The New Hotness

A local hospital contacted Mary’s quilting group to sew up cloth covers to prolong the life of their medical-grade N95 masks. Their recommended pattern, the Fu Face Mask from the FreeSewing group, comes in three sizes:

Freesewing - Fu Mask — Freesewing – Fu Mask

N.B.: Use their original PDF, because a JPG picture probably won’t come out at the right size.

Also N.B.: Used by itself, this is not a medical-grade filter mask.

The patterns do not include the usual 1/4 inch seam allowance around the outside, so I cranked out 3D printed plastic cutting templates.

If you’re not interested in 3D printing, 2D print the PDF file on cardboard, sketch a seam allowance, and cut it out, as quilters have been doing since slightly after home printers happened.

The plan of attack:

Convert mask outlines into a bitmap image (GIMP)
Create Bezier curves by tracing outlines (Inkscape)
Save curves as SVG files
Convert SVG into solid model (OpenSCAD)
Add stiffening ribs &c
Save as STL solid model
Slice into G-Code file (Slic3r)
Fire the M2!

So, we begin …

Import the PDF into The GIMP, delete the text & suchlike, convert to monochrome, and save the pattern outlines as a PNG file:

Fu Facemask - outlines — Fu Facemask – outlines

It turns out Inkscape can directly import the PDF, but it valiantly tries to convert all the text and the incidental graphic elements, none of which will be useful in this situation. It’s easier to delete them in The GIMP and make a bank shot off a PNG file.

Update: Scruss’s comment provides a much simpler workflow!

Import the PNG into Inkscape and trace one outline with the Bezier curve tool:

Fu Mask - Inkscape Bezier trace — Fu Mask – Inkscape Bezier trace

If you squint really carefully, you’ll see Bezier control handles sticking out of the nodes. I laid three nodes along the top arc and four along the right side, but do what’cha like; the Insert key or Shift+I inserts and Delete removes nodes. It’s easier to center a node in the middle of the PNG line with snapping turned off: Shift+drag while mousing or globally with #.

You could unleash the bitmap auto-tracer, but it generates a bazillion uselessly tiny Bezier curves.

When you’re happy, select and copy the path with Ctrl+C, paste it into a shiny new Inkscape document (Ctrl+N) with Ctrl-V, save it with a catchy file name like Fu Mask - Small - nominal.svg, and close that document to return to the document with the PNG outlines and the original path.

Select the original path again, create a dynamic offset with Ctrl+J, open the XML editor with Ctrl+Shift+X (which automagically selects the proper SVG element), and change the inkscape:radius value from 0 to 6.35 (mm, which everyone should use) to get a 1/4 inch seam allowance:

Fu Mask - Inkscape XML Editor - Offset radius — Fu Mask – Inkscape XML Editor – Offset radius

The path will puff out with curved corners:

Fu Mask - Inkscape offset — Fu Mask – Inkscape offset

Copy into a new document, save as Fu Mask - Small - seam allowance.svg, and close.

Repeat that process for each of the three mask sizes to create three pairs of SVG files: the nominal mask outline and the corresponding seam allowance outline for each size.

The OpenSCAD program imports the SVG files, removes the nominal outline from within the seam allowance to leave the outline, adds stiffening ribs, and stamps an ID letter on both sides of the central button:

Fu Mask Cutting Template - Small - solid model — Fu Mask Cutting Template – Small – solid model

Choose one of the three sizes with the OpenSCAD customizer, save the resulting model as an STL file, repeat for the three sizes, and you’re done.

This process can convert any outline paths in SVG files into cutting templates, so, should the Fu Mask not suit your fancy, Use The Source.

For convenience, the STL files are on Thingiverse.

From the comments, a Washington hospital uses a similar pattern: their PDF with assembly instructions.

The OpenSCAD source code as a GitHub Gist:

	// Fu Mask cutting templates
	// Ed Nisley – KE4ZNU – 2020-03
	// Mask patterns from:
	// https://freesewing.org/blog/facemask-frenzy/
	// More info on my blog:
	// https://softsolder.com/2020/03/29/fu-mask-cutting-templates/

	/* [Mask Size] */

	Name = "Small"; // [Small, Medium, Large, Test]

	/* [Hidden] */

	Templates = [ // center ID letter and file name
	["S","Small"],
	["M","Medium"],
	["L","Large"],
	["T","Test"], // for whatever you like
	];

	T_ID = 0; // Template indexes
	T_NAME = 1;

	BarThick = 4.0; // template thickness
	HubOD = 20.0; // center button diameter

	// These should match slicer values

	ThreadThick = 0.25;
	ThreadWidth = 0.40;

	Protrusion = 0.1; // make clean holes

	//— Build it

	t = Templates[search([Name],Templates,1,1)[0]]; // find template index

	Dir = "./";

	FnOuter = str(Dir,"Fu Facemask – ",t[T_NAME]," – seam allowance.svg");
	FnInner = str(Dir,"Fu Facemask – ",t[T_NAME]," – nominal.svg");

	difference() {
	linear_extrude(BarThick,convexity=5) {

	intersection() {
	import(FnOuter,center=true);
	union() {
	square([200.0,5.0],center=true); // horizontal bar
	square([5.0,200.0],center=true); // vertical bar
	}
	}

	circle(d=HubOD); // central button

	difference() { // cutting template!
	import(FnOuter,center=true);
	import(FnInner,center=true);
	}

	}

	translate([0,0,BarThick – ThreadThick]) // top ID recess
	cylinder(d=HubOD – 6*ThreadWidth,h=ThreadThick + Protrusion);

	translate([0,0,-Protrusion]) // bottom ID recess
	cylinder(d=HubOD – 6*ThreadWidth,h=ThreadThick + Protrusion);

	}

	translate([0,0,2*BarThick/3]) // top ID
	linear_extrude(height=BarThick/3,convexity=2)
	text(text=t[T_ID],size=10,
	font="Arial:style:Bold",halign="center",valign="center");

	mirror([1,0,0]) // bottom ID
	linear_extrude(height=BarThick/3,convexity=2)
	text(text=t[T_ID],size=10,
	font="Arial:style:Bold",halign="center",valign="center");

view raw Fu Mask Cutting Template.scad hosted with ❤ by GitHub

Verily, there’s nothing like a good new problem to take your mind off all your old problems …

The New Hotness

Fu Mask Cutting Templates