segmented turning - Wonthaggi Woodcrafters

SEGMENTED TURNING
What is it?
Segmented turning is the creation of a bowl, vase or similar object by the assembly of small wooden segments. The segments are
assembled into rings and the number of segments required for a ring will determine the angle of each of the segments. Numerous
rings are assembled on top of each other to create the bowl or vase.
This computer software described in this document facilitates the calculation of the segment length of each ring.
Terminology?
Before more detail is given on creating a design and performing the calculations certain terms must be defined to provide the
necessary language.
The Segment:
The segment is fully defined by four properties. The width
defines how broad the segment is and is also the width of the strip of timber the segment is cut from. The thickness defines the
height or how thick the segment is. The length of the segment determines how many segments make up a ring. Knowing the length,
which we calculate by using some basic trigonometry and the angle, the segment is fully determined. The angle indicated in the
figure above is often referred as the mitre angle or the cutting angle. This angle is half the actual wedge angle of which the segment
is part of. See figure below.
To determine the length of the segment you must first establish the number
of segments you wish for a ring. For example, for a 12 segment ring the
wedge angle will be 360 divided by 12 or 30 degrees. The mitre angle is half
the wedge angle and is therefore 15 degrees.
By trigonometry, the length is;
2 x radius x TAN(mitre angle)
By applying this simple formula to every ring in the object we wish to build
we obtain all the necessary segment lengths.
The computer software described in the following pages does all the
calculation of lengths and prints a clear table of values for all rings in a
project.
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Read on…
The Project: A Bowl
Before attempting any project it is necessary to draw a plan. 1mm graph paper is a good choice. Draw the profile or shape of the
bowl or vase you wish to make. Draw only the right hand side making the left side of the graph paper correspond to the centre of
the object. See the plan below for a simple bowl containing seven rings and two types of timber, say a red and white timber.
Begin by drawing the base. The base for this bowl will be ⅓ the diameter of the bowl’s rim, say 100mm for a 300mm size bowl.
Number the rings 1 to 7 from the bottom to the top. Draw the bowl’s shape making the thickness 10mm. The bowl will be made of
5 rings 15mm thick (white timber) and 2 rings 5mm thick (red timber). In addition, you need to determine the number of segments
for each ring. For our simple bowl we will have each ring made from the same number of segments. We will choose 12 segments,
making the mitre angle of the segment 15 degrees (180/12)
For each ring, draw the thickness of the ring and width of timber required as a rectangle around the outline of the bowl. Allow 5mm
excess on the inside and outside of the bowl. Also measure the radius of each segment from the plan (the distance from the centre
to the outer edge of the segment). See plan drawing below.
We are now ready to use the Segmented Turning Helper software to enter the necessary data from our plan and use the software
to calculate the segment length for each ring.
Given the input data, timber, number of rings, mitre angle, thickness, width and radius, obtained from the plan drawing, the
Segmented Turning Helper software will calculate the length of the segments for each ring. In addition the software will calculate
the vernier length, stock length for each ring and a complete stock list.
The software will also provide a computer rendered profile of the object similar to the diagram in the plan drawing. One or two
related images for the project may also be specified and printed with the calculated lengths. See the following documentation for a
full description on how to use the software.
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Using the Computer Program Segmented Turning Helper
Assuming the software has been installed on your computer and ready to run, double click the Segmented
Turning Helper icon on your computer desktop. The program will open and appear as shown below.
Opening Screen
Click the New Project icon
and enter the number of rings in the project.
The screen is refreshed to display a table with the required number of rows as well as a row for the base.
Enter the base size as; Base Diameter x Height mm. For example; Base 100x20 mm
Next enter the
timber name
and/or color in the
Timber/Color
column. In the
sample project
rings 1,2,3,5,7 use a
white timber. Rings
4 and 6 use a red
timber. You will
need to select
appropriate
timbers for these
different colors and
generally would
enter the timber name rather than the color. For example, ash and Jarrah for white and red respectively.
The computer screen should now look something like the above.
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Enter the number of segments for each ring. The mitre angle is automatically calculated by the software.
Now enter the segment thickness, width and radius values for each ring. All dimensions are in millimeters. These values are all
obtained from the plan drawing. The computer screen should now be similar to the image below and the software has all the data
required for calculating the segment length.
Click the Calculate button to calculate the segment lengths.
The last three columns in the table are filled by the results of the calculations performed by the software updating the table as
follows.
The segment length column gives the length of the segments for each ring. The vernier length values are a little smaller than the
segment length values and represent the perpendicular distance from the blade to the stop block.
