Laser Cutter: Improving the Red-Dot Pointer

The red-dot pointer on the OMTech laser cutter has the same problem as my laser aligner for the Sherline mill: too much brightness creating too large a visual spot. In addition, there’s no way to make fine positioning adjustments, because the whole mechanical assembly is just a pivot.

The first pass involved sticking a polarizing filter on the existing mount while I considered the problem:

OMTech red dot pointer - polarizing filter installed
OMTech red dot pointer – polarizing filter installed

The red dot pointer module is 8 mm OD and the ring is 10 mm ID, but you will be unsurprised to know the laser arrived with the module jammed in the mount with a simple screw. Shortly thereafter, I turned the white Delrin bushing on the lathe to stabilize the pointer and installed a proper setscrew, but it’s obviously impossible to make delicate adjustments with that setup.

Making the polarizing filter involves cutting three circles:

OMTech red dot pointer - polarizing filter
OMTech red dot pointer – polarizing filter

Rotating the laser module in the bushing verified that I could reduce the red dot to a mere shadow of its former self, but it was no easier to align.

Replacing the Delrin bushing with a 3D printed adjuster gets closer to the goal:

Pointer fine adjuster - solid model
Pointer fine adjuster – solid model

Shoving a polarizing filter disk to the bottom of the recess, rotating the laser module for least brightness, then jamming the module in place produces a low-brightness laser spot.

The 8 mm recess for the laser module is tilted 2.5° with respect to the Y axis, so (in principle) rotating the adjuster + module (using the wide grip ring) will move the red dot in a circle:

Improved red-dot pointer - overview
Improved red-dot pointer – overview

The dot sits about 100 mm away at the main laser focal point, so the circle will be about 10 mm in diameter. In practice, the whole affair is so sloppy you get what you get, but at least it’s more easily adjusted.

The M4 bolt clamping the holder to the main laser tube now goes through a Delrin bushing. I drilled out the original 4 mm screw hole to 6 mm to provide room for the bushing:

Improved red-dot pointer - drilling bolt hole
Improved red-dot pointer – drilling bolt hole

The bushing has a wide flange to soak up the excess space in the clamp ring:

Improved red-dot pointer - turning clamp bushing
Improved red-dot pointer – turning clamp bushing

With all that in place, the dimmer dot is visually about 0.3 mm in diameter:

Improved red-dot pointer - offset
Improved red-dot pointer – offset

The crappy image quality comes from excessive digital zoom. The visible dot on the MDF surface is slightly larger than the blown-out white area in the image.

The CO₂ laser hole is offset from the red laser spot by about 0.3 mm in both X and Y. Eyeballometrically, the hole falls within the (dimmed) spot diameter, so this is as good as it gets. I have no idea how durable the alignment will be, but it feels sturdier than it started.

Because the red dot beam is 25° off vertical, every millimeter of vertical misalignment (due to non-flat surfaces, warping, whatever) shifts the red dot position half a millimeter in the XY plane. You can get a beam combiner to collimate the red dot with the main beam axis, but putting more optical elements in the beam path seems like a Bad Idea™ in general.

The OpenSCAD source code as a GitHub Gist:

