The final jig runs in the left miter slot on the table saw
and is used to cut the pieces a specific length at the required 10° angle.
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| Table Saw & Jig |
With one exception it’s pretty straightforward to
build. I started with a piece of
hardwood cut to fit snugly in the miter slot in the table saw and screwed it to
a particle board base. One problem I did
run into was that I was having a hard time making the runner run smoothly
along the entire length of the miter slot.
The runner would have a good fit and then jam depending on where it was
in the miter slot. Some close examination
and measuring revealed the miter slot was not a consistent width from end to
end. There were also little scallops of
varying size remaining all along the slot from when it was machined.
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| Scallops in Table Saw Miter Slot |
The fix was to take a 10” metal file hold it flat against
the sides of the miter slot and carefully file the face getting rid of the high
spots.
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| Filing Edge of Miter Slot |
It took less than 10 minutes and made all the
difference. I just had to hold the file
square and keep an even pressure on it from one end of the slot to the other.
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| Filed Miter Slot |
With the runner mounted to the base I marked the saw blade
path through the jig, mounted front and rear braces keeping the screws away
from the blade path. Hitting a screw
with my good cutoff blade would be an expensive mistake and make me say bad
words. I then made the
initial cut in the jig going only far enough into the jig so as to be able to cut
the segments. The last piece to be mounted to
the jig is the fence. This is the most
critical part and the one that you can count on taking some time to get just
right. Process is to mark the 10 degree
angle off of square to the saw blade, align the fence to the line and on the
right side screw the fence to the base with one screw. That screw that will act as a pivot and allow
for the fine tuning to come. Next, swing
the left side of the fence down so the angle is just slightly more than the 10°
and on the left side of the fence screw a stop block to the base below the
fence. The photo below will help make
this all clear.
The fence will still rotate because it’s only fastened at
one end. This is planned so that the
needed fine adjustments can be made.
Next, clamp the fence to the block to hold it in place for our test
cuts.
Prior to making the test cuts I added a couple of toggle
clamps. The one on the outside sets the
segment Outer Face Length and the other clamps the blank in place when it gets
short. The second clamp also acts as a
guard to help keep my fingers out of the blade path.
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| Both Toggle Clamps Installed |
Before setting up the jig I ripped a scrap piece of plywood
1½” wide for my test blank. The right toggle
clamp is then used to set the stop block for the Outer Face Length at about
1¼”. I did add a thin spacer under the
stop block to help keep sawdust from getting trapped and impacting the
accuracy. Cutting process is to first trim the end of the blank to get one angle.
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| End of Blank Trimmed to 10 Degree Angle |
Second, flip the blank over and cut the first segment.
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| First Test Segment Cut |
Third, is to flip the blank over and cut the second
segment. The process is repeated until
you have cut 18 pieces to be put together for a test fit.
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| Second Segment Cut |
Why 18 pieces and not 12 since I am making a bowl with 12
segments per layer? It’s because the
bowl is an open segmented piece.
Remember for an open segmented piece each solid segment covers 20
degrees plus the 10 degree gap equals a total arc covered of 30 degrees. So, 12 pieces times 30 degrees each equals
the 360 degrees in a circle. However,
for our accuracy test we need to make a solid ring. With each piece cut at 20 degrees I need 18
pieces to equal 360 degrees. Making the
test fit requires clamping up the 18 pieces in a ring. For me using a hose clamp works the
best.
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| First Set of Segments Test Fitted |
Getting a perfect fit requires some trial and error since
there are so many cuts. Just think, for
the above ring there are 18 pieces with two angled cuts. This means that whatever error you have per cut gets
multiplied by 36. This is why a tiny error in each cut becomes a real problem. Because I set the stop block so the fence was
just a bit more than 10 degrees I can sneak up on the correct angle by adding
thin shims between the stop block and the fence. Playing cards to start with and masking tape
at the end. It is an iterative process
of cutting pieces, assembling for a test fit and adjusting the fence as
needed. When the ring fits together with
no gaps the fence is securely screwed to the base. The good thing is that you only have to do
this once. When the jig is adjusted
perfectly the pieces go together in a circle and there are no gaps at any of
the joints like this ring.
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| Final Test Ring - No Gaps! |
Next up – Material Selection, Preparation and Setup
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