diff --git a/examples/README.md b/examples/README.md
index 246c894..c2de1ec 100644
--- a/examples/README.md
+++ b/examples/README.md
@@ -3,11 +3,20 @@
 Click on image to go to code directory.
 
 
+## Metric spacers M3,M4,M6,M8,M16
+
+| Example | Execution Time | File size |
+|---|---|---|
+|[Metric spacers M3,M4,M6,M8,M16](metric-spacers)|1s|9451kB|
+
+[![Metric spacers M3,M4,M6,M8,M16](fig/metric-spacers.png)](metric-spacers)
+
+
 ## ADZ Nagano sensor cover
 
 | Example | Execution Time | File size |
 |---|---|---|
-|[ADZ Nagano sensor cover](adz-sensor-cover)|1s|10MB|
+|[ADZ Nagano sensor cover](adz-sensor-cover)|0.5s|10MB|
 
 [![ADZ Nagano sensor cover](fig/adz-sensor-cover.png)](adz-sensor-cover)
 
@@ -25,7 +34,7 @@ Click on image to go to code directory.
 
 | Example | Execution Time | File size |
 |---|---|---|
-|[ATX Bench power supply mod](atx-bench-supply)|1s|4540kB|
+|[ATX Bench power supply mod](atx-bench-supply)|0.5s|4540kB|
 
 [![ATX Bench power supply mod](fig/atx-bench-supply.png)](atx-bench-supply)
 
diff --git a/examples/fig/metric-spacers.png b/examples/fig/metric-spacers.png
new file mode 100644
index 0000000..4ddf86c
Binary files /dev/null and b/examples/fig/metric-spacers.png differ
diff --git a/examples/generate_examples.go b/examples/generate_examples.go
index ae764c2..2e87eaf 100644
--- a/examples/generate_examples.go
+++ b/examples/generate_examples.go
@@ -36,6 +36,12 @@ var examples = []struct {
 	ExecutionTime string
 }{
 	// Add new examples here!
+	{
+		Name:      "Metric spacers M3,M4,M6,M8,M16",
+		Dir:       "metric-spacers",
+		resultSTL: "spacers.stl",
+		view:      defaultView,
+	},
 	{
 		Name:      "ADZ Nagano sensor cover",
 		Dir:       "adz-sensor-cover",
diff --git a/examples/metric-spacers/spacers.go b/examples/metric-spacers/spacers.go
new file mode 100644
index 0000000..bc866b1
--- /dev/null
+++ b/examples/metric-spacers/spacers.go
@@ -0,0 +1,66 @@
+package main
+
+import (
+	"math"
+
+	"github.com/soypat/sdf"
+	"github.com/soypat/sdf/form3"
+	"github.com/soypat/sdf/form3/obj3/thread"
+	"github.com/soypat/sdf/helpers/matter"
+	"github.com/soypat/sdf/render"
+	"gonum.org/v1/gonum/spatial/r3"
+)
+
+type spacer struct {
+	// Metric diameter.
+	D float64
+	// Height of spacer.
+	H float64
+}
+
+func main() {
+	spacers := []spacer{
+		{D: 3, H: 7},
+		{D: 3, H: 12},
+		{D: 4, H: 7},
+		{D: 6, H: 30},
+		{D: 8, H: 30},
+		{D: 16, H: 10},
+	}
+	var sdfs []sdf.SDF3
+	var x float64
+	for i := range spacers {
+		s, err := spacers[i].sdf(matter.PLA)
+		if err != nil {
+			panic(err)
+		}
+		s = sdf.Transform3D(s, sdf.Translate3D(r3.Vec{X: x}))
+		sdfs = append(sdfs, s)
+		x += spacers[i].D * 3
+	}
+	s := sdf.Union3D(sdfs...)
+	err := render.CreateSTL("spacers.stl", render.NewOctreeRenderer(s, 300))
+	if err != nil {
+		panic(err)
+	}
+}
+
+func (s spacer) sdf(material matter.Material) (sdf.SDF3, error) {
+	const tol = 0.03
+	holeCorrected := material.InternalDimScale(s.D * (1 + tol))
+	ftf := math.Ceil(holeCorrected*1.4) - .15 // Face to face hex distance
+	hexRadius := getHexRadiusFromFTF(ftf)
+	sp, err := thread.HexHead(hexRadius, s.H, "")
+	if err != nil {
+		return nil, err
+	}
+	hole, err := form3.Cylinder(s.H, holeCorrected/2, 0)
+	if err != nil {
+		return nil, err
+	}
+	return sdf.Difference3D(sp, hole), nil
+}
+
+func getHexRadiusFromFTF(ftf float64) (radius float64) {
+	return ftf / math.Cos(30.*math.Pi/180.) / 2
+}
diff --git a/helpers/matter/3dprint.go b/helpers/matter/3dprint.go
index df13776..51b8420 100644
--- a/helpers/matter/3dprint.go
+++ b/helpers/matter/3dprint.go
@@ -2,12 +2,22 @@ package matter
 
 import "github.com/soypat/sdf"
 
+type Material interface {
+	Scale(s sdf.SDF3) sdf.SDF3
+	InternalDimScale(real float64) float64
+}
+
 var (
 	// PLA (polylactic acid) is the most widely used plastic filament material in 3D printing.
-	PLA = ViscousMaterial{shrink: 0.2e-2, pullShrink: .45} // 0.2% shrinkage
+	PLA = Viscoelastic{shrink: 0.3e-2, pullShrink: .45} // 0.3% shrinkage
 )
 
-type ViscousMaterial struct {
+type Ideal struct{}
+
+func (Ideal) Scale(s sdf.SDF3) sdf.SDF3             { return s }
+func (Ideal) InternalDimScale(real float64) float64 { return real }
+
+type Viscoelastic struct {
 	// shrink is the thermal contraction shrinkage of a material once the material
 	// cools to room temperature after the heated bed is turned off.
 	shrink float64
@@ -16,13 +26,12 @@ type ViscousMaterial struct {
 }
 
 // Scale scales a 3D
-func (m ViscousMaterial) Scale(s sdf.SDF3) sdf.SDF3 {
-	scale := 1 / (1 - m.shrink) // is this correct?
-	return sdf.ScaleUniform3D(s, scale)
+func (m Viscoelastic) Scale(s sdf.SDF3) sdf.SDF3 {
+	return sdf.ScaleUniform3D(s, 1+m.shrink)
 
 }
 
-func (m ViscousMaterial) InternalDimScale(real float64) float64 {
+func (m Viscoelastic) InternalDimScale(real float64) float64 {
 	if real <= 0 {
 		panic("InternalDimScale only works for non-zero dimensions")
 	}