// Laser cutter red-dot module fine adjust
// Ed Nisley KE4ZNU 2022-09-22
Layout = "Show"; // [Build, Show]
/* [Hidden] */
ThreadThick = 0.25;
ThreadWidth = 0.40;
HoleWindage = 0.2;
Protrusion = 0.1; // make holes end cleanly
inch = 25.4;
ID = 0;
OD = 1;
LENGTH = 2;
function IntegerMultiple(Size,Unit) = Unit * ceil(Size / Unit);
//----------------------
// Dimensions
PointerOD = 8.0 + 0.2; // plus loose turning fit
Aperture = 5.0; // clear space for lens
SkewAngle = 2.5;
MountRing = [10.0,16.0,8.0]; // OEM laser module holder
GripRim = [Aperture,MountRing[OD] + 2*1.5,3.0]; // finger grip around OD
NumSides = 24;
//----------------------
// Useful routines
module PolyCyl(Dia,Height,ForceSides=0) { // based on nophead's polyholes
Sides = (ForceSides != 0) ? ForceSides : (ceil(Dia) + 2);
FixDia = Dia / cos(180/Sides);
cylinder(r=(FixDia + HoleWindage)/2,
h=Height,
$fn=Sides);
}
//----------------------
// Holder geometry
module Holder() {
difference() {
union() {
cylinder(d=GripRim[OD],h=GripRim[LENGTH],$fn=NumSides);
PolyCyl(MountRing[ID],MountRing[LENGTH] + GripRim[LENGTH],NumSides);
}
translate([0,0,-Protrusion]) // close enough without skew angle
PolyCyl(Aperture,2*MountRing[LENGTH],NumSides);
translate([0,0,MountRing[LENGTH]/2 + GripRim[LENGTH]])
rotate([0,SkewAngle,0])
translate([0,0,-MountRing[LENGTH]/2])
PolyCyl(PointerOD,2*MountRing[LENGTH],NumSides);
}
}
//----------------------
// Build it
if (Layout == "Show") {
Holder();
}
if (Layout == "Build") {
Holder();
}

Tour Easy: Chain Drop Pin

Every now and again, an upshift to the large chainring on my Tour Easy would go awry and drop the chain off the outside, where it would sometimes jam between the pedal crank and the spider. In the worst case the flailing chain would also jam in the TerraCycle idler, but I fixed that a while ago.

Contemporary chainrings (i.e., anything made since the trailing decades of the last millennium) generally have a chain drop pin positioned against the crank specifically to prevent such chain jamming.

Making a chain drop pin is no big deal if you’ve got a lathe and an M4 tap:

Tour Easy - DIY Chain Drop pin
Tour Easy – DIY Chain Drop pin

A closer look:

Tour Easy - DIY Chain Drop pin - detail
Tour Easy – DIY Chain Drop pin – detail

That’s a 10 mm length of 5/16 inch brass rod drilled with a recess to fit the head of a 10 mm M4 socket-head cap screw.

The pin should be a micro-smidgen shorter, as it just touches the crank, but, if anything, moving the chainring inward by one micro-smidgen improved the upshifts and I’m inclined to go with the flow.

Should’a done it decades ago …

Mini-Lathe: Adapting a Five Inch Four Jaw Chuck Adapter Plate

The kludge required to trim the coaster rims disturbed the silt enough to reveal a long-lost 5 inch 4 jaw chuck that fit neither the old South Bend lathe nor the new mini-lathe. In any event, the chuck does have an adapter plate on its backside, it’s just not the correct adapter plate for the spindle on my mini-lathe.

Making it fit required enlarging an existing recess to fit the spindle plate, a straightforward lathe job with the plate grabbed in the 3 jaw chuck’s outer jaws:

5 inch 4 jaw chuck - boring spindle recess
5 inch 4 jaw chuck – boring spindle recess

Carbide inserts don’t handle interrupted cuts very well, but sissy cuts saved the day. The plate is kinda-sorta cast iron, so the “chips” are dust and a vacuum snout reduces the mess; you can see some chips inside the bore.

A faceplate for the mini-lathe lathe located three holes matching the spindle plate, after I noticed the amazing coincidence of both parts having 26 mm bores. Making an alignment tool from a scrap of 3/4 inch (!) Schedule 40 PVC pipe was an easy lathe job:

5 inch 4 jaw chuck - adapter plate alignment
5 inch 4 jaw chuck – adapter plate alignment

Transfer-punching those holes produced pips on the chuck side of the adapter plate:

5 inch 4 jaw chuck - spindle bolt spotting
5 inch 4 jaw chuck – spindle bolt spotting

I thought about freehanding the holes, but came to my senses:

5 inch 4 jaw chuck - adapter plate drilling
5 inch 4 jaw chuck – adapter plate drilling

Of course, the Sherline lacks enough throat for the plate, so each hole required clamping / locating / center-drilling / drilling / finish drilling. With all three drilled, hand-tapping the threads was no big deal:

5 inch 4 jaw chuck - rebuIlt adapter plate
5 inch 4 jaw chuck – rebuIlt adapter plate

Those are M8×1.25 studs from LMS (although the ones I got look like the 30 mm version), with the long end sunk in the adapter plate to put the other end flush with the nut on the far side of the spindle plate:

5 inch 4 jaw chuck - installed - spindle nuts
5 inch 4 jaw chuck – installed – spindle nuts

And then it fits just like it grew there, although the jaws don’t have much clearance inside the interlock cover:

5 inch 4 jaw chuck - installed - front view
5 inch 4 jaw chuck – installed – front view

Now I’m ready for the next set of coasters and, if the jaws stick out too far, I can gimmick the interlock switch for the occasion.

If the truth be known, I ordered two sets of those studs along with the 4 inch 4 jaw chuck intended for the mini-lathe, so, if anything, I’m now over-prepared.

The description of the 4 inch chuck seems inconsistent with its listed dimensions, which may be why I ended up with the larger chuck in the first place. You can never have enough chucks: all’s well that ends well.

Acrylic Coasters: Edge Finishing, Round 4

Lacking a 4-jaw chuck for the lathe, this should suffice:

Coaster Epoxy Rim - chuck-in-chuck setup
Coaster Epoxy Rim – chuck-in-chuck setup

Which is just the Sherline 4-jaw chuck chucked in the lathe’s 3-jaw chuck, with both chuck Jaw 1 positions lined up and marked on the acrylic disk fixture. The picture is a recreation set up after the fact, because I lack a good picture of the overall scene.

Now it’s easy enough to center the fixture, stick the coaster in place with reasonable accuracy, then tweak the Sherline chuck to center the coaster:

Coaster Epoxy Rim - turning setup
Coaster Epoxy Rim – turning setup

Because the bottom layer is a laser-cut disk, eyeballometrically aligning its edge to a simple pointer worked surprisingly well:

Coaster Epoxy Rim - locating mirror edge
Coaster Epoxy Rim – locating mirror edge

Turning the OD down to match the bottom disk meant I could finally get decent results with zero drama:

Coaster Epoxy Rim - turned samples
Coaster Epoxy Rim – turned samples

From the bottom, this one has a 3 mm mirror, the 3 mm fluorescent green frame + petals, and a 1.6 mm top sheet:

Coaster Epoxy Rim - turned 6 petal mirror
Coaster Epoxy Rim – turned 6 petal mirror

This one has a 3M double-sided tape with low-surface-energy adhesive layers between the mirror and the fluorescent blue frame + petal, with epoxy between the top layer and the frame.

Coaster Epoxy Rim - turned 4 petal
Coaster Epoxy Rim – turned 4 petal

If I never tell anybody, they’ll think the slightly granular look if the tape was deliberate; it looks OK to me.

And, for completeness, the crash test dummy from the start of this adventure:

Coaster Epoxy Rim - turned 6 petal black
Coaster Epoxy Rim – turned 6 petal black

I don’t know how to avoid the bubbles, as the usual torch-the-top and pull-a-vacuum techniques pop bubbles at the epoxy-air interface. These bubbles are trapped under the top acrylic sheet, even though I was rather painstaking about easing the layer down from one side to the other while chasing bubbles along.

Maybe I can define bubbles as Part of the Art?

Definitely fancier than chipboard, although not nearly as absorbent.

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
Coaster Epoxy Rim – turning fixture rim

So the fixture needs a spacer:

Coaster Epoxy Rim - cutting chuck 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
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
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
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

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
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
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?

Acrylic Coasters: Edge Finishing, Round 1

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

Cut Acrylic Coaster - bottom
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
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
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
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.