Posts Tagged 3018 CNC

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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GRBL Error 33: Arc Coordinates vs. Decimal Places

The LinuxCNC G2/G3 arc command doc has this to say about numerical precision:

When programming arcs an error due to rounding can result from using a precision of less than 4 decimal places (0.0000) for inch and less than 3 decimal places (0.000) for millimeters.

So I normally set GCMC to produce three decimal digits, because its default of eight digits seems excessive for my usual millimeter measurements, and assume whatever G-Code sender I use won’t chop digits off the end in passing. Mistakenly setting bCNC to round at two decimal places showed what happens with fewer digits, as bCNC’s default is four decimal digits.

A closer look at the coordinates in the lower right part of the spreadsheets (from yesterday) shows the limited accuracy with two decimal digits:

Spreadsheet - GCMC 2 digit - full path - detail
Spreadsheet – GCMC 2 digit – full path – detail

The red blocks mark the first failing arc, where the relative error falls out of tolerance. If GRBL were less fussy (which it should not be), then the next arcs would proceed as shown.

Rounding to three decimal digits pushes the errors into to the third place, with the yellow highlight marking the worst errors:

Spreadsheet - GCMC 3 digit - detail
Spreadsheet – GCMC 3 digit – detail

As you should expect, the smallest arcs have the largest relative errors, although they’re now well within GRBL’s (and LinuxCNC’s, for that matter) limits.

Rounding to four decimal digits makes the errors vanishingly small:

Spreadsheet - GCMC 4 digit - detail
Spreadsheet – GCMC 4 digit – detail

So, by and large, don’t scrimp on the decimal digits … but we knew that already.

I’d been setting GRBL to produce three decimal places, but now I’m using four. Adding a few characters to each G-Code command reduces the number of commands fitting into GRBL’s buffer space, but bCNC normally keeps it around 90% full, so the path planner should remain perfectly happy.

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Homage Tek Circuit Computer: Yellow Variation

An on-sale pack of yellow Astrobrights card stock tempted me:

Homage Tek CC - Yellow Astrobrights paper
Homage Tek CC – Yellow Astrobrights paper

The somewhat wrecked cursor comes from my collection of discards, because I haven’t yet figured out how to mill the outline and engrave the hairline on raw stock.

The paper isn’t quite the same color as my Genuine Pickett Model 110-ES circular slide rule:

Homage Tek CC vs Pickett 110ES colors
Homage Tek CC vs Pickett 110ES colors

Nor, of course, are the ticks and legends nearly as fine as you get with real engraving, but it’s probably Close Enough™ for anybody other than a Real Collector™.

The Pilot V5RT ink bleeds less on Astrobrights card stock than on the previous, somewhat coarser, card stock:

Tek CC - Yellow Astrobrights paper - bare
Tek CC – Yellow Astrobrights paper – bare

An automagic color adjustment bleaches the yellow and makes the black ink much more visible.

Laminating the paper crisps the contrast a bit, although it’s more obvious in person:

Tek CC - Yellow Astrobrights paper - laminated
Tek CC – Yellow Astrobrights paper – laminated

You can see tiny air bubbles over the darkest part of the ticks and letters.

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Diamond-Drag Styrene Engraving: Scraped Enamel

For the first time in a loooong time, I applied Testors Gloss Enamel paint to styrene plastic:

Engraving Testpiece D - Testors Enamel - red
Engraving Testpiece D – Testors Enamel – red

Two coats of black paint produced the larger areas along the inner scales and completely filled those engraved lines:

Engraving Testpiece D - Testors Enamel - red black applied
Engraving Testpiece D – Testors Enamel – red black applied

With exactly the correct paint on exactly the correct material, it cured into a non-removable layer. Being enamel, however, the last coat requires two or three days for a full cure, so this isn’t a short-attention-span project.

