In Techniques

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CNC "Woodturning"

By Randy Johnson

A rotary indexing head allows a CNC machine to
create 3-dimensional shapes in the round. It’s an accessory
that can be added to most CNC machines. Some companies
even make it as a stand alone machine. A rotary
indexing head looks similar to a standard woodturning
lathe, but its approach to shaping wood is quite different.
In fact, it’s more like milling than woodturning. One
of the best features of a rotary indexing head is its ability
to create shapes that aren’t easily turned on a standard
wood lathe, such as this hexagonal chisel handle. Intricate
round relief carvings are also possible. Because it’s CNC
based, a rotary indexing head is capable of great precision
and easy repeatability. However, since the shaping is done
with a router bit in small increments (as small as 1/50" per
pass), the milling process can take a while to complete.
Machining this chisel handle took about 2-1/2 hours, but
its unique shape was intriguing to design and mill. It also
makes an attractive addition to my tool box.



You need to think a little differently.

Think flat

CNC turnings usually start out as a flat design, so the first step is to “unwrap” the cylindrical profile. CNC design software
uses a variety of drawing tools that assist this process. One tool automatically calculates the flat design’s width, based on the
maximum diameter you specify for the turning. Another tool takes complex shapes such as the hexagonal cross section of
this handle and converts it into the flat shape.

Click any image to view a larger version.

Think round

The design software converts (wraps) the flat design into its cylindrical shape to give you a preview of the final piece.

Think parts

Each part of a CNC turning is created separately. The parts are then joined to create the final design. The basic steps used to
design this chisel handle appear below.

Basic Parts Creation

Each 3-dimensional part is created using a line drawing of its cross section to extrude (or “sweep”) the shape along a path. The
tapered tenon and the round pommel are extruded across the width of their designs, while the body of the handle is extruded
along the length of its design. The handle’s contoured hexagonal body is created using three different cross sections and a
software tool (or “gadget”) that automatically unwraps the hexagonal shape into its corresponding flat shape.

Parting tabs are added to the ends of the final design to connect the part to the unmachined ends of the billet. The tabs are
created using the same drawing tools used to create the tapered tenon and the round pommel.

Basic Machine Steps

Step 1: Create a cylinder

The first step is to round off corners of the
billet to create a cylinder. The CNC design
software includes a gadget that automatically
calculates the cutting paths needed to
remove the corners, based on the dimensions
of the square billet and the finished diameter
of the cylinder. To create the hexagonal chisel
handle I started with a 2" square billet and
rounded it to 1-3/4" diameter using a 1/4" dia.
bullnose bit. Rounding this 22" long cylinder
takes about 20 minutes.

Step 2:
Rough rout the shape

The same 1/4" bullnose bit roughs out the
handle’s hexagonal shape. In this case, the
cutting passes are programmed to run the
length of the cylinder and remove a 1/8"
deep x 1/8" wide a strip of material with each
pass. The last pass leaves 1/32" of material to
be removed in the next step. Roughing out
this shape takes about 25 minutes.

Step 3:
Finish rout the final shape

The final (finishing) pass removes the last
1/32" of material and leaves a smooth
surface. To do this, the 1/4" bullnose bit is
programmed to “step over” each previous
pass by only 1/50". This tiny step-over
leaves a surface that’s easy to clean up
with 180 grit sandpaper. This finishing pass
takes about 50 minutes to complete.

Step 4:
Add details

The surface of a CNC turning can be
embellished with additional details, including
lettering. For this chisel handle I combined
a 60° V-bit and a script-style font to create
a look similar to metal engraving or laser
etching. These finely detailed 3/4" tall
letters demonstrate the precision of a CNC’s
operation. Routing them takes only a minute.

Twists are also possible

This story originally appeared in American Woodworker February/March 2012, issue #158.



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