengths, because I can’t move the EMT in precise increments and it’d never stay in that position anyway. Instead, I should slide shims under the three lowest corner feet to raise them enough to match the left rear corner.
2018-01-04
MPCNC: Emergency Stop / Feed Hold / Resume Pendant

The Protoneer CNC Shield has pin headers for GRBL’s Feed Hold and Resume inputs, so it seemed appropriate to put big buttons on the far end of the cable:

MPCNC – E-stop Hold Resume switch box

The Emergency Stop Push Button Switch Station arrived for ten bucks delivered halfway around the planet.

There’s not much to the wiring inside the box:

MPCNC – E-Stop switch box – interior

I drilled a hole to fit the 6 pin Aviation Wire Connectors I got for this very purpose:

MPCNC – E-stop switch box – drilling

You could CNC machine a precise D-hole, but let’s stay realistic about the application. Applying a deburring tool enlarged the 9/16 inch hole enough to force the 16 mm threads into it, with the drill press holding the connector perpendicular to the box while I hand-turned the chuck to screw it in.

Although I like the Protoneer CNC Shield, I really really dislike using header pins as connectors:

MPCNC – Protoneer Wiring – SSR

Those pins are much too delicate.

The DC-DC solid state relay input connects to the Arduino’s +5 V power supply through the red mushroom disconnect switch. The mushroom is normally closed to turn on the SSR and connect the power brick’s +24 V supply to the motors; it opens when slapped. GRBL will continue about its business, but without any power to the steppers the MPCNC will stop dead in its tracks. Turn the mushroom cap clockwise to unlatch and reset.

The disconnect switch should also kill AC power to the router, when I get around to adding one to the mix, probably through a DC-AC SSR.

AFAICT, the cable should come out of the box on the end with the mushroom switch, putting the “normal” pushbuttons closer to me. I did it the other way around, because I want the panic button to be the most easily reached thing on the benchtop. If I have time to think about it, I can reach around the mushroom to the Hold switch.

2018-01-03
Reading a Vernier Height Gage
The first time around, I simply set both pairs of MPCNC rails to equal heights using my height gage (*) as a reference, rather than as a measurement tool:

MPCNC – Rail height measurement

By now, I assume all the plastic bits have shaken themselves down and the rails have settled into their more-or-less permanent locations, so it’d be useful to measure the actual rail heights and adjust as needed. The scale along the vertical bar of the height gage gives the height of the top surface of the projecting arm above the bench:

Brown and Sharpe 585 Height Gage

Normally, the gage base would sit on a surface plate. Building an MPCNC on a big granite slab would certainly cut down on the shakes from overly enthusiastic acceleration settings!

The nicely reshaped and polished lathe bit transfers the top surface of the gage arm to the top of the MPCNC rail, so whatever height shows up on the vernier gives the rail height. The exact value, of course, doesn’t really matter in this situation, but when you need an actual measurement, it’s got you covered.

The two brackets slide along the height gage, with the thumbscrews on the right locking them in position. To measure a height, you loosen both thumbscrews, slide the whole affair to put the arm bracket at about the right height, tighten the top thumbscrew to anchor the adjusting bracket, twirl the knurled wheel to precisely position the arm bracket, then read the height from the scale.

This requires reading a vernier height gage scale:

Vernier Height Gage – 132.20 mm

The other scale on the other side has inches, but nobody uses those any more. Right?

Things I didn’t get quite right the first time around:
- The numbers along the right side are in centimeters
- The smallest lines on that scale mark 0.5 mm increments
- The numbers on the vernier have units of 1/50 mm = 0.02 mm
So, to read the scale:
- Multiply centimeters by 10 to get millimeters: 130
- Add the number of whole millimeters below the 0 vernier index: 2
- Add a half millimeter if needed: 0
- Find the matching vernier increment: 10
- Multiply the increment by 2: 20
- Slap the decimal point two places left and add: 132.20
OK, try this one:

Vernier Height Gage – 159.84 mm

As I see it:
- Read 15 cm
- Count 9 ticks
- Add the 0.5 mm tick
- Match vernier tick 17, multiply and slap decimal = 0.34 mm
- Add: 150 + 9 + 0.5 + 0.34 = 159.84 mm
There, now, that wasn’t so hard, was it?

There’s obviously a parallax issue between the edge of the vernier scale and the main scale; it’s easier to get it right in person than in the photograph.

I pronounced the reading as “160 minus point 5 is 159 and a half plus point 34 is point 84”, but I also take eight photographs as I work my way around the MPCNC frame to review any suspicious results.

Obviously, reading a digital height gage would be much easier & faster, but we don’t want to deskill the workforce, do we?

The maker’s mark on my height gage says it’s a Brown & Sharpe 585 with a 19 inch scale; B&S has long since been Borged. Back in the day, this painstakingly applied etching distinguished it from all the other height gages in the shop:

Brown and Sharpe 585 Height Gage – D.E 1-I-3 etching

We’ll never know the rest of the story.

(*) When Starrett spells it “gage”, it’s good enough for me.
2018-01-02
Roadside Jewelry

I spotted a piece of jewelry during a recent walk:

Headlight Condenser – rear

The other side shows off The Shiny Bit:

Headlight Condenser – front

I seem to have swapped the “front” and “rear” labels; the flat side faces the LED / HID bulb.

