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

Acrylic Coasters: Edge Finishing, Round 3

Although I could turn the coaster fixture’s OD, the lathe jaws are slightly longer than the fixture is thick:

Coaster Epoxy Rim – turning fixture rim

So the fixture needs a spacer:

Coaster Epoxy Rim – cutting chuck spacer

The ID is bigger and the OD is smaller than the fixture, so it won’t get in the way of further proceedings:

Coaster Epoxy Rim – 3-jaw lathe setup

The pad on the live center came from the cookie cut from the fixture, with a just slightly off-center 3 mm hole poked into it to hold the point away from the coaster.

A ring of carpet tape on the fixture provides traction holding the coaster in place:

Coaster Epoxy Rim – carpet tape

That turned out to be more trouble than it was worth; scissoring a pair of strips to fit the OD works just fine.

In any event, the live center applies enough pressure to keep the adhesive happy.

The fixture disk is sacrificial, so it now has a notch around its front face where the cutter cleared the coaster.

Although I intended to shim the fixture against the chuck jaws to center the coaster, my first attempt at manually centering the thing on the fixture was Close Enough™ that I just turned the OD to see how well the whole process worked:

Coaster Epoxy Rim – turned 6 petal black

The edge finish is arguably not Good Enough™, but it looks much better in person. In particular, the difference between the transparent acrylic top layer and the black acrylic frame around the petals is much more prominent in the photo, perhaps due to scatter from the overhead desk light.

This was the original crash test dummy acrylic coaster, so more care will be in order for the next set. In particular, shimming the fixture requires removing and replacing it for each adjustment, which can easily become a non-converging process.

Next up: I like little chucks …

2022-08-24
Acrylic Coasters: Edge Finishing, Round 2

Because the Sherline mill can’t cut all the way around a 4 inch OD coaster clamped to its table, I set up the 4-jaw chuck on the rotary table and centered the nicely round fixture:

Coaster Epoxy Rim – centering fixture plate

Admittedly, the centering need not be so precise, but practice makes perfect.

A few strips of double-stick tape affixed the test coaster, with too many clamps applied to settle it in place:

Coaster Epoxy Rim – Sherline clamp setup

A few sissy cuts demonstrated the tape lacked sufficient stickiness to hold the coaster in place against the milling cutter’s uplift. I managed to mill most of the perimeter with those clamps in place, moving each one from just ahead of the cutter to just behind the cutter.

That way lies both madness and organic damage.

There are better tapes and better adhesives, all trading off a really sticky fixture against difficulty extracting an undamaged part.

A more complex circular fixture with built-in mechanical edge clamps extending around a major part of the perimeter seems like entirely too much of a diversion for a couple of obscene-gerund coasters.

A live center in a lathe tailstock applies pressure in exactly the right place to hold a circular object against a fixture while slicing off the entire perimeter, with the only problem being centering the object.

Maybe shimming the fixture against one chuck jaw will suffice?

2022-08-23
Acrylic Coasters: Edge Finishing, Round 1

Assembling acrylic pieces inside an epoxy-filled frame produces nice results:

Cut Acrylic Coaster – bottom

The gotcha: epoxy oozes from between the layers to form a slobbery edge.

I tried introducing a similar coaster to Mr Disk Sander with reasonable results:

Coaster Epoxy Rim – disk sanded rim

The coaster on the bottom has its original generous epoxy slobber around the acrylic disks.

Assembling the layers inside a mold seems fraught with messiness, particularly if I eventually want to get it out of the mold.

Using a finer abrasive disk would certainly help, but the whole process requires intense concentration and is utterly unforgiving of mistakes.

I figured I could attach the coaster to a lathe fixture and turn the rim, so I made a fixture from scrap acrylic:

Coaster Epoxy Rim – cutting fixture plate

The lathe chuck inside jaws fit inside the hole and I set up to turn the OD to a nice even diameter:

Coaster Epoxy Rim – turning fixture rim

The fixture sat flush against the middle step of the jaws with plenty of clearance from the outer step, so I could turn the OD without whacking the carbide insert.

I planned to grab the OD and turn the ID to a (reasonably) concentric finish, but the outer jaws have an absolute diameter limit a few millimeters less than the 4 inch = 101.4 mm coaster OD.

After some increasingly desperate attempts, I concluded that, lacking a 4-jaw lathe chuck, there was no way to mount the coaster on the fixture and have it sit it even approximately centered on the spindle axis.

