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Broom Handle Screw Thread: Replacement Plug

Somehow, we wound up with a broom handle and a broom head, the former missing a threaded stub that was firmly lodged in the latter. A few minutes of Quality Shop Time sawed off the end of the handle and unscrewed the stub to produce this array of fragments:

Broken broom handle thread

Broken broom handle thread

It’s a cylindrical Thing tailor-made for (or, back in the day, by!) a lathe. My lathe has quick-change gears that can actually cut a 5 TPI thread, but that seems like a lot of work for such a crude fitting. Instead, an hour or so of desk work produced this:

Broom Handle Screw - solid model - overview

Broom Handle Screw – solid model – overview

Some after-the-fact search-fu revealed that the thread found on brooms and paint rollers is a 3/4-5 Acme. Machinery’s Handbook has 13 pages of data for various Acme screw threads, making a distinction between General Purpose Acme threads and Stub Acme Threads: GP thread depth = 0.5 × pitch, Stub = 0.3 × pitch. For a 5 TPI thread = 0.2 inch pitch, that’s GP = 0.1 inch vs. Stub = 0.06 inch.

I measured a 5.0 mm pitch (which should be 5.08 mm = 0.2 inch exactly) and a crest-to-root depth of 1.4 mm = 0.055 inch, which makes them look like 3/4-5 Stub Acme threads. But, I didn’t know that at the time; a simple half-cylinder 2.5 mm wide and 1.25 mm tall was a pretty close match to what I saw on the broken plastic part.

Although OpenSCAD’s MCAD library has some screw forms, they’re either machine screws with V threads or ball screws with spheres. The former obviously weren’t appropriate and the latter produced far too many facets, so I conjured up a simpler shape: 32 slightly overlapping cylinders per turn, sunk halfway in the shaft at their midpoint, and tilted at the thread’s helix angle.

Broom Handle Screw - thread model closeup

Broom Handle Screw – thread model closeup

The OpenSCAD source code has a commented-out section that removes a similar shape from the shaft between the raised thread, but that brought the rendering to its knees. Fortunately, it turned out to be unnecessary, but it’s there if you want it.

With the shaft diameter set to the “root diameter” of the thread and the other dimensions roughly matching the broken plastic bits, this emerged an hour later:

Broom handle screw plug - as built

Broom handle screw plug – as built

The skirt thread was 0.25 to 0.30 mm thick, so the first-layer height tweak and packing density adjustments worked fine and all the dimensions came out perfectly. The cylindrical thread form doesn’t have much overhang and the threads came out fine; I think the correct straight-sided form would have more problems.

The hole down the middle accommodates a 1/4-20 bolt that applies enough clamping force to keep the shaft in compression, which ought to prevent it from breaking in normal use. I intended to use a hex bolt, but found a carriage bolt that was exactly the right length and had a head exactly the same diameter as the shaft, so I heated it with a propane torch and mushed its square shank into the top of the hexagonal bolt hole (the source code now includes a square recess):

Broom handle screw plug - in handle

Broom handle screw plug – in handle

The dimples on the side duplicate the method that secured the original plastic piece: four dents punched into the metal handle lock the plastic in place. It seems to work reasonably well, though, and is certainly less conspicuous than the screws I’d use.

Screwing it in place shows that it’s slightly too long (I trimmed the length in the source code):

Broom handle installed

Broom handle installed

It’s back in service, ready for use…

The OpenSCAD source code:

// Broom Handle Screw End Plug
// Ed Nisley KE4ZNU March 2013

// Extrusion parameters must match reality!
//  Print with +1 shells and 3 solid layers

ThreadThick = 0.25;
ThreadWidth = 2.0 * ThreadThick;

HoleWindage = 0.2;

function IntegerMultiple(Size,Unit) = Unit * ceil(Size / Unit);

Protrusion = 0.1;			// make holes end cleanly

//----------------------
// Dimensions

PI = 3.14159265358979;

PostOD = 22.3;              // post inside metal handle
PostLength = 25.0;

