Machine Shop, Software

MPCNC: Drag Knife Holder

My attempt to use a HP 7475A plotter as a vinyl cutter failed due to its 19 g pen load limit:

HP 7475A knife stabilizer - big nut weight
HP 7475A knife stabilizer – big nut weight

The MPCNC, however, can apply plenty of downforce, so I tinkered up a quick-and-dirty adapter to put the drag knife “pen” body into the MPCNC’s standard DW660 router holder:

MPCNC - DW660 adapter drag knife holder - fixed position
MPCNC – DW660 adapter drag knife holder – fixed position

That’s using the DW660 adapter upside-down to get the business end of the knife closer to the platform. The solid model descends from the linear-bearing Sakura pen holder by ruthless pruning.

It didn’t work well at all, because you really need a spring for some vertical compliance and control over the downforce pressure.

Back to the Comfy Chair:

Drag Knife Holder - DW660 Mount - solid model
Drag Knife Holder – DW660 Mount – solid model

A trio of the lightest springs from a 200 piece assortment (in the front left compartment) pushes the upper plate downward against the drag knife’s flange:

MPCNC - DW660 adapter drag knife holder - spring loaded
MPCNC – DW660 adapter drag knife holder – spring loaded

There’s a bit more going on than may be obvious at first glance.

The screws slide in brass tubing press-fit into the upper plate, because otherwise their threads hang up on the usual 3D printed layers inside the (drilled-out) holes. Smaller free-floating brass tubing snippets inside the springs keep them away from the screw threads; the gap between the top of the tubing and the screw head limits the vertical compliance to 3 mm. The screws thread into brass inserts epoxied into the bottom disk, with a dab of low-strength Loctite for stay-put adjustment.

I bored the orange PETG disk to a nice slip fit around the knife body:

DW660 drag knife holder - boring body
DW660 drag knife holder – boring body

The upper plate also required fitting:

DW660 drag knife holder - boring plate
DW660 drag knife holder – boring plate

A few iterations produced reasonably smooth motion over a few millimeters, but it’s definitely not a low-friction / low-stiction drag knife holder. It ought to be good for some proof-of-concept vinyl cutting, though.

The OpenSCAD source code as a GitHub Gist:

// Drag Knife Holder for DW660 Mount
// Ed Nisley KE4ZNU - 2018-09-26
Layout = "Show"; // Build, Show, Puck, Mount, Plate
/* [Extrusion] */
ThreadThick = 0.25; // [0.20, 0.25]
ThreadWidth = 0.40; // [0.40]
/* [Hidden] */
Protrusion = 0.1; // [0.01, 0.1]
HoleWindage = 0.2;
inch = 25.4;
function IntegerMultiple(Size,Unit) = Unit * ceil(Size / Unit);
ID = 0;
OD = 1;
LENGTH = 2;
//- Adjust hole diameter to make the size come out right
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,$fn=Sides);
}
//- Dimensions
KnifeBody = [12.0,16.0,2.0]; // body flange -- resembles HP plotter pen
WallThick = 3.0; // minimum thickness / width
Screw = [4.0,8.5,8.0]; // holding it all together, OD = washer
Insert = [4.0,6.0,10.0]; // brass insert
Plate = [KnifeBody[ID],KnifeBody[OD] + 3*Screw[OD],4.0]; // spring reaction plate
PlateGuide = [4.0,4.8,Plate[LENGTH]]; // ... guide tubes
NumScrews = 3;
ScrewBCD = 2*(KnifeBody[OD]/2 + Screw[OD]/2 + 0.5);
NumSides = 9*4; // cylinder facets (multiple of 3 for lathe trimming)
// Basic shape of DW660 snout fitting into the holder
// Lip goes upward to lock into MPCNC mount
Snout = [44.6,50.0,9.6]; // LENGTH = ID height
Lip = 4.0; // height of lip at end of snout
PuckOAL = Snout[LENGTH] + Lip; // total height
Key = [Snout[ID],25.7,PuckOAL]; // rectangular key
module DW660Puck() {
translate([0,0,PuckOAL])
rotate([180,0,0]) {
cylinder(d=Snout[OD],h=Lip/2,$fn=NumSides);
translate([0,0,Lip/2])
cylinder(d1=Snout[OD],d2=Snout[ID],h=Lip/2,$fn=NumSides);
cylinder(d=Snout[ID],h=PuckOAL,$fn=NumSides);
intersection() {
translate([0,0,0*Lip + Key.z/2])
cube(Key,center=true);
cylinder(d=Snout[OD],h=Lip + Key.z,$fn=NumSides);
}
}
}
module MountBase() {
difference() {
DW660Puck();
translate([0,0,-Protrusion]) // knife holder body
PolyCyl(KnifeBody[ID],2*PuckOAL,NumSides);
translate([0,0,PuckOAL - KnifeBody[LENGTH]/2]) // ... half of flange, loose fit
PolyCyl(KnifeBody[OD] + 2*HoleWindage,KnifeBody[LENGTH],NumSides);
for (i=[0:NumScrews - 1]) // clamp screws
rotate(i*360/NumScrews)
translate([ScrewBCD/2,0,-Protrusion])
rotate(180/8)
PolyCyl(Insert[OD],2*PuckOAL,8);
}
}
module SpringPlate() {
difference() {
cylinder(d=Plate[OD],h=Plate[LENGTH],$fn=NumSides);
translate([0,0,-Protrusion]) // knife holder body
PolyCyl(KnifeBody[ID],2*PuckOAL,NumSides);
translate([0,0,Plate[LENGTH] - KnifeBody[LENGTH]/2]) // ... half of flange, snug fit
PolyCyl(KnifeBody[OD],KnifeBody[LENGTH],NumSides);
for (i=[0:NumScrews - 1]) // clamp screws
rotate(i*360/NumScrews)
translate([ScrewBCD/2,0,-Protrusion])
rotate(180/8)
PolyCyl(PlateGuide[OD],2*PuckOAL,8);
}
}
//-----
// Build it
if (Layout == "Puck")
DW660Puck();
if (Layout == "Plate")
SpringPlate();
if (Layout == "Mount")
MountBase();
if (Layout == "Show") {
MountBase();
translate([0,0,2*PuckOAL])
rotate([180,0,0])
SpringPlate();
}
if (Layout == "Build") {
translate([0,Snout[OD]/2,0])
MountBase();
translate([0,-Snout[OD]/2,0])
SpringPlate();
}
view raw Drag Knife Holder - DW660 Mount.scad hosted with ❤ by GitHub