Ed

By turns: tinker, engineer, husband, author, amateur raconteur, recumbent cyclist, father, ham radio geek. So many projects, so little time!

Homepage: http://softsolder.com

Sherline: Diamond Drag Engraving Tool Holder

Although I shouldn’t have used a hardened shaft for the case, the rest of the diamond drag tool holder worked out well enough:

Sherline Diamond Drag Holder - assembled
Sherline Diamond Drag Holder – assembled

The dimension doodle shows what’s inside and gives some idea of the sizes:

Sherline Diamond Drag Holder - dimension doodles
Sherline Diamond Drag Holder – dimension doodles

From left to right, it’s an M6×1.0 setscrew to adjust the spring preload, a spring harvested from a cheap clicky ballpoint pen, a machined cap, a 3 mm rod (which should be a hardened & ground shaft, but isn’t) surrounded by a pair of LM3UU linear bearings, a machined coupler, and the stub of a diamond engraving tool’s shank.

Tapping 15 mm of M6×1.0 thread inside of the case took an unreasonable amount of grunt. Next time, brass.

The setscrew gets a little boss to hold the spring away from the adjacent threads in the case:

Sherline Diamond Drag Holder - setscrew spring boss
Sherline Diamond Drag Holder – setscrew spring boss

The little machined cap has a somewhat longer spring guide to prevent buckling:

Sherline Diamond Drag Holder - shank cap spring guide
Sherline Diamond Drag Holder – shank cap spring guide

The spring fits snugly on the slightly enlarged section inside the last few coils, with the rest being a loose fit around the guide. When the spring is fully compressed, it’s just slightly longer than the guide and can’t buckle to either side.

The cap gets epoxied onto the 3 mm rod with some attention to proper alignment:

Sherline Diamond Drag Holder - shank cap alignment
Sherline Diamond Drag Holder – shank cap alignment

The other end of the rod has a 3 mm thread, which would be a serious non-starter on a hardened rod.

The shortened diamond tool shank gets epoxied into the gizmo connecting it to the now-threaded rod, again with some attention paid to having it come out nicely coaxial:

Sherline Diamond Drag Holder - diamond tool alignment
Sherline Diamond Drag Holder – diamond tool alignment

The LM3UU bearings got epoxied into the case, because I don’t have a deep emotional attachment to them.

Unscrew diamond tool, push spring onto cap, drop rod through bearings, crank setscrew more-or-less flush with the end of the case, screw diamond in place with some weak threadlock, add oil to rod, work it a few times to settle the bearings, and it’s all good.

A quick spring rate measurement setup, with a brass tube holding the diamond point off the scale pan:

Sherline Diamond Drag Holder - installed
Sherline Diamond Drag Holder – installed

The spring rate works out to 230 g + 33 g/mm for deflections between 1.0 mm (263 g) and 3.5 mm (346 g), so it’s in the same ballpark as the diamond tools on the MPCNC and CNC 3018.

Note: WordPress just “improved” their post editor, which has totally wrecked the image alignment. They’re all set to “centered” and the editor says they are, but they’re not. It’s a free blog and I’m using one of their ancient / obsolete / unsupported themes, so I must update the theme. Bleh.

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Mini-Lathe vs. Case-Hardened Shaft

While doodling a drag knife holder for the Sherline, I figured a 3/8 inch shaft would hold all the parts and fit neatly into a standard Sherline tool holder, which it did:

Sherline Diamond Drag Holder - installed
Sherline Diamond Drag Holder – installed

Having recently upcycled a 3/8 inch shaft from the Thing-O-Matic into a pen holder for the CNC 3018-XL, I cut off another section with an abrasive wheel, then tried to face it off:

Hardened shaft facing - abrasive step
Hardened shaft facing – abrasive step

Although the mini-lathe’s carbide insert gnawed at the shaft’s case-hardened shell, it obviously wasn’t making much progress against that step.

Back to the abrasive cutoff saw:

Hardened shaft facing - abrasive flattening
Hardened shaft facing – abrasive flattening

Which looked better, although it still wasn’t quite perpendicular to the shaft axis.

Back to the lathe:

Hardened shaft facing - lumpy face
Hardened shaft facing – lumpy face

Well, it’s better, but it sure ain’t pretty.