I do, however, have a 4-jaw chuck for the Sherline mill, normally used with the rotary table.

Next up: Round 2.

2022-08-22
OMTech 60 W Laser: Mirror Cleaning

While I was puttering around inside the laser cabinet, I figured it was time to check the mirrors for cleanliness. The first two mirrors looked fine, but Mirror 3 needed help:

OMTech 60W laser mirror 3 cleaning – before

It turns out OMTech used molybdenum rather than gold-plated silicon or copper, trading off some reflectivity to reduce damage from over-enthusiastic cleaning with a vigorous circular motion.

A first pass with an optical wipe removed most of the crud:

OMTech 60W laser mirror 3 cleaning – during

Gentle touch-up with a little more isopropyl alcohol cleared the rest:

OMTech 60W laser mirror 3 cleaning – after

The focus lens required similar attention, but there is no way to get meaningful pictures of a transparent lens.

Realigning the mirrors went well (top before, middle during, lower after):

Beam Alignment Targets- 2022-08-06

The diagonal results at Mirror 3 show the XY axes aren’t quite square, but AFAICT it’s close enough. The rightmost tape shows good beam centering in the nozzle and the Focus target shows excellent Z alignment over about 50 mm of travel.

Done!

2022-08-19
Layered Paper Coaster

A long-forgotten pad of Art Paper in assorted colors came to the surface:

Layered Coaster – tweaked

An angled view shows off the layering a little better:

Layered coaster – side view

Done manually with LightBurn’s Offset tool: shrink the frame’s interior openings (which lie outside the frame) by 1 mm per step, then cut each shape into a different color. The black layer is a complete disk, stuck atop a plain chipboard disk for stiffening.

In the cold light of day, I think I offset the green layer by 2 mm.

It’s not a particularly useful coaster, because you want a flat surface under your drink, but it does look pretty. Nowhere close to that good, but I like it.

The next time around, I’ll automate the process by stepping the sash width by 1 mm and saving each SVG image separately.

2022-08-18
OMTech 60W Laser: Repurposing the HV Power Supply Water Protect Input

For reference, the input terminals on the OMTech anonymous 60 W HV laser power supply:

OMTech 60W HV power supply – terminals

AFAICT, that’s the default layout for all similar power supplies.

The H and L pins are the High- and Low-active enable inputs that, when it’s working right, control the laser output. The KT332 controller (and, most likely, all RuiDa controllers) produce a low-active output, so you just wire the controller’s output to the L input and you’re done.

That was the original failure that got me to this point: the power supply ignored its L input and turned the beam on at whatever power the PWM signal on the IN terminal called for. Having that happen was surprising, having it happen with the cabinet lid open was … disturbing.

The P input is intended for the Water Protect signal from the flow sensor on the laser cooling plumbing. When the water is flowing, the IN terminal will be low and the power supply will pay attention to the L input.

The power supply arrived with a jumper between the P input and the G ground / common terminal:

OMTech 60W HV power supply – Water Protect jumper

The jumper holds the P input low = active, meaning the power supply thinks the water is always flowing.

It turns out that the Water Protect signal goes only to the controller. When it’s inactive = no water flowing, the controller will refuse to fire the laser and also sound an alarm. Running the signal directly to the power supply would result in a puzzling failure-to-fire with no diagnostic from the controller.

I removed that jumper and added a (green) wire from the Lid Interlock signal at the controller:

OMTech KT332 controller – Lid Interlock input – added wire

To the power supply’s P input:

OMTech 60W HV power supply – Water Protect as Lid Interlock

In principle, if this power supply fails the same way as the previous one (with its L input always active), then at least it won’t fire with the lid up.

Believing that may display a childish naivety, but at least the thing seems marginally safer than it was before.

2022-08-17
Laser-Cut Coaster Kerf Compensation

Having written some parametric coaster generators, I did this for a Digital Machinist article:

Chipboard Kerfs – as cut – composite

That’s the top and bottom of a 40 mm diameter chipboard dollhouse coaster. I made it that small to emphasize the laser kerf: a scant 3 mm across the scorched path on the top and barely 1 mm wide through the bottom, with tabs holding the pieces in place.

The SVG images include the overall frame, as seen above, and the separate pieces for kerf compensation:

Miniature Coaster – on platform

Embiggening the pieces by 0.15 mm all around produces a very snug fit:

Chipboard Kerfs – compensated – composite

I must eventually try that trick with wood, but at least I managed to get the process down without wasting entire veneer sheets.

2022-08-16