The values in the Vernier Length column are used to measure the distance from the blade to the stop to give the correct segment
length. The stop I have contrived is a digital depth gauge easily adjusted to measure the vernier length.
See the section ‘Cutting Accurate Segments’ for clarification of the Vernier length.
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Click the printer button to print the table of values.
The printer button will display the printer dialog
window.
Select the printer and click the Print button at the bottom of the
dialog to print..
The print preview button gives a preview of the printout. Click this button to preview before printing. The computer
screen should show a table similar to that shown below.
From the print preview screen you can print the table of values by clicking the small printer icon at the top left of the print preview
screen. If the printout is contained in more than one page click the 2/3/4 or 6 page icon to preview all pages. Alternatively, at the
top right of the preview screen is a numeric up/down control that allows you to preview the designated page.
Click the magnifier icon to enlarge the preview. Select the enlargement size.
Click the close button to return to the main software window.
Use this printout, together with the plan drawing to cut the ring segments and build your project.
At this point you should save the full table of values. Click the save button on the main menu bar to do this. See the next section.
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Click the save button to save all table data. The first time you save you will need to supply a file name. For the sample
project described in this document the file name is ‘Project#1’
To open an existing project click the open file icon.
Supply the file name of the project to open.
You can use the open command if you wish to modify an
existing project or just print a new copy of the table of values.
The new command will initialize a new project. Specify the number of rings you
wish for the project by using the up/down numeric control and click the OK
button
Click the settings icon to open the settings window.
Name of Creator
Enter your name in the creator box.
The name you enter will be shown on the main window and is
also printed in the table of values. The default or initial value
is ‘unknown author’.
Linking number of segments and mitre angle
Click the check box to link number of segments and mitre
angle. This is generally true for closed segmented projects. In
open segmented projects it may be desirable to not link these
two values.
If you link the number of segments and mitre angle the
computer software will automatically generate the mitre angle
when you enter number of segments or vice versa.
The third setting specifies a value used in determining the stock list. For example, if one ring is to be cut from 40mm wide stock and
another ring from 42mm wide stock it is most likely that you will use the 42mm wide stock for both rings – assuming both rings are
to be made from the same timber and from the same thickness.
Specify a value, like 5mm to allow the software to group similar timber widths of the same timber and timber thickness in
determining the stock list.
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The Tools command of the menu bar at the top of the Segmented Turning Helper
software exposes various useful features. These are explained below.
In many projects the timber name/color, mitre angle, number of segments, timber
thickness may be the same for all rings in a project. In these situations you may set
the value for every row in the table (corresponding to every ring in the project) to
the desired value.
Click Tools, click Set Column Value, then click the small down arrow to open the the available list to choose from. Click the desired
column name from one of the following values;
Timber/Color
Number of Segments
Mitre Angle
Thickness
Enter its related value in the value box. Click OK.
The designated column is filled with the specified value.
Note that specifying a mitre angle automatically calculates
the number of segments, and specifying the number of
segments will automatically calculate the mitre angle, if the
two values are linked – see Settings.
For example, specifying 12 segments per ring will fill the
#Segments column with 12 and Mitre Angle column with 15
degrees, if the columns are linked. If the two columns are
not linked the #Segments column only if filled with
the value of 12.
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The Tools/Set Number of Rings option will allow you to
change the number of rings in an existing project. Enter
the new value and press OK.
If the current project has more rings than the new value
rings at the bottom of the table will be removed and if any
data was present for those rings it will be lost. For
example, if the current project contains 20 rings and you
modify the project to contain 18 rings, the bottom two
rings, ring 19 and 20 will be discarded.
This command will calculate the segment lengths and is equivalent to pressing the calculate button.
The Show Reference Image 1 command will allow you to link a reference image to
your project. Initially, a blue box is displayed on top of the Segment Terms window.
Right click the box to pop up a context menu. Click the Add Image command to
specify and display the image you wish to add. When you click the Add Image
command a file dialog is displayed allowing you to locate the image on your hard disk.
Once you have located the image and clicked the OK button of the File Dialog the
image is displayed in the blue box on the screen.
For example;
The image may appear distorted if the width/height ratio of the
image does not match the width/height ratio of the initial blue
box. Adjust the aspect ratio and size of the image by floating the
mouse over the borders of the box and when a double headed
arrow appears click the mouse left button down and while holding
the button down stretch the box as desired.
The image will automatically be printed when you print your
project.
As the previous option but allows a second reference image to be included.