It’s “non-removable” unless you’re willing to abrade the surface:

Engraving Testpiece D - Testors Enamel - scrape sand - overview
Engraving Testpiece D – Testors Enamel – scrape sand – overview

Sanding tends to remove too much plastic, particularly when confronted with raised walls & suchlike along the grooves. The darkest scale down the middle was engraved with 300 g downforce and is deep enough to retain all its paint:

Engraving Testpiece D - Testors Enamel - sanded - 250 300 g - detail
Engraving Testpiece D – Testors Enamel – sanded – 250 300 g – detail

As expected, paint scrapers produce better results:

Engraving Testpiece D - Testors Enamel - scrape - 250 300 g - detail
Engraving Testpiece D – Testors Enamel – scrape – 250 300 g – detail

There’s not much visible difference between the 250 g and 300 g scales.

All the scraped lines are over 0.1 mm wide, with the heavier downforce producing maybe 0.12 mm.

The double-coated lines are flush with the (scraped) surface and visibly matte. The single-coated regions have the usual glossy enamel finish remaining deep in the lines & numbers, with a thin matte outline flush with the surrounding surface. It’s basically impossible to photograph those features, at least for me.

The colors are crisp & vivid: enamel paint is the way to go!

The next testpiece should run downforce variations from 300 through 500 g and speeds from 1000 to 2400 mm/min. Scraping off the raised plastic before painting should deliver a better ahem painting experience without much surface damage; the trick will be clearing all the debris from the engraved lines.

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Diamond-Drag Styrene Engraving: Scraped Sharpie

Applying only two Sharpie colors to the third quadrant of the engraving testpiece produces a more restrained result:

Diamond on styrene C - scraped red-black Sharpie - start
Diamond on styrene C – scraped red-black Sharpie – start

Instead of sanding the surface, I used a paint scraper to remove everything down to the engraved grooves. The scraper in the upper right is a Rubbermaid 54807, which is apparently no longer available. If I ever buy a new scraper, I’ll spring for a carbide blade.

A dirt speck under the plastic sheet can still obliterate the markings, though:

Diamond on styrene C - scraped red-black Sharpie - first clearing
Diamond on styrene C – scraped red-black Sharpie – first clearing

Overall, the results look just like a real slipstick:

Diamond on styrene C - scraped red-black Sharpie - 225 300 g scale detail
Diamond on styrene C – scraped red-black Sharpie – 225 300 g scale detail

The upper scale was engraved at 225 g downforce, the lower at 300 g, with corresponding differences in width & depth.

Seen at higher magnification with omnidirectional light through the microscope, the tick marks have more detail:

Diamond on styrene C - scraped red-black Sharpie - 225 300 g line detail
Diamond on styrene C – scraped red-black Sharpie – 225 300 g line detail

The upper ticks are 0.1 mm wide and the lower ticks a scant 0.2 mm wide. Both ticks on the sanded Sharpie sample were close to 0.1 mm, which suggests:

  • Scraping removes less plastic
  • The grooves have a flat-ish bottom and side walls roughly matching the slightly worn 60° diamond tool

Sharpie ink is, of course, soluble in alcohol:

Diamond on styrene C - scraped red-black Sharpie - alcohol wipe
Diamond on styrene C – scraped red-black Sharpie – alcohol wipe

That’s not unexpected, as I’ve been removing Sharpie with alcohol forever, but it’s worth keeping in mind. I don’t know if spraying a clear topcoat (Krylon FTW!) would provide good sealing with enough wear resistance.

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Diamond-Drag Styrene Engraving: Sanded Sharpie Colors

Attacking another quadrant of the engraving testpiece with All The Sharpies produced a cheerful mess:

Diamond on styrene B - Sharpie colors
Diamond on styrene B – Sharpie colors

The “300 g” notation is wrong: the innermost scale is on the middle deck, which I engraved with 250 g of downforce, and reads through a window on the top deck. The next scale outward, the inner half of the green block on the left, would be on the upper deck at 300 g, just beyond the innermost scale.

