Month: May 2022

Home Ec, Machine Shop

Laser-cut Shop Wipes

Wiping down a tool or wiping up a mess with a small rag and then throwing it out simplifies cleanup:

Shop wipes
Shop wipes

Long ago, I applied scissors to old towels / t-shirts / whatever to get randomly sized squares, but when Mary began using rotary cutters for her sewing projects I immediately saw the light. A few times a year, I lower the scrap box level and restock the shop wipes boxes.

A laser cutter is even better:

Shop Rags - LB camera layout
Shop Rags – LB camera layout

Flatten the rag on the honeycomb, drag a few rectangles into place, and fire the laser:

Shop wipes - laser cut
Shop wipes – laser cut

Something like 50 mm/s at 60% power works for all the fabrics I’ve tried, from worn-out towels and dead sweatpants to napkins and t-shirts. Thinner fabrics can be stacked, but wrinkles and seams get in the way of clean cuts.

Rounded-corner rectangles are easy enough to draw and the scrap cloths have different shapes, so I don’t see much point in saving a file with any specific layout. Your scrap box may be more orderly.

A clean cut lets the outer cloth just lift away:

Shop wipes - on honeycomb
Shop wipes – on honeycomb

The wipes give off a distinct smell of charred cloth, but running them through the clothes washer in a big mesh bag with everything else solves that problem.

Obviously, one couldn’t possibly justify a laser cutter to make shop wipes, but if you happen to have one just standing around, well …

Machine Shop, Oddities

OMTech 60 W Laser: Expanded Clutter Space

The OMTech laser manual specifically warns against allowing clutter to accumulate atop the cabinet:

It is highly recommended to have an extra work table nearby in order to avoid placing objects on or directly adjacent to the machine, which could become a fire or laser hazard.

OMTech USB570c Cabinet Laser Engraver User Manual

The Basement Shop lacks the floor space for their recommended “extra work table”, so the laser cabinet now sports a pair of wings:

OMTech Laser - side shelf - end view
OMTech Laser – side shelf – end view

They’re a pair of those battered maple shelves, cut to fit the length of the cabinet:

OMTech Laser - side shelf
OMTech Laser – side shelf

They’re a convenient 9 inches wide, just right for general clutter. That stubby screwdriver encroaching on the lid shows I haven’t been entirely successful.

Each white shelf bracket has three self-tapping machine screws driven into the wood and a single 4 mm SHCS through a hole drilled into the cabinet with a nyloc nut & washer on the inside. If I understand the somewhat abbreviated instruction sticker correctly, I installed them upside-down in order to put the longer end under the wood where it would do the most good; they’re entirely rigid enough for the purpose.

Some of the same indoor urethane finish I slobbered on the gate signs tidied the sawed ends.

Machine Shop, Software

OMTech 60 W Laser: Plant Markers

While calibrating the laser’s scan offset, I also tried various fonts:

Offset cal - text - overview
Offset cal – text – overview

Putting two lines of the most-readable font inside an outline reverse-engineered from a few handwritten samples let me cut out a bunch of plant markers from white-on-black Trolase acrylic:

Plant Markers - cutting
Plant Markers – cutting

Which look downright dignified in real life:

Plant markers - African Violet
Plant markers – African Violet

Admittedly, sweet potato slips don’t require such extensive documentation:

Plant Markers - sweet potatoes
Plant Markers – sweet potatoes

Cutting the sheet flat on the honeycomb platform worked well, modulo Sadler’s warning about cutting acrylic, and a few smudges on the back of the markers will go unnoticed.

This was actually an excuse to use LightBurn’s Variable Text feature, so the tags contain formatting codes:

Plant Markers - Variable Text template
Plant Markers – Variable Text template

The codes give the position and format for text fields in a CSV file containing one line for each tag:

Austrocylindropuntia subulata,Eve’s Pin Cactus
Euphorbia,abyssiniaca
possibly G. Carinata,var. Verucosa
African Violet,Maui
Sansevieria trifasciata,Mother in law’s tongue
Plectranthus,'Mona Lavender'

The rules governing quoted strings and suchlike remain to be explored, but single quotes in the CSV file pass through unchanged.

Putting a tab at the point of the marker will prevent it from falling free when cut out, should you want to try raising the sheet above the platform to reduce the amount of crud accumulating on the back side.