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The Segment Terms window is displayed to the right of the data table. Because of the
size of your computer screen you may not see all the contained terms. To show the full
window click Tools/Show Segment Terms. To collapse the Segment Terms window and
return to the Data Table window, click it with the mouse left button.
The Segmented Turning Helper software will attempt to draw a profile of the project and may be viewed by clicking Tools/Show
Project Drawing. The project drawing is automatically printed when you print or print preview the project.
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Page 1
Page 1 of the printout give all input data and the
calculated segment length, vernier length and total length
for each of the rings in the project. Below the table is the
stock list where same timbers of equal thickness and with
width variance of 5mm (or some nominated value of your
own as specified in Settings) are grouped and printed as a
required timber stock list.
Page 2 shows the project drawing and any reference
images linked to the project.
Note: The printout is for project John’s Bowl2.seg which is
provided in the Projects folder of the Segmented Turning
Helper software.
This completes the software documentation. The following
pages discuss some general features of segmented turning
and table saw use.
Page 2
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For readers who are new to segmented turning please continue to read the rest of this document as it provides information and tips
for cutting your segments. The description below employs a table saw and assumes that you are capable of using your table saw
effectively and safely.
The most crucial part of segmented turning is getting
your rings to fit accurately without gaps between the
segments. You can make perfect rings only if you cut
very accurate angles and lengths. Segments can be cut
on a band saw, with a drop saw or most accurately with
a table saw.
Using a table saw is the preferred method as it
provides most accuracy. You can use a high quality
mitre gauge to help get the angles right or you can
construct a sliding table for various commonly used
angles, such as 15, 10 and 7.5 degrees. Using a high
quality saw blade will also reduce burrs and tears.
Photo 1 shows the sliding table on the left specially
constructed to cut 15 degree mitre angles and a second Photo 1
sliding table on the right to which a depth gauge has
been affixed to act as a stop whose position can quickly
be adjusted.
Because of the geometry of the situation, the vernier
length is slightly shorter than the segment length. The
vernier length corresponds to the height of the
superimposed triangle (the perpendicular distance
between the table saw and the stop). The segment
length corresponds to the hypotenuse of the triangle.
The mathematics relating these two values is;
VernierLength = SegmentLength * Cos(Mitre Angle)
Photo 2
Initially, the depth gauge is extended to meet the blade and the readout set to zero. For each ring set the depth gauge to the
calculated vernier length.
The segmented Turning Helper software calculates both segment length and vernier length values. You should use the vernier
length value in setting the cutting distance for the segment. This distance could be measured off with a ruler and your own stop
clamped in position. Other arrangements are also possible but some simple to use device should be built to enable the vernier
measurement to be set quickly and accurately. The photographs above show what I built.
If you are cutting your segments accurately you can glue the whole ring at once. Otherwise glue half ring sections together. When
dry, check the matching ends of the half circles and square the ends on a disk sander, using 120 grit disk, if necessary.
PVA, polyvinyl acetate glues are very common and very good for gluing wood. You could also use Titebond II which sets relatively
quickly. Apply the glue to the face being glued preferably with a small brush. Bring the corresponding segments together, slide the
two segments being glued, once or twice, and fix into position by bringing the two points of the segments together accurately.
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Completed rings or half ring should be left to dry for 4 to 6 hours before being handled. When gluing segments I use non-stick
baking paper allowing the paper to be easily peeled off when the assembly of segment is dry. The underside of the ring, that is the
side which will be glued onto the base or a previously assembled ring should be inspected and cleaned of any excess glue or
unevenness. Sanding of the bottom of the ring may be required if the ring is uneven. I use a belt sander to do this. The two surfaces
being glued as you begin to stack your rings should be perfectly clean and flat.
After each ring has been constructed and glued onto the partially completed object and left to dry it should be placed on the lathe
and the face trued up before adding the next ring. A very fine cut, with a bowl gouge, across the face or sanding with 120 grit block
across the face should be sufficient. I use a sanding block made with 120 grit paper glued to a piece of MDF of size 300x100x20 mm.
Turn your bowl or vase progressively as you stack the rings. My preference
is to glue three or four rings at a time and then turn both the inside and
outside of the object, as shown in photo 3.
Photo 3 Partially Turned Bowl
References:
1.
2.
For further information about cutting segments refer to Alan Mc Naught’s article, Cutting Segments on a Table Saw,
Australian Woodworker, July/Aug 2013, Issue No 170.
See the document; www.turnedwood.com/framesled.html for instructions on constructing a mitre saw sled.
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