Change threading and screws to use 2*$slop

Per [my argument](BelfrySCAD#1679 (comment)), I believe this is the proper method. There is only one mating surface for each radius, so by following the documentation for `$slop`, we should only be adding one `$slop` for radius and not two.

Author: coryrc

screws.scad

@@ -593,7 +593,7 @@ module screw(spec, head, drive, thread, drive_size,
             assert(is_undef(user_thread_len) || (is_finite(user_thread_len) && user_thread_len>=0), "Must specify nonnegative thread length");
    sides = max(pitch==0 ? 3 : 12, segs(nominal_diam/2));
    rad_scale = _internal? (1/cos(180/sides)) : 1;
-   islop = _internal ? 4*get_slop() : 0;
+   islop = _internal ? 2*get_slop() : 0;
    head_height = headless || flathead ? 0 
                : counterbore==true || is_undef(counterbore) || counterbore==0 ? struct_val(spec, "head_height")
                : counterbore;
@@ -746,7 +746,7 @@ module screw(spec, head, drive, thread, drive_size,
 //   The screw hole can be threaded to receive a screw or it can be an unthreaded clearance hole.  
 //   The tolerance determines the dimensions of the screw
 //   based on ISO and ASME standards.  Screws fabricated at those dimensions will mate properly with standard hardware.
-//   The $slop argument makes the hole larger by 4*$slop to account for printing overextrusion.  It defaults to 0.
+//   The $slop argument makes the hole larger by 2*$slop to account for printing overextrusion.  It defaults to 0.
 //   .
 //   You can generate a screw specification from {{screw_info()}}, possibly create a modified version, and pass that in rather than giving the parameters.
 //   .

