The New Hotness

About the third time I removed the mini-lathe’s change gear cover by deploying a 4 mm hex wrench on its pair of looong socket head cap screws, I realized that finger-friendly knobs were in order:

LMS Mini-lathe cover screw knobs - installed — LMS Mini-lathe cover screw knobs – installed

A completely invisible length of 4 mm hex key (sliced off with the new miter saw) runs through the middle of the knob into the screw, with a dollop of clear epoxy holding everything together:

LMS Mini-lathe cover screw knobs - epoxied — LMS Mini-lathe cover screw knobs – epoxied

The 2 mm cylindrical section matches the screw head, compensates for the 1.5 mm recess, and positions the knobs slightly away from the cover:

LMS Mini-lathe cover screw knob - solid model — LMS Mini-lathe cover screw knob – solid model

They obviously descend from the Sherline tommy bar handles.

I built three of ’em at a time to get a spare to show off and to let each one cool down before the next layer arrives on top:

LMS Mini-lathe cover screw knobs - on platform — LMS Mini-lathe cover screw knobs – on platform

The top and bottom surfaces have Octagram Spiral infill that came out nicely, although it’s pretty much wasted in this application:

LMS Mini-lathe cover screw knob - Slic3r first layer — LMS Mini-lathe cover screw knob – Slic3r first layer

I have no explanation for that single dent in the perimeter.

The cover hangs from those two screws, which makes it awkward to line up, so I built a shim to support the cover in the proper position:

LMS Mini-lathe cover support shim - Slic3r preview — LMS Mini-lathe cover support shim – Slic3r preview

Nope, it’s not quite rectangular, as the change gear plate isn’t mounted quite square on the headstock:

LMS Mini-lathe - cover alignment block — LMS Mini-lathe – cover alignment block

I decided ~~when~~ if that plate eventually gets moved / adjusted / corrected, I’ll just build a new shim and move on. A length of double-sticky tape holds it onto the headstock.

Mounting the cover now requires only two hands: plunk it atop the shim, press it to the right so the angled side settles in place, insert screws, and it’s done.

A short article by Samuel Will (Home Shop Machinist 35.3 May 2016) pointed out that any chips entering the spindle bore will eventually fall out directly into the plastic change gears and destroy them. He epoxied a length of PVC pipe inside the cover to guide the swarf outside, but I figured a tidier solution would be in order:

LMS Mini-lathe - change gear shield — LMS Mini-lathe – change gear shield

The solid model looks just like that:

LMS Mini-lathe cover shaft shield - Slic3r preview — LMS Mini-lathe cover shaft shield – Slic3r preview

The backside of the shield has three M3 brass inserts pressed in place. I marked the holes on the cover by the simple expedient of bandsawing the base of the prototype shield (which I needed for a trial fit), lining it up with the spindle hole, and tracing the screw holes (which aren’t yet big enough for the inserts):

LMS mini-lathe - cover hole template — LMS mini-lathe – cover hole template

Yeah, that’s burned PETG snot around 10 o’clock on the shield. You could print a separate template if you prefer.

The various diameters and lengths come directly from my lathe and probably won’t be quite right for yours; there’s a millimeter or two of clearance in all directions that might not be sufficient.

Don’t expect the cover hole to line up with the spindle bore:

LMS mini-lathe - view through cover and spindle — LMS mini-lathe – view through cover and spindle

I should build an offset into the shield that jogs the holes in whatever direction makes the answer come out right, but that’s in the nature of fine tuning; those holes got filed slightly egg-shaped to ease the shield a bit to the right and it’s all good.

Heck, having the spindle line up pretty closely with the tailstock seems like enough of a bonus for one day.

The OpenSCAD source code as a GitHub Gist:

	// Tweakage for LMS Mini-Lathe cover
	// Ed Nisley – KE4ZNU – June 2016

	Layout = "Shaft"; // Knob Shim Shaft

	use <knurledFinishLib_v2.scad>

	//- Extrusion parameters must match reality!
	// Print with 2 shells and 3 solid layers

	ThreadThick = 0.20;
	ThreadWidth = 0.40;

	HoleWindage = 0.3; // extra clearance to improve hex socket fit

	Protrusion = 0.1; // make holes end cleanly

	inch = 25.4;

	//———————-
	// Dimensions

	//- Knobs for cover screws

	HeadDia = 8.5; // un-knurled section diameter
	HeadRecess = 2.0; // … length inside cover surface + some clearance

	SocketDia = 4.0; // hex key size
	SocketDepth = 10.0;

	KnurlLen = 15.0; // length of knurled section
	KnurlDia = 20.0; // … diameter at midline of knurl diamonds
	KnurlDPNom = 12; // Nominal diametral pitch = (# diamonds) / (OD inches)