Put gently, the mini-lathe’s lack of rigidity doesn’t help in the least. The compound was a-reelin’ and a-rockin’ on every revolution and eventually turned a slight tilt into a distinct radial step.

Memo to Self: Dammit, use a brass rod!

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COVID-19: Elephant Sighting

As far as this engineer can tell, here’s about all you need to know about the COVID-19 pandemic:

Total Deaths = Total Cases recorded two weeks earlier

This also works forward in time: given the total number of cases “today”, I (and you) can predict the total number of deaths in two weeks, give or take a few days.

Run the numbers for Italy, because it has a relatively long timeline and trustworthy data:

  • 2020-03-01: 1694 cases → 2020-03-15: 1809 deaths
  • 2020-03-02: 2036 cases → 2020-03-16: 2158 deaths
  • 2020-03-03: 2502 cases → 2020-03-17: 2503 deaths

As the numbers become difficult to comprehend, the time difference slows to 16 days instead of 14:

  • 2020-03-06: 4636 cases → 2020-03-22: 4825 deaths
  • 2020-03-07: 5883 cases → 2020-03-23: 6077 deaths

On 2020-03-23, Italy had 63,927 confirmed cases. Prediction: Easter will not be celebrated in the usual manner.

Consider the data for the US, also in March 2020:

  • 2020-03-05: 175 cases → 2020-03-19: 174 deaths
  • 2020-03-06: 252 cases → 2020-03-20: 229 deaths
  • 2020-03-07: 353 cases → 2020-03-21: 292 deaths

Pop quiz: Given that the US has 32,761 total cases as of today (2020-03-22), estimate the total deaths in two weeks.

New York State will have similar statistics, although it’s too soon to draw conclusions from today’s 20,875 confirmed cases.

In addition to the Wikipedia articles linked above, you may find these sites useful:

Exhaustive tracking and mapping from Johns Hopkins (the GUID gets to reach the JHU data): https://www.arcgis.com/apps/opsdashboard/index.html#/bda7594740fd40299423467b48e9ecf6

Comprehensive COVID-19 tracking, with logarithmic graph scales: https://www.worldometers.info/coronavirus/

More raw data: https://virusncov.com/

CDC National cases, with a per-day graph down the page: https://www.cdc.gov/coronavirus/2019-ncov/cases-updates/cases-in-us.html

New York State COVID-19 info: https://coronavirus.health.ny.gov/home

Perhaps more useful for me than you, but the Dutchess County information: https://www.dutchessny.gov/Departments/DBCH/2019-Novel-Coronavirus.htm

The current recommendation: remain home unless and until you develop COVID-19 symptoms requiring urgent medical attention. Should that happen to me, I fully expect there will be no medical attention to be found and, certainly, all available medical equipment will be oversubscribed.

Speaking strictly as an Olde Farte looking at the data, the future looks downright grim.

On the upside, it’s amazing how little an order to remain home changed my daily routine: so many projects, so little time.

Memo to Self: Wash your hands!

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Drag Knife Calibration: Downforce and Speed

The drag knife faceplant suggested I must pay a bit more attention to fundamentals, so, with a 60° drag knife blade sticking out a reasonable amount, the next step is to see what effect the cutting “depth” (a.k.a. downforce) and speed have on the outcome.

A smidge of GCMC code later:

Drag Knife Cal - depth - overview - Camotics sim
Drag Knife Cal – depth – overview – Camotics sim

It’s not obvious, but each pattern steps downward by 0.5 mm from left to right. With the spring force equal to 375 g + 57 g/mm, the downforce ranges from 400 to 520 g over the five patterns.

Laminated scrap, meet drag knife:

Drag Knife Cal - Depth - as cut
Drag Knife Cal – Depth – as cut

Pulling up on the surrounding scrap left the patterns on the sticky mat:

Drag Knife Cal - Depth - extracted
Drag Knife Cal – Depth – extracted

Which suggested any cutting force would work just fine.