FlangeOD = 24.0;            // stop flange
FlangeLength = 3.0;

PitchDia = 15.5;            // thread center diameter
ScrewLength = 20.0;

ThreadFormOD = 2.5;         // diameter of thread form
ThreadPitch = 5.0;

BoltOD = 7.0;               // clears 1/4-20 bolt
BoltSquare = 6.5;          	// across flats
BoltHeadThick = 3.0;

RecessDia = 6.0;			// recesss to secure post in handle

OALength = PostLength + FlangeLength + ScrewLength; // excludes bolt head extension

$fn=8*4;

echo("Pitch dia: ",PitchDia);
echo("Root dia: ",PitchDia - ThreadFormOD);
echo("Crest dia: ",PitchDia + ThreadFormOD);

//----------------------
// Useful routines

module Cyl_Thread(pitch,length,pitchdia,cyl_radius,resolution=32) {

Cyl_Adjust = 1.25;                      // force overlap

    Turns = length/pitch;
    Slices = Turns*resolution;
    RotIncr = 1/resolution;
    PitchRad = pitchdia/2;
    ZIncr = length/Slices;
    helixangle = atan(pitch/(PI*pitchdia));
    cyl_len = Cyl_Adjust*(PI*pitchdia)/resolution;

    union() {
        for (i = [0:Slices-1]) {
            translate([PitchRad*cos(360*i/resolution),PitchRad*sin(360*i/resolution),i*ZIncr])
                rotate([90+helixangle,0,360*i/resolution])
                    cylinder(r=cyl_radius,h=cyl_len,center=true,$fn=12);
        }
    }
}

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);
}

module ShowPegGrid(Space = 10.0,Size = 1.0) {

  Range = floor(50 / Space);

	for (x=[-Range:Range])
	  for (y=[-Range:Range])
		translate([x*Space,y*Space,Size/2])
		  %cube(Size,center=true);

}

//-------------------
// Build it...

ShowPegGrid();

difference() {
    union() {
        cylinder(r=PostOD/2,h=PostLength);
        cylinder(r=PitchDia/2,h=OALength);
        translate([0,0,PostLength])
            cylinder(r=FlangeOD/2,h=FlangeLength);
        translate([0,0,(PostLength + FlangeLength)])
            Cyl_Thread(ThreadPitch,(ScrewLength - ThreadFormOD/2),PitchDia,ThreadFormOD/2);
    }

    translate([0,0,-Protrusion])
        PolyCyl(BoltOD,(OALength + 2*Protrusion),6);
    translate([0,0,(OALength - BoltHeadThick)])
        PolyCyl(BoltSquare,(BoltHeadThick + Protrusion),4);

//    translate([0,0,(PostLength + FlangeLength + ThreadFormOD)])
//        Cyl_Thread(ThreadPitch,(ScrewLength - ThreadFormOD/2),PitchDia,ThreadFormOD/2);

	for (i = [0:90:270]) {
		rotate(i)
			translate([PostOD/2,0,PostLength/2])
				sphere(r=RecessDia/2,$fn=8);
	}
}
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  1. #1 by madbodger on 2013-04-01 - 08:19

    Good use of primitives and hierarchy! Actually plotting the 3D surface directly would be some truly annoying math.

    • #2 by Ed on 2013-04-01 - 08:32

      some truly annoying math

      Which would produce lovely results far below the printer’s resolution!

      I’m in awe of the guys who do exotic math with OpenSCAD objects, but … all I need is a helix that engages a broom head tightly enough to not fall out. [grin]

  2. #3 by Bill Rutiser on 2013-04-01 - 09:34

    Is there a nut on the inside end of the bolt?

    • #4 by Ed on 2013-04-01 - 10:01

      Got it in one!