	DiamondDepth = 1.5; // … depth of diamonds
	DiamondAspect = 4; // length to width ratio

	KnurlID = KnurlDia – DiamondDepth; // dia at bottom of knurl

	NumDiamonds = ceil(KnurlDPNom * KnurlID / inch);
	echo(str("Num diamonds: ",NumDiamonds));

	NumSides = 4*NumDiamonds; // 4 facets per diamond

	KnurlDP = NumDiamonds / (KnurlID / inch); // actual DP
	echo(str("DP Nom: ",KnurlDPNom," actual: ",KnurlDP));

	DiamondWidth = (KnurlID * PI) / NumDiamonds;

	DiamondLenNom = DiamondAspect * DiamondWidth; // nominal diamond length
	DiamondLength = KnurlLen / round(KnurlLen/DiamondLenNom); // … actual

	TaperLength = 0*DiamondLength;

	//- Shim to support cover

	CoverTopThick = 2.0;
	ShimThick = 10.0;
	ShimCornerRadius = 2.0;

	ShimPoints = [[0,0],[60,0],[60,(13.5 – CoverTopThick)],[0,(14.5 – CoverTopThick)]];

	//- Shaft extension to keep crap out of the change gear train

	ID = 0;
	OD = 1;
	LENGTH = 2;

	Shaft = [24.0,30.0,41.0]; // ID=through, OD=thread OD, Length = cover to nut seat
	ShaftThreadLength = 3.0;
	ShaftSides = 6*4;

	ShaftNut = [45,50,16]; // recess around shaft nut, OD = outside of cover

	Insert = [3.5,5.0,8.0]; // 3 mm threaded insert

	NumCoverHoles = 3;
	CoverHole = [Insert[OD],35.0,12.0]; // ID = insert, OD = BCD, LENGTH = screw hole depth

	ShaftPoints = [
	[Shaft[ID]/2,0],
	[ShaftNut[OD]/2,0],
	[ShaftNut[OD]/2,Shaft[LENGTH]],
	[ShaftNut[ID]/2,Shaft[LENGTH]],
	[ShaftNut[ID]/2,Shaft[LENGTH] – ShaftNut[LENGTH]],
	[Shaft[OD]/2, Shaft[LENGTH] – ShaftNut[LENGTH]],
	[Shaft[OD]/2, Shaft[LENGTH] – ShaftNut[LENGTH] – ShaftThreadLength],
	[Shaft[ID]/2, Shaft[LENGTH] – ShaftNut[LENGTH] – ShaftThreadLength],
	];

	//———————-
	// 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);
	}

	//- Build things

	if (Layout == "Knob")
	difference() {
	union() {
	render(convexity=10)
	translate([0,0,TaperLength]) // knurled cylinder
	knurl(k_cyl_hg=KnurlLen,
	k_cyl_od=KnurlDia,
	knurl_wd=DiamondWidth,
	knurl_hg=DiamondLength,
	knurl_dp=DiamondDepth,
	e_smooth=DiamondLength/2);
	color("Orange") // lower tapered cap
	cylinder(r1=HeadDia/2,
	r2=(KnurlDia – DiamondDepth)/2,
	h=(TaperLength + Protrusion),
	$fn=NumSides);
	color("Orange") // upper tapered cap
	translate([0,0,(TaperLength + KnurlLen – Protrusion)])
	cylinder(r2=HeadDia/2,
	r1=(KnurlDia – DiamondDepth)/2,
	h=(TaperLength + Protrusion),
	$fn=NumSides);
	color("Moccasin") // cylindrical extension
	translate([0,0,(2*TaperLength + KnurlLen – Protrusion)])
	cylinder(r=HeadDia/2,h=(HeadRecess + Protrusion),$fn=NumSides);

	}

	translate([0,0,(2*TaperLength + KnurlLen + HeadRecess – SocketDepth + Protrusion)])
	PolyCyl(SocketDia,(SocketDepth + Protrusion),6); // hex key socket
	}

	if (Layout == "Shim")
	linear_extrude(height=(ShimThick)) // overall flange around edges
	polygon(points=ShimPoints);

	if (Layout == "Shaft")
	difference() {
	rotate_extrude($fn=ShaftSides,convexity=5)
	polygon(points=ShaftPoints);
	for (i=[0:NumCoverHoles-1])
	rotate(i*360/NumCoverHoles)
	translate([CoverHole[OD]/2,0,-Protrusion])
	rotate(180/8)
	PolyCyl(Insert[OD],15,8);
	}

view raw LMS Mini-lathe cover screw knob.scad hosted with ❤ by GitHub

The original doodle with more-or-less actual dimensions and clearances and suchlike:

The New Hotness

Mini-Lathe: Cover Screw Knobs and Change Gear Protector