Flushed with success, I cut some speed variations at the minimum depth of Z=-0.5 mm = 400 g:

Drag Knife Cal - Speed - 0.5 mm - as cut
Drag Knife Cal – Speed – 0.5 mm – as cut

The blade cut through the top laminating film, the paper, and some sections of the bottom film, but mostly just scored the latter.

Repeating at Z=-1.5 mm = 460 g didn’t look much different:

Drag Knife Cal - Speed - 1.5 mm - as cut
Drag Knife Cal – Speed – 1.5 mm – as cut

However, the knife completely cut all the patterns:

Drag Knife Cal - Speed - 1.5 mm - extracted
Drag Knife Cal – Speed – 1.5 mm – extracted

As far as I can tell, the cutting speed doesn’t make much difference, although the test pattern is (deliberately) smooth & flowy like the Tek CC deck outlines. I’d been using 1000 mm/min and 2000 mm/min seems scary-fast, so 1500 mm/min may be a good compromise.

The GCMC source code as a GitHub Gist:

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Drag Knife Blade Extension

The battered corner of my bench scale shows it’s been knocking around for quite a while, but the drag knife blade tip seems pretty close to the first 0.5 mm division:

Drag Knife Blade - 0.5 mm
Drag Knife Blade – 0.5 mm

The blade extends from the LM12UU holder for the MPCNC.

Scribbling the blade across a scrap of laminated yellow card stock (about 0.4 mm thick) showed it didn’t cut all the way through the bottom plastic layer, even with the spring mashed flat.

So I screwed it out to 0.7 mm:

Drag Knife Blade - 0.7 mm
Drag Knife Blade – 0.7 mm

The scale isn’t quite parallel to the blade axis and maybe it’s sticking out 0.8 mm; setting a drag knife’s blade extension obviously isn’t an exact science.

In any event, another scribble slashed all the way through the laminated deck without gashing the sacrificial cardboard atop my desk, which seems good enough.

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Drag Knife Blade Lengths

The distances from the sharp tip to the top end of the edge, measured parallel to the shank axis:

  • 60° = 1.3 mm
  • 45° = 0.7 mm
  • 30° = 0.6 mm

Here, the angle goes upward from the paper / Tek CC deck / whatever to the shank axis, so the 60° blade at the top of the picture has the longest blade edge.

Drag Knife Blades - unused 60 45 30 degree
Drag Knife Blades – unused 60 45 30 degree

That’s for one trio of blades from a single eBay seller. I expected no consistency between sellers and that’s exactly what I got when I sorted my collection by peering through the microscope:

Drag Knife Blades - inconsistent cap colors
Drag Knife Blades – inconsistent cap colors

Red seems consistently 45°, but blue & yellow caps can cover either 30° or 60° blades. The actual blade angle lies mostly within ±5° of the nominal value, with 45° between 40° and 50°, but I doubt my few samples span the QA space.

The flat shaping the backside of the blade should put the point 0.25 mm from the shank axis and, because the blades are 1.0 mm ⌀, also 0.25 mm from the OD. A few spot measurements suggest the point offset can be up to 0.4 mm from the axis, so any fancy calculations you might think of making seem pretty much irrelevant.

There’s not much practical difference between the 30° (“window tint”) and 45° (“vinyl”) blades, particularly given the angle and offset tolerances, but 60° blades (“card stock”) seem better suited to cutting the 0.3 mm to 0.4 mm thick laminated Tek Circuit Computer decks than the 45° blades I’ve been using.

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Round Soaker Hose Splint

One of two new round rubber soaker hoses arrived with a slight crimp, enough to suggest it would crumble at an inopportune moment. Rather than return the hose for something that’s not an obvious failure, I clamped the crimp:

Round Soaker Hose Splice - top
Round Soaker Hose Splice – top

Unlike the clamps for the punctured flat soaker hoses, this one doesn’t need to withstand much pressure and hold back a major leak, so I made the pieces a bit thicker and dispensed with the aluminum backing plates:

Round Soaker Hose Splice - bottom
Round Soaker Hose Splice – bottom

The solid model is basically the same as for the flat hoses, with a slightly oval cylinder replacing the three channels:

Round Soaker Hose Splice - OpenSCAD model
Round Soaker Hose Splice – OpenSCAD model

The OpenSCAD source code as a GitHub Gist:

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