threading.scad

@@ -174,7 +174,7 @@
 //   anchor = Translate so anchor point is at origin (0,0,0).  See [anchor](attachments.scad#subsection-anchor).  Default: `CENTER`
 //   spin = Rotate this many degrees around the Z axis after anchor.  See [spin](attachments.scad#subsection-spin).  Default: `0`
 //   orient = Vector to rotate top towards, after spin.  See [orient](attachments.scad#subsection-orient).  Default: `UP`
-//   $slop = The printer-specific slop value, which adds clearance (`4*$slop`) to internal threads.
+//   $slop = The printer-specific slop value, which adds clearance (`2*$slop`) to internal threads.
 // Example(2D):
 //   projection(cut=true)
 //       threaded_rod(d=10, l=15, pitch=1.5, orient=BACK);
@@ -327,7 +327,7 @@ module threaded_rod(
 //   anchor = Translate so anchor point is at origin (0,0,0).  See [anchor](attachments.scad#subsection-anchor).  Default: `CENTER`
 //   spin = Rotate this many degrees around the Z axis after anchor.  See [spin](attachments.scad#subsection-spin).  Default: `0`
 //   orient = Vector to rotate top towards, after spin.  See [orient](attachments.scad#subsection-orient).  Default: `UP`
-//   $slop = The printer-specific slop value, which adds clearance (`4*$slop`) to internal threads.
+//   $slop = The printer-specific slop value, which adds clearance (`2*$slop`) to internal threads.
 // Examples(Med):
 //   threaded_nut(nutwidth=16, id=8, h=8, pitch=1.25, $slop=0.05, $fa=1, $fs=1);
 //   threaded_nut(nutwidth=16, id=8, h=8, pitch=1.25, left_handed=true, bevel=false, $slop=0.1, $fa=1, $fs=1);
@@ -500,7 +500,7 @@ module threaded_nut(
 //   anchor = Translate so anchor point is at origin (0,0,0).  See [anchor](attachments.scad#subsection-anchor).  Default: `CENTER`
 //   spin = Rotate this many degrees around the Z axis after anchor.  See [spin](attachments.scad#subsection-spin).  Default: `0`
 //   orient = Vector to rotate top towards, after spin.  See [orient](attachments.scad#subsection-orient).  Default: `UP`
-//   $slop = The printer-specific slop value, which adds clearance (`4*$slop`) to internal threads.
+//   $slop = The printer-specific slop value, which adds clearance (`2*$slop`) to internal threads.
 // Example(2D):
 //   projection(cut=true)
 //       trapezoidal_threaded_rod(d=10, l=15, pitch=2, orient=BACK);
@@ -647,7 +647,7 @@ module trapezoidal_threaded_rod(
 //   anchor = Translate so anchor point is at origin (0,0,0).  See [anchor](attachments.scad#subsection-anchor).  Default: `CENTER`
 //   spin = Rotate this many degrees around the Z axis after anchor.  See [spin](attachments.scad#subsection-spin).  Default: `0`
 //   orient = Vector to rotate top towards, after spin.  See [orient](attachments.scad#subsection-orient).  Default: `UP`
-//   $slop = The printer-specific slop value, which adds clearance (`4*$slop`) to internal threads.
+//   $slop = The printer-specific slop value, which adds clearance (`2*$slop`) to internal threads.
 // Examples(Med):
 //   trapezoidal_threaded_nut(nutwidth=16, id=8, h=8, pitch=2, $slop=0.1, anchor=UP);
 //   trapezoidal_threaded_nut(nutwidth=16, id=8, h=8, pitch=2, bevel=false, $slop=0.05, anchor=UP);
@@ -769,7 +769,7 @@ module trapezoidal_threaded_nut(
 //   anchor = Translate so anchor point is at origin (0,0,0).  See [anchor](attachments.scad#subsection-anchor).  Default: `CENTER`
 //   spin = Rotate this many degrees around the Z axis after anchor.  See [spin](attachments.scad#subsection-spin).  Default: `0`
 //   orient = Vector to rotate top towards, after spin.  See [orient](attachments.scad#subsection-orient).  Default: `UP`
-//   $slop = The printer-specific slop value, which adds clearance (`4*$slop`) to internal threads.
+//   $slop = The printer-specific slop value, which adds clearance (`2*$slop`) to internal threads.
 // Example(2D):
 //   projection(cut=true)
 //       acme_threaded_rod(d=10, l=15, pitch=2, orient=BACK);
@@ -881,7 +881,7 @@ module acme_threaded_rod(
 //   anchor = Translate so anchor point is at origin (0,0,0).  See [anchor](attachments.scad#subsection-anchor).  Default: `CENTER`
 //   spin = Rotate this many degrees around the Z axis after anchor.  See [spin](attachments.scad#subsection-spin).  Default: `0`
 //   orient = Vector to rotate top towards, after spin.  See [orient](attachments.scad#subsection-orient).  Default: `UP`
-//   $slop = The printer-specific slop value, which adds clearance (`4*$slop`) to internal threads.
+//   $slop = The printer-specific slop value, which adds clearance (`2*$slop`) to internal threads.
 // Examples(Med):
 //   acme_threaded_nut(nutwidth=16, id=3/8*INCH, h=8, tpi=8, $slop=0.05);
 //   acme_threaded_nut(nutwidth=16, id=3/8*INCH, h=10, tpi=12, starts=3, $slop=0.1, $fa=1, $fs=1, ibevel=false);
@@ -968,7 +968,7 @@ module acme_threaded_nut(
 //   anchor = Translate so anchor point is at origin (0,0,0).  See [anchor](attachments.scad#subsection-anchor).  Default: `CENTER`
 //   spin = Rotate this many degrees around the Z axis after anchor.  See [spin](attachments.scad#subsection-spin).  Default: `0`
 //   orient = Vector to rotate top towards, after spin.  See [orient](attachments.scad#subsection-orient).  Default: `UP`
-//   $slop = The printer-specific slop value, which adds clearance (`4*$slop`) to internal threads.
+//   $slop = The printer-specific slop value, which adds clearance (`2*$slop`) to internal threads.
 // Example(2D): The straight gray rectangle reveals the tapered threads.  
 //   projection(cut=true) npt_threaded_rod(size=1/4, orient=BACK);
 //   right(.533*INCH/2) color("gray") rect([2,0.5946*INCH],anchor=LEFT);
@@ -1108,7 +1108,7 @@ module npt_threaded_rod(
 //   anchor = Translate so anchor point is at origin (0,0,0).  See [anchor](attachments.scad#subsection-anchor).  Default: `CENTER`
 //   spin = Rotate this many degrees around the Z axis after anchor.  See [spin](attachments.scad#subsection-spin).  Default: `0`
