Machine Shop, Software

CNC 3018-Pro: Platter Fixtures

Up to this point, the Sherline has been drilling 3.5 inch hard drive platters to serve as as reflecting bases for the vacuum tubes:

LinuxCNC - Sherline Mill - Logitech Gamepad
LinuxCNC – Sherline Mill – Logitech Gamepad

The CNC 3018-Pro has a work envelope large enough for CD / DVD platters, so I mashed the Sherline fixture with dimensions from the vacuum tube code, added the 3018’s T-slot spacing, and conjured a pair of fixtures for a pair of machines.

Because I expect to practice on scrap CDs and DVDs for a while:

Platter Fixtures - CD on 3018
Platter Fixtures – CD on 3018

And a 3.5 inch hard drive platter version:

Platter Fixtures - hard drive platter on 3018
Platter Fixtures – hard drive platter on 3018

The holes sit at half the 3018’s T-slot spacing (45 mm / 2), so you can nudge the fixtures to the front or rear, as you prefer.

The alignment dots & slots should help touch off the XY coordinate system on the Sherline, although it can’t reach all of a CD. Using bCNC’s video alignment on the hub hole will be much easier on the 3018.

After fiddling around with the 3018 for a while, however, the CD fixture doesn’t have many advantages over simply taping the disc to a flat platen. Obviously, you’d want a sacrificial layer for drilling, but it’s not clear the OEM motor / ER11 chuck would be up to that task.

The OpenSCAD source code as a GitHub Gist:

// Machining fixtures for CD and hard drive platters
// Ed Nisley KE4ZNU February ... September 2016
// 2019-08 split from tube base models
PlatterName = "CD"; // [3.5inch,CD]
CNCName = "3018"; // [3018,Sherline]
PlateThick = 5.0; // [5.0,10.0,15.0]
RecessDepth = 4.0; // [0.0,2.0,4.0]
//- Extrusion parameters must match reality!
/* [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);
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,$fn=Sides);
}
ID = 0;
OD = 1;
LENGTH = 2;
//----------------------
// Dimensions
P_NAME = 0; // platter name
P_ID = 1; // ... inner diameter
P_OD = 2; // ... outer diameter
P_THICK = 3; // ... thickness
PlatterData = [
["3.5inch", 25.0, 95.0, 1.75],
["CD", 15.0, 120.0, 1.20],
];
PlatterSides = 3*4*5; // polygon approximation
B_NAME = 0; // machine name
B_OC = 1; // ... platform screw OC, use small integer for slot
B_STUD = 2; // ... screw OD clearance
BaseData = [
["3018", [5.0, 45.0], 6.0], // slots along X axis
["Sherline", [1.16*inch,1.16*inch], 5.0], // tooling plate
];
//----------------------
// Drilling fixture for disk platters
module PlatterFixture(Disk,Machine) {
PI = search([Disk],PlatterData,1,0)[P_NAME]; // get platter index
echo(str("Platter: ",Disk));
Platter = [PlatterData[PI][P_ID],
PlatterData[PI][P_OD],
PlatterData[PI][P_THICK]];
BI = search([Machine],BaseData,1,0)[B_NAME]; // get base index
echo(str("Machine: ",Machine));
AlignOC = IntegerMultiple(Platter[OD],10);
echo(str("Align OC: ",AlignOC));
AlignSlot = [3*ThreadWidth,10.0,3*ThreadThick];
StudClear = BaseData[BI][B_STUD]; // ... clearance
StudOC = [IntegerMultiple(AlignOC + 2*StudClear,BaseData[BI][B_OC].x), // ... screw spacing
BaseData[BI][B_OC].y];
echo(str("Stud spacing: ",StudOC));
NumStuds = [2,1 + 2*floor(Platter[OD] / StudOC.y)]; // holes only along ±X edges
echo(str("Stud holes: ",NumStuds));
BasePlate = [(20 + StudOC.x*ceil(Platter[OD] / StudOC.x)),
(10 + AlignOC),
PlateThick];
echo(str("Plate: ",BasePlate));
PlateRound = 10.0; // corner radius
difference() {
hull() // basic plate shape
for (i=[-1,1], j=[-1,1])
translate([i*(BasePlate.x/2 - PlateRound),j*(BasePlate.y/2 - PlateRound),0])
cylinder(r=PlateRound,h=BasePlate.z,$fn=4*4);
for (i=[-1,0,1], j=[-1,0,1]) // origin pips
translate([i*AlignOC/2,j*AlignOC/2,BasePlate.z - 2*ThreadThick])
cylinder(d=4*ThreadWidth,h=1,$fn=6);
for (i=[-1,1], j=[-1,1]) { // alignment slots
translate([i*(AlignOC + AlignSlot.x)/2,
j*Platter[OD]/4,
(BasePlate.z - AlignSlot.z/2 + Protrusion/2)])
cube(AlignSlot + [0,0,Protrusion],center=true);
translate([i*Platter[OD]/4,
j*(AlignOC + AlignSlot.x)/2,
(BasePlate.z - AlignSlot.z/2 + Protrusion/2)])
rotate(90)
cube(AlignSlot + [0,0,Protrusion],center=true);
}
for (i=[-1,1], j=[-floor(NumStuds.y/2):floor(NumStuds.y/2)]) // mounting stud holes
translate([i*StudOC.x/2,j*StudOC.y/2,-Protrusion])
rotate(180/6)
PolyCyl(StudClear,BasePlate.z + 2*Protrusion,6);
translate([0,0,-Protrusion]) // center clamp hole
rotate(180/6)
PolyCyl(StudClear,BasePlate.z + 2*Protrusion,6);
translate([0,0,BasePlate.z - Platter[LENGTH]]) // disk locating recess
rotate(180/PlatterSides)
linear_extrude(height=(Platter[LENGTH] + Protrusion),convexity=2)
difference() {
circle(d=(Platter[OD] + HoleWindage),$fn=PlatterSides);
circle(d=Platter[ID] - HoleWindage,$fn=PlatterSides);
}
translate([0,0,BasePlate.z - RecessDepth]) // drilling recess
rotate(180/PlatterSides)
linear_extrude(height=(RecessDepth + Protrusion),convexity=2)
difference() {
circle(d=(Platter[OD] - 10),$fn=PlatterSides);
circle(d=(Platter[ID] + 10),$fn=PlatterSides);
}
}
}
//----------------------
// Build it
PlatterFixture(PlatterName,CNCName);
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