      The through hole is unthreaded, so the carriage bolt and nut (atop a washer) compress the whole plastic plug. With a bit of luck, the bolt should take the strain and this fix will last forever…

  3. #5 by Erik on 2013-04-01 - 10:58

    I perpetrated a similar repair a couple of years ago – the broken screw threads were epoxied to a foot-long chunk of 1/2-13 all-thread (it was a reasonably snug fit). Once that cured, the end of the broom handle was filled with epoxy and the all-thread inserted into there. I used Methyl Acrylic epoxy so it would stick to the plastic. Fumes were horrible – this really needs to cure outside for a week before coming in. It held until the broomhead completely failed – it was one of those rubber “miracle pet hair brooms”, which actually worked really well for five cats.

    • #6 by Ed on 2013-04-01 - 13:17

      It held until the broomhead completely failed

      The perfect definition of a job well done!

      Speaking of toxic chemicals, it seems methylene chloride is the least awful solvent for the PLA filament used in the M2. I wonder if that’s still available as paint stripper? Talk about use-it-outdoors stuff: yuch!

      • #7 by Erik on 2013-04-01 - 18:40

        Yes, it’s still used as (a) paint stripper and (b) for disolving the caffeine out of the bean when making decaf coffee (though other solvents are used, perhaps more oten, today – liquid CO2 comes to mind). In fact, you can use it to measure the caffeine in your beans – dissolve the stuff, decant the solution, and dry it. You’ll want to use more than a normal amount of coffee. This gets complicated in the case of tea, because you’ll end up dissolving theobromide as well. Naming oddity – theobromide contains no bromine. Go figure.

        • #8 by Ed on 2013-04-01 - 18:58

          disolving the caffeine out of the bean

          I’ll leave that to Raj, the man with the coffee plantation… he should know!

          theobromide contains no bromine

          I thought that name sounded familiar. From Wikipedia: theobromine is derived from Theobroma, the name of the genus of the cacao tree, (which itself is made up of the Greek roots theo (“God”) and broma (“food”), meaning “food of the gods”)

      • #9 by Red County Pete on 2013-04-01 - 19:12

        I’m still not sure that “least awful” is the right description for methylene chloride… Years back, I went through the toxic control books at work (semiconductors use lots of fun chemicals…), and the short answer on protective gloves was: nothing works for long. Losing skin to the stuff wasn’t much fun…

        There are some other plastics out there now, so a re-research should be done, but it’s still vile stuff. Still sold at Home Depot; just look for the can that’s not bulging and shows no rust spots. IIRC, Jasco sells it as a stripper.

        • #10 by Ed on 2013-04-01 - 21:11

          nothing works for long

          That’s what I found with all the affordable gloves for home use: methylene chloride just wiped ’em right out. It was easier to not use gloves and just try to get as little on my hands as possible, but I didn’t like it one little bit.

          some other plastics out there now

          I got three kilos of PLA for the M2, so it’ll be a while and, in any event, there’s not much else for filament-based 3D printers. I may eventually get some ABS just because it’s easily solvent-bondable with acetone (which looks downright tasty by comparison).

        • #11 by Red County Pete on 2013-04-01 - 22:57

          Sorry, posting while tired. (More Linux installation stuff–I know all kinds of ways I can’t boot the P4 into Slack, but have one good way that does. FWIW, a Y2K Sony will not boot from a usb stick, but a RH7 distro on the same machine can write the CD-ROMs I need.) Whee.

          I meant “plastics” to be about glove materials. According to OSHA, there are a couple of inner glove materials (polyethylene and one other) which kind of work, if they’re protected by neoprene or nitrile. I think nitrile came out after my research.

          On filament plastic, I could like the ABS. Acetone is OK as a solvent, assuming you keep proper fire precautions. I keep mine on the far end of the barn from the heaters… I keep a small amount of carburator cleaner, but treat it with respect bordering on fear. I’ve destroyed some skin with that crud–takes about 6 months for the blister/dead skin cycle to end.

          • #12 by Ed on 2013-04-02 - 08:24

            protected by neoprene or nitrile

            IIRC, ordinary blue nitrile gloves disintegrated in minutes. My big bulky neoprene-coated gloves were too awkward, soooo … [sigh]

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