 //   orient = Vector to rotate top towards, after spin.  See [orient](attachments.scad#subsection-orient).  Default: `UP`
-//   $slop = The printer-specific slop value, which adds clearance (`4*$slop`) to internal threads.
+//   $slop = The printer-specific slop value, which adds clearance (`2*$slop`) to internal threads.
 // Example(2D):
 //   projection(cut=true)
 //       buttress_threaded_rod(d=10, l=15, pitch=2, orient=BACK);
@@ -1223,7 +1223,7 @@ module buttress_threaded_rod(
 //   anchor = Translate so anchor point is at origin (0,0,0).  See [anchor](attachments.scad#subsection-anchor).  Default: `CENTER`
 //   spin = Rotate this many degrees around the Z axis after anchor.  See [spin](attachments.scad#subsection-spin).  Default: `0`
 //   orient = Vector to rotate top towards, after spin.  See [orient](attachments.scad#subsection-orient).  Default: `UP`
-//   $slop = The printer-specific slop value, which adds clearance (`4*$slop`) to internal threads.
+//   $slop = The printer-specific slop value, which adds clearance (`2*$slop`) to internal threads.
 // Examples(Med):
 //   buttress_threaded_nut(nutwidth=16, id=8, h=8, pitch=1.25, left_handed=true, $slop=0.05, $fa=1, $fs=1);
 function buttress_threaded_nut(
@@ -1324,7 +1324,7 @@ module buttress_threaded_nut(
 //   anchor = Translate so anchor point is at origin (0,0,0).  See [anchor](attachments.scad#subsection-anchor).  Default: `CENTER`
 //   spin = Rotate this many degrees around the Z axis after anchor.  See [spin](attachments.scad#subsection-spin).  Default: `0`
 //   orient = Vector to rotate top towards, after spin.  See [orient](attachments.scad#subsection-orient).  Default: `UP`
-//   $slop = The printer-specific slop value, which adds clearance (`4*$slop`) to internal threads.
+//   $slop = The printer-specific slop value, which adds clearance (`2*$slop`) to internal threads.
 // Example(2D):
 //   projection(cut=true)
 //       square_threaded_rod(d=10, l=15, pitch=2, orient=BACK);
@@ -1432,7 +1432,7 @@ module square_threaded_rod(
 //   anchor = Translate so anchor point is at origin (0,0,0).  See [anchor](attachments.scad#subsection-anchor).  Default: `CENTER`
 //   spin = Rotate this many degrees around the Z axis after anchor.  See [spin](attachments.scad#subsection-spin).  Default: `0`
 //   orient = Vector to rotate top towards, after spin.  See [orient](attachments.scad#subsection-orient).  Default: `UP`
-//   $slop = The printer-specific slop value, which adds clearance (`4*$slop`) to internal threads.
+//   $slop = The printer-specific slop value, which adds clearance (`2*$slop`) to internal threads.
 // Examples(Med):
 //   square_threaded_nut(nutwidth=16, id=10, h=10, pitch=2, starts=2, $slop=0.1, $fn=32);
 function square_threaded_nut(
@@ -1527,7 +1527,7 @@ module square_threaded_nut(
 //   anchor = Translate so anchor point is at origin (0,0,0).  See [anchor](attachments.scad#subsection-anchor).  Default: `CENTER`
 //   spin = Rotate this many degrees around the Z axis after anchor.  See [spin](attachments.scad#subsection-spin).  Default: `0`
 //   orient = Vector to rotate top towards, after spin.  See [orient](attachments.scad#subsection-orient).  Default: `UP`
-//   $slop = The printer-specific slop value, which adds clearance (`4*$slop`) to internal threads.
+//   $slop = The printer-specific slop value, which adds clearance (`2*$slop`) to internal threads.
 // Example(2D): Thread Profile, ball_diam=4, ball_arc=100
 //   projection(cut=true) ball_screw_rod(d=10, l=15, pitch=5, ball_diam=4, ball_arc=100, orient=BACK, $fn=24, blunt_start=false);
 // Example(2D): Thread Profile, ball_diam=4, ball_arc=120
@@ -1616,8 +1616,8 @@ module ball_screw_rod(
 //   .
 //   If internal is true then produce a thread mask to difference from an object.  When internal is true the rod
 //   diameter is enlarged to correct for the polygonal nature of circles to ensure that the internal diameter is the
-//   specified size.  The diameter is also increased by `4 * $slop` to create clearance for threading by allowing a `2 *
-//   $slop` gap on each side.  If bevel is set to true and internal is false then the ends of the rod will be beveled.
+//   specified size.  The diameter is also increased by `2 * $slop` to create clearance for threading by allowing a
+//   `$slop` gap on each side.  If bevel is set to true and internal is false then the ends of the rod will be beveled.
 //   When bevel is true and internal is true the ends of the rod will be filled in so that the rod mask will create a
 //   bevel when subtracted from an object.  The bevel is at 45 deg and is the depth of the threads.
 //   .
@@ -1661,7 +1661,7 @@ module ball_screw_rod(
 //   anchor = Translate so anchor point is at origin (0,0,0).  See [anchor](attachments.scad#subsection-anchor).  Default: `CENTER`
 //   spin = Rotate this many degrees around the Z axis after anchor.  See [spin](attachments.scad#subsection-spin).  Default: `0`
 //   orient = Vector to rotate top towards, after spin.  See [orient](attachments.scad#subsection-orient).  Default: `UP`
-//   $slop = The printer-specific slop value, which adds clearance (`4*$slop`) to internal threads.
+//   $slop = The printer-specific slop value, which adds clearance (`2*$slop`) to internal threads.
 // Example(2DMed): Example Tooth Profile
 //   pitch = 2;
 //   depth = pitch * cos(30) * 5/8;
@@ -1741,7 +1741,7 @@ module generic_threaded_rod(
       assert(is_undef(bevel2) || is_num(bevel2) || is_bool(bevel2) || bevel2=="reverse", "bevel2/bevel must be a number, boolean or \"reverse\"");
     sides = quantup(segs(max(r1,r2)), starts);
     rsc = internal? (1/cos(180/sides)) : 1;    // Internal radius adjusted for faceting
-    islop = internal? 2*get_slop() : 0;
+    islop = internal? get_slop() : 0;
     r1adj = r1 * rsc + islop;
     r2adj = r2 * rsc + islop;
 