I removed the excess marker with a 320 (-ish) grit abrasive sanding block, producing a remarkable amount of gray dust in the process:

Diamond on styrene B - sanded
Diamond on styrene B – sanded

The general idea was to find out what the colors looked like when confined to narrow engraved slots:

Engraving Testpiece B - Sharpie colors - 2x600 dpi
Engraving Testpiece B – Sharpie colors – 2×600 dpi

It’s enlarged a factor of two from the 600 dpi scanned image by the simple expedient of changing it to 300 dpi, then assuming all the downstream image handling will Do The Right Thing, which could happen.

I sanded it before fully appreciating how even the smallest particle of crud under the styrene sheet ruins the result:

Engraving Testpiece B - debris oversanding
Engraving Testpiece B – debris oversanding

In this section, the scale with green numbers and black ticks was engraved at 300 g and is slightly less abraded than the adjacent scale at 225 g. Guesstimating the depth at 0.13 mm, 0.15 mm at most, the sanding block doesn’t remove much plastic at all … just enough to remove the scales.

The lines are all about 0.1 mm wide and, to the naked eyeball, look about the same as the lines on my K&E Deci-Lon slipstick:, done on a real production line with an actual engraving tool and somebody who knew what he (I’m sure) was doing:

KE Deci-Lon Slide Rule - scale detail
KE Deci-Lon Slide Rule – scale detail

The red CI scale reads right-to-left and, under magnification, you can see where the red ink made its way into the adjacent tick marks. I doubt they were using a pen, but it might be a mechanized roller or dauber.

All in all, sanding works, but it’s messy and poorly controlled.

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Diamond-Drag Styrene Engraving: Line Width

Engraving all the Tek Circuit Computer scales on a single sheet of styrene plastic with a diamond drag tool produced a test piece with plenty of lines and characters:

Diamond on styrene - engraving test - in action
Diamond on styrene – engraving test – in action

I covered one quarter with good old black Sharpie, a lacquer crayon, and well-aged black acrylic wall paint:

Diamond on styrene - engraving test - raw color fill
Diamond on styrene – engraving test – raw color fill

Applying a sanding block removed the rubble + scribbles and brought the surface down to the engraved patterns:

Diamond on styrene - engraving test - 225 250 300g 2400mm-min
Diamond on styrene – engraving test – 225 250 300g 2400mm-min

The lacquer crayon doesn’t seem to adhere well to styrene:

Diamond on styrene - 225 250 g 2400mm-min - lacquer crayon
Diamond on styrene – 225 250 g 2400mm-min – lacquer crayon

A closer look shows I probably sanded off too much of the surface, perhaps above some grit below the sheet, because those lines almost vanish:

Diamond on styrene - 225 250 g 2400mm-min - lacquer crayon
Diamond on styrene – 225 250 g 2400mm-min – lacquer crayon

The crayon may adhere better to deeper lines. These are obviously too shallow and the pigment seems to come off in chunks:

Diamond on styrene - 300g 2400mm-min - lacquer crayon
Diamond on styrene – 300g 2400mm-min – lacquer crayon

The acrylic trim paint filled its patterns, despite having turned into a gummy mass during decades on the shelf:

Diamond on styrene - 225g 2400mm-min - acrylic paint
Diamond on styrene – 225g 2400mm-min – acrylic paint

The Sharpie ink, being basically a thin liquid, completely filled its patterns and (apparently) soaked into the rough side walls. The lines seem to be 0.1 mm wide at 225 g downforce:

Diamond on styrene - 225g 2400mm-min - Sharpie
Diamond on styrene – 225g 2400mm-min – Sharpie

They’re less uniform at 250 g:

Diamond on styrene - 250g 2400mm-min - Sharpie
Diamond on styrene – 250g 2400mm-min – Sharpie

A 300 g downforce produces (somewhat) more uniform 0.15 mm wide lines and slightly distorted characters:

Diamond on styrene - 300g 2400mm-min - Sharpie
Diamond on styrene – 300g 2400mm-min – Sharpie

I have no way to measure the actual engraving depth. If the 60° diamond tool had a perfect point, which it definitely doesn’t, then a 0.15 mm wide trench would be 0.13 mm deep. I’ve obviously sanded off some of the surface, so those lines could be, at most, 0.1 mm deep.

All in all, the engraving came out better than I expected!

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