It looked even better after extraction and casual cleaning:

Headlight Condenser – sunlit

It seems someone with a relatively new car had a fairly high energy accident just north of Red Oaks Mill. The remainder of the debris consisted of shattered engineering plastic. We’ll never know the rest of the story.

Both lens surfaces have a slight nubbly finish, perhaps to produce some side light around the main beam. The rectangular opening apparently shaped the low beam and doesn’t appear movable, so perhaps the car had separate headlights for the high beams.

I’m not quite sure what to do with a chipped condenser lens, so it’s sitting on the windowsill (in a sun-safe orientation) along with many other glittery bits of glass I’ve collected over the years.

2017-12-30
Pogo Pins
A Pogo Pin reference may be useful:
- P.. and R.. refer to Pin and Receptacle (a.k.a. socket), respectively
- Pxx and Rxx = nominal pin diameter in 0.01 mm units: P50 = 0.48 mm
For pins, the suffix -hn indicates pin head shape, the most useful of which may be:
- B1: 45° cone
- J1: dome end
- Dx: large dome, also 1D
- Gx: cylinder
- Ex: large 90° cone, sometimes 1E
- T2 – large chisel
For sockets, the suffix -ntl gives:
- n – entry shape: 1 = shaped entry, 2 = straight entry
- t – termination: C = crimp, S = solder, W = wire
- l – length of wire in 100 mm units: 7 = 700 mm
From what I can find on eBay, all pins have 6 mm travel with typically 75 / 100 / 180 g spring force.

A picture ripped from the reference to forestall link rot:

P75 Spring Test Probes

Memo to Self: US-based eBay sellers charge three times more than Chinese sellers, but deliver in one-third the time.

[Update: Simon sends a link to Everett Charles Technologies, a pogo-pin manufacturer providing “Probably much more information than anyone should ever want”. Of course, eBay / Amazon junk may not meet any particular specs, so scale your expectations accordingly.]
2017-12-29
MPCNC: Plotter Pen Holder Spring Constant

Watching the MPCNC plot Spirograph patterns led me to wonder about how much force the printed drag knife holder applies to the pen:

Spirograph – liquid ink pen – detail

The HP 7475A plotter spec calls for 19 g = 0.67 oz of downward force on the pen, so, in an ideal world, one might want to use one’s collection of aging plotter pens in a similar manner.

Plotter pen, meet digital scale:

MPCNC – Plotter pen force test

Stepping the pen downward in 0.1 mm increments produced a set of numbers and a tidy linear fit graph:

MPCNC Plotter Pen Holder – Spring Constant

I hereby swear I’m not making things up: the spring constant really is a nice, round 100 g/mm!

I set plot_z = -1.0 in the GCMC program, with Z=0.5 touched off atop a defunct ID card on the paper surface to compensate for any tabletop warp / bow / misalignment, plus any errors from the tool length probe. An eyeballometric scan against a straightedge shows pretty nearly no misalignment, which means the holder mashes the pen against the paper with about 100 g of force, five times the HP spec.

A distinct case of pen abuse rears its ugly head.

It’s time to conjure a height probe for the tool holder.

2017-12-28
Spirograph Random Numbers: What Are The Odds?
The GCMC Spirograph Generator program chooses parameters using pseudo-random numbers based on a seed fed in from the Bash script, so I was surprised to see two plots overlap exactly:

Overlaid pattern – G-Code simulator

The two overlapping traces are the 15 inward-pointing wedges around the central rosette.

The first one:
```
(PRNG seed: 38140045)
(Paper size: [16.50in,14in])
(PlotSize: [15.50in,13.00in])
(Stator 3: 150)
(Rotor  4: 40)
(GCD: 10)
(Offset: -0.94)
(Dia ratio: -0.27)
(Lobes: 15)
(Turns: 4)
(Plot scale: [5.11in,4.29in])
(Tool change: 1)
T1
M6
```
The second one:
```
(PRNG seed: 74359295)
(Paper size: [16.50in,14in])
(PlotSize: [15.50in,13.00in])
(Stator 3: 150)
(Rotor  4: 40)
(GCD: 10)
(Offset: -0.93)
(Dia ratio: -0.27)
(Lobes: 15)
(Turns: 4)
(Plot scale: [5.12in,4.30in])
(Tool change: 3)
T3
M6
```
The Offset isn’t quite the same, but the pen width covers up the difference.

With only four Stators and 17 Rotors, the probability of picking the same pair works out to 0.25 × 0.059 = 1.4%. You can sometimes get the same number of Lobes and Turns from several different Stator + Rotor combinations, but these were exact matchs with the same indices.

The Pen Offset within the Rotor comes from a fraction computed with ten bit resolution, so each Offset value represents slightly under 0.1% of the choices. If any four adjacent values look about the same, then it’s only eight bits of resolution and each represents 0.4%.

The Rotor and Stator set the Diameter ratio, but the sign comes from what’s basically a coin flip based on the sign of a fraction drawn from 256 possibilities; call it 50%.

Overall, you’re looking at a probability of 28 ppm = 0.0028%, so I (uh, probably) won’t see another overlay for a while …

I don’t know how to factor the PRNG sequence into those numbers, although it surely affects the probability. In this case, two different seeds produced nearly the same sequence of output values, within the resolution of my hack-job calculations.

Whatever. It’s good enough for my simple purposes!
2017-12-27