@@ -2004,7 +2004,7 @@ module generic_threaded_rod(
 //   anchor = Translate so anchor point is at origin (0,0,0).  See [anchor](attachments.scad#subsection-anchor).  Default: `CENTER`
 //   spin = Rotate this many degrees around the Z axis after anchor.  See [spin](attachments.scad#subsection-spin).  Default: `0`
 //   orient = Vector to rotate top towards, after spin.  See [orient](attachments.scad#subsection-orient).  Default: `UP`
-//   $slop = The printer-specific slop value, which adds clearance (`4*$slop`) to internal threads.
+//   $slop = The printer-specific slop value, which adds clearance (`2*$slop`) to internal threads.
 function generic_threaded_nut(
     nutwidth,
     id,
@@ -2068,7 +2068,7 @@ module generic_threaded_nut(
         difference() {
             _nutshape(nutwidth,h, shape,bevel1,bevel2);
             if (pitch==0) 
-               cyl(l=h+extra, d1=full_id1+4*get_slop(), d2=full_id2+4*get_slop(),
+               cyl(l=h+extra, d1=full_id1+2*get_slop(), d2=full_id2+2*get_slop(),
                    chamfer1=ibevel1?-IBEV*full_id1:undef,
                    chamfer2=ibevel2?-IBEV*full_id2:undef);
             else