Machine Shop, Science

OMTech 60 W Laser: Wood Cutting

Just to see how the OMTech 60 W laser cuts wood:

Laser cut wood samples
Laser cut wood samples

From left to right:

  • 5.3 mm oak plywood: 10 mm/s 70% (1/4 inch)
  • 7.7 mm plywood: 6 mm/s 70% (from OMTech crate)
  • 19 mm pine: 2 mm/s 70-80% (3/4 inch)
  • 20 mm oak: 2 mm/s 70% (3/4 inch lovely wood)
  • 19 mm maple: 2 mm/s 80% (3/4 inch shelving)
  • 20 mm plywood: fail at 2 mm/s 90% two passes

I thought a pine plank would cut faster than oak, but they’re equally stubborn.

Maple requires slightly more power, with the glued butt joints between the slabs putting up a stiff resistance.

A sheet of 3 mm MDF cuts well at 20 mm/s 60% and I expect 3 mm plywood might need similar numbers.

A pervasive odor of burned wood seems to be the only downside; if you think a wood stove is a good idea, you’ll love laser cutting the stuff. Sanding the blackened perimeter and sealing the surface surely helps, but it’s feasible only for the kind of simple convex shapes you don’t really need a laser to cut.

Machine Shop

OMTech 60 W Laser: Engraving Scan Offset Adjustment

The fuzzy edges engraved on the acrylic test sample showed the need for scan offset adjustment:

Please Close The Gate - acrylic test piece
Please Close The Gate – acrylic test piece

The problem arises from the finite delay between the controller turning the laser beam on and the rise time of the death ray energy at the focal point.

LightBurn can produce a calibration coupon (on Trolase laminated acrylic) to help explore the multidimensional parameter space:

Offset cal - zero offset - overview
Offset cal – zero offset – overview

The “Interval” value is the vertical (Y-axis) scan line spacing. The laser spot diameter is, at absolute best, about 0.2 mm on the focal plane, with the actual engraved line being smaller due to the energy distribution across the beamwidth and the power required to visibly damage the material, so a 0.1 mm interval should result a little bit of overlap between adjacent scan lines.

A closer look shows the serrated edges on the left and right sides of the engraved squares:

Offset cal - zero offset - detail
Offset cal – zero offset – detail

Peering at it through a measuring magnifier suggests the offset is a bit over 0.2 mm at 400 mm/s, corresponding to a 500 µs delay between laser turn-on in the rightward direction and turn-off in the leftward direction.

The LightBurn Scanning Offset Adjustment is half the measured distance, with an Initial Offset parameter to adjust the starting point of the first scan line. You measure the distance at each speed and fill in the table accordingly.

Iterating through offsets, speeds, powers, and intervals produces a series of test coupons slicing through the parameter space:

Offset cal - iteration
Offset cal – iteration

All in all, a 0.1 mm offset at 400 mm/s with 14% power (about 8 W) and 0.075 mm interval looks pretty good:

Offset cal - final offset - detail
Offset cal – final offset – detail

Engraving various fonts:

Offset cal - text - overview
Offset cal – text – overview

A closer look (left coupon on the top):

Offset cal - text detail
Offset cal – text detail

LightBurn linearly interpolates between table entries of offset values at specific speeds, so you must fill in several lines to give it something to munch on. The top text came from an offset table with two entries at 400 and 500 mm/s, which obviously wasn’t quite sufficient. The bottom text comes from a three-entry table:

LightBurn scan offset table
LightBurn scan offset table

Which produces a better result, even at 500 mm/s and 20% power (12 W) on scrap acrylic:

Close Gate - test acrylic - overview
Close Gate – test acrylic – overview

A closer look:

Close Gate - test acrylic - detail
Close Gate – test acrylic – detail

Much better!

Home Ec, Machine Shop, Software

Garden Cart Handle Pivot

For reasons not relevant here, I was tapped to replace the plastic parts attaching the handle to a garden cart:

Garden Cart - handle attachment
Garden Cart – handle attachment

The owner tried to contact the “manufacturer” to no avail; repair parts are simply not available, even if the name painted on the cart had a meaningful relationship to anything else.

Well, I can fix that:

Garden Cart - handle repair parts
Garden Cart – handle repair parts

Fortunately, another cart in the fleet provided the missing bits so I could reverse-engineer their measurements.

The solid model looks about like you’d expect:

Garden Cart Handle - show view
Garden Cart Handle – show view

Printing the two halves with those nice (yellow) bosses in place wasn’t feasible. They were exactly 1 inch in diameter, so I just parted two cookies from the end of a stout acetal rod after drilling a hole for the 2-¼ inch 5/16-18 bolt.

The two pieces took nigh onto three hours with five perimeters and 50% infill:

Garden Cart Handle - slicer preview
Garden Cart Handle – slicer preview

While delivering and installing the parts, I got volunteered to haul plants to cars with one of the carts during the upcoming Spring Plant Sale. That’ll teach me to stay in the Basement Shop …

The OpenSCAD source code as a GitHub Gist:

// Garden Cart Handle Pivot
// Ed Nisley KE4ZNU 2022-05
Layout = "Show"; // [Show,Build]
/* [Hidden] */
ThreadThick = 0.25;
ThreadWidth = 0.40;
HoleWindage = 0.2;
Protrusion = 0.1; // make holes end cleanly
inch = 25.4;
function IntegerMultiple(Size,Unit) = Unit * ceil(Size / Unit);
ID = 0;
OD = 1;
LENGTH = 2;
//----------
// Dimensions
// Handle lies along X axis
HandleOD = (7/8) * inch;
BoltOD = (5/16) * inch;
Washer = [BoltOD,1.0 * inch,2.0]; // just for Show
Disk = [BoltOD,62.0,(3/16) * inch];
ClampBase = [(1 + 7/8)*inch,(1 + 1/8)*inch,2.0];
Kerf = 2.0;
CornerRadius = 1.0;
PivotOA = [Disk[OD],Disk[OD],HandleOD + 2*ClampBase.z + 2*Disk[LENGTH]];
//----------------------
// 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(d=(FixDia + HoleWindage),h=Height,$fn=Sides);
}
//----------------------
// Set up parts
module Handle() {
translate([-2*PivotOA.x,0,0])
rotate([0,90,0])
PolyCyl(HandleOD,4*PivotOA.x,24);
}
module Bolt() {
translate([0,0,-PivotOA.z])
PolyCyl(BoltOD,2*PivotOA.z,12);
}
module Pivot() {
difference() {
union() {
hull()
for (i=[-1,1], j=[-1,1]) // rounded block
translate([i*(ClampBase.x/2 - CornerRadius),j*(ClampBase.y/2 - CornerRadius),-PivotOA.z/2])
cylinder(r=CornerRadius,h=PivotOA.z,$fn=4*8);
for (k=[-1,1])
translate([0,0,k*(PivotOA.z/2 - Disk[LENGTH]/2)])
rotate(180/36)
cylinder(d=Disk[OD],h=Disk[LENGTH],$fn=36,center=true);
}
Handle();
Bolt();
cube([2*ClampBase.x,2*ClampBase.y,Kerf],center=true); // slice through center
}
}
//----------
// Build them
if (Layout == "Show") {
rotate([90,-45,0]) {
Pivot();
color("Green")
translate([2*PivotOA.x - PivotOA.x/2,0,0])
Handle();
color("Red")
Bolt();
color("Yellow")
for (k=[-1,1])
translate([0,0,k*(PivotOA.z/2 + Washer[LENGTH])])
rotate(180/36)
cylinder(d=Washer[OD],h=Washer[LENGTH],$fn=36,center=true);
}
}
if (Layout == "Build") {
Offset = 5.0;
intersection() {
translate([-(PivotOA.x/2 + Offset),0,PivotOA.z/2])
Pivot();
translate([-2*PivotOA.x,-2*PivotOA.y,0])
cube([4*PivotOA.x,4*PivotOA.y,PivotOA.z/2],center=false);
}
intersection() {
translate([(PivotOA.x/2 + Offset),0,PivotOA.z/2])
rotate([180,0,0])
Pivot();
translate([-2*PivotOA.x,-2*PivotOA.y,0])
cube([4*PivotOA.x,4*PivotOA.y,PivotOA.z/2],center=false);
}
}
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Home Ec, Machine Shop, Software

Laser-cut Cutworm Collars

Mary, having had considerable trouble with cutworms in her gardens, routinely deploys cardboard collars around new plants:

Cutworm Collars - assembled
Cutworm Collars – assembled

It seems cutworms trundle around until they find an edible plant, chew through the stem and topple the plant, then trundle off without taking another bite. A small cardboard barrier prevents them from sensing the plant: apparently, motivation to climb a short wall hasn’t yet evolved.

Up to this point, Mary applied scissors to tissue boxes, but I proposed an alternative with an adjustable fit to any plant:

Laser Cutting Cutworm Collars
Laser Cutting Cutworm Collars

A splayed cardboard box rarely lays flat, a condition enforced by a few MDF stops used as clamps.

Come to find out no two tissue boxes have identical dimensions, even boxes from the same brand / retailer, so lay out duplicates of the collar template to match your stockpile.

That was fun!

The SVG image as a GitHub Gist:

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view raw Cutworm Collar.svg hosted with